'Hi mill 11' lllllllHIl !i!!!!!:!!!il i;llinilUIIIIIHlllli;!Miilllil>l;!!l:l!li;ili iiliiiiiiiiiiliifl ih l!!i i ! !li:ili!ili'l';lljllllinii(!l Class _S_5i3_L Book . L 7 5 CQESRIGHT DEPOSm Bgricultural Science Series L. H. BAILEY, Editor THE NATURE AND PROPERTIES OF SOILS AGRICULTURAL SCIENCE SERIES UNDER THE EDITOESHIP OF L. H. BAILEY THE NATURE AND PROPERTIES OF SOILS, by T. Lyttleton Lyon and Harry 0. Buclcman THE NATURE AND PROPERTIES OF SOILS A COLLEGE TEXT OF EDAPHOLOGY T. LYTTLETON LYON FEOFESSOB OF SOIL TECHNOLOGY, COENELL UNIVEESITT HARRY O. BUCKMAN PEOFESSOE OF SOIL TECHNOLOGY, COENELL UNIVEESITT THE MACMILLAN COMPANY 1922 All rights reserved FEINTED IN THE UNITED STATES OF AMERICA Copyright, 1922, By the MACMILLAN COMPANY. Set up and electrotyped. Published April, 1922. APR 19 1922 0)C!.A659657 TABLE OF CONTENTS CHAPTER PAGE I. Some Conceptions of the Soil and Its Relation to Plants 1 II, Soil Forming Processes 16 III. The Geological Classification of Soils 38 IV. The Soil Particle and Certain Important Relations 66 V. The Organic Matter op the Soil 99 VI. The Colloidal Matter of the Soil 127 VII. Soil Structure and Its Modification 139 VIII. The Forms of Soil Water and Their Characteristics 151 IX. The Water op the Soil in Its Relation to Plants . 184 X. The Control of Soil Moisture 202 XI. Soil Heat 223 XII. Soil Air 247 XIII. The Absorptive Properties of Soils 263 XIV. The Soil Solution 275 XV. The Removal op Nutrients from the Soil by Cropping AND Leaching 289 XVI. Chemical Analysis op Soils 311 XVII. Alkali Soils 328 XVIII. Soil Acidity 345 XIX. Liming the Soil 362 XX. Soil Organisms, Carbon, Sulfur and Mineral Cycles 384 XXI. Soil Organisms — the Nitrogen Cycle 409 XXII. Commercial Fertilizer Materials 442 XXIII. The Principles of Fertilizer Practice 471 XXIV. Farm Manure 499 XXV. Green Manure 535 XXVI. The Maintenance of Soil Fertility 552 Index op Authors 561 Index op Subject Matter 567 V NATURE AND PROPERTIES OF SOILS CHAPTER I SOME CONCEPTIONS OF THE SOIL AND ITS RELA- TIONS TO PLANTS Due to the action of climatic agencies the outer solid por- tions of the earth readily pass into a loose and disintegrated condition. This layer, although superficial and insignifi- cant in comparison to the bulk of the earth, has performed and is still performing a marvelous function. Life on the earth has been slowly but steadily developing and changing until we see about us the forms that characterize our age. This evolution has depended to no small degree on this super- ficial layer of decomposed rock with its admixture of de- caying organic matter which together form the soil. In this medium many and varied organisms have lived and from it have drawn, wholly or in part, their sustenance, leaving as a recompense a contribution of organic debris, which in its turn has given rise to reactions of almost unbelievable com- plexity. Like the life which it has sustained and nourished, the soil has been changing and evolving. The soil of today is not the soil of yesterday nor will it be the soil of tomorrow. It is never still. It is continually seeking a mechanical and chemical adjustment with the forces which surround it or 1 2 NATURE AND PROPERTIES OF SOILS are active within its precincts. Such an equilibrium it never attains and thus the evolution goes on and on. It is this continual change and this endless response to environment that makes the soil useful to plants. The disintegrating rock and the decaying organic additions are thus converted into a mechanical support for plants, while at the same time they are forced to liberate the nutrients essential to plant growth. In the light of its origin and function the soil may be defined as a mixture of broken and weathered fragments of rock and decaying organic matter, which covers the earth in a thin layer and supplies mechanical support and in part sustenance to plants. This debris of rock and plant residue, teeming with its microscopic life and ever restless in its endless efforts at equilibrium, is the arable soil from which man must obtain his bread. As the light of investigation is thrown on it, new changes, new functions and new and unsuspected re- lationships are brought to view until the story of the soil may be retold with a clearer insight into those processes that render it useful to man. 1. Composition of the soil. — The soil as defined is com- posed of two general classes of material, mineral and organic. The former in most cases makes up from 90 to 99 per cent, by weight of the dry substance of a soil, the organic matter, except in the case of peat and muck, being in rela- tively smaller amounts. In spite of the low proportion of organic matter its presence is vital, not only because of its influence physically but because of the nutrients, especially nitrogen, that it carries. The mineral portion of a soil functions as a frame-work and as a source of certain chem- ical elements, which are necessary to proper crop growth and development. It must be realized at the very outset that the two main constituents in a normal soil exist in very intimate relation- SOME CONCEPTIONS OF THE SOIL ship, reactions occurring not only within each group but between the groups as well. Unless such interactions take place it is unlikely that the mixture will ever be in a con- dition either chemically, physically or biologically to sus- tain plant growth. These reactions, although very complex, take place with surprising ease and rapidity. As a con- sequence the study of this complex, heterogeneous and highly dynamic mass that we call the soil is often be- set with difficulties that completely baffle our pres- ent facilities for its study. 2. Soil-forming rocks.' — In any study of soil origin or composition, how- ever cursory, the geological phases of the problem im- mediately force attention. This is due to the bearing that certain geological phe- nomena have on soil condi- tions and crop growth. In the soil we find that the inorganic materials have originated from the com- mon rocks. The best known country rocks are of course involved because they present the greatest outcrop surface and of necessity must contribute most to the mineral fabrication of the soil. They are classified under three heads — igneous, sedi- mentary and metamorphic. The most important types from the standpoint of soil formation are the following : * For excellent nontechnical discussions of rocks and minerals: — Pirsson, L. V., Eocks and Eock Minerals; New York, 1915. Merrill, G. P., Eocks, Eock Weathering and Soils; New York, 1906. ORGANIC- 10% Fig. 1. — Volume composition of a loam soil when in good condition for plant growth. The air and water in a soil are variable and their proportion determines to a considerable degree the productiv- ity. 4 NATURE AND PROPERTIES OF SOILS Igneous Sedimentary Metamorphic Granite Limestone Marble Syenite Dolomite Schist Diorite Shale Slate Gabbro Sandstone Quartzite Basalt Conglomerate Gneiss The mineralogical complexity of rocks has an important bearing on the question of soil formation and soil composi- tion. The fragments of any soil are, for the most part, dis- tinguishable as separate minerals rather than as mineral aggre- gates. For example, a soil from a granite would be char- acterized by separate grains of quartz, orthoclase, micro- cline and perhaps mica rather than by fragments of the orig- inal granite itself. Again, it is the composition of the easily decomposable minerals rather than the composition of the bulk rock that determines what simplifications shall occur, what new substances shall arise in the soil and what elements shall be liberated for plant use. 3. Soil minerals. — Although hundreds of minerals have been identified, comparatively few are common or important ^ in rock formation. As a consequence, the list of im- portant minerals found in soils will be correspondingly cur- tailed, although enough are always present, especially in the finer portions, to make the soil very complex mineralogically. The minerals as to origin may be divided into two groups: (1) those that persist from the original rock and (2) those that are produced by the decomposition of the original min- erals, during soil formation. For example, the quartz grains ' The following table indicates the approximate proportions of the common minerals in the earth's crush to a depth of ten miles: Feldspars 57.8% Clay 1,0% Amphibole and Py- Carbonates 5 roxene 16.0 Limonite 2 Quartz 12.7 All others 8.2 Mica 3.6 Kecalculatcd from Clarke, F. W., Data of Geochemistry ; U. S. Geol. Survey, Bui. 695, pp. 32-33. 1920. SOME CONCEPTIONS OF THE SOIL of soil almost always come directly from the original rock as do particles of orthoclase, biotite, and apatite. Hematite, the kaolinite group and the chlorite and epidote groups generally originate in soils through weathering. The fol- lowing list of minerals is by no means complete, yet it includes the more important forms from the soil and plant standpoint. A LIST OF THE MOST IMPORTANT SOIL MINERALS.* (The elements in bold type are those necessary for plant nutrition.) 1. Quartz 2. Orthoclase and Microcline feldspar 3. Muscovite mica 4. Biotite mica 5. Plagioclase feldspar 6. Calcite and Dolomite 7. Hornblende and Augite 8. Olivine 9. Apatite 10. Kaolinite group 11. Serpentine and Talc 12. Chlorite group 13. Epidote group 14. Hematite 15. Limonite group SiOs KAlSiaOs KH^AlgSigOio KHMgFeAl/sigOia Ca and Na aluminum silicates CaCO,, and (Ca, Mg) CO3 Ca, Mg, Fe aluminum silicates (Mg, Fe),SiO, Ca^ (P0J3(C1, F) Typified by kaolinite. H.Al^Si^Og Hydrated Mg silicates Hydrated Mg, Fe aluminum silicates Hydrated Ca, Fe aluminum silicates Fe^Og Typified by limonite 2 FCoOg. 3 H.O * Below are some of the most important mineralogical investigations of soil: McCaughey, W. G., and Williams, H. F., The Microscopic De- termination of Soil-Forming Minerals; U. S. Dept. Agr., Bur. Soils, Bui. 91. 1913. Plummer, J. K., Petrography of Some North Carolina Soils and Its Relationship to their Fertilizer Requirements, Jour. Agr. Res., Vol. V, No. 13, pp. 569-581. 1915. Robinson, W. O., The Inor- ganic Composition of Some Important American Soils; U. S. Dept. Agr., Bui. 122. Aug., 1914. 6 NATURE AND PROPERTIES OF SOILS 4. Importance of soil minerals. — Quartz is found in al- most all soils, making up often from 80 to 90 per cent, of the composition, although a range from 40 to 70 per cent, is more common. Its universal presence is due to its hard- ness and insolubility. Quartz is a make-weight material, however, as it probably contributes but little to plant nutri- tion. In the form of sand, quartz has a great influence on the friability of soil, improving and maintaining the phys- ical condition to a marked degree. Orthoclase, microcline, muscovite and, to a lesser degree, biotite are important because of their potash content.^ They decompose, often rather readily, into kaolinite and similar products, thus liberating potassium in soluble form. The plagioclase feldspars also give rise to kaolinite. They carry, however, sodium and calcium. The latter element^ plays an important role in soil both as a nutrient and as an amend- ment. When not sufficiently active it must be applied in some form. Calcite and dolomite also carry calcium. Horn- blende and augite bear calcium as well as magnesium and iron. Olivine is a magnesium and iron silicate. The oxida- tion of the iron of the above minerals gives rise to hematite, so common as a red coloring matter of soil. Practically all of the phosphorus of the soil, either organic or inorganic, has its origin in apatite, yet this mineral occurs but sparingly either in rock or soil. It makes up but 6 per cent, of igneous rocks. This accounts for the small percent- age of phosphoric acid in most soils and explains why it is often added in fertilizers.^ *PIummer, J. K., Availahility of Potash in Some Common Soil- forming Minerals, Jour. Agr. Res., Vol. XIV, No. 8, pp. 297-315. Aug., 1918. de Turk, E., Potassium-hearing Minerals as a Source of Potassium for Plant Growth; Soil Sci., Vol. 8, No. 4, pp. 269-301. 1919. ^Shorey, E. C. et al., Calcium Com,pounds in Soils; Jour. Agr. Res., Vol. VII, No. 3, pp. 57-77. Jan., 1917. ^ Fry, W. H., Condition of Phosphoric Acid Insoluble in Hydro- chloric Acid; Jour. Ind. and Eng. Chem., Vol. V, No. 8, pp. 664- 665. 1913. SOME CONCEPTIONS OF THE SOIL 7 The members of the kaolinite group are decomposition prod- ucts resulting from the decay of the feldspars and similar minerals. While kaolinite itself shows no nutrients in its formula, it often carries considerable calcium, potassium, magnesium and phosphorus by absorption. Moreover, its close association with other decomposition products such as serpentine, talc, chlorite and epidote tends to accentuate its importance in plant nutrition. The plasticity and cohesion imparted to a soil by the presence of the kaolinite group and its associated minerals are of great practical importance as is also the capacity to hold, either physically or chemically, the bases already mentioned. Hematite and limonite are simple iron compounds and usually occur in the soil as a result of the decomposition of certain iron-bearing minerals such as biotite, hornblende and augite. These iron compounds impart the red and yellow colors so characteristic of certain southern soils. Most of the soluble iron of the soil has its source in these minerals. Hema- tite and limonite are produced by the same general processes as are the kaolinite group and are found in very intimate contact with the serpentine, epidote, chlorite and kaolinite. 5. Soil organic matter. — One of the essential differences between a normal fertile soil and a mass of rock fragments lies in the organic content of the former. The organic matter practically all comes from plants and animals that have in- vested the surface of the soil and the soil material. Through the agency of bacteria and other organisms with which the soil is liberally supplied, this organic tissue quickly loses its original form, and becomes the dark incoherent material so noticeable in fertile soils. The decay is not one of immediate simplification, as might be supposed. The split-off compounds react not only with materials of a similar origin but also with the decomposing mineral fragments. This tendency pro- vides the intimate relationship between the organic and in- organic constituents of the soil already emphasized as an ex- 8 NATURE AND PROPERTIES OF SOILS ceedingly desirable condition. Incidentally the soil is ren- dered thereby very much more difficult to study, especially chemically. The incorporation of organic matter in any soil, either by natural or artificial means, tends, if the proper decay occurs, to make the soil more friable. The water capacity is markedly increased and the vigor of the bacterial and chemical activ- ities stimulated to a marked degree. As these two latter actions progress, some of the organic matter passes into simple combinations, allowing certain elements to become available to crops. Nitrogen, which is held in the soil largely in organic combination, emerges in the form of ammonia, nitrites and nitrates. It is from a salt of nitric acid that most plants absorb their nitrogen. Small amounts of sulfur, phosphorus, potassium and calcium are liberated from the tissue as decay proceeds. The largest product of organic decay, however, is carbon dioxide (CO2), which in the soil becomes important as a solvent for minerals, thus hastening the decomposition processes. 6. Factors for plant growth. — The growth and develop- ment of a plant depends on two sets of factors, the internal and external. The latter may be classified as follows: (1) mechanical support, (2) heat, (3) light, (4) oxygen, (5) water, and (6) nutrients.^ With the exception of light, the soil supplies, either wholly or in part, all of these conditions. Mechanical support is a function entirely of the soil. The comparatively loose and friable condition presented by most soils allows ample foothold to the ramifying roots. Air and water are easily supplied because of the open condition of the soil, and its large pore spaces. Temperature depends almost wholly on climatic relationships. The water ^ Nutrients are materials from which food may be elaborated once they have been absorbed by plants. The energy for this synthetic proc- ess comes from the sun. A food is any substance from which the plant may obtain energy for its normal processes. A large proportion of the materials absorbed by plants are nutrients. SOME CONCEPTIONS OF THE SOIL 9 of the soil acts as a plant nutrient in itself and functions also as a solvent for other materials. By its circulation it not only promotes solution but it continually brings nutrient elements in contact with the absorbing surfaces of the roots. The two prime functions of the soil are thus realized through the factors discussed above — mechanical support and a suffi- cient supply of certain nutrient elements under favorable conditions. 7. Nutrient elements.^ — Although the physical condition of the soil exerts a far-reaching influence on plant growth, the relationships involved are more readily understood than those which have to do with plant nutrition. Moreover, the solubility of the necessary nutrients is very closely related to the complex processes of soil formation. Ten elements^ are usually considered as necessary for plant growth. If one is lacking, normal development will not occur. They may be classified as follows: From air or water From the soil Carbon Nitrogen Calcium Oxygen Phosphorus Magnesium Hydrogen Potassium Sulfur Nitrogen Iron Plants obtain most of their carbon and oxygen directly from the air by photosynthesis and respiration. The hydro- gen comes, at least partially, from water. All of the other elements, except a small amount of nitrogen utilized directly from the air by certain plants, are obtained from the soil. It must not be inferred, however, that the bulk of the plant ^ For an excellent discussion of the functions of plant nutrients, see Kussell, E. J., Soil Conditions and Plant Growth, Chap. II, pp. 30-46; New York. 1915. * It may be possible that manganese and silicon and possibly chlorine and fluorine function as nutrients. They as well as sodium, aluminum, titanium, barium, strontium, and certain rarer elements are found in plant ash. 10 NATURE AND PROPERTIES OF SOILS tissue is fabricated from the soil. Quite the reverse is true. Fresh plant tissue generally carries only from .5 to 2.5 per cent, of mineral material. In spite of this, it is the mineral elements of nutrition that generally limit crop growth since a plant can always obtain, except in cases of drought or disease, unlimited amounts of carbon, hydrogen and oxygen. 8. Primary nutrient elements. — While all of the seven soil nutrients must be available that plants may grow normally, only four or five are likely to become limiting factors. The others are almost always in great sufficiency. These few, nitrogen, phosphorus, potassium, calcium and occasionally sulfur, receive as a consequence especial attention. They may limit growth because they are actually lacking or be- cause their availability is low. These conditions often occur in the same soil. Combined nitrogen exists in the soil to a large degree as a part of the partially decayed organic matter present therein.^ As decay proceeds, small quantities of this nitrogen appear as ammonia in combination with some acid radical such as the chloride or sulfate or with the hydroxal group. Later, it is changed through further bacterial action to the nitrate form, united with some bases such as calcium or po- tassium. It is from this latter combination that most plants obtain the greater part of their nitrogen. These inorganic nitrogen compounds, present at any one time in a soil, are but a small proportion of the total soil nitrogen. The air both above the soil and that circulating within its pores has been the original source of all the combined nitrogen. Nat- ural processes have facilitated the combination which has been necessary for such a transfer. The encouragement of such ^ Certain rocks, particularly those of a sedimentary nature, carry considerable nitrogen. When such rocks weather, this nitrogen tends to become available. The organic matter, therefore, does not absolutely control the amount of nitrogen in a soil. Hall, A. D., and Miller, N. H. J., The Nitrogen Compounds of the Fundamental Bocks; Jour. Agri. Sci., Vol. II, Part 4, pp. 343-345. July, 1908. SOME CONCEPTIONS OF THE SOIL 11 fixation processes, especially those of a biological nature, is a feature of practical soil improvement. Phosphorus has its origin in the mineral apatite (Cag- (P04)3(C1,F)) and exists in the soil not only in this form but as tri-calcium phosphate (Ca3(P04)2), iron and alum- inum phosphates (FeP04 and AIPO4) and in certain other inorganic complexes. It also exists in organic combinations of a constantly varying nature. It probably is utilized by the plant as a simple phosphate such as the mono- or di-calcium salt (CaH^CPO^), and Ca.JlAl^O,)^^. Potassium, as already stated, occurs in the soil in orthoclase and microcline (KAlSigOg), in mica, especially muscovite (HjKAlaSigOia), and in other aluminum silicates, both hy- drated and non-hydrated. These complex forms supply potash to the soil solution and thus to the plant at a more or less rapid rate in the bicarbonate, carbonate, chloride, nitrate, and sulfate forms. Calcium, while necessary in the soil as a nutrient, also functions as an amendment in that it seems to preserve a proper soil reaction. It is possible that this relationship is as much nutritive as strictly chemical. Calcium exists in the soil in many minerals, of which calcite, plagioclase feldspar, horn- blende and augite are perhaps the most important. It is carried as an absorbed compound by kaolinite and similar materials. Calcium becomes available in the soil as the ni- trate, bicarbonate, chloride, phosphate, and sulfate. Sulfur is found in the soil in rather small amounts and generally forms a part of the organic matter. Inorganically it usually occurs as a sulfate combined with the common bases. In this form it is available to plants. The original source^ of most of the soil sulfur has been pyrite (FeSg), the ' Considerable sulfur is brought to the soil in atmospheric precipita- tion. From 5 to 150 pounds an acre a year have been reported. Wilson, B. D. Sulfur Supplied to the Soil in Eain Water, Jour. Amer. Soe, Agron., Vol. 13, No. 5, pp. 226-229. 1921. 12 NATURE AND PROPERTIES OF SOILS commonest sulfide of this element. Although sulfur is no more abundant in the average soil than phosphorus, it is generally not considered as an extremely important fertilizing constituent. It is interesting to note at this point the amounts of the above elements in ordinary mineral soils. Generally the nitro- % 5% 10% \^% 80.11 SiOs rc^Oj+AlgOj— 9.01 HazO 2.0 ■ K2O 1.5 ■ CaO .61 MgO- P205- SO5- N — -.51 -.11 -.11 ORGAN IC- FiG. 2. — Chemical composition of a representative productive soil. gen (N) may range from .1 to .2 per cent., the phosphoric acid (expressed as PgOg) from .05 to .30 per cent, and the potash (expressed as KgO) from 0.5 to 2.0 per cent. Of the plant nutrients in the soil nitrogen, although usually present in small quantities, is relatively more available than is phosphoric acid or even potash. Phosphoric acid may be in the minimum because of its unavailability as well as because of the small quantity. Potash is commonly present in rela- SOME CONCEPTIONS OF THE SOIL 13 tively large amounts. Its occurrence in complex and insoluble silicates makes its availability of vital consideration. The presence of abundant organic matter may have much to do with the liberation of sufficient potash for vigorous plant growth. The amount of lime (expressed as CaO) in soils is difficult to state with any degree of satisfaction because of a very wide range in composition. Some soils carry only a fraction of a per cent., while others, especially those formed under conditions where an originally high calcium content has been maintained or where calcium has accumulated, show as much as 10 or 12 per cent. The variability of the sulfur is much less. A range from .02 to .30 per cent, of sulfur (expressed as SO3) will include most soils. It is interesting at this point to note the average composi- tion of thirty-five representative American surface soils^, which were studied by the United States Bureau of Soils dur- ing a systematic investigation of the arable lands of the United States east of the Rocky Mountains. A comparison of these data with those setting forth the composition of the litho- sphere ^ may be made with profit. (Table I, page 14.) It is immediately noticeable that silicon, aluminum, and iron make up the greater portion of both soil and lithosphere and that the nitrogen, sulfur and phosphorus are particu- larly low in both cases. Magnesium, calcium, sodium, and potassium occur in fair amounts, especially in the earth's crust. It is noticeable also that the soil is much higher than the lithosphere in silicon, nitrogen, organic matter, and car- bon but much lower in all of the other constituents. These differences have developed as a result of the losses and gains during soil formation. ^Robinson, W. O. et al., Variations in the Chemical Composition of Soils; U. S. Dept. Agr., Bui. 551. June, 1917. * The Lithosphere refers to the solid portion of the earth, in this case to a depth of ten miles. Clarke, F. W., Data of Geochemistry ; U. S. Geol. Survey Bui. 695, p. 33. 1920. 14 NATURE AND PROPERTIES OF SOILS Table I Comparison of the Chemical Composition of American Surface Soils with that of the Lithosphere. 35 American Composition of Constituents ^ Surface Soils lithosphere SiO^ 84.67 59.77 AI3O3 6.73 14.89 TiO^ .66 .77 Fe^Os 2.53 6.25 MnO .06 .09 Na,0 .49 3.25 K26 1.03 2.98 CaO .40 4.86 MgO .27 3.74 P2O3 .09 .28 SO3 .09 .28 Nitrogen .07 a — Organic Matter 2.61b — Carbon 1.51c .03 (a) Average of 22 soils only, (b) Average of 13 soils only, (c) Calculated from the organic matter. 9. The soil and the plant. — As the soil considered agri- culturally is essentially a medium for crop production, its rational study has to do with the consideration and applica- tion of such scientific principles as have a bearing on prac- tical soil management. Anything that makes clearer the relationships between soil and crop has a proper place. Un- less a scientific phase has a crop relation, either directly or indirectly, it need receive but scant consideration. The com- position of the soil, its chemical and biological changes, its physical peculiarities and its reaction to certain additions must receive especial attention. More knowledge of the soil ^ Soils contain many other elements, although in small amounts, such as chlorine, barium, csesium, chromium, lithium, molybdium, rubidium, vanadium, etc. Robinson, W. O., The Inorganic Constituents of Some Important American Soils; U. S. Dept. Agr., Bui. 122. Aug., 1914. SOME CONCEPTIONS OP TPIE SOIL 15 will mean better systems of management and will allow the farmer to fulfill to a greater degree his duty to himself and to the State — the production of paying crops and the passing on to the next generation of a soil depleted as little as possible in fertility. CHAPTER II SOIL-FORMING PROCESSES The forces which have to do with soil formation are largely climatic in nature. They promote the physical and chemical breaking down of rock masses, they intermix there with the decaying organic matter and they shift the products from place to place. Even after the soil is apparently at rest and has become an effective agency in plant production, these same forces are still much in evidence. The physical and chemical evolutions through which mineral and organic mate- rials at or near the earth's surface are passing due to natural forces are spoken of as weathering} Erosion and deposition are terms referring to the natural translocations which soils and soil materials are frequently forced to undergo. If a soil represents a condition more stable than the rock, the rock change is in that direction. If a soil presents con- stituents or conditions not wholly stable to the forces effective at that particular time, it in turn seeks a change by an altera- tion or an elimination. A cycle of development is thus set up proceeding from youth to adolescence and even into old age. According to conditions, soils may age rapidly or slowly. Rejuvenation may even occur, while cases of arrested develop- ment may exist for short periods. 10. Soil-forming processes classified. — While weather- ing, with the changes in form and composition which inva- riably accompany it, profoundly affects topography, it is very * The term weathering is somewhat misleading since it comprehends forces other than those generally considered as weather. All of the forces involved, however, depend upon climatic conditions. 16 SOIL-FORMING PROCESSES 17 superficial in comparison to the earth's bulk. Nevertheless, the weathered mantle, in spite of its comparative insignifi- cance, presents an effective medium for plant growth. The agencies of formation, therefore, demand more than the brief mention just given. These forces are geologic when the soil is being evolved, but once the soil materials are in place, the actions become localized and the influences may be considered as soil processes rather than more broadly geological. The soil-forming processes^, while diverse both in action and product, may be classified under two heads, mechanical and chemical. The former is often designated as disintegra- tion, the latter as decomposition. • SOIL-FORMING PROCESSES I. Mechanical (disintegration) A. Erosion and deposition. Water, ice and wind.^ B. Temperature change. Differential expansion of minerals, exfoliation and frost. C. Biological influences. Plants and animals. II. Chemical (decomposition) A. Oxidation and deoxidation. B. Carbonation and decarbonation. C. Hydration and dehydration. D. Solution. 11. The mechanical action of water. — From the time that that water as rain beats down upon the solid earth until it is finally discharged into the ocean, there to pound as waves upon the bordering lands, it is moving, sorting, and rework- ing the products of weathering. Water to erode must be ' For a complete and detailed discussion of soil formation, see Merrill, G. P., Bocl'S, Eoclc Weathering and Soils; New York. 1906. Also, Emerson, H. L., Agricultural Geology; New York. 1920. ^ Gravity is generally included in this group. While indirectly of great significance in soil formation, its direct action is not of great importance and is adequately disposed of in paragraph 27. 18 NATURE AND PROPERTIES OF SOILS armed. Its cutting power, therefore, depends on the amount of sediment that it carries and on its velocity of flow. Erosion by water deserves particular attention, as its denud- ing effects are very rapid when geologically viewed. Most of the changes in topography are due to such activity. The material swept away is partly in suspension and partly in solution.^ The Appalachian Mountains, whose uplift was complete in Carboniferous times, have lost vastly more of their mass than now remains in view. While most of the debris from the ancient erosive cycles has been changed to rock or has become a noticeable charac- teristic of ocean water, remnants persist. To these remnants rivers, lakes and oceans are making, year by year, substantial additions. The cutting, carrying and depositing activity of streams produce alluvial soils of which the Mississippi flood plain is a well known example. Deltas built into oceans, lakes and gulfs represent stream activity under different condi- tions, while uplifted continental shelves are often bedded with erosive products. The delta and marine soils of the Atlantic and Gulf coastal plains afford examples of the latter types of soil production. Even the pounding, grinding and sorting activities of waves in ocean and lake are no mean factors in the mechanics of soil formation. 12. Glacial action. — Ice at the present time, especially in temperate regions, is of little importance in soil forma- tion. Nevertheless, at a comparatively recent date geolog- ically, it had much to do with the preparation and deposition of soil materials over great areas in central and northern North America, northern Europe and the British Isles. Dur- ing the Great Ice Age immense continental glaciers succes- sively invaded these regions, much as the ice cap is over- * The chemical denudation by streams is generally spoken of as corro- sion. Abrasion is applied to the wear of the stream load upon its channel and of the particules in suspension upon themselves. Erosion is a broader term including corrosion and abrasion as well as trans- portation. SOIL-FORMING PROCESSES 19 riding Greenland to-day. Of great thickness and weight and impelled southward by tremendous pressure, these ice sheets swept away the old soil mantle and ground the underlying rocks with irresistible energy. The heterogeneous debris, im- bedded in the ice, only served to enhance the cutting power of the slowly moving mass. Hundreds of square miles were covered and as the ice was often several thousand feet thick, mountains as well as hills were over-ridden. (See Fig. 3.) In the melting back of these tremendous ice sheets, the accumulated debris was of necessity left behind. When the ice retreat was rapid, the deposit was comparatively thin and uniform. When a halt occurred, the material was left in irregular hummocks. It is hardly necessary to state that the soil developed from the former deposit is the more important agriculturally, due to its level topography and wide extent. The area of the latter is fortunately small. The streams flowing from the ice fronts were no insignificant feature of the glacial phenomena. Such streams were heavily laden with sediment, which was distributed far and wide in regions miles beyond the ice front. In whatever manner the glacial debris was laid down it is necessary to note that such deposits were soil material, not soil. Chemical action in all its complexity and the interven- tion of plants and animals, especially the former, were neces- sary before a true soil could be born, a soil still in its youth and covering in the United States alone over 500,000 square miles. (See Fig. 3, page 20.) 13. The influence of wind. — AVind, like water and ice, has both cutting and carrying power. The fluting of rocks, the polishing of stones, and the undermining of cliffs are of such frequent note as to require but brief mention. There seems no escape from the conclusion that wind is engaged in rock disintegration. Its geological function in arid regions seems similar to that of running water in humid lands. It is, however, as a transporting agency of fine materials NATURE AND PROPERTIES OF SOILS ^^^lO' Fig. 3. — Sketch map of North America showing the approximate south- ward extension of the great ice sheets and the three centers of accumulation. SOIL-FORMING PROCESSES ^1 that the wind is of especial importance in soil formation. The movement of sand and dust in both humid and arid regions is almost incessant. In desert storms 200 tons of materials liave been known to float over every acre of land. The finer particles travel for miles in a very short time. Southern Italy has received as much as one inch of dust from Africa during a single storm. The movement of sand dunes is but another evidence of the transporting power of air in motion. Wind as an agency in soil formation would perhaps receive much less attention were it not for the existence of large areas of a certain silty soil called loess. This soil exists along the Rhine both in France and Germany, in southern Russia, in Roumania, in China and in central United States. This material, as well as the adobe of our arid Southwest is con- sidered as largely wind laid. Since the loess is highly fertile and of great agricultural importance, added attention is thus directed towards wind as a soil-forming agency. (See Fig. 4.) 14. Change in temperature. — Variations of temperature, especially if sudden or wude, greatly augment the denuding actions of water, ice, and wind. Rocks and soil become heated during the day and at night often cool much below the tem- perature of the air. This warming and cooling is particularly effective as a disintegrating agent. Rocks are mineral aggre- gates, the minerals varying in their coefficients of expansion. With every temperature change differential stresses are set n\), which eventually must produce cracks and rifts, since the minerals never assume their original position. Incipient focii for further physical and chemical change are thus established. Although the expansion coefficient of rock is low, it must be remembered that very large surfaces are involved. Moreover, it is the multiplicity of the rifts rather than their magnitude that is important. The influence of temperature change is manifested on rocks in another way. Due to slow conduction the outer surface of a rock often maintains a markedly different temperature 22 NATURE AND PROPERTIES OF SOILS than the inner and more protected portions. This differential heating tends to set up lateral stresses which may cause the surface layers to peel away from the parent mass. This phe- nomena is spoken of as exfoliation. The differential expansion S^Vrn DAKOTA . M^^^^SOTA NEBRASK# \ i^"^ Fig. 4. — Approximate distribution of loess in central United States. of the rock minerals of course plays a part in this disintegra- tion, although exfoliation readily occurs in rocks which are more or less homogeneous. While this form of weathering may go on alone, it is much accelerated by chemical action and the prying of freezing water. SOIL-FORMING PROCESSES 23 One peculiarity of pure water is that its maximum density occurs at 39.2 deg. F. From this point the volume increases as the temperature is lowered. Ice, which forms at 32 deg. F., thus occupies a greater space than the water from which it was derived. The force developed by freezing is equivalent to about 150 tons to the square foot or a pressure of 141 atmospheres. The cracks and crevices of surface rocks in humid regions are from time to time filled with moisture. Rocks below the surface contain water continuously. The change of this water from a liquid to a solid always produces marked disintegi'ation. Mountain-top rubble, talus slopes, alluvial fans, and similar formations are evidences of such action. The load of sediment carried by streams is often due to the prying action of temperature change, especially where crevice water is present. This action of temperature is by no means ended when a soil is produced. Freezing and thawing is of tremendous im- portance in bettering the physical condition, especially of heavy soils. It is to such forces that the farmer owes the good tilth of his land. In addition it must be noted that the rapidity of chemical change is largely a function of temper- ature. The concentration of the soil solution and the avail- ability of the nutrient elements thus come under the influence of this apparently simple force. 15. Plants and animals. — While plants and animals unite their activities with the processes already mentioned, their influence is conflned largely to the soil and the soil material. Simple plants such as mosses and lichens grow upon exposed rock, there to catch dust and dirt until a thin film of highly organic material accumulates. Higher plants sometimes exert a prying effect on rock, which results in some distintegration. Such influences, however, are of but little import in soil for- mation compared to the drastic activities of water, wind, ice and temperature change. In the soil, roots by their ramifications promote aeration 24 NATURE AND PROPERTIES OF SOILS and drainage, as well as an accumulation and distribution of organic materials. Lichens, mosses, and algge play their parts in a similar manner. It must be noted, however, that while plants tend to preserve and improve the soil tilth, their action in this respect is not wholly physical. Decay due largely to bacterial action is necessary before the accumulated organic matter can improve to any marked degree the physical con- dition of the soil. This is only one of the many examples illustrating the cooperation of physical and chemical changes incident to soil formation. Animals influence the soil physically by their burrowing propensities. Gophers, squirrels, ants, and the like mix and open up the soil, thus providing for the circulation both of air and water. Other soil forces, both physical and chemical, are markedly encouraged thereby. Earth worms produce similar effects. They not only pass great quantities of soil through their bodies, but they carry much to the surface. This has been estimated as amounting to one or two surface inches in a decade. Man also is producing important physical changes on the soil and soil material. The plowing under of green-manures, crop residues and farm manure, the addition of lime and fertilizers and the tillage incident to cropping have much to do with the physical changes, which are con- tinually occurring in the soil. 16. Oxidation and deoxidation. — Scarcely has the disin- tegration of rock begun than its decomposition is also appar- ent. This is especially noticeable in humid regions where the chemical and physical processes of soil formation are par- ticularly active and markedly accelerate each other. Of the chemical forces, oxidation is usually, especially near the sur- face, the first to be noticed. It is particularly manifest in rocks carrying iron in the sulfide, carbonate or silicate forms. The sulfide, although widespread, is less important in pro- moting rock decay than the other combinations. The oxida- tion of iron in any form is indicated by a discoloration of the SOIL-FORMING PROCESSES 25 affected roek, which from the first is streaked with iron oxide. The mica, amphibole, pyroxene and garnet groups are par- ticularly affected, until, as the process continues, these min- erals waste away into unrecognizable forms so weakening the rock as to cause it to crumble easily. The way is now open for vigorous chemical and physical changes of all kinds. Oxi- dation may be illustrated chemically, using olivine as the mineral decomposed. It is to be noted that the first step is the assumption of water and the production of serpentine and ferrous oxide. The latter quickly changes to the susquioxide. 3MgFeSi04+2H20=H,Mg3Si209-fSi02+3FeO Olivine Water Serpentine Silica Ferrous Oxide 4FeO + Oo = 2Fe203 (red) Ferrous Oxygen Ferric Oxide Oxide Deoxidation is the reverse of oxidation, being a reduction of the amount of oxygen present in the compound. With hematite it might occur as follows : 2Fe203 — O2 = 4FeO Ferric Oxide Oxygen Ferrous Oxide In a similar way, other oxides and salts may be reduced by the withdrawal of oxygen. This action occurs in poorly drained soils or in soil very rich in organic matter. It is generally apparent in forest soils just below the organic sur- face layer. Here the leaching downward of small quantities of organic acids has been sufficient to develop a definite grey- ish zone, varying both in color and depth. The bleaching of sands, shales, sandstones, and clays may often be due to deoxidation rather than the actual removal of ferric iron. No great importance need be attached to deoxidation either in soil formation or in the chemical processes which continue to affect the soil after it is definitely developed. 26 NATURE AND PROPERTIES OF SOILS 17. Carbonation. — The process of oxidation is almost al- ways accompanied by the action of carbon dioxide. This gas is a constituent of the air and is a product of the organic decay which vigorously progresses in most soils. It occurs in large amounts in rain water, especially in warm climates. It increases the solvent action of water by actively engaging in chemical reactions, producing carbonates and bicarbonates with the various rock and soil bases. The decomposition of orthoclase and muscovite mica into kaolinite and carbonates is as follows: 2KAlSi,0s + 2HoO + CO, = H^AUSi^Og + K0CO3 + 4Si02 Orthoclase Water Carbon Kaolinite Potassium Silica Dioxide Carbonate 2H2KAl3Si30i. + CO, + 4H2O = SH.Al^SiaOg + K2CO3 Muscovite Carbon Water Kaolinite Potassium Dioxide Carbonate Under certain conditions decarbonation may occur. When- ever the processes of weathering produce either mineral or organic acids carbonates are rapidly decomposed. The presence of unsaturated aluminum silicates may also rapidly promote decarbonation by absorbing the base and liberating the acid radical. This latter reaction is of especial importance in soil. 18. Hydration. — All the chemical transformations above discussed depend on the presence of a certain amount of water, especially if rapid changes are to occur. The illus- trative reactions already cited indicate this. Oxidation pro- ceeds but slowly in a dry atmosphere, water being necessary as a catalytic agent. In the carbonation of the potash of orthoclase and mica, water enters into the reactions, produc- ing not only kaolinite but also potassium hydroxide, which is later changed to the carbonate. Water functions in the chemical changes of rock and soil SOIL-FORMING PROCESSES 27 ill another way — as water of combination.^ The process is called hydration. While hydration usually proceeds or ac- companies oxidation and carboiiation, thus making them pos- sible, it often, unlike these transformations, occurs at great depths and may be practically the only change that the rock minerals have undergone. Many minerals, especially the oliv- ine, feldspar and mica groups, are so affected. They become soft and lose their luster and elasticity on the assumption of this chemically combined water. Considerable increase of bulk occurs during the transition of the rock to soil. The latter change has no small physical significance. This hydra- tion is particularly effective in encouraging other kinds of chemical decay. In addition to the examples already cited, the change of hematite to limonite, which occurs to a greater or less degree in every soil wliere the sesquioxide is present, is worthy of note : 2Fe203 -f- 311,0 = 2Feo03 . 3H,0 Hematite Water Limonite (yellow) When the products of weathering dry out due to varying weather conditions, dehydration may occur. Thus limonite may readily reduce to a lower hydrate or to hematite. 19. Solution. — It is quite evident that while weathering and erosion produce many compounds of a very complex char- acter, there is a tendency toward simplification and, as water is universally present, some solution occurs. Such bases as calcium, magnesium, sodium and potassium are found in the water that circulates in rocks, soil materials and soils. These bases, when in solution, are generally combined as chlorides, phosphates, nitrates, carbonates, and the like. Carbon dioxide intensifies to a marked degree the solvent action of water and consequently increases its power as a weathering agent. The * Note carefully the difference between hydration and the production of an hydroxide. The former is the more important as a soil phenome- non. 28 NATURE AND PROPERTIES OF SOILS atmosphere carries about .03 per cent, of carbon dioxide by volume, while considerable amounts are brought down on rocks and soil by rain and snow. Traces of nitric and sul- furic acid are also found in rain water. The carbon dioxide produced within the soil by decaying organic matter keeps the concentration of this gas high at points where it can act most effectively. Solution/ accelerated both by mechanical and chemical means, is of particular importance in two directions. In the first place, it allows a continual loss of plant nutrients not only as the soil is being formed but after it becomes a proper medium for plants. This constant drain accounts for the deficiency of certain elements in the soil and the need in cer- tain eases of such additions as lime and fertilizers. On the other hand, this solution, however wasteful, is necessary since plants absorb nutrients from the soil only in soluble form. The concentration and composition of the materials in the soil water is thus a function of solution, which is a culmina- tion of the activities of the soil processes already discussed. 20. General statement of soil formation. — By a very complicated coordination the mechanical and chemical forces of weathering reduce the solid rock to small fragments and mix therein the necessary organic matter. The process slowly proceeds until a suitable medium for the growth of higher plants is produced. As a rule, the chemical processes are in- complete and all stages of decay are exhibited. This is for- tunate, as solution may thereby continue to renew the nutrients in the soil-water for a long period and thus maintain the continuous productivity of the soil. The products of disintegration and decomposition are com- monly classified into two general groups, sedentary and trans- * While the formula for water is generally given as H^O the molecule is not as simple as this, being at low temperature as high as (HjO)!, The remarkable power of water as a solvent may be due to extra oxygen valences as well as to the high dielectric constant which favors ioniza- tion, thus hastening chemical reaction. SOIL-FORMING PROCESSES 29 ported. The former remains in place, being the rock residuum in which organic matter accumulates. Residual clay is an example. The second group, on the other hand, in addition suffers transportation and is represented by the soils arising from glacial drift, alluvial accumulations, aeolian deposits, and the like. In the first case, the soil is derived from a single lithologic unit ; in the second place, the assorted and blended materials are from many sources. A general statement of the formation of a residual soil ^ is obviously the easier to Fig. 5. — The gradual transition of country rock into residual soil by weathering in. situ. make. Such a statement adequately covers every process in the production of a transported soil except the disintegra- tion, assortment, and solution due to translocation, (See Fig. 5.) "The changes that a rock undergoes in forming a residual soil are first a physical breaking down, accompanied by certain chemical transformations, which consist in the hydration of a portion of the feldspars, micas and similar minerals; the ^ Buekman, H. O., The Formation of Residual Clay; Trans. Amer. Cer. Soc, Vol. XIII, p. 362. Feb., 1911. 30 NATURE AND PROPERTIES OF SOILS oxidation and hydration of a part of the combined iron ; and a carbonation and solution of a large proportion of the soluble bases. These processes are hastened and the whole mass evolved into a soil by the admixture and decay of certain amounts of organic matter. ' ' ^ 21. Variation of soil formation with climate. — It may be seen readily that the activity of the various soil-forming agencies will fluctuate with climate. A comparison of weath- ering and erosion in an arid and a humid region will illustrate the point at issue. Under arid conditions, the physical forces will dominate and the resultant soil will be coarse. Tempera- ture changes, wind action and the influence of animals will be almost the sole agents. In a humid region, however, the forces are more varied and practically the full quota will be at work. Chemical decay will accompany disintegration and the result will be shown in the greater fineness of the product. The separate minerals will also show the change of color and loss of luster so characteristic of chemical action. A granite, for example, is a very insoluble rock, compared with a lime- stone, and in a humid region, where chemical agencies are dominant, it will be markedly more resistant. If, however, these rocks are exposed in an arid region, where physical weathering is potent, the results will be entirely different. The limestone, being homogeneous, will not be affected mark- edly by temperature changes, but the stresses set up in granite must ultimately reduce it to fragments. Arid soils, besides being rather coarse, are generally rather uniform, there being little difference between soil and subsoil. The soils of humid regions are usually of fine texture, par- ticularly in residual sections, since the chemical agencies have ^ It is well to remember that synthetic processes as well as forces of simplification and dissolution are active in soil formation. The soil features that result are of two kinds, hereditary and acquired. The former develop through geological forces, the latter through the activity of true soil processes. SOIL-FORMING PROCESSES 31 been so active. Various colors may develop because of oxida- tion, hydration, and the presence of organic matter. Such soils usually are not excessively deep, and are likely to be underlaid by subsoils heavier than the surface. The general physical condition and tilth of arid soil is uniformly better than that of regions of plentiful rainfall. Chemically, because of less leaching, the arid soils contain more of the important mineral elements. The following analyses bring out the differences in a striking manner : Table II COMPARATWE ANALYSES OF ARID AND ' HUMID SOILS^ Arid Soils Humid Soils CONSTITUENTS AVKRAGE OF Average of 313 Samples 466 Samples Insoluble 77.82 88.24 AI2O3 7.89 4.30 Fe,03 5.75 3.13 CaO 1.36 .11 K2O .73 .22 P3O. .12 .11 MgO 1.41 .23 Volatile 4.94 3.64 It is immediately apparent that the arid soil is poorer in silica than the humid soil, but richer in iron and alumina, in- dicating a less vt^eathered condition of the feldspars. Due to a greater amount of leaching, the humid soil is much lovt^er in phosphoric acid, lime, magnesia, and potash. The humus in arid soils is somewhat lower than in the soils under better ^Hilgard, E. W., Die Boden arider und humider Lander; Internat. Mitt. Bodenkunde, Bd. I, pp. 415-529. 1912. 32 NATURE AND PROPERTIES OF SOILS conditions of rainfall, as one would naturally expect. The amount of easily soluble material is higher in arid regions, due to the lack of rain and the tendency for soluble salts to accumulate. Biologically, organisms are active at greater depths^ in arid than in humid regions, because of the loose structure of arid soils and because of their good aeration. Such soils are seldom water-logged except from improper ir- rigation. In humid regions bacterial action is limited very largely to the surface foot of soil, since only there are the aeration and the food conditions adequate. The intensity of biological activity in arid soils is very largely governed by moisture, and when moisture conditions are satisfied, bacterial changes may be expected to take place rapidly. 22. Special cases of soil formation. — Having compared the weathering of granite and limestone under different cli- matic conditions, it is interesting to note the quantitative chem- ical changes of these rocks as they are reduced residually to soil under humid conditions. The following analyses- indicate the elements that are likely to be lost to the greatest extent during the process. (See Tables III and IV, page 33.) The soil resulting from the decay of the granite was a deep red clay, with numerous quartz grains present. The soil from the limestone was very plastic and high in silicate silica. Leaching has probably gone on to a very great extent in both soils. It is noticeable in both cases that the bases, such as calcium, magnesium, sodium, and potassium, have suffered severe losses. The carbonate has almost wholly disappeared from the limestone clay, indicating that a residual soil from such a rock will probably need an application of lime. (See Figs 6 and 7, pages 34 and 35.) ^Lipman, C. B., The Distribution and Activities of Bacteria in Soils of the Arid Region; Univ. Calif., Pub. in Agr. Sci., Vol. I, No. 1, pp. 1-20. 1912. ' Merrill, G. P., Weathering of Micaceous Gneiss; Bui. Geol. Soe. Amer., Vol. 8, p. 160. 1879. SOIL-FORMING PROCESSES Table III FRESH GRANITE AND ITS RESIDUAL CLAY 33 Constituents EOCK Soil Percentage LOST^ SiOa 60.69 45.31 52.45 A1303 16.89 26.55 .00 Fe303 9.06 12.18 14.35 CaO 4.44 .00 100.00 MgO 1.06 .40 74.70 K,0 4.25 1.10 83.52 Na^O 2.82 .22 95.03 P2O3 .25 .47 .00 Ignition .62 13.75 gain Table IV VIRGINIA LIMESTONE AND ITS RESIDUAL CLAY ^ Constituents Rock Soil Percentage * Lost SiOg 7.41 57.57 27.30 Al,03 1.91 20.44 .00 Fe^Og .98 7.93 24.89 CaO 28.29 .51 99.83 MgO 18.17 1.21 99.38 K,0 1.08 4.91 57.49 NaaO .09 .23 76.04 P2O5 .03 .10 68.78 CO2 41.57 .38 99.15 H2O .57 6.69 gain *The percentage loss of any constituent is calculated as follows: A X 100 = X 100 — X = % Lost. BX 'Diller, J. A = % any constituent in residual material. B = % same constituent in fresh rock. C = % of the constant constituent in residual soil. D = % of the constant constituent in fresh rock. S.. Educational Series of Bock Specimens; U. S. Geol. Survey, Bui. 150, p. 385. 1898. 34 NATURE AND PROPERTIES OF SOILS The analyses indicate that the soil from the granite does not differ greatly from the original rock, except in the loss of bases, assumption of water, and increase of organic matter. The soil from the limestone presents greater differences, due '60.6ci=z==^z2i::::"_v_".:::::"^ ,38.71 AlgO'; CaO + l ^'^ MtjO KgO .4 1 ( 4.Z l.l (2.8C 1.2. NazO 60 CZ3 ' ' GRANITE ■{^RESIDUAL SOIL 1^.7 Fig. 6. — Diagram showing the composition of fresh granite and its residual soil. Note the marked hydration of the soil. to the disappearance of the calcium carbonate. The analyses of the two soils resemble each other rather closely in spite of their widely different sources. Since weathering, especially residual weathering, causes a loss of basic materials and SOIL-P^ORMING PROCESSES 35 thereby favors the accumulation of silica, alumina and iroii, all soils as they age tend to approach each other in chemical composition. Yet, owing to a differe^ice in the adjustment of the forces at work and to the time element, no two soils % S'°2 {5X6 F«2 05+12.90=] Ca0+r46.0l MgO 1 1.71 coe 1^'; 4t.6C 41 LIMESTONE RESIDUAL S Proteoses — > Peptones — > Peptides /^ ^Amino-acids * Sehreiner, O. and Shorey, E. C, The Isolation of Harmful Substances from Soils; IT. S. Dept. Agr., Bur. Soils, Bui. 53, 1909; also Buls. 47, 70, 74, 77, 80, 83, 87, 88, and 90. See also, Sullivan, -M. X., Oripin of Vanillin in Soil; Jour. Ind. & Eng. Chem., Vol. 6, No. 11, pp. 919-921, 1914. Kelley, W. P., The Organic Nitrogen of Hawaiian Soils; Jour. Amer. Chem. Soc, Vol. XXXVI, No. 2, pp. 429-444, Feb., 1914. Walters, E. H., Proteoses and Peptones in Soils; Jour. Ind. & Eng. Chem., Vol. 7, No. 10, pp. 860-863, 1915. Lathrop, E. C, Protein Decomposition in Soils; Soil Sci., Vol. I, No. 6, pp. 509-532, June, 1916. 108 NATURE AND PROPERTIES OF SOILS Hentriacontane — CsJiei Histidine — C^HpOgNg Dihydroxystearic acid — Trithi()ll)eiizaldeliyde — C,J1,,0, (C,33CSH)3 Succinic acid- — CJleO^ Creatinine — C4H.ON3 Picoline carboxylic acid — Salicylic Aldehyde — C.H.O^N C,H,OHCOH 57. Relation of organic compounds to plants. — So far as the plant is concerned, organic compounds may be divided into three groups: those that are beneficial, those that are neutral, and those that are toxic or harmful in their effects. As an example of the first group, histidine and creatinine ^ may be mentioned. Here is a case in which the compounds in the soil organic matter may exert a stimulating effect on plant growth, supplementing the nitrates ^ to a certain extent. That the nitrogen of the soil organic matter may be utilized by plants is well summarized by the publications of Hutchin- son and Miller.^ As an example of a harmful compound aris- ing from the decomposition of the organic matter, dihydroxy- stearic acid may be mentioned as one of the best known. This compound was the first to be isolated and identified by the Bureau of Soils and is very toxic. The discovery of such compounds in the soil has revived the old theory of toxicity,* by which the infertility of certain soils was accounted for. Root excretions were also held to be detrimental to succeeding crops of the same kind. The toxic materials of the soil organic matter largely originate under ^Skinner, ,J. J., Effect of Histidine and Arginine as Soil Constituents ; Eighth Internal. Cong. App. Chem., Vol. XV, pp. 253-264, 1912. Also, Beneficial Effects of Creatinine and Creatine on Growth; Bot. Gaz., Vol. 54, No. 2, pp. 152-163, 1912. ^Schreiner, O., and Skinner, .T. ,J., Nitrogenous Soil Constituents and Their Bearing upgn Soil Fertility; U. S. Dept. Agr., Bur. Soils, Bui. 87, p. 68, 1912. Also, Schreiner, 0., and Others, A Beneficial Organic Con- stituent of Soils; Creatinine ; U. S. Dept. Agr., Bur. Soils, Bui. 83, p. 44, 1911. ^ Hutchinson, H. B., and Miller, N. H. J., The Direct Assimilation of Inorganic and Organic Forms of Nitrogen hi/ Higher Plants; Jour. Agr. Sci., Vol. 4, Part 3, pp. 282-302, 1912. *See Schreiner, 0., and Eeed, H. S., Some Factors Influencing Soil Fertility; U. S. Dept. Agr., Bur. Soils, Bui. 40, pp. 36-40, 1907. THE ORGANIC MATTER OF THE SOIL 109 conditions of poor drainage and aeration. The toxicity of such compounds as dihydroxystearie acid, picoline carboxylic acid and aldehydes may, therefore, be overcome by oxidation.^ Good soil aeration is a factor in dealing with such conditions. Fertilizers, according to Schreiner and Skinner,^ seem to decrease the harmful effects of such compounds; nitrogenous fertilizers overcoming some toxic materials, and phosphoric acid or potash neutralizing others. Robbins ^ has shown that soil organisms have the power of causing the disappearance of certain toxic materials in the soil, such as cumarin, vanillin, pyridine, and quinoline. While. Schreiner found twenty soils, out of a group of sixty taken in eleven states of this country, to contain dihydroxy- stearie acid, this does not necessarily mean that this or sim- ilar compounds are serious detrimental factors. It is very likely that such compounds are merely products of improper soil conditions, and are to be considered as concomitant with depressed crop yield. When such conditions are righted, the so-called toxic matter will disappear, as has been shown by the researches of Davidson.* Good drainage, lime, tillage, aeration, and oxidation, are so efficacious in this regard that permanent organic soil toxicity need never be a factor in soils rationally managed, ^Schreiner, O., and Others, Certain Organic Constituents of Soils in delation to Soil Fertility ; U. S. Dept. Agr., Bur. Soils, Bui. 47, p. 52, 1907. Also, Schreiner, O., and Reed, H. S., The Bole of Oxidation in Soil Fertility; U. S. Dept. Agr., Bur. Soils, Bui. .56, p. 52, 1906. * Schreiner, O., and Skinner, J. J., Organic Compounds and Fertilizer Action; U. S. Dept. Agr., Bur. Soils, Bui. 77, 1911. Also, Experi- mental Study of the Effect of Some of the Nitrogenous Soil Constituents on Growth; Plant World, Vol. 16, No. 2, pp. 45-60, Feb., 1913. ' Robbins, W. J., The Cause of the Disappearance of Cum-arin, Vanillin, Pyridine and Quinoline in the Soil; Ala. Agr. Exp. Sta., Bui. 195, .June, 1917. Also, The Destruction of Fa7iillin in the Soil by the Action of Soil Bacteria; Ala. Agr. Exp. Sta., Bui. 204, June, 1918. Robbins, W. J., and Massey, A. B., Tlie Effect of Certain Environmental Conditions on the Bate of Destruction of Fanillin by a Soil Bacterium; Soil Sci., Vol. X, No. 3, pp. 237-246, Sept., 1920. * Davidson, J., A Comparative Study of the Effects of Cumarin and Vanillin on Wheat Grown in Soil, Sand and Water Culture; Jour. Amer. Soc. Agron., Vol. 7, No. 4, pp. 145-158, 1915. 110 NATURE AND PROPERTIES OF SOILS 58. Simple products of organic decomposition. — As the processes of chemical and biological change of the soil organic matter proceed, the simple compounds already noted begin to appear. This change is of course coordinate with a certain amount of synthetic action, but compounds thus built up must ultimately succumb to the agencies at work and suffer a splitting-up and reduction to simple bodies. Carbon dioxide is one of the most important of these compounds, always being a product of bacterial activity. Its importance has already been noted in the discussion of weathering. Here it heightens the solvent power of water and tends to increase the amount of nutrient material carried in the soil solution. Carbonation is a direct result of its presence. With increased organic matter in any soil, greater bacterial action and an increase in the carbon dioxide evolved may well be expected. In fact, the carbon dioxide production of a soil is considered by some authors ^ to be a measure of bacterial activity. With this increase in carbon dioxide, the soil air is markedly reduced in its free oxygen and an alteration in bacterial and plant relationships may thereby be induced. The following figures by Wollny ^ show the composition of the soil atmosphere and the effects of additional organic ma- terial on the carbon dioxide content : Table XXI Soils Percentage by Volume of CO, Atmospheric air .04 2.54 1.06 9.74 20.96 Soil air (average 19 analyses) A sandy soil 18.33 19.72 Sandy soil plus manure 10.35 ^Stoklasa, J., and Ernest, A., trher den Ursprung, die Menge, und die Bedeutung des KoMendioxyds im Boden; Centrlb. Bakt., II, 14, Seite 723-736, 1905. ^Wollny, E., Die Zersetsung der Organischen Stoffe; Seite 2, Heidel- berg, 1897. THE ORGANIC MATTER OF THE SOIL 111 While carbon dioxide may be evolved by the splitting-up of both carbohydrate and nitrogenous bodies, ammonia re- sults only from the latter. It is really the first extremely simple nitrogenous body produced. It can be utilized by some plants as a source of nitrogen, as is also true of certain products of partial decomposition such as urea, but ordinarily it must undergo oxidation. This oxidation results in nitrites (NO2) and ultimately in nitrates (NO3), the latter usually being considered as the chief source of the nitrogen utilized by plants. Other simple products, such as methane (CH^), hydrogen disulphide (HoS), carbon disulphide (CSg), and the like, may also result. They are relatively unimportant, however, as regards the plant, in comparison with the role played by car- bon dioxide, ammonia, the nitrites, and the nitrates. The production of the nitrates from ammonia is very closely cor- related with good soil conditions, especially optimum moisture and adequate aeration. The proper handling of the soil, then, will not only tend to eliminate toxic matter and prevent its further formation but will encourage the proper decay of the soil organic matter and the production of simple compounds which will function directly or indirectly as nutrients. 59. Carbonized materials of soil. — After the extraction of the soil for the study of the ordinary organic compounds, a considerable mass of material remains, which is insoluble in water, alkali, and other ordinary solvents. By the extrac- tion of a large amount of soil, Schreiner and Brown ^ were able to study this material. They found it susceptible to di- vision into six groups, as follows: (1) plant tissue, (2) insect and other organized material, (3) charcoal particles, (4) lig- nite, (5) coal particles, and (6) materials resembling natural hydrocarbons, as bitumen, asphalt, and the like. Such ma- * Schreiner, 0., and Brown, B. E., Occurrence and Nature of Carbon- ized Material in Soils; U. S. Dept. Agr., Bur. Soils, Bui. 90, 1912. 112 NATURE AND PROPERTIES OF SOILS terial was found not only near the surface of the soil but at depths of fifteen or twenty feet. The exact origin of this material is problematical. Forest and prairie fires, infiltration, mild oxidation, and lignifica- tion might be mentioned. Of a certainty the agencies of dis- tribution are the natural forces engaged in physical weather- ing. Such material can be divided into two general groups, organized and unorganized; in the former, the normal struc- ture remains intact, while in the latter the original features have been obliterated. Part of it belongs, therefore, in the original plant tissue group ; a part of it with the partially de- cayed material ; while some must be included with the simple products of decomposition. This carbonized material is im- portant, as it makes up no inconsiderable part of the soil organic matter. It is very resistant, and consequently lends stability to the organic constituents. 60. The determination of soil organic matter.^ — A num- ber of methods have been proposed for the direct or indirect determination of the organic matter in soils, but none has proved entirely satisfactory, since the composition of this ma- terial is so indefinite and complicated and so likely to change while under investigation. Other soil constituents also tend to interfere with the determination. Three general methods seem worthy of mention, as they have been used very widely in soil analyses and at least give comparative, if not absolutely accurate, results. They will be discussed in the inverse order of their value. Loss of ignition.^ — This is a simple method which designs to burn off the organic matter and determine its loss by dif- ference. Five grams of dry soil are placed in a crucible and ignited at a low red heat until the organic matter is all oxi- ^Soil organic matter as here used refers only to the original and partially decayed organic constituents. Carbon dioxide^ methane, nitrites, nitrates and similar compounds are, therefore, not included in this term. ^ Wiley, H. W., Official and Provisional Methods of Analysis; U. S. Dept. Agr., Bur. Chem., Bui. 107, p. 19, 1908. THE ORGANIC MATTER OF THE SOIL 113 dized. The cold mass is moistened with ammonium carbonate and heated to a temperature of 150°C. in order to expel the excess of ammonia and replace the carbon dioxide. The change in weight is rated as loss on ignition. This method is open to the objection that, besides the loss of organic matter, a certain amount of water of combina- tion, and all ammoniacal compounds, nitrates, carbon dioxide, and some alkali chlorides, if the temperature is carried too high, are driven off. The method, therefore, gives high results, especially in the presence of large amounts of hydrated sili- cates such as are likely to occur in residual soils. Notwith- standing these objections, this method has been used to a very great extent in soil analysis.^ Chromic acid me^/ioc?.— This method, proposed by Wolff, has been modified and improved by various chemists. War- ington and Peake - have perhaps done more with the method than any other investigators. In the United States the modi- fication by Cameron and Breazeale ^ has been very generally accepted.* It consists in the treatment of the soil sample with sulfuric acid, and chromic acid, or potassium bichromate. The organic matter, in the presence of the sulfuric acid and an oxidizing agent, evolves carbon dioxide until, if the mix- * Eather offers a modification to this method which seems to obviate some of its difficulties. The soil is first extracted with dilute HCl and HF to remove the hydrated aluminum silicates, the organic matter being little influenced thereby. The sample is then ignited in the usual manner. Rather, J. B., An Accurate Loss-on-Ignition Method for the Betermination of Orqanic Matter in Soils; Jour. lud. and Eng. Chem., Vol. X, No. 6, pp. 439-442, June, 1918. ^ Warington, R., and Peake, W. A., On the Betermination of Carbon in Soils; Jour. Chem. Soc. (London), Trans., Vol. 37, pp. 617-625, 1880. ^Briggs, L. J., and others, The Centrifugal Methods of Meclianical Soil Analysis; U. S. Dept. Agr., Bur. Soils, Bui. 24, pp. 33-38, 1904. Also, Cameron, F. K., and Breazeale, J. F., The Organic Matter in Soils and Subsoils; Jour. Amer. Chem. Soc, Vol. 26, pp. 29-45, 1904. ••Waynick offers a simplification of this method: Waynick, D. D., A Simplified Wet CombiLstion Method for the Betermination of Carbon in. Soils; Jour. Ind. and Eng. Chem., Vol. XI, No. 7, pp. 634-637, 1919. 114 NATURE AND PROPERTIES OF SOILS ture is boiled, practically all of the carbon is thus driven off. This gas is drawn through a train of absorption bulbs, caught in a solution of potassium hydroxide, and thus weighed. A second determination is now made on a new sample of soil, leaving out the chromic acid. The carbon dioxide given off under such conditions is that of an inorganic nature. The weight of this gas substracted from the total carbon dioxide leaves the organic carbon dioxide. The data from the use of the chromic acid method may be expressed as organic carbon or as organic matter. Multiply- ing the carbon dioxide by .471 or the carbon by 1.724 is con- sidered as giving an approximate figure for the organic mat- ter. The results obtained with the chromic acid method are usu- ally lower than those from ignition or combustion, due par- tially to the oxidation resistance of the carbonized matter, already discussed. This material, while it succumbs to igni- tion, resists the action of the sulfuric and chromic acids to a very large degi'ee. The water of hydration is, of course, not a factor in the chromic acid method. Bomh Combustion. ^ — Two grams of soil, .75 gram of mag- nesium powder, and 10 grams of sodium peroxide (Na^Oa) are thoroughly mixed in a closed dry calorimeter bomb. The mixture is then exploded by heating, all of the carbon of the soil being changed to the carbonate form by the reaction. The fused charge is now removed to a flask and by treating with acid, the carbon in the form of carbon dioxide may be driven off into a Parr apparatus and measured under stand- ard conditions of temperature and pressure. The amount of inorganic carbonate carbon in the soil must ^ Wiley, H. W., Official and Provisional Methods of Analysis; U. S. Dept. Agr., Bur. Chem., Bui. 107, p. 234, 1908. There are a number of other methods of complete combustion. Very often the combustion is carried on in a current of oxygen over hot cuprous oxide. The organic carbon may thus be very accurately determined. THE ORGANIC MATTER OF THE SOIL 115 be determined on a separate sample and deducted from the figure obtained by the combustion above described. This will give the organic carbon of the soil in terms of carbon dioxide. The percentage of organic carbon may now be calculated as well as the approximate amount of organic matter (C X 1.724 = organic matter or CO2 X -471 := organic matter.)^ 61. Determination of soil humus. — Humus ^ is a term ap- plied to that portion of the organic matter which can be re- moved with ammonium hydroxide after the soil has been treated with hydrochloric acid and washed free thereof. The common method of humus estimation is that proposed by Grandeau.^ The sample of soil is first washed with acid in order to remove the bases in combination with the organic mat- ter. It is next treated with ammonia, which will then dissolve out the humous materials. The method is based on the fact that when a soil is lacking in active basic material, certain parts of the organic matter are soluble in an alkali. The dark humous extract obtained with the ammonia is called Matiere Noire and is supposed to be the most active part of the soil organic matter. This method has undergone several modifications * of which * Wiley presents the following comparisons of the three methods dis- cussed above: Soil Ignition Combustion (c X 1.724) Chromic acid (c X 1.724) Old pasture New pasture 9.27 7.07 5.95 6.12 4.16 2.44 4.84 3.32 Arable soil 2.03 Wiley, H. W., Principles and Practices of Agricultural Analysis, Vol. 1, pp. 352-354, Easton, Pa., 1906. ^ The term ' ' humus ' ' is used in a number of different ways. Conti- nental Europeans make it synonymous with organic matter. In some cases it is used to indicate all of the partially decayed material of the soil. The restricted meaning employed in this text is less confusing as it coincides with the chemical interpretation. Grandeau believed the organic matter thus dissolved was a determining factor in soil fertility. "Grandeau, L., Traiti d' Analyse de Matieres Agricoles; I, p. 151, 1897. * A comparison of the various methods is found as follows : Alway, 116 NATURE AND PROPERTIES OF SOILS that of Hilgard ^ and that of Houston and McBride - seem most important. In the procedure an attempt is made to keep the concen- tration of the ammonia in contact with the soil constant dur- ing the extraction. Consequently the sample, after treatment with the acid, is washed into a 500 cubic centimeter flask, which is filled to the mark with 4 per cent, ammonia. Diges- tion is allowed to proceed for twenty-four hours, with fre- quent shakings. The solution is then filtered and evaporated to dryness. The residue is weighed, after drying thoroughly at 100° C, and then ignited, the loss being considered as humus. This method is open to serious criticism in that it is wholly arbitrary and subject to considerable inaccuracy through manipulation and the ignition of the humic residue. There is also some doubt whether the figures obtained have any direct relation to the fertility of the soil.^ 62. The organic matter and nitrogen of representative soils. — The amount of organic matter in soils varies so widely according to the nature of the soil and climate conditions that it is difficult to present representative figures. Excluding peat and muck, which are 20 to 80 per cent, organic, the aver- age mineral surface soil is found to contain from .50 per cent, to 18 or 20 per cent, of organic matter. Some surface soils of West Virginia,* averaging 2.88 per cent, organic matter, F. J., and others, The Determination of Humus; Neb. Agr. Exp. Sta., Bui. 115, June, 1910. ^ Hilgard, E. W., Humus Determination in Soils; U. S. Dept. Agr., Div. Chem., Bui. 38 (edited by H. W. Wiley), p. 80, 1893. ^Houston, H. A., and McBride, F. W., A Modification of Grandeau's Method for the Determination of Humus; U. S. Dept. Agr., Div. Chem., Bui. 38 (edited by H. W. Wiley), pp. 84-9^, 1893. See also. Smith, O. C, A Proposed Modification of the Official Method of Determining Humus; Jour. Ind. and Eng. Chem., Vol. 5, No. 1, pp. 35-37, Jan., 1913. 'Gortner, R. A., The Organic Matter of the Soil; III. On the Pro- duction of Humus from Manures; Soil Sci., Vol. Ill, No. 1, pp. 1-8, Jan., 1917. Carr, R. H., Is the Humus Content of the Soil a Guide to Fer- tility; Soil Sci., Vol. Ill, No. 6, pp. 515-524, June, 1917. * Salter, E. M., and Wells, C. F., Analyses of West Virginia Soils; W. Va. Agr. Exp. Sta., Bui. 168, Dec, 1918. THE ORGANIC MATTER OF THE SOIL 117 range from .73 per cent, to 15.14 per cent., while similar fig- ures on the Russian Tschernozen ^ vary from 3.45 to 16.72 with an average of 8.07 per cent. The subsoil of course runs lower in every case. The following figures, while far from representative, are suggestive: Table XXII PERCENTAGE OF ORGANIC MATTER (C X 1.724) IN CERTAIN REPRESENTATIVE SOILS OF THE UNITED STATES. Description Surface Subsoil 8 Residual soils — Robinson - 3 Glacial and loessial soils — Robin- son ^ 1.76 4.59 2.86 3.83 7.46 .64 144 2 Kansas till soils Call ^ 6 Nebraska loess soils — Alway * 30 Minnesota till soils — Rost and Alway ^ 1.98 1.96 1.88 As the soil nitrogen is carried almost wholly by the organic matter, and is a true organic constituent of the soil, its con- sideration at this point is opportune. The nitrogen ^ of soils varies with the organic matter and may range in surface mineral soils from .01 to .60 per cent. West Virginia ^ soils, ^ Kossowitsch, P., Die Schwarzerde ; Internat. Mitt. f. Bodenkiinde, Band I, Heft 3-4, S. 316, 1912. ^ Robinson, W. O., The Inorganic Composition of Some Important American Soils; U. a Dept. Agr., Bui. 122, 1914. ' Call, L. E., ei al^, Soil Survey of Shawnee County, Kansas; Kans. Agr. Exp. Sta., Bui. 200, 1914. * Alway, F. J., and McDole, G. R., The Loess Soils of the NeirasTca Portion of the Transition Region: I. Hygroscopiciti/, Nitrogen and Organic Carbon; Soil Sei., Vol. I, No. 3, pp. 197-238, Mar., 1916. ° Rost., C. O., and Alway, F. J., Minnesota Glacial Soil Studies; I. A Comparison of the Soils of the Late Wisconsin and lowan Drifts; Soil Sci., Vol. XI, No. 3, pp. 161-200, Mar., 1921. *Soil nitrogen is determined by either the Kjeldahl or the Gunning method. These will be described later. See paragraph 165. 'Salter, R. M., and Wells, C. F., Analyses of West Virginia Soils; W. Va. Agr. Exp. Sta., Bui. 168, Dec, 1918. 118 NATURE AND PROPERTIES OF SOILS for example, while averaging .147 per cent, nitrogen, range from .043 to .539. Louisiana ^ soils average .049 per cent, with a range from .001 to .109. In muck and peat the amount of nitrogen is much higher, attaining in some cases 3 per cent. The following figures indicate the nitrogen contents that may be expected in average soils: Table XXIII PERCENTAGE OF NITROGEN IN CERTAIN REPRESENTATIVE SOILS OF THE UNITED STATES Description Soil Subsoil 71 Cecil soils of North Carolina - . . . . 165 Norfolk soils of North Carolina ^. . 16 Loess soils of Central U. S.* 381 Kentucky soils ^ .048 .039 .154 .120 .338 .024 .020 .083 .070 30 Minnesota till soils ^ .092 While the ratio between the respective amounts of soil nitrogen and organic' matter is no more constant than that between the organic carbon and the organic matter (C X 1.724 = organic matter), it is of some general value. If ^ Walker, S. S., Chemical Composition of Some Louisiana Soils as to Series and Texture; La. Agr. Exp. Sta., Bui. 177, Aug., 1920. ^Williams, C. B., et at., Beport on the Piedmont Soils, Particularly with Beference to their Nature, Plant-food Requirements and Adaptor bility to Different Crops; Bui. N. C. Dept. Agr., Vol. 36, No. 2, Feb., 1915. ^Williams, C. B., et al., Beport on Coastal Plain Soils, Particularly with Beference to their Nature, Plant-food Bequirements and Suitability for Different Crops; Bui. N. C. Dept. Agr., Vol. 39, No. 5, May, 1918. *Eobinson, W. O., et al., Variation in the Chemical Composition of Soils; U. S. Dept. Agr., Bui. 551, June, 1917. Alway, F. J., and McDole, G. E., The Loess Soils of the Nebraska Portion of the Transition Re- gion: I. Hygroscopicity, Nitrogen and Organic Carbon; Soil Sei., Vol. I, No. "3, pp. 197-238, Mar., 1916. Also, Bennett, H. H., Soils and Agriculture of the Southern States, pp. 332-353 ; New York, 1921. " Averitt, S. D., The Soils of Kentucky; Ky. Agr. Exp. Sta., Bui. 193, July, 1915. "Eost, C. O., and Alway, F. J., Minnesota Glacial Soil Studies: I. A Compariso7i of the Soils of the Late Wisconsin and the loivan Drifts; Soil Sci., Vol. XI, No. 3, pp. 161-200, Mar., 1921. THE ORGANIC MATTER OF THE SOIL 119 the percentage of nitrogen in the soil is multiplied by 20, a rough idea of the amount of organic matter may be obtained (N X 20 == organic matter). The following data from Rost and Alway ^ illustrate not only the variations in organic matter and nitrogen that may be expected in the surface and subsurface of different soils, but the correlation between the organic matter and nitrogen just mentioned: Table XXIV AVERAGE PERCENTAGE OF ORGANIC MATTER (C X 1.724) AND NITROGEN IN THIRTY REPRESENTATIVE MINNESOTA TILL SOILS FROM THREE SERIES. THE FIGURES FOR EACH OF THE THREE SOIL TYPES ARE AVERAGES OF TEN ANALYSES. Dkpth Forest Carrington Loam Upland Prairie Carrington Silt Loam Lowland Prairie Fargo Silt Loam Organic Matter Nitro- gen Organic Matter Nitro- gen Organic Matter Nitro- gen 1 — 6 inches. . . 7—12 " 13 24 " 25 36 '' 5.34 2.41 1.38 .86 .253 .119 .078 .041 7.96 6.00 S.ll 1.31 .373 .285 .165 .062 13.08 8.00 3.24 1.39 .616 .385 .150 .054 The following tentative classification of mineral soils on the basis of their percentages of organic matter and nitrogen is offered for generalized field use : Table XXV Description Percentage of Organic Matter Percentage op Nitrogen Low .0— 3.0 3.0— 6.0 6.0 10.0 above 10.0 00— 10 Medium High .10— .25 25 40 Very high above .40 ^ Rost, C. O., and Alway, F. J., Minnesota Glacial Soil Studies: I. A Comparison of the Soils of the Late Wisconsin and the lotvan Drifts; Soil Sci., Vol. XI, No. 3, pp. 161-200, Mar., 1921. 120 NATURE AND PROPERTIES OF SOILS 63. The humus content of soils is of com-se lower than the organic matter contained in them. It likewise varies according to climate and region, not only in amount, but also in composition. The following data from Hilgard ^ and Alway - illustrate these points : Table XXVI THE COMPOSITION OF CALIFORNIA ARID AND HUMID SOILS. (hilgard) Description Humus in Soil (Percentage) Nitrogen in Humus (Percentage) Nitrogen in Soil (Percentage) 41 Arid uplands soils 15 Sub irrigated arid soils. . 24 Humid soils .91 1.06 4.58 15.23 8.38 4.23 .135 .099 .166 Table XXVII COMPARATIVE COMPOSITION OF SEMI-ARID (wAUNETA) AND HUMID (weeping water) LOESS SOILS OF NEBRASKA. ( ALWAY ) Organic Matter (Percentage) Humus (Percentage) Nitrogen (Percentage) Wauneta Weeping Water Wauneta Weeping Water Wauneta Weeping Water 1st foot. 2nd ".. 3rd "... 4th "... 5th "... 6th "... 2.77 1.38 1.09 .79 .55 .45 4.98 3.02 1.38 .83 .45 .36 1.02 .65 .48 .34 .26 .26 2.34 1.29 .55 .27 .23 .19 .136 M2 .065 .046 .038 .030 .236 .154 .083 .059 .043 .038 ^Hilgard, E. W., Soils, pp. 136-137; New York, 1911. For further data regarding Hilgard 's conclusions see: Alway, F. J., and Bishop, E. S., Nitrogen Content of the Humus of Arid Soils; Jour. Agr. Pies., Vol. 5, No. 20, pp. 909-916, Feb., 1916. ' Alway, F. J., et ah, The Loess Soils of the Nebraska Portion of the Transition Becjion: I. Hygroscopicity, Nitrogen and Organic Carbon; Soil Sci., Vol. I, No. 3, pp\ 197-238, Mar., 1916. //. Humus, Humus-Ni- trogen and Color; Soil Sci., Vol. I, No. 3, pp. 239-258, Mar., 1916. THE ORGANIC MATTER OF THE SOIL 121 It is eviden«t that Immid soils not only contain the greater amounts of organic matter, but also excel in humus. The humus of the arid regions, however, is richer in nitrogen, due to the character of the decomposition going on. As a conse- quence the nitrogen in the soil of humid regions is not greatly in excess of that in the soils of drier climates. The percentage of humus not only decreases in the lower depths of the soil, but also changes in composition, becoming poorer in nitrogen the deeper the soil. 64. The influence of organic matter on the soil. — The effects of organic matter on soil and plant conditions are as numerous as they are complex. Some of the influences are direct, others are indirect. As the specific gravity of organic matter is low, the first effect of its addition would be to lower the specific gravity of the soil. The organic matter tends also to spread the individual particles of soil farther apart, especi- ally in a clay. Such action will markedly influence the volume weight. The loosening effects of organic matter are especially ap- parent in such soil as clay* On the other hand, because or- ganic matter has a higher cohesive and adhesive power than sand, it performs the function of a binding material with the latter soil, a condition much to be desired in a material pos- sessing such loose structure. As the water capacity of organic matter is very high, a soil rich in organic constituents usually possesses a high water- holding power. This makes possible greater volume changes both on drying and in the presence of excessive moisture. The granulating effects of wetting and drying and freezing and thawing are, therefore, accelerated. The increased water ca- pacity of the soil, resulting from the presence of organic ma- terials, is of great importance in drought resistance, while the black color imparted by the humus tends to raise the heat absorptive, power of the soil. The better tilth induced by the presence of organic matter 122 NATURE AND PROPERTIES OF SOILS in any soil tends to facilitate ease in drainage and to encour- age good aeration. These two conditions are of course neces- sary for the promotion of soil sanitation. Root extension and bacterial activity are thus increased. It is of especial impor- tance that the splitting-up of the organic matter shall take place in the presence of plenty of oxygen, in order that toxic compounds may not be generated and that products highly favorable to plant growth should be formed. The soil organic matter, however, functions in other ways than those strictly physical and chemical. Its degradation products may serve as nutrients for higher plants. Bacteria and other soil organisms are also furnished a source of energy thereby and the production of carbon dioxide is much in- creased. This carbon dioxide, as well as the organic acids generated, tends to raise the capacity of the soil-water as a solvent, and thus the amount of mineral material available to the crop is greatly increased. The general effect of organic matter, then, is to better the soil as a foothold for plants, and to increase either directly or indirectly the available nutri- ent supply for the crop. 65. Maintenance of soil orgunic matter.^ — The mainte- nance of a proper supply of organic matter in a soil is a ques- tion of great practical importance, as productivity is gov- erned very largely by the organic content of the soil. This maintenance of the soil organic matter depends on two factors : (1) the source of supply and methods of addition; and (2) the promotion of proper soil conditions in order that the ^Snyder, H., Effect of the Rotation of Crops upon the Humus Content and the Fertility of Soils; Minn. Agr. Exp. Sta. Bui. 53, June, 1897. The Production of Humus in Soils; Minn. Agr. Exp. Sta., Bui. 89, Jan., 1905. Morse, F. W., Humus in New Hampshire Soils; N. H. Agr. Exp. Sta., Bui. 138, .June, 1908. Hopkins, C. G., Phosphorus and Humus in Relation to Illinois Soils; 111. Agr. Exp. Sta., Cire. 116. Feb., 1908. Thatcher, R. W., The Nitrogen and Humus Problem of Dry Farming; Wash. Agr. Exp. Sta., Bui. 105, June, 1912. Fippin, E. O., Nature, Effects and Maintenance of Humus in the Soil; Cornell Reading Course for the Farm, Vol. Ill, No. 50, Oct., 1913. Loughridge, R. H., Humus of California Soils; Calif. Agr. Exp. Sta., Bui. 242, Jan., 1914. THE ORGANIC MATTER OF THE SOHj 123 organic matter may perform its legitimate functions. The source of supply will be considered first. The organic matter of the soil may be increased in a nat- ural way by the plowing under of green crops. This is called green-manuring and is a very satisfactory practice. Such crops as rye, buckwheat, clover, peas, beans, and vetch lend themselves to this method of soil improvement. Not only do these crops increase the actual organic content of a soil, but in the case of legumes the nitrogen may also be in- creased in amount, if the nodule bacteria are present and active. Green-manures to be effective must be hardy, rapid in growth, succulent, and should produce abundant foliage. Rye and oats are particularly valuable from this standpoint. Such legumes as cowpeas, vetch, field peas, soybeans, and velvet beans are adapted to summer growth. Red clover or sweet clover, being a biennial, may be seeded one year and turned under the next spring. Oats and peas or rye and peas make a very good combination for fall green-manuring. Hairy or winter vetch may be seeded with rye in the autumn and used as a green-manure in the spring. In the South green-manur- ing crops may be utilized to much better advantage than in the northern states as the longer growing season permits the use of a green-manure following the normal harvest. Due to the tendency of bare soil to lose nutrients by leach- ing, especially in the summer and fall, it is always best to keep the land covered with vegetation of some kind. Cover- or catch-crops are used for this purpose, especially on sandy land, although they are profitable on heavier soils as well. Wheat on sandy land may be followed by cowpeas, which not only conserve nitrates but fix nitrogen from the air in addition. Rape, cowpeas, vetch, and soybeans are sometimes seeded in com at the last cultivation. When a soil receives clean culti- vation a part of the year, as is practiced very frequently in orchards, it is very desirable that a crop be plowed under oe- 124 NATURE AND PROPERTIES OF SOILS casionally to replace the organic matter lost by oxidation. Whether such catch-crops are pastured or turned under, they tend to increase the soil organic matter. Weeds, which spring up after the crop is harvested, are often valuable as cover- and catch-crops and when turned imder aid in maintaining the organic content of the land. Crop residues form no inconsiderable portion of the organic matter produced on the land. If such materials as straw, stubble, cornstalks, and the like are incorporated in the soil, much will be accomplished towards the upkeep of the organic matter. The burning of straw and cornstalks, especially in the Middle West, entails an enormous waste of carbon as well as of nitrogen. The value of crop residues has been demon- strated very conclusively by the Illinois Experiment Station ^ on their outlying experimental farms. At Bloomington, for instance, the turning under of crop residues for five years increased the wheat yields 4.4, 7.9 and 5.9 bushels in 1911, 1912 and 1913 respectively. Farm manure is one of the most important by-products on the farm and is especially valuable because of its organic mat- ter. Although only about one-fourth of the organic materials of the original food given the animal ever reaches the land, the use of such a by-product is worth while, since the carbon it contains comes from the air and not from the soil. The main losses that the carbon of the crop undergoes when thus util- ized are due to the digestive influences of the animal and to the leaching and fermentation which goes on in the manure. While sufficient manure ordinarily can not be produced from the crops grown on the farm to maintain the organic matter of its soil, the use of farm manure with green-manure and crop residues in a proper rotation is fundamental in good soil man- agement. 66. Organic matter and soil conditions. — Improper soil 'Hosier, J. G., and Gustafson, A. F., Soil Physics and Management, p. 171; Philadelphia and London, 1917. THE ORGANIC MATTER OF THE SOIL 125 conditions not only prevent tlie proper decay of organic mat- ter, but also tend to encourage the production of products in- imical to plant growth. Therefore, in order that organic ma- terials added to any soil may produce the proper decomposi- tion products and perform their normal functions, soil con- ditions in general must be of the best. Tile drainage should be installed, if necessary, in order to promote aeration and 50% LOSS ORGANIC MATTER Fig. 22.— Diagram showing tlie practical sources of the soil organic matter and the cycle through which its constituents pass. Note that the carbon, oxygen and hydrogen come very largely from air and water and that fixation of nitrogen may occur if the crop is a legume. Only about 25 per cent, of the organic matter fed to animals ever reaches the soil in farm manure under average con- ditions. granulation. Lime should be added if basic materials are lacking, for it promotes bacterial activity as well as plant growth. The addition of fertilizers will often be a benefit, as will also the establishment of a suitable rotation. The rotation of crops not only prevents the accumulation of toxic materials, but also, by increasing crop growth, makes pos- sible a larger addition of organic matter by green-manuring. 126 NATURE AND PROPERTIES OF SOILS 67. Resume. — An understanding of the complex organic relationships witliin the soil is of great practical value, as it determines to a large degree the yield of crops, their rota- tion order and their fertilization. Moreover, tillage operations must be varied according to the organic nature of the soil. Unless a system of soil management is adopted which will at least partially keep up the organic matter of the soil, crop yields may be expected to decrease materially in a few years. Good soil management seeks to adjust the addition of or- ganic matter, the physical and chemical condition of the soil, and the losses through cropping and leaching, in such a way that paying crops may be harvested while impairing the or- ganic supply of the soil as little as possible. Any system of agriculture that tends permanently to lower the organic mat- ter of the land is impractical and improvident, as well as un- scientific. CHAPTER VI THE COLLOIDAL MATTER OF THE SOIL^ Research in physics and physical chemistry is each day making it clearer that the properties of matter are by no means entirely determined by chemical composition. Matter varies in its physical character and its chemical activities with its fineness of division. Coarsely divided substances function much differently when they become molecular complexes and still more diversely when their aggregates are divided into their molecular and ionic components. Because of the par- ticular properties exhibited by material in a fine state of di- vision, approaching but not attaining a molecular simplifica- tion, a special name is utilized. A substance in such a con- dition is said to be colloidal or in the colloidal state. 68. The colloidal state - arises when one form of matter (either a gas, liquid, or solid) in a very fine state of division ^ Colloidal chemistry is now so well understood that it will be necessary to develop only those phases which have a direct bearing on soil phenomena. ^Sonie of the following general references may prove helpful: Eamann, E., Kolloidstudien bei Bodenlundlichen Arbeiten; Kolloid- chemische Beihef te ; Band II, Heft 8/9, Seite 285-303, 1911. Niklas, H., Die Kolloidchemie und Hire Bedeutung filr Bodenlcunde, Gcologie, und Mineralogie ; Internat. Mitt, fiir Bodenkunde, Band II, Heft 5, Seite 383-403, 1913. Bancroft, W. D., The Theory of Colloid Chemistry ; Jour. Phys. Chem., Vol. 18, No. 7, pp. 549-558, 1914. Taylor, W. W., The Chemistry of Colloids; New York, 1915. Burton, E. F., The Physical Properties of Colloidal Solutions; London, 1916. Zsigmondy, E,, The Chemistry of Colloids, Part I; trans, by E. B. Spear, New York, 1917. Wiegner, G., Boden und Bodenbildung ; Dresden and Leipzig, 1918. Bancroft, W. D., Applied Colloidal Chemistri/ ; New York, 1921. Thatcher, E. W., Chemistry of Plant Life; Chap. XV, New York, 1921. 127 128 NATURE AND PROPERTIES OF SOILS is distributed through a second, which may also be a gas, a liquid, or a solid. The material in the finely divided state is called the dispersed phase, while the matter containing it is designated as the continuous or dispersive medium. A very good example of a colloidal system occurs when very fine clay particles (solids) are suspended in water (liquid) or when an emulsion of oil and water is formed, the oil under certain conditions becoming the dispersed material, hetero- geneously disposed. The particles of material in a colloidal state in these cases are so small that they will not sink as long as conditions are stable. Moreover, they exhibit the Brownian movement,^ the oscillations increasing very rapidly as the size decreases. Such particles are molecular complexes and the solution is heterogeneous. In this respect a colloidal solu- tion differs from a true solution, which is homogeneous, the particles being molecules and often ions. 69. Size of colloidal particles. — The size of the particles of matter in a colloidal state vary with the material and with the conditions of formation. The diameters of material in a colloidal state are considered to range from 100 |i [i ^ (.0001 m.m.) to 1 n fx (.000001 m.m.). Above 100 \x \i suspended material is usually sinkable, while below 1 |i [x the particles generally become single molecules and a true solution is at- tained. Theoretically it would seem possible to pass from a suspension to a true solution without a break by a progres- sive subdivision of particles. There seems to be a discontinu- ity, however, between the colloidal state and a true solution. As the molecular complexes subdivide, they at last go into solution and may reprecipitate as coarser complexes, thus ^ Small particles, even those well within the range of ordinary micro- scopic vision, exhibit, when suspended in a liquid, an oscillating motion around a central position. This movement^ which is called the Brownian, is inversely proportional to the size of the particle. It is probably due to the bombardment of the molecules and ions of the liquid in which the particle is suspended. The Brownian movement is very slow for particles of a diameter of .001 mm. ^A micron (/i) = .001 mm. or 10-^ mm. A millimicron (liU/t)=: .000001mm. or 10* mm. THE COLLOIDAL MATTER OF THE SOIL 129 maintaining a considerable gap between the two states of matter.^ 70. The phases of a colloidal state. — As already empha- sized, two phases are necessary for a colloidal state — a dis- persive medium and a material that will heterogeneously disperse therein. Threee materials may function as a dis- persive medium — a liquid, a solid, or a gas. In the same way, with each dispersed material there are three possibilities — a liquid, a solid, or a gas. This gives eight general phases to be considered in colloidal chemistry.^ The liquid-solid and the liquid-liquid phases are by far the most important as far as soil materials are concerned. The dispersed materials of soil colloids are the minerals either in a hydrous or non-hydrous condition and the organic mat- ter in various stages of decay. The dispersive medium is of course the soil solution. 71. Colloids vs. crystalloids. — It must not be inferred, because the colloidal state is often wrongly contrasted with the crystalloidal, that material in a colloidal condition is al- ways amorphous. It is often crystalline. Moreover, it may be animate, as some bacteria are minute enough to function col- loidally. It is obvious also that the same chemical material may exist either in the colloidal or non-colloidal state. For example, silicic acid, hydrated ferric oxide, gold, carbon black, •Bancroft, W. D., Applied Colloidal Chemistry, p. 18.3; New York 1921. ' The eight phases with examples are : Solid in solid Carbon in steel. Liquid in solid water of crystallization Gas in solid gases in minerals Solid in liquid colloidal solution of metals Liquid in liquid emulsions of oil in water Gas in liquid air in water, foam Solid in gas smoke in air Liquid in gas clouds Gas in gas noncolloidal, merely a mixture of molecules. After Burton, E. F., The Physical Properties of Colloidal Solutions, p. 10; London, 1916. 130 NATURE AND PROPERTIES OF SOILS and other materials, may or may not be colloidal, according to circumstances. The fineness of division is the explanation of colloidal properties. In order to place such a discussion on a more understandable basis, a few additional illustrations may not be amiss. The following materials, which may exist in a colloidal condition, are for convenience grouped under two general heads, organic and inorganic : Organic : Gelatin, agar, caramel, albumin, starch jelly, humus, some bacteria, carbon black, and tannic acid. Inorganic : Gold, silver, hydrated ferric oxide, arsenious sulphide, zinc oxide, silver iodide, Prussian blue, and the like. 72. The properties of colloidal materials. — In general, there are certain properties which materials in a colloidal state exhibit and by which they are distinguished from true solutions. In the first place, since they are not in true solu- tion, they exert little or no effect on the freezing point and the vapor pressure of liquids. Some colloids have absolutely no effect on these properties, while others, as they allow a certain small amount of true solution to take place, do possess such influences to a slight degree. Secondly, colloids do not pass readily through semi-permeable membranes, such as parchment paper or pig's bladder. Their diffusive powers are low. This serves as an easy way of separating colloidal and non-colloidal material. Thirdly, heat and the addition of electrolytes will serve to coagulate certain colloids, a prop- erty which again serves to distinguish them sharply from a true solution. Fourthly, colloidal material has great ab- sorptive power, not only for water, but also for gases and materials in solution, a quality of extreme importance in soil phenomena. Many colloids are coagulated by the addition of an elec- trolyte,^ the phenomenon often being spoken of as floccula- ^ An electrolyte is any substance which has the ability when in solution to carry an electric current, the substance suffering decomposition there- by. The current is carried by the liberated ions. Hydrochloric acid, for example, dissociates into ionic hydrogen and ionic chlorine, the THE COLLOIDAL MATTER OF THE SOIL 131 tion.^ A very good example is afforded by treating a colloidal clay suspension with a little calcium hydroxide. The tiny particles almost immediately coalesce into floccules, and be- cause of their combined weight, sink to the bottom of the containing vessel, leaving the supernatant liquid clear. The same action will take place in the soil itself, but of course with less rapidity and under conditions less noticeable to the eye. Some dispersed materials, when thus separated from their dispersive medium, will reassume the colloidal state with ease when an opportunity is offered. In other cases, the col- loidal condition is difficult to restore. Gelatin is an example of the first group and is called a reversible colloid. Ferric hydrate is an example of the more or less irreversible type. Just why this phenomenon of flocculation or agglutination takes place is rather difficult to state. It is found that cer- tain colloids, when subjected to the proper electric current, will migrate to either the positive (anode) or the negative (cathode) pole. These particles evidently carry a charge of electricity. Hydrated ferric oxide, aluminium hydrate, and basic dyes, for example, move toward the cathode and carry a positive charge; while arsenious sulphide, silicic acid, gold, silver, humus and acid dyes move toward the anode and are negative. It is assumed that as long as the colloidal particles remain charged, they repel each other and the colloidal state persists. When an electrolyte is added, which develops by ionization a dominant opposite charge, it is supposed to cause a neutralization of the repellent electricity carried by the colloidal particles, and flocculation occurs. Certain colloids may flocculate certain others, as the gela- tinization of silic acid by hydrated ferric oxide. At times one colloid may protect another, probably by surrounding it former carrying a positive and the latter a negative charge of electricity (H*-i-C-). KNO3 gives K*+ NO3-. The ionization varies with the substance, the dilution and certain other conditions. *See Wolkoflf, M. I., Flocculation of Soil Colloidal Solutions; Soil Sci., Vol. I, No. 6, pp. 585-601, June, 1916. A good bibliography is appended. 132 NATURE AND PROPERTIES OF SOILS with a protective film. Such a case may be shown by adding gelatin to a clay suspension. When a colloid such as hy- drated ferric oxide is flocculated, it loses to a certain extent its colloidal properties, and assumes the characteristics of non-colloidal materials. 73. Soil colloids and their generation.^ — In soils there seem to exist two very general and indefinite groups of col- loidal materials, besides all gradations and variations: (1) vis- cous, gelatinizing and reversible colloids, and (2) non-viscous, non-gelatinizing, easily coagulable and irreversible colloidal matter. The decaying organic materials in the soil and the mineral matter contribute liberally to both groups. Both of these groups, with their bewildering variations and grada- tions, play important parts in the physical and chemical phe- nomena of the normal soil. The organic colloidal matter in a soil rich in decomposing tissue is obviously of great importance. Such material is very heterogeneous, very complex, and constantly changing. As yet very little study of the organic soil colloids has been made because of the difficulties presented by the problem. Humus colloids may be viscous or non-viscous, as the case may be, and may or may not be thrown down by calcium hy- droxide. The absorptive power of these colloids for water, gases, and such materials as calcium, magnesium, and potas- sium is very highly developed — as much so, probably, as that of the inorganic colloids. These organic colloids are not only added as a part of the original plant tissue but are also formed during the tearing-down and splitting-off processes ^Van Bemmelen, J. M., Bis Absorption; Seite 114-115, Dresden, 1910. Also, Die Absorptionsverhindungen und das Absorptsvermogen der Ackererde; Landw. Ver. Stat., Band. 35, Seite 69-136, 1888 ; Way, J. T., On Deposits of Soluble or Gelatinous Silica in the Lower Beds of the Chalk Formation; Jour. Chem. Soc, Vol. 6, pp. 102-106, 1854. War- ington, R., On tJi,e Fart Taken by Oxide of Iron and Alumina in the Adsorptive Action of Soils; Jour. Chem. Soc, 2d ser.. Vol. 6, pp. 1-19, 1868. Cushman, A. S., The Colloid Theory of Flasticity; Trans. Amer. Cer. Soc, Vol. 6, pp. 65-78, 1904. Ashley, H. E., The Colloid Matter of Clay and its Measurements ; U. S. Geol. Survey, Bui. 388, 1909. THE COLLOIDAL MATTER OF THE SOIL 133 incident to bacterial activity, during which, compounds are thrown off in such a state of division as to assume the condi- tion that has been designated as colloidal. Of course the chem- ical forces of weathering are also operative in this process of organic colloidal production. While some inorganic soil colloids, as silicic acid and hy- drated ferric oxide, are rather simple chemically, most of the mineral colloidal material is extremely complex. The soil, especially when of a clayey nature, always contains large amounts of complicated hydrated aluminum silicates of con- stantly varying constitution.^ Such material, whether simple or complex, arises from ordinary weathering reactions and develops in the soil as the latter is built up. A simple ex- ample may be cited. When a feldspar undergoes decomposi- tion the following reaction may be used to illustrate the pos- sible change that takes place : 2KAlSi308 + 2H2O + CO2 = H.ALSi^Oo + 4SiOo + KXOg Orthoclase Water Carbon Kaolinite Silica Potassium Dioxide Carbonate Kaolin almost always originates in this w^ay, an alkali car- bonate and silica being formed at the same time. The proc- ess is essentially one of hydration and carbonation ; the car- bon dioxide by reacting with the alkali permits the process to go on. The silica may go to one or more of three possible destinations, according to conditions, — to free quartz, to col- loidal silica or to make up complex colloidal hydrated alu- minum silicates. The last mentioned condition seems the most ^ The Bureau of Soils have prepared a colloidal solution from soil by passing a well shaken mixture of soil and water through a Sharpies centrifuge. The colloidal matter was separated from its dispersive medium by means of a porcelain filter. This ultra-clay seemed to be a mixture of various colloids and consisted mainly of hydrated alu- minum silicates with varying amounts of ferric hydroxide, silicic acid, organic matter and possiblj^ aluminum hydroxide. Moore, C. J., Fry, W. H., and Middleton, H. E., Methods for Deter- mining tJie Amounts of Colloidal Material in Soils; Jour. Ind. ami Eng. Chem., Vol. 13, No. 6, pp. 527-530, June, 1921. 134 NATURE AND PROPERTIES OF SOILS probable fate of the silica as the process is strongly one of hydration. 74. Influence of colloidal material ^ on soil properties. — ^ The amount of matter in a colloidal state in soils is extremely variable, ranging from almost nothing in sand to a very large percentage in heavy plastic clays. There is no satisfactory means of finding the amount of colloidal material in soil. All of the available methods depend for their expression on the intensity of certain qualities, supposed to be developed by colloid content. This indicates that the methods are largely comparative rather than exact or strictly analytical in nature. Ashley's method depends on the absorption of certain dyes to indicate the relative amount of material in a colloidal state. The difficulty in this method, however, lies in choosing the most effective dye and regulating its concentration. Moreover, different colloids vary so much in absorp- tive capacity for the same dye, that only roughly comparative results have thus far been possible. Mitscherlich uses the absorptive capacity of the soil for water vapor as a colloidal index. In this method the air-dry soil in a thin layer is brought to absolute dryness over phosphorus pentoxide. It is then placed in a desiccator over a 10 per cent, solution of sulfuric acid and the condensation is hastened by a partial vacuum. The sulfuric acid is used in order to prevent the deposition of dew on tlie soil. After exposure for about twenty-four hours, the soils are found to have taken up their maximum moisture of condensation, which is called the hygro- scopic water. The soil is then weighed, and the increase, figured to a percentage based on dry soil, is taken as a measure of colloidal content. The reverse process may also be followed, by exposing air-dry soil in a saturated atmosphere and afterwards drying over phosphorus pentoxide. The hygroscopicity of the soil, or its hygroscopic coefficient, is thus the basis for colloidal comparison. Ashley, H. E., The Colloid Matter of Clay and Its Measurement; U. S. Geol. Survey, Bui. .388, 1909. Eodewald, H., und Mitscherlich, A. E., Die Bestimmung der Hygro- sliopisitdt; Landw. Ver. Stat., Band 59, Seite 433-441, 1903. Also, Mitscherlich, E. A., und Ploess, R., Ein Beitrage sur Bestimmung der Eygrosl'opizitdt und zur Bewertung der physikolischen Bodenanalyse; Internat. Mitt. f. Bodenkunde, Band 1, Heft 5, Seite 463-480, 1912. Ehrenberg, P., und Pick, H., Beitrage sur Physilcalischen Bodenunter- suchung ; Zeit. f. Forst- und Jagdwesen, Band 43, Seite 35-47, 1911. Also, Vageler, P., Die Bodewald-Mitscherliclische TJieorie der Hygro- skopizitdt vom Standpunkte der Colloidchemie und ihr Wert sur Beur- teitung der Boden; Fuhling's Landw. Zeit., Band 61, Heft 3, Seite 73-83, 1912. Stremme, H., and Aarnio, B., Die Bestimmung des Gehaltes anorga/n- ischer Kolloide in Zersetzten Gesteinen und deren tonigen TJnlagerungs- produkten; Zeitsch. f. Prak. Geol., Band 19, Seite 329-349, 1911. Tempany, H. A., Shrinkage in Soils; Jour. Agr. Sci., Vol. VIII, Pt. 3, pp. 312-330, June, 1917. Beaumont, A. B., Studies in the Reversibility of the Colloidal Condi- tion of Soils; Cornell Agr. Exp. Sta., Memoir 21, Apr., 1919. THE COLLOIDAL MATTER OF THE SOU. 135 As may naturally be inferred the influence of the colloidal matter on soil conditions, especially as related to plants, is extremely important. This influence is exerted in a number of ways, modifying the physical and chemical as well as the biological activities within the soil. One important attribute imparted to soil by colloid develop- ment is high absorptive power. This power extends not only to condensation of gases, but also to water and to materials in solution. The water of condensation on dry soil particles when exposed to a saturated atmosphere is largely determined by the colloidal content. The absorptive capacity for mate- rials in solution affects both bases and acid radicals, although the former is usually more strongly influenced. This has a very important bearing on the economic use of fertilizers and on the loss of plant nutrients from the soil. Colloidal mate- rial may also function as a catalyst^ in that it may force certain reactions that otherwise might proceed but slowly. Since an adjustment is always taking place between the soil colloidal material and the soil solution as far as soluble constituents are concerned, it is readily seen that not only the concentration but also the composition of the latter is at least partially a function of the colloidal matter of the soil. Colloidal matter, moreover, does not exert the same absorptive power for all material but is capable of a certain amount of selection. For example, if ammonium sulfate is added to a soil, the ammonia is strongly taken up, which tends to release the sulfate ion. The continuous use of such a fertilizer on a soil low in active bases will ultimately result in an acid con- dition. This is another example of the practical importance of the soil colloidal matter. The movement of air and water in the soil is strongly in- fluenced by colloidal materials. In a fine soil in which the individual pore spaces are normally very minute the develop- ^ A catalyst is a material capable of hastening or retarding a chemical reaction, the catalytic agent itself not entering into the reaction. 136 NATURE AND PROPERTIES OF SOILS ment of colloidal matter may seriously interfere with aeration and capillary movement of water. The loosening of a clay soil tends to ameliorate such conditions and to counteract 1000 2000 5000 4000 5000 c.c. Fig, 23. — Curves showing the absorption of PO4 in parts per million by various soils from a solution of mono-calcium phosphate containing 200 parts to the million of PO4. The volume of the percolate is used as the abscissas. Such absorption is a rough measure of the colloidal content of a soil. the unfavorable influence of the colloidal condition of the soil. Such a structural condition is largely ascribed to the plasticity and cohesion^ of the soil, which are in turn, of ^ Any material which allows a change of form without rupture and which will retain this form when the pressure is removed, is said to be plastic. Putty with a proper admixture of oil is a very good example of a plastic body. As is well known, various materials differ in plasticity. Very closely correlated with plasticity, but not in exact similarity, is cohesion. By the cohesion of a soil is meant the tendency that its particles exhibit in sticking together and in conserving the mass intact. THE COLLOIDAL MATTER OF THE SOIL 137 course, governed by the amount and the quality of colloidal matter present/ In general it is found that, other conditions being equal, an increase of certain types of colloidal matter increases plas- ticity; in other words, the ease with which a soil may be worked into a puddled condition becomes greater. This is a rather undesirable quality when too pronounced, and in clays, in which it is most likely to be developed because of the pres- ence of large amounts of mineral colloids, some means of decreasing the colloidal influence is advisable. This great plasticity is developed because the colloids, especially those of a gelatinous and viscous nature, facilitate the ease with which the particles may move over one another and yet cohere sufficiently to prevent disruption of the mass. In general, also, the greater the plasticity of a soil, the greater is the cohesion when dry. In soils, then, in which certain kinds of colloidal materials are very high, clodding may occur if the soil is tilled too dry because of the great tendency of the par- ticles to cohere. Cohesion and plasticity, as factors in soil structure, soil granulation, and tilth will receive further atten- tion later. It must not be inferred from the preceding discussion that the generation of colloidal matter is always detrimental to soil conditions. In sandy soils the presence of such material is extremely beneficial as it tends to bind the soil together, promotes granulation, and prevents loss of plant nutrients by leaching. It is only in heavy soils in which excessive amounts of mineral colloids may develop that a detrimental condition is likely to exist. This occurs because of a high cohesion and plasticity, because of the absorption of plant nutrients and because of tendencies toward acidity. The addition of organic ^ Davis, N. B., TJie Plasticity of Clay; Trans. Amer. Cer. Soc, Vol. 16, pp. 65-79, 1914. Cushman, A. S., TJie Colloid Theory of Plasticity; Trans. Amer. Cer. Soc, Vol. 6, pp. 65-78, 1904. Also, Ashley, H. E„ The Colloid Matter of Clay and Its Measurement ; U. S. Geol. Survey, Bui. 388, 1909. 138 NATURE AND PROPERTIES OF SOILS matter and the development of non-plastic organic colloids will do much to alleviate such conditions. 75. Resume.— The attempt to explain natural phe- nomena from the standpoint of crystalloidal chemistry alone is a failure. Nature has chosen to reveal herself, largely in colloidal form. Such a condition of matter is the rule and not the exception. Whether the sky, the ocean, or the land is dealt with, the larger part of the natural phenomena are plausibly explained only through knowledge of colloidal chem- istry. In general, the more complex the material ^nd the more intricate the reactions to which it is subjected, the more likely it is that the colloidal state will result. Proteid materials, for example, whether in plants or animals, are almost always col- loidal. It is to be expected, therefore, that the soil with its complicated organic and inorganic components and its rapid and complex reactions should generate colloidal matter and that material in such a state should play a prominent part in soil and plant activities. CHAPTER VII SOIL STRUCTURE AND ITS MODIFICATION The structural condition of the soil is very important to plant growth, since the circulation of air and water so nec- essary to normal development is controlled thereby. The struc- tural condition may be loose or compact, hard or friable, gran- ulated or non-granulated, as the case may be. Of these con- ditions granulation, especially in heavy soils, is of vital im- portance, since it is really a summation of all favorable struc- tural conditions. By granulation is meant the drawing to- gether of the small particles around suitable nucleii, so that a crumb structure is produced. The grains thus cease to function singly. The importance of such a structural condi- tion on a heavy soil is obvious. The soil becomes loose because of the larger units, air moves more freely, and water not only drains away readily wiien in excess, but responds with celerity to the osmotic pull of the plant. 76. Soil structure types. — The structural condition of a soil can generally be attributed directly to its textural nature as can readily be seen by comparing sandy and clayey soils. For convenience of discussion two general structural groups may be established: (1) single-grained, and (2) compound- grained. In the former the particles function more or less separately and the soil is, as a consequence, rather open and friable. In the latter group the particles, being small, tend to stick together and the units instead of being solid are aggre- gates, their size and character as well as their relations to each other being a determining factor in the physical condition of the soil. As most soils are mixtures of large, medium, and 139 140 NATURE AND PROPERTIES OF SOILS small particles, it is only the coarse sandy soils on the one hand and very fine clayey soils on the other that ideally repre- sent these two groups. Most soils, especially loams, present combinations of the single and compound grain structures. Single-grain structure as found in sandy soils has certain obvious advantages, such as looseness, friability, good aera- tion, and drainage and easy tillage. On the other hand, such soils are often too loose and open and lack the capacity to absorb and hold sufficient moisture and nutrient materials. They are, as a consequence, likely to be droughty and lacking in fertility. There is only one method of improving in a prac- tical field way^ the structure of such a soil — the addition of organic matter. Organic material, if it undergoes favorable decomposition when incorporated with the soil, will not only act as a binding material for the particles but will also in- crease the water capacity. Nitrogen also is added and if the organic matter is properly supplemented with fertilizers and lime, the soil fertility will usually be markedly improved. A sandy soil high in organic matter is almost ideal from a struc- tural standpoint. The modification of the structural condition of a heavy soil is not such a simple problem as in the case of a sandy one. In the latter the plasticity and cohesion is never high even after the addition of large amounts of organic materials that rapidly develop into a colloidal state. In clays and similar soils the potential plasticity and cohesion ^ are always high * In the greenhouse or garden, structure may be modified by mixing different soils. This is not practicable in the field. * There are no satisfactory methods of determining either the plasticity or the cohesion of soils. For plasticity determination, see: Atterberg, A., Dis Plastizitdt der Ton; Internat. Mitt. f. Bodenkunde, Band I, Heft 1, Seite 10-43, 1911. Kinnison, C. S., A Study of the Atterberg Plasticity Method; Trans. Amer. Cer. Soc, Vol. 16, pp. 472-484, 1914. For methods of estimating cohesion : A good description of Schiibler's apparatus is found on page 104 of Bodenkunde, by E. A. Mitscherlich, published by Paul Parey, Berlin, in 1905. Haberlandt, H., Uber die Kohdreszenz, Verhalinisse ver- schiedener Bodenarten ; Forsch. a. d. Gebeite d. Agri.-Physik., Band I, Seite 148-157, 1878. Also, Wissenschaftlich praTctische Untersuchungen SOIL STRUCTURE AND ITS MODIFICATION 141 due to the presence of large amounts of complex hydrated aluminum silicates in a colloidal condition. The more plastic a soil becomes, the more likely it is to puddle/ especially if worked when wet. Moreover, a soil of high plasticity is prone to become hard and cloddy when dry, due to the cohesive ten- dencies of the small particles. Heavy soils must, therefore, be treated very carefully, especially in tillage operations. If plowed too wet, puddling occurs, the aggregation of particles is broken down, and an unfavorable structure is sure to re- sult. If plowed too dry, great lumps are turned up which are difficult to work down into a good seed-bed. In a sandy soil, no such difficulties are encountered.^ Granulation or the production of a compound-grain struc- ture is the only means of correcting the physical condition of a heavy fine-grained soil. In this process the small particles are drawn towards innumerable suitable nucleii and a porous structure is developed. The size of the individual pore spaces is thereby increased and air and water drainage is facilitated. The structural condition in reality simulates a single-grain state with this important difference, however: the particles are porous and not solid. Unless a hea\'y soil possesses at least some granulation, it is more or less unfit for agricultural operations. (See Fig. 24.) 77. Granulation. — While it is possible to list the factors auf dem Gebeite des Pflanzenbaues ; Band I, Seite 22, 1875. Puchner, H., Untersuchungen iiber die Kohareszenz der Bodenarten; Forsch. a. d. Gebiete d. Agri.-Physik., Band 12, Seite 195-241, 1889. Atterberg, A., Die Konsistenz und die Bindigkeit der Boden; Internat. Mitt. f. Boden- kunde, Band II, Heft 2-3, Seite 149-189, 1912. Cameron, F. K., and Gallagher, F. E., Moisture Content and Plvysical Condition of Soils; V. S. Dept. Agr., Bur. Soils, Bui. 50, 1908. *Wlien a soil in a plastic condition has been kneaded until its pore space is much reduced and it has become practically impervious to air and water, it is said to be puddled. The development of gelatinous and viscous colloidal materials seems to be the controlling factor in such a condition, the pore space of a puddled soil being largely filled with such material. When a soil in this condition dries, it becomes hard and dense. * Sandy soils are often plowed rather wet in order to render them more compact than they normally would be. 142 NATURE AND PROPERTIES OF SOILS that bring about granulation in a soil, it is difficult to state specifically just why this phenomenon takes place. It has been suggested that much of the granule formation in the soil is due to the contraction of the moisture around the particles when, for any reason, the moisture content is reduced. It is known that the soil particles tend to be drawn together by this reduction in the soil-moisture, due to the pulling power of the thinned films. If to this condition is added a material which tends to exert not only a drawing power on loss of moisture, but also a bind- FiG. 24.— A well granulated soil and a puddled soil. Organic matter plays an important role in structural condition. ing and cementing power when dry, all the essentials for suc- cessful granulation are present. This second force is found in. the colloidal material existing in considerable quantities in heavy soils. Such materials have already been shown to deter- mine the cohesion of the soil. The influence of the colloidal material is considered by many authorities as the more im- portant in the structural adjustments of the soil. It is evident that if cohesion and plasticity are to function in granulation — or, in other words, locally in the soil instead of generally and uniformly as when clodding or puddling occurs — a certain moisture content must be maintained. In a soil subject to such a condition, the cohesive forces being SOIL STRUCTURE AND ITS MODIFICATION 143 localized, the internal strains and pressures are unequal and a tendency arises for the mass to divide along lines of weak- ness into groups of particles. The binding capacity of col- loidal material, as well as of salts deposited from the soil solution, tends to make such a crumb structure more or less permanent. The moisture content most favorable for granu- lation seems to be that which is optimum for plant growth.^ 78. — Forces facilitating granulation.- — Granulation is nothing more or less than a favorable condition brought about by the force exerted by a variable water film and the pulling and binding capacities of colloidal material, operating at numberless localized foci. It is evident that any influence or change in the soil which will cause a greater localization of these operative forces will promote the aggregation of the particles. The addition of materials from extraneous sources is also a practice that may tend to develop lines of weakness and thus cause a more intense activity of the forces at work. The conditions, additions, and practices tending to develop or facilitate a granular structure in soils may be listed under six heads: (1) wetting and drying of the soil, (2) freezing and thrawing, (3) addition of organic matter, (4) action of roots and animals, (5) addition of lime and (6) tillage. Only the last two need additional consideration. 79. Granulating influence of lime.^ — One of the effects of lime in the soil, especially of the oxide and hydroxide forms, ^ Cameron, F. K., and Gallagher, F. E., Moisture Content and Physical Condition of Soils; U. S. Dept. Agr., Bur. Soils, Bui. 50, p. 8, 1908. ^ Fippin, E. O., Some Causes of Soil Granulation; Trans. Amer. Soc. Agron., Vol. 2, pp. 106-121, 1910. Czermak, W., Bin Beitrag zur Erkent- uis der Verdnderungen der Sog physilcalischen Bodeneigensliaften durch Frost, Eitse, und die Beigabe einiger SaJze; Landw. Ver. Stat., Band 76, Heft 1-2, Seite 73-116, 1912. Also, Ehrenberg, P., und Eomberg, G. F. von, Zur Frostwirbung auf den Erdboden; Jour. f. Landw. Band 61, Heft 1, Seite 73-86, 1913. 'Lime in a strictly chemical sense refers only to calcium oxide (CaCf). The term is used here with an agricultural meaning, including all cal- cium and magnesium compounds which are ordinarily added to the soil to correct acidity, thus including not only calcium oxide but calcium hydroxide and calcium carbonate [Ca(OH)o and CaCOa] as well. 144 NATURE AND PROPERTIES OF SOILS is a flocculating action. This agglomeration, as already ex- plained, is the drawing together of the finer particles of a soil mass into granules. When calcium hydroxide is mixed with water containing fine particles in suspension there is almost immediately a change in the arrangement of the par- ticles. They first draw together in light, fluffy groups, or floc- cules, which then rapidly settle so that the supernatant liquid is left clear or nearly so. This phenomenon is termed flocculation, because of the peculiar appearance of the aggregates. This flocculating tendency when lime is added goes on in the soil as well as with suspensions, although more slowly. In general, the lime tends to satisfy the absorptive capacity of the colloidal material and by throwing down these colloids develops lines of weakness. The cohesive power of the soil is thus localized and agglomeration must necessarily occur. The various forms of lime differ in their flocculating capacities, calcium oxide and hydroxide being very active, while calcium carbonate is relatively inactive in this regard. It must not be inferred that lime is generally added for its flocculating influence. It is used primarily for other reasons, the amounts applied being in general too small to have very much influence on the structural condition of the soil. War- ington,^ however, reports a statement of an English farmer to the effect that by the use of large quantities of lime on heavy clay soil, he was enabled to plow with two horses instead of three. It is generally true that soils rich in lime are well granulated, and maintain a much better physical condition than soils of the same texture that are low in lime. 80. Tillage.- — Tillage aims to accomplish three primary ^ Warington, E., Physical Properties of Soils, p. 33, Oxford, 1900. ' For a very complete review of the theory and practice of plowing and cultivation, with a complete bibliography: Sewell, M. C, Tillage: A Bevieto of the Literature; Jour. Amer. Soc. Agron., Vol. II, No. 7, pp. 269-290, Oct., 1919. The following books upon the mechanics of tillage may prove helpful: Davidson, J. B., and Chase, L. W., Farm Machinery and Farm Motors; New York, 1908. The Oliver Ploiv Boole; South Bend, Ind., 1920. SOIL STRUCTURE AND ITS MODIFICATION 145 purposes: (1) modification of the structure of the soil; (2) disposal of rubbish or other coarse material on the surface, and the incorporation of manures and fertilizers into the soil ; and (3) the deposition of seeds and plants in the soil in position for growth. The most prominent of these purposes is the modification of the soil structure. This affects the retention and movement of moisture and air, the absorption and retention of heat, and either promotes or retards the growth of organisms. The creation of a soil-mulch is merely a change in the structure of the soil at such times and in such a manner as may prevent the evaporation of moisture. In fine-textured soils, in which 1 Z 5 Fig. 25. — Three types of plow bottoms; 1, stubble; 2, sod; 3, general purpose. the granular structure is most desired, tillage may have an important influence on the formation or destruction of gran- ules. As has been pointed out, any treatment that increases the number of lines of weakness in the soil structure facili- tates the activities of the moisture films and the colloidal mate- rials in producing soil granules. Tillage shatters the soil and breaks it into many small aggregates, which may be drawn together and loosely cemented as a result of the evaporation of moisture. The more numerous the lines of weakness pro- duced, the more pronounced is the granulation; and, con- versely, the fewer the lines of weakness produced, the more coarse and cloddy is the structure. According to their mode of action, tillage implements may Kamsower, H. C, Equipment for the Farm and Farmstead; Boston, 1917. King, F. H., Physics of Agriculture; Chap. XI, Madison, Wis., 1910. 146 NATURE AND PROPERTIES OF SOILS be grouped as follows: plows, cultivators, packers and crushers, 81. The action of the plow. — The moldboard plow brings about its effects because of the differential stresses set up in the furrow slice as it passes over the share and the moldboard. The soil in immediate contact with the plow sur- face is retarded by friction, and the layers above tend to slide over one another much as the leaves of a book when they are bent. If the soil is in just the proper condition, maximum granulation results ; but if the moisture is too high or too low, puddling or clodding may follow, especially on a heavy soil. Not only does a shearing occur, but this shearing is differ- ential, due to the slope of the share and especially to the curve of the moldboard. When the soil is to be turned over with the least expenditure of energy, the share is sloping and is set to deliver a slanting cut, and the moldboard is long and gently inclined. This allows the furrow slice to be turned with little granulation and with a minimum effort. When maxi- mum granulation and pulverization are desired, the mold- board is short and sharply turned, and the share is less slop- ing and the cutting edge less slanting. Such conditions make for the development of more friction and the generation of those internal twisting and shearing stresses necessary for good granulation. The sharper the bending of the furrow slice, the greater are the internal stresses set up. Various types of moldboards and shares designated for special soils and particular operations are on the market. (See Fig. 25.) The disc plow is a sharp rolling disc set at such an angle that it slices off and turns over the soil, pulverizing it fairly effectively somewhat after the manner of the moldboard plow. One advantage of the disc plow is its lighter draft, due to a rolling rather than a sliding friction in the soil. In prac- tice it is especially effective on very dry, hard soil. While the plow is the very best pulverizing agent when optimum soil-moisture conditions prevail, it is also a most SOIL STRUCTURE AND ITS MODIFICATION 147 effective puddling agent when the soil is wet. Therefore, care in the judging of optimum conditions for plowing is a most important feature in the maintenance and encouragement of soil granulation. A careful study of the moisture con- ditions in a clay soil is especially necessaiy in order to de- termine just what is the correct moisture content for good plowing. That this condition must be gauged carefully and immediate use made of the advantages it offers is shown by its narrow limits. A few days may suffice for the moisture to pass through and beyond such a condition. A clay soil is so Fig. 26. — A six-shovel cultivator. difficult to handle at best that no opportunities such as are offered by optimum moisture conditions should be lost. More- over, a heavy soil plowed too dry or too wet does not regain its normal granular condition for several seasons. Such care is unnecessary with a sandy soil. 82. Cultivators, packers and crushers. — The many types of cultivators may be grouped under three heads: (1) cultivators proper, (2) levelers and harrows, and (3) seeder cultivators. The action of all these implements is the same in that they stir the soil, at the same time loosening the struc- ture and cutting off weeds. AVhile the action is much shal- lower than with the plow, the same attention should be paid to moisture conditions. Particularly is this true in pulveriza- 148 NATUKE AND PROPERTIES OF SOILS tion immediately after plowing. When the moisture condi- tions are optimum, the clods are more easily shattered and the formation of a suitable seed-bed is speedily accomplished. The cultivators proper are well represented by the ordinary corn cultivator whether equipped with shovels, knives or discs. Under the leveler and harrow type may be placed the spike and spring-tooth harrow, the various kinds of weeders, the acme harrow and the disc harrow. The latter may be equip- ped with solid, cut-away, or spading discs. The grain drill, either of the press or disc type, is a representative of the seeder cultivators, which considerably influence the structural condition of the soil although such action is not their primary purpose, (See Fig. 26.) Packing and crushing are ordinarily performed by the same implement, since any tool that compacts does a certain amount of crushing; and, conversely, any implement that crushes the soil does some compacting. Such an implement as the culti- packer cultivates, packs and promotes granulation in one operation. The difficulty of establishing a rigid classification is evident. Rollers may be of the solid or barrel type, the corrugated type, or the bar type. The subsurface packer is also included in this group. Rollers tend to force the soil particles nearer together and smooth the surface. If at the same time they establish a soil-mulch so much the better. The rolling of the land after seeding is an attempt to stimulate the capillary movement of the water and to hasten germination by bring- ing the seed in closer contact with the soil. The planker, drag, or float is a common type of single crusher. It is generally broad and heavy, without teeth and is dragged over the soil. The lumps are rolled under its edges and ground together in such a manner as effectively to reduce their size. The soil is leveled and smoothed at the same time. This implement may be used instead of a roller in many cases. (See Fig. 27.) SOIL STRUCTURE AND ITS MODIFICATION 149 83. Soil tilth. — The previous data and discussion have clearly shown the very great importance of structure in the successful handling of the soil in the field. Since good phy- sical condition will reflect itself on crop yield it is evident that structure must ultimately be considered in .relation to all plant growth. This relationship is usually expressed by the term tilth. While structure refers to the arrangement of the particles in general, and granulation to a particular aggre- gate condition, tilth goes one step farther and includes the plant. Tilth, then, refers to the physical condition of the soil Fig. 27. — A planker or drag, useful in the crushing of clods. as related to crop growth. It may be poor, medium, good, or excellent, according to circumstances. Good tilth may de- mand in many soils maximum granulation, in others only a medium development. Tillage operations by influencing the structure of the soil aim to develop optimum tilth. Optimum tilth always implies the presence of water since the best phys- ical relationships cannot be developed without such moisture conditions. 84. Summary. — The factors which control the struc- tural condition of the soil to the greatest extent are plasticity and cohesion, their influence intensity being due directly to the presence of certain kinds of materials, especially hydrated aluminum silicates, in a colloidal state. As plasticity and cohesion increase the tendencies of a soil to puddle when wet 150 NATURE AND PROPERTIES OF SOILS and to clod when dry are augmented. Therefore in heavy soils a modification in these factors is advisable, through a careful control of moisture and a bettering of the granular structure of the soil. Grranulation, while due to some extent to the influence of the water film, is traceable largely to col- loidal matter both mineral and organic. It is really a con- centration of the forces of cohesion and plasticity around num- berless localized foci. Granulation takes place under the in- fluence of wetting and drying, freezing, plants and animals, addition of lime and organic matter, and tillage operations, especially plowing. The farmer exerts a modifying influence on structure most efficiently by increasing the organic content of the soil and by plowing. He is, of course, aided and abetted by natural forces. Efficient tillage requires good judgment in the selection of proper implements and mechanical skill in their operation. It demands besides an understanding of the properties of soils and a knowledge of their plant relationships. Sandy soils are easily handled provided sufficient organic matter is main- tained. Such cannot be said of clayey soils. Due to the high cohesion and plasticity of heavy soils the moisture zone for successful tillage is particularly narrow. The ability to detect when this zone has been reached in a clay soil is one of the essentials of its successful management. Another essential is the effective widening of such a zone by granulation oper- ations. The optimum moisture condition for tillage is generally near the optimum condition for plant growth — a happy coinci- dence, since by regulating the moisture content for plant devel- opment conditions are rendered most favorable for all soil ac- tivities. It is thus possible to produce in one operation that desideratum in all soil physical operations, an optimum tilth. CHAPTER VIII TEE FORMS OF SOIL-WATER AND THEIR CHARACTERISTICS ' A SOIL, in order to function as a medium for plant growth, must contain a certain amount of water. This moisture pro- motes the innumerable chemical and biological activities of the soil, it acts as a solvent and carrier of nutrients, and in addi- tion it functions as a nutrient itself. The amount, character, and control of the soil-moisture must evidently be reckoned with in any study of soil and plant relationships, whether they are of a practical or a theoretical nature. The productivity of a soil is often a direct function of its moisture condition. 85. Forms of soil-water. — As has already been demon- strated, a soil of a given volume weight has a definite pore space which may be occupied largely by air or by water, or shared by both, as the case may be. Of course, an ideal soil for growth is one in which there is both air and water, the proportions depending on the texture and the structure of the soil and the character of the crop. Assuming for the time being, however, that the pore space is almost entirely filled with w^ater, or, in other words, that the soil is saturated, three forms of w^ater are found to be present — hygroscopic, capillary and gravitational. These forms differ not only in the amount and proportion of the solutes which they carry but also in the positions that they occupy in their relation to the larger soil particles and the accompanying colloidal complexes. ^ Keen, B. A., Belations Existing Between the Soil and Its Water Content; Jour. Agr. Sci., Vol. X, Part 1, pp. 44-71, Jan., 1920. A good review of the subject. 151 152 NATURE AND PROPERTIES OF SOILS If an absolutely dry soil is exposed to a moist atmosphere, it will absorb moisture rather rapidly until the colloidal sur- faces are in equilibrium with the air as far as water vapor is concerned. Other conditions being equal, maximum water will be taken up from an atmosphere which is saturated with moisture. The moisture thus taken up is called hygroscopic water, its amount being determined quite largely by the mag- nitude of the colloidal material present in the soil. On adding more water, it will be found that the absorptive power of the soil has been by no means satisfied by the hygro- scopic water. Moisture will still be taken up by the colloidal complexes and it will also collect in the interstices between the soil particles. This water which is above and beyond the hygroscopic is generally called the capillary. That part held by the colloidal complexes is very similar in characteristics to the hygroscopic water in that it is tightly held and is more or less immovable. That portion in the interstices, especially the larger spaces, is in the form of a film, is loosely held, and responds to capillary action. While typical capillary water is much different from hygroscopic moisture, it grades into the latter with no sharp line of demarcation. Once the capillary capacity of the soil is satisfied, a third form of water may appear. This water is but slightly in- fluenced either by the colloidal complexes or the larger soil particles and consequently is free to respond to the pull of gravity. It is called the free or gravitational moisture and is the water which passes through the soil and appears in streams and rivers bearing in solution the tremendous amounts of soluble salts which are every year lost from the land. 86. Hygroscopic water. — The hygroscopic water in a soil has been spoken of as the water of condensation, or ab- sorption. It is, however, quite distinct from water condensed on a surface colder than the moist atmosphere in which it is placed. All bodies possess the power, to a greater or less de- gree, of absorbing water even when at the same temperature THE FORMS OF SOIL-WATER 153 as the air with which they are in contact, provided, of course, that the air contains water-vapor. Such condensation is largely a function of the surface exposed. One of the characteristics peculiar to colloidal materials is a high absorptive power for water, whether it is presented in the form of a liquid or vapor. This capacity is due to the tremendous surface exposed by matter in a colloidal state, which not only may hold the moisture physically but may even force it into loose chemical combination.^ The hygro- scopic water is probably not in the form of a film around the particles but in a much more intimate relationship. That which is held physically is probably, in part at least, in a con- dition of solid solution. If any of the hygroscopic water'Ms held chemically, the bond is probably a rather loose one. A large proportion of the hygroscopic moisture is obviously not in a liquid state and consequently is immovable as such. When a hygroscopically saturated soil is exposed to a partially saturated air, a portion of the hygroscopic moisture will be lost through vaporization. In order to expel the remainder of the hygroscopic water, the soil must be heated. For con- venience of determination, it is generally assumed that all of the hygroscopic moisture will be driven from an air-dry soil by heating it for four or five hours at a temperature of 100° or 110° C. This is only an assumption, however, as some of the moisture in intimate relationship with the colloidal com- plexes probably still remains. The amount of energy necessary to expel the hygroscopic moisture from the soil is very great, since its only movement is thermal and because it is held so closely. As so much energy is expended in removing this water, it is reasonable to *See, Bouyoucos, G. J., Classification and Measurement of the Dif- ferent Forms of Water in the Soil by Means of the Dilatometer Metlwd; Mich. Agr. Exp. Sta., Tech. Bui. 36, Sept., 1917. Eelationship between the Unfree Water and the Heat of Wetting of Soils and its Significance ; Mich. Agr. Exp. Sta., Tech. Bui. 42, Mar., 1918. A New Classification of the Soil Moisture; Soil Sei., Vol. XI, No. 1, pp. 33-47, Jan., 1921. 154 NATURE AND PROPERTIES OF SOILS expect that a certain amount of heat of condensation will be apparent when it is resumed.^ Patten ^ and Bouyoucos ^ offer the following quantitative data concerning this point : Table XXVIII HEAT EVOLVED BY WETTING SOILS DRIED AT 110° C. Soil Calories to a Kilo of Dry Soil Quartz sand Norfolk sand Hagerstown loam Miami silt loam Cecil clay Superior clay Muck (25% organic matter) Peat 000 347 1108 1742 3376 5158 6413 22185 87. Determination of the hygroscopic coefficient. — The methods for the determination of the maximum liygro- scopicity of a soil, or, in other words, the hygroscopic coeffi- cient, are simple in outline. The soil, in a thin layer, is ex- posed to an atmosphere of definite humidity under conditions of constant temperature and pressure. Complications arise from the necessity of using a very thin layer of soil, from the difficulty of controlling humidity, and from the tendency of capillary water to form in the soil interstices before the hygro- scopic capacity is satisfied. The question of how long the exposure should take place has not been definitely settled. It ^ The tremendous heat of wetting is probably due to the latent heat of water, to the attraction that soils have for water and to the condition into which the water is transformed. The heat of condensation is so large as to suggest the probability of a change in the aggregation of the moisture thus absorbed. ^Patten, H. E., Heat Transference in Soils; U. S. Dept. Agr., Bur. Soils, Bui. 59, p. 34, 1909. ' Bouyoucos, G. J., BelationsMp betu-een the Vnfree Water and the Beat of Wetting of Soils and its Significance; Mich. Agr. Exp. Sta., Tech. Bui. 42, Mar. 1918. THE FORMS OF SOIL-WATER 155 is evident, therefore, that not only must any method be more or less arbitrary but that its value can only be comparative. In the actual procedure/ the sample of soil may be air- dried or dried at 100° or 110° C. If the former method is followed, the sample after exposure is heated for four or five hours at 100° or 110° C, the loss being considered as hygro- scopic water. If oven-dried soil is utilized, the gain in weight due to the exposure to the moist air is the hygroscopic mois- ture. If a saturated air is made use of, the gain is maximum hygroscopicity, from which can be calculated the percentage of hygroscopic water based on dry soil, called the hygroscopic coefficient. If a partially saturated air is utilized, a sample of stock soil, the hygroscopic coefficient of which is known, is exposed at the same time. The determination on the known sample shows what proportion of possible hygroscopic water has been taken up. From this the hygroscopic coefficient of the unknown soil sample can be calculated.- 88. Hy^oscopic capacity of soils. — Since hygroscopic- ity depends almost directly on the colloidal nature of the soil, it is evident that texture, external factors being under con- trol, will be an important factor in determining the hygro- scopic coefficient. When the organic matter of soils is more or less the same in amount, the inorganic colloids seem to con- "■ Hilgard, E. W., Soils; pp. 196-201, New York, 1911. This method ia practically the same as that used for the comparative estimation of the colloidal content of the soil^ the hygroscopic coefficient being the comparative figure obtained. See note to paragraph 74 of this text. Bouyoucos determines the hygroscopic coefficient in an approximate way by means of the dilatometer method. The dilatometer is an apparatus which measures the expansion of water on freezing. If a given amount of soil and water is reduced below zero, the expansion attained will reveal the amount of water remaining unfrozen, due to its soil relationships. Bouyoucos finds that the amount of moisture unfrozen after supercooling to —4° C. (slightly more freezes at -78° C.) correlates fairly well with the hygroscopic coefficient. Bouyoucos, G. J., A New Classification of Soil Moisture; Soil Sci., Vol. XI, No. 1, pp. 33-47, Jan., 1921. ^Alway, F. J., and Clarke^ V. L., Use of Two Indirect Methods for the Determination of the Hiigroscopic Coefficients of Soils; Jour. Agr. Kes., Vol. VII, No. 8, pp. 345-351, Nov., 1916. 156 NATURE AND PROPERTIES OF SOILS trol the hygroscopicity. The following; figures from Briggs and Schantz/ by whom the hygroscopic coefficient was deter- mined by exposing air-dry soil at 20° C. to a saturated atmo- sphere and then drying at 110° C, illustrate this point. The organic matter was not a serious disturbing factor. Table XXIX HYGROSCOPIC CAPACITY OF VARIOUS SOILS EXPRESSED IN PER- CENTAGE BASED ON DRY SOIL'^ Soils Coarse sand .... Fine sand Sandy loam . . . . Fine sandy loam Loam Clay loam Clay Percentage Hygroscopic OF Clay Coefficient 1.6 .5 3.9 1.5 7.5 3.5 12.9 6.6 14.4 9.6 22.0 11.4 32.5 13.2 ' Briggs, L. J., and Schantz, H. L., The Wilting Coefficient for IHf- ferent Plants and Its Indirect Determination ; U. S. Dept. Agr., Bur. Plant Ind., Bui. 230, p. 65, Feb., 1912. See also, Loughridge, R. H., Investigations in Soils Physics; Calif. Agr. Exp. Sta., Rep. of Work of the Agr. Exp. Stations of Calif, for 1892-3-4, pp. 76-77. Ammon, Georg., Vntersuchungen iiber das Condeiisationsvermogen der Bodenconstituenten fur Gase; Forseh. a. d. Gebiete d. Agri.-Physik., Band II, Seite 1-46, 1879. Dobeneek, A. F., von, Untersuchungen iiber das Ahsorptionsvermogen und die Hygroskopizitdt der BodenTconstituenten ; Forseh. a. d. Gebiete d. Agri.-Physik., Band XV, Seite 163-228, 1892. * During the many years of soil investigation, especially where the problems had to deal either directly or indirectly with moisture, five methods of water expression have been evolved, their use depending on the nature of the work and on the points to be expressed. They may be listed under two general heads: A. Percentage expression 1. Percentage on a dry basis 2. Percentage on a wet basis B. Volume expression 1. Cubic inches to the cubic foot of soil 2. Percentage by volume 3. Surface inches A soil carrying 25 per cent, of water on the dry soil basis contains 20 per cent, on the moist basis (soil plus water). The former method is THE FORMS OF SOIL-WATER 157 Apparently, the finer the soil, the higher the hygroscopic coefficient. This is due to the fact that most of the inorganic colloidal matter is carried by the finer separates. In consid- ering the hygroscopicity, however, the influence of the organic matter must not be forgotten. Organic colloidal matter has a veiy marked influence on absorption, and as the organic matter of the soil increases, the hygroscopicity rises rapidly. The following data from Beaumont^ is interesting in this respect : Table XXX THE HYGROSCOPIC COEFFICIENT^ COMPARED TO CERTAIN OTHER SOIL FACTORS Soil Clay % Igni- tion % Humus % Hygro- scopic Coeffi- cient % Dunkirk silty clay loam, surface Dunkirk silty clay loam, subsoil Clyde clay loam, surface Vergennes clay, subsoil 12.9 20.0 20.1 74.5 5.08 3.05 14.54 5.79 1.26 .20 4.34 .49 3.80 5.77 18.90 17.40 In comparing the two Dunkirk soils it is apparent that the colloidal clay is the dominant factor in determining the mag- preferable in that the basis for calculation is not a changeable one as is the weight of moist soil. The dry basis is practically always used in soil work. Where two soils of different volume weight are compared, the per- centage relationship does not give a true idea of the relative amounts of water present. A volume expression should then be used. If a cubic foot of soil, weighing 100 pounds, contains 10 pounds of water it would be carrying (10 X 27.6) or 276 cubic inches of water. This would equal (276-^1728) X 100 or 15.9 per cent, by volume or (10->-.5.2) = 1.92 surface inches. ^ Beaumont, A. B., Studies iii the Eeversibility of the Colloidal Condi- tion of Soils; Cornell Agr. Exp. Sta., Memoir 21, pp. 501-504, April, 1919. * Moisture content in this text unless otherwise indicated will always be expressed on the dry soil basis. 158 NATURE AND PROPERTIES OF SOILS nitude of the hygroscopic coefficient. With the Clyde and Vergennes, however, the organic colloidal matter is dominant, since the Clyde with only 20 per cent, of clay has a higher hygroscopic figure than the Vergennes which carries 74.5 per cent, of that separate. The Clyde clay loam and the Dunkirk subsoil have the same amount of clay, yet the former pos- sesses a hygroscopic coefficient over three times larger. Two external conditions seem to be important in determin- ing the amount of hygroscopic water in soils — (1) humidity and (2) temperature. It has been definitely established that the higher the humidity the higher the content of hygro- scopic moisture. An air-dry soil will, therefore, contain less moisture in a dry atmosphere than in one carrying large amounts of water-vapor. When the soil is in contact with a saturated air it will take up hygroscopic water to its full capacity and be at the point spoken of as the hygroscopic coefficient. As the soil air is generally considered to be satu- rated or almost saturated with water- vapor, ^ except in the surface layers or during periods of protracted drought, a soil in normal condition may be considered, for all practical pur- poses, to be at its maximum hygroscopicity. An increase of the temperature of the saturated atmosphere seems to increase hygroscopicity. With a partially saturated air the influence seems to be in the opposite direction.^ This, however, is not an important practical point. The hygroscopic coefficient, defined as the maximum hygro- scopic water that a soil will hold, is controlled largely by the texture and organic content of the soil. It may vary from a very low figure in a sandy soil to as high as 15 per cent, for a clay high in organic matter. With a muck or peat, the per- ^ Eussell, E. J., and Applyard, A., The Atmosphere of the Soil: Its Composition and Causes of Variation; Jour. Agr. Sei., Vol. VII, Part 1, p. 5, 1915. ^ For a full discussion of this point, see Lipman, C. B., and Sharj), Li. T., a Contribution to the Subject of the Hygroscopic Moisture of Soils; Jour. Phys. Chem., Vol. 15, No. 8, pp. 709-722, Nov., 1911. THE FORMS OF SOIL-WATER 159 centage would be considerably higher, in some cases reaching 50 or 60 per cent. It must always be kept in mind, however, that the point designated as the hygroscopic coefficient is more or less arbitrary and that there is no sharp line of demarca- tion between the moisture designated as hygroscopic and that which lies near it, but is called capillary, 89. The capillary water.^ — The moisture above the hygroscopic coefficient but not free to respond to gravity is generally spoken of as the capillary water. The portion of this moisture lying in contact or in the immediate neighbor- hood of the hygroscopic water is probably capable of only sluggish diffusion movement if any.^ This part of the capillary moisture is held largely by the colloidal matter and may be considered as transitional between the true hygroscopic and the more active capillary portion. Although so closely related to the hygroscopic water in general properties and character- istics, the soil does not assume it by absorption from vapor- laden air. This separates it at least analytically from the hygroscopic form of moisture. Moreover, it is probably largely in the liquid state, which is hardly true of all of the hygroscopic water. The more active capillary water exists in the large inter- stices and as a film over the particles and the colloidal com- plexes. It is held rather loosely by the soil, yet strongly enough to counteract gravitation. This part of the capillary moisture, being more or less beyond colloidal influence, is free to respond to the forces active in true solutions and, there- fore, may move from place to place as equilibrium stresses may demand. While the inner portion of the capillary water is held by the absorptive power of the colloidal surfaces, the outer and freer portion is maintained by the surface tension ^ The colloidal conceptions regarding soil-moisture has made it advis- able to give the term capillary a broader significance than its root meaning justifies. ^Bouyoucos, G. J., A Neiv Classification of the Soil Moisture; Soil Sci., Vol. XI, No. 1, pp. 33-47, Jan., 1921. 160 NATURE AND PROPERTIES OF SOILS of the water film. The distinctive characteristics of these two portions of the capillary water are due to their controls — colloidal in one case, surface tensional in the other.^ While the outer portion of the capillary water is undoubt- edly in the form of a more or less continuous film from par- ticle to particle, the bulk of such moisture probably exists normally in the interstices between the soil grains. Such a condition arises because of the pressure developed by the force of surface tension. The pressure due to surface tension, however it may be expressed, varies with the curvature of the film and is proportional to twice the surface tension di- vided by the radius. The less the radius the greater the cur- vature and, therefore, the greater the stress developed by sur- face tension.^ The situation so far as the soil is concerned may be ex- plained in an empirical way as follows : Suppose that two par- ticles, each carrying a capillary water film, be brought into such contact that the films coalesce. There are now two distinct surfaces, that at A, A' (see Fig. 28), with the curva- * Bouyoucos classifies these two types of capillary water as free (the more active) and capillary-absorbed (the inner group). The distinction is made on the basis of his dilatometer results, the portion which freezes at about 0°C being considered as the more active or free. Bouyoucos, G. J., A New Classification of the Soil Moisture; Soil Sci., Vol. XI, No. 1, pp. 33-47, Jan., 1921. ^Surface tension is the tension of a liquid surface by virtue of which it acts like an elastic enveloping membrane, tending always to contract to the minimum area. While molecules in the interior portion of the liquid are attracted in all directions and are thus at equilibrium, those on the surface are attracted by an overbalancing force toward the interior. In measurement, surface tension is considered as the force with which the surface on one side of a line, one centimeter long, pulls against that on the other side of the line. It is generally expressed in dynes. The pressure due to surface tension varies with the curvature of the film. It is usually expressed as: ' p^2T r where P is the pressure; T, surface tension; and r, the radius of the drop. As the radius becomes less, the curvature increases and the pres- sure due to surface tension increases. An increase of T will increase the pressure, P. THE FORMS OF SOIL- WATER 161 ture of the original film, and that at B, which is very acute and which naturally must exert a very great outward pull. Under the stress of this pull developed by the surface tension acting in this film of very great curvature, the water is drawn into the space between the particles, where it becomes thicker than the capillary film about the particles. The readjustment continues until the forces developed by the two films become equal. An equilibrium is now established. In the soil the tendency towards adjustment is somewhat similar in so far Fig, 28. — A conventional diagram showing the coalescence and read- justment of the outer capillary water film of two particles when brought in contact. At the left is shown the condition before the adjustment with a sharp angle at B; on the right, the films are at equilibrium with a thickening at B due to movement from A and A'. as the outer capillary water is concerned. Complete equilib- rium is probably never reached, however, due to constantly disturbing factors. 90. The determination of the amount of capillary water in the soil. — The capillary water in a sample of field soil may be determined by making a moisture test in the ordinary way for the total water contained,^ after the gravi- * A moisture determination on a sample of field soil is generally carried out as follows: — 100 grams of the sample, after thorough mixing, is weighed into a suitable weighing dish and air-dried. The sample is then placed in an oven and heated at 100°C or 110°C for four or five hours. It is then cooled in a disiccator and weighed. The loss in weight is water. The moisture is calculated as percentage based on the dry mat- ter of the soil. If the weight of the water lost was 20 grams, the percentage of moisture would be (20 -^ 80) X 100 or 25 per cent based on dry soil. 162 NATURE AND PROPERTIES OF SOILS tational water has had time to drain away. This represents the hygroscopic plus the capillary water. A determination of the hygroscopic coefficient on another sample yields a figure which, when subtracted from the total water, will give the capillary water present in the soil. The capillary water at various points in a soil column may be obtained by subtracting the hygroscopic coefficient from the various percentages of moisture present, since the hygroscopic moisture is little in- fluenced by height of column or ordinary structural condi- tions. The determination cited above may or may not give the maximum water-holding capacity of a soil. To fill such a need a laboratory method has been devised by Hilgard,^ which attempts to show the maximum retentive power of a soil for water. A small perforated brass cup is used, having a diameter of about 5 centimeters and capable of containing a soil column 1 centimeter in height. A short column is used, since it is only under such conditions that a soil may retain against gravity the greatest amount of water. Also the soil is able to expand or contract, as the case may be, on the assumption of water until an equilibrium is reached. A filter-paper disc is often placed in the metal cup, and the soil is poured in, gently jarred down, and stroked off level with the top of the cup. The cup is then set in water and the soil is allowed to take up its maximum moisture. After draining, the weight of the wet soil plus the cup, together with the weights pre- viously obtained, will allow a calculation of the total water retained based on the absolutely dry soil. If the maximum capillary water is desired, the hygroscopic coefficient may be subtracted from the maximum water retained. Since this method is a laboratory procedure and the soil used is not in its normal structural state, the results cannot be accurately applied to field conditions. While the figures » Hilgard, E. H., Soils, p. 209, New York, 1911. THE FORMS OF SOIL-WATER 163 obtained may be fairly accurate for a sand, they are certainly much too high for heavy soils. Comparisons with field soils have shown the data obtained by the above method to be from 30 to 130 per cent, too high.^ 91. The capillary capacity of soils. — As might nat- urally be expected, the factors that tend to vary the amount of capillary water in a soil are several and their study is rather complex due to the secondary influences that they may generate and to the variable nature of the capillary moisture. These factors may be discussed under four heads: (1) surface tension, (2) texture, (3) structure and (4) organic matter. Any condition that will influence surface tension will ob- viously influence the forces active in the outer portion of the capillary water. A rise in temperature, for example, if the soil is capillarily saturated, will allow some of the water to become gravitational. A lowering of temperature would cause an opposite change. This theory has been verified by certain experiments by King,- in which he found, other conditions being constant, a very decided influence on capillary water through change of temperature. AVollny ^ has shown that a depression of .65 per cent, in sand to as high as 3.7 per cent, in kaolin may occur from a rise in temperature of twenty degrees. While surface tension may be greatly varied by the presence of salts in solution, the soil-water is generally so dilute that the condition is not very important * in determining * Alway, F. J., and McDole, G. R., The Fclation of Movement of Water in a Soil to its Hyqroscopicity and Initial Moistness; Jour. Agr. Ees., Vol. X, No. 8, pp. 391-428, 1917. Israelson, O. W., Studies on Capacities of Soils for Irrigation Water; Jour. Agr. Res., Vol. XIII, No. 1, pp. 1-36, 1918. ^ King, F. H., Fluctuations in the Level and Bate of Movement of Ground Water; U. S. Dept. Agr., Weather Bur., Bui, 5, pp. 59-61, 1892. ' Wollny, E., Untersuchungen iiber die WasserJcapacitdt der Bodenarten; Forsch. a. d. Gebiete der Agri.-Physik, Band 9, Seite 361-378, 1886. * Karraker, P. E., Effect on Soil Moisture of Changes in the Surface Tension of the Soil Solution Brought About By Addition of SolublSi Salts; Jour. Agr. Ees., Vol. 4, No. 2, pp. 187-192, May, 1915. 164 NATURE AND PROPERTIES OF SOILS capillary capacity except in arid or semi-arid regions. In fact, changes in surface tension through any cause are of little practical importance. The finer the texture of a soil the higher is its capillary capacity. This is due to the presence of colloidal material and to the greater number of angles in which capillary water may be held. The amount of .jnternal surface exposed by a fine-textured soil is immensely larger than in one of a sandy character. While texture influences both the inner and outer capillary water the structure of the soil has more to do with the active film-like portion. As a clayey soil is granulated the interstitial spaces are enlarged and an increased capillary capacity results. At the same time, compacting a sand will cause a rise in the capillary capacity of that^- soil by increasing not only the actual effective surface, but also the number of angles possible for capillary concentration. Further compact- ing will then cause a decrease. Organic matter, especially when well decayed, is commonly recognized as having great capillary capacity, far excelling the mineral portion of the soil in this respect. Its porosity affords an enormous internal surface, while its colloids exert an affinity for moisture which raises its water capacity to a very high degree. Its tendency to swell on wetting is but a change in condition incident to an approach to its maximum moisture content, and has a very marked influence on the structure of the soil. The water-holding capacity of muck and peat may range as high as 300 or 400 per cent, based on the dry matter present. Assuming a hygroscopic coefficient of 50 per cent., the capillary figure is still very high. Besides this direct effect, organic matter exerts a stimulus toward better granulation, a condition in itself favorable to increased water-holding power. The capillary water in any soil, other conditions being equal, tends to vary with the height of the column. This comes about from the effect of gravity on the outer portion of the capillary THE FORMS OF SOIL-WATER 165 film, tending to give more water at the base of the column. The condition may be explained em- pirically as follows : If a number of par- ticles carrying maximum capillary films are brought together vertically, the weight of a large portion of the conducting film is thrown momentarily on the surfaces at the top. The capillary spaces at this point immediately lose water downvvard, so that they may assume a greater curvature and thus support this extra weight thrown on them. This curvature must be sufficient to balance the curvature pressure of the par- ticles below plus the weight of the water in the connecting films. The particles be- neath are at the same time undergoing a similar adjustment with a set of particles farther below, losing water in order to allow a change of curvature. The action continues in this manner in an attempt to establish equilibrium, thus giving more water at the bottom of the column. If the amount of capillary water is too great to be supported, enough is lost by gravity to bring about an equilibrium (see Fig. 29). The above illustration, however, does not apply strictly to soil conditions, since only part of the capillary water is in a true film form and free to move with extreme ease. Moreover, rain water is applied from above, where also occurs rapid evaporation. Thus at any particular time the moisture content of a field soil might be higher near the surface than farther down in the soil Fig. 29. — Diagram showing in a con- ventional way the adjustment tendency of the outer capillary water in a long column and the appearance of free water if the weight is too great. 166 NATURE AND PROPERTIES OF SOILS or vice versa as the case may be. As the capillary water in a soil is reduced there is a tendency for the soil column to be more nearly uniform, providing, of course, that the equi- librium forces have had time to act and are not too much influenced by other factors. While representative data regarding the moisture-holding capacity of soils are difficult to give, the following figures from Always indicate the general effect of texture and organic matter. The maximum water capacity was determined in the laboratory and the maximum field capacity was obtained by sampling the soils very shortly after irrigation. Table XXXI THE MAXIMUM WATER CAPACITY OF VARIOUS SURFACE SOILS AS DETERMINED IN THE LABORATORY AND UNDER FIELD CONDITIONS, RESPECTIVELY " Soils Organic Matter % Hygro- scopic Co- efficient % Field Water Capacity % Maximum Water Capacity, Laboratory Method % Sand 1.22 1.07 1.55 4.93 2.22 1.1 1.7 3.3 10.0 10.1 10.2 12.9 11.7 12.8 19.6 31.5 31.3 39.2 47.6 37.0 Sand 27.1 Sandy soil, residual. Red loam, residual . . Silt loam, loess Silt loam, loess Black adobe 34.2 49.0 56.8 60.9 60.3 The effect of texture on water capacity is very apparent, a rough correlation existing also between the water retained and the hygroscopic coefficient. The influence of organic matter ^ Alway, F. J., and McDole, G. K., The delation of Movement of Water m a Soil to its Hygroscopicity and Initial Moistnessi; Jour. Agr. Ees., Vol. X, No. 8, pp. 391-428, 1917, * Note again that moisture percentages are always expressed on dry- soil weight. THE FORMS OF SOIL-WATER 167 is clearly shown by the two loess silt loams. Perhaps most important of all is the marked discrepancy between the actual field capacity and the arbitrary and artificial laboratory method. The normal water-holding capacity of a mineral soil, varying with texture and organic matter, seems to range from Xv X \ \ ^N » >v S \ > . s 40 \ \ \ t \^ \ C2 % < ^0 \ N . V H ' t \ '' U \ \ \ \ \ \ 1 20 \ N . \ \ \^ \ *x^ ^>>^,^^\ o -ii/Jb V 10 *~" h-o^N ^^^^ "v^ «) <■• ^v tu '^Cv u Z '~"~— ~. 10 15 20 25 50 %WATER Fig. 30. — Diagram showing the distribution of moisture in capillary columns of soil of different textures. The end of each column rests in free water. (Buckingham, E., Bur. Soils, Bui. 38, 1907.) about 10 to 50 per cent, based on dry soil. Muck and peat of course run much higher, 400 per cent, being not uncommon.^ * Briggs and McLane have perfected a method of comparing soils on the basis of their capacity to hold water against a definite and constant centrifugal force of one to three thousand times the force of gravity. The soils, in thin layer, are placed in perforated brass cups which fit into a centrifugal machine capable of developing the above force, and are whirled until equilibrium is reached. The resultant moisture per- centage is designated as the moisture equivalent. It really represents the capillary capacity of a soil of minimum column length when subject to a constant and known force or pull. The finer the soil, the greater of course is the moisture equivalent. The authors found that 1 per cent. of clay or organic matter represented a retentive power of about .62 168 NATURE AND PROPERTIES OF SOILS 92. Capillary movement of water. — It has already been shown that different thicknesses of capillary films tend to equalize in the soil due to the pulling forces developed by the angle of curvature between the particles.^ It is evident that differences in curvatures must be the motive force in the capil- lary movement of soil-water. Let it be supposed, for conveni- ence, that three equal spheres when brought in contact contain unequal amounts of water in the angles of curvature (see Fig. 31). In this case the greater pull would exist at A, since the angle here is more acute. Consequently water must move per cent., while 1 per cent, of silt corresponded to a retention of only .13 per cent, of water. Kepresentative data is as follows: Soils Norfolk coarse sand. . . Norfolk fine sandy loam. Yazoo loam Waverly silt loam Houston clay loam. . . . Houston clay Organic Matter % .9 1.3 1.3 2.0 3.7 1.4 Sands Silt Clay % % % 87.9 7.3 4.8 73.4 18.1 8.5 25.8 64.1 10.1 14.9 62.9 22.2 30.9 42.5 26.6 10.0 56.6 33.4 Moisture Equivalent % 4.6 6.8 18.9 24.4 32.4 38.2 Briggs, L. J., and McLane, J. W., The Moisture Equivalent of Soils; U. S. Dept. Agr. Bur. Soils, Bui. 45, 1907. ^ An ingenious method for measuring quantitatively the capillary pull exerted by a moist soil has been devised by Lynde and Dupre. The apparatus consists of a glass funnel joined to a thick-walled capillary tube by means of a piece of rubber tubing, a water seal being used at this point. The lower end dips into mercury. The soil to be studied is placed in the funnel, and after being saturated is connected by means of a wick of cheesecloth or filter paper to the water column previously established in the capillary tube. If no break occurs between the soil and the capillary water column, the apparatus is ready for use. The excess water having drained away, there is a thinning of the films on the soil surface due to evaporation. Equilibrium adjustments now take place, which result in the drawing upward of the water column. The mercury follows, and the strength of the pull may be measured by the length of the mercury column. The old method of measuring capil- lary power by the water movement through a dry soil is vitiated by two conditions — the length of time necessary, and the fact that the maximum lift cannot be obtained due to excessive friction. This new method uses a wet soil, requires only a short time, and gives a more nearly accurate idea of the power of the capillary pull. It does not, however, THE FORMS OP SOIL-WATER 169 through the connecting film until the pull at A and that at B become the same. Such an adjustment might go on over a large number of films, and if one end of the column was ex- posed to an evaporation of just the correct rate and the other end was in contact witli plenty of moisture, large quantities of water would be moved by capillarity. This capillary movement may go on in any direction in the soil, since it is largely independent of gravity; yet under natural field conditions the adjustment tends to take place very largely in a vertical direction, due to evaporation and absorption by plants. When a soil is exposed to evaporation, the surface films are thinned and water moves upward to adjust the tension. This explains why such large quantities of soil-water may be lost so rapidly from an exposed soil. Capillary adjustment may go on downward, also, as is the ease after a shower. Here the rapidity of the adjustment is aided by gravity and movement of the water of percolation. The capillary adjustment in a soil tends to take place whether or not the soil column is in contact with free water. If no gravity water is present, the adjustment is merely from a moist soil to a drier one. In studying the rate and height of capillary movement of water in any soil, especially in the yield data regarding rate of movement, — a factor of vital importance to plant growth. Lynde and Dupre, in their results, confirm the statements already made regarding the relation of texture to capillary power : Soil Medium sand. . . Fine sand Very fine sand. Silt Clay Diameter op Grains in Millimeters .50 - .25 .25 - .10 .10 - .05 .05 - .005 .005- — Lift of Water Column, in Feet 1.78 4.05 9.99 26.80 Lynde, C. J., and Dupre, H. A., On a New MetJiod of Measuring the Capillary Lift in Soils; Jour. Amer. Soc. Agron., Vol. 5, No. 2, pp. 107-116, 1913. 170 NATURE AND PROPERTIES OF SOILS laboratory, the maintenance of a supply of free water is usually provided for, since this allows a nearer approach to the maximum capillary capacity for any point in the column and also gives the most rapid capillary adjustment. To persons familiar with the habits of growing plants, it is evident that capillary movement must play an important part in their nutrition, since the rootlets are unable to bring their absorptive surfaces in contact with all the interstitial spaces in which the bulk of the available water is held. Con- sequently a consideration of the movement of capillary mois- FiG. 31. — Conventional diagram showing the mechanics of the movement of the film portion of the capillary water. The readjustment takes place in the direction of (A) due to the tension developed by the greater film curvature at that point. ture is necessary, not only as to its mechanics, but also in respect to the factors influencing its rate and height of move- ment. These factors are as follows: (1) surface tension and viscosity; (2) thickness of capillary film; (3) texture; and (4) structure. Surface tension and viscosity. — As the force developed by surface tension is the activating factor in capillary adjust- ment, any change in the former will influence this movement. Theoretically, a rise in temperature or the presence of soluble salts would decrease the rapidity of the capillary activity of soil-water. In a normal soil, however, the change of surface tension is generally not sufficient to have any very great prac- tical influence. Viscosity, on the other hand, is much more important. If the viscosity of water at 0° C. is taken as 100, THE FORMS OF SOIL-WATER 171 its viscosity at 25° is 50 and at 30°, 45. Tliis explains to a large degree the increased rate of capillary movement due to temperature rise.^ The distance of such adjustment would, however, be lessened somewhat. Salts in solution would tend to check the rate of capillary movement both through in- creased viscosity and the influence on surface tension.- It would only be in alkali soils, where the concentration of soluble salts is very great, that any considerable retardation would occur. Thickness of capillary film. — It has been repeatedly noticed, in the studj^ of the capillary adjustment between two soils that the lower the percentage of water, the slower is the move- ment. This indicates that the thickness of the outer capillary film, which connects the interstices in which lies the bulk of the movable soil-water, is an important factor in the rate of movement. The above phenomena may be empirically explained as fol- lows: Let it be supposed that a withdrawal of water occurs at A (see Fig. 32), the interstitial space between two of the particles, the water surface being represented by the line aa'. There is an immediate increase in the curvature of this sur- face, and water tends to flow through the capillary film chan- nel (cc'c'') toward this area of greater tension. If water * Bouyoucos has shown that the movement in a soil column of uniform moisture is from the warmer portion toward the colder. The movement from a moist layer to a dryer one goes on more rapidly than when the moist soil is cool and the dry soil warm. Bouyoucos, G. J., Effect of Temperature on Movement of Water Fapor and Capillary Moisture in Soils; Jour. Agr. Ees., Vol. V, No. 4, pp. 141-172, Oct., 1915. *Wollny, E., Untersuchungen iiber die Eapillare Leitung des Wassers in Boden. Forsch. a. d. Gebiete d. Agr.-Physik, Band 7, "Seite 269-308, 1884. Also, Forsch. a. d. Gebiete d. Agri.-Physik, Band 8, Seite 206-220. 1885. Briggs, L. J., and Lapham, M. H., Capillary Studies; U. S. Dept. Agr. Bur. Soils, Bui. 19, ppj. 5-18, 1902. Karraker, P. E., Eff'ect on Soil Moisture of Changes in the Surface Tension of the Soil Solution brought about by the Addition of Soluble Salts; Jour. Agr. Ees., Vol. IV, No. 2, pp. 187-192, May, 1915. Davis, E. O. E., The Eff'ect of Soluble Salts on the Physical Proper- ties of Soils; U. S. Dept. Agr. Bur. Soils, Bui. 82, pp. 23-31, 1911. 172 NATURE AND PROPERTIES OF SOILS continues to be withdrawn at A, this adjustment goes on with considerable ease until the film channel (cc'c'O becomes so thin as to cause its surface now (bb'b") to approach very closely to the surface of the soil particle and the inner capil- lary water. The sluggishness of the water movement becomes a factor at this point, impeding the capillary adjustment to- ward A. This point of sluggish capillary movement has been designated by Widtsoe^ as the point of lento-capillarity, and Fig. 32. — Conventional diagram for the explanation of the effect of the thickness of water film about the soil particles and their colloidal complexes on the ease of capillary adjustment. is expressed in percentage based on the dry weight of the soil. It lies near the transition zone between the inner and outer capillary water. The amount of capillary water delivered at any one point, therefore, will obviously be influenced by the thickness of the film and may consequently be taken as a measure of rate of adjustment. A short soil column should deliver more water than a longer one, due to the thicker films at the surface of the former. King,^ in studying the evaporation from the sur- faces of sand columns of different lengths, their bases being in contact with free water, obtained some significant data. ^Widtsoe, J. A., and McLaughlin, W. W., The Movement of Water in Irrigated Soils; Utah Agr. Exp. Sta., Bui. 115, pp. 223-231, 1912. " King, F. H., Prvnciples and Conditions of the Movements of Ground Water; U. S. Geol. Survey, 19th Ann. Kept, Part II, p. 92, 1897- 1898. Also Briggs, L. J., and Lapham, M. H., Capillary Studies; U. S. Dept. Agr. Bur. Soils, Bui, 19, pp. 24-25, 1902. THE FORMS OF SOIL-WATER 173 He found, for example, that a six-inch column would deliver six times more water to its surface in a given time than a thirty-inch column operating under the same conditions. In air-dry soil it is obvious that, before capillarity may function, a continuous film must be present. Such a condi- tion is impossible unless some of the more active capillary moisture is in the soil. The water content in a soil must often be rather high before capillarity is a noticeable phenomenon. This condition is taken advantage of in the use of soil-mulches, where a loose dry layer of soil on the surface may check evaporation by impeding capillary rise. The presence of oily substances on the soil grains may also be of some importance in this respect. Texture. — In soils of fine texture not only is the amount of film surface exposed greater than in coarse soils but the curvature of the films is also greater, due to the shorter radii. The effective pressure exerted by the films is consequently much higher in fine-grained soil. Both the greater exposure of surface and the increased pressure serve to raise the fric- tion coefficient and retard the rate of flow. The finer the texture of the soil, other factors being equal, the slower is the movement of capillary water. Water should, therefore, rise less rapidly from a water-table through a column of clay than through a sand or a sandy loam. The distance to which water may be drawn by the effective capillary power of a soil, equilibrium being established, de- pends on the number of interstitial angles. The greater the number of angles, the greater is the total pulling power of the films. As a silt soil contains a larger number of such angles, its capillary pull is greater than that of sand, and con- sequently the ultimate movement would be of greater scope. The finer the texture, then, the slower is the rate of capillary movement but the greater is the distance. The relation of texture to rate and height of capillary move- ment in air-dry soil is shown by the following unpublished 174 NATURE AND PROPERTIES OF SOILS data, obtained in the laboratory of tbe Department of Soil Technology, Cornell University: Table XXXII EFFECT OF MOISTURE ON RATE AND HEIGHT OF CAPILLARY RISE FROM A WATER-TABLE THROUGH AIR-DRY SOIL SOIL 1 HouE 1 Day 2 Days 3 Days 4 Days 5 Days Sandy soil Clayey soil Silt loam Inches 3.5 .5 2.5 Inches 5.0 5.7 14.5 Inches 5.9 8.9 20.6 Inches 6.8 10.9 24.2 Inches 6.8 12.2 26.2 Inches 6.9 13.3 27.4 It is seen that the movement in sand is rapid, one-half of the total rise being attained in one hour. The maximum height is reached in about three days. The silt loam in this case seems to be of just about the proper textural condition for a fairly rapid rise, yet it exerts enough capillary pull to attain a good distance above the water-table. The friction in the clay is greater, however, and this results in a slower rate. Structure has already been shown to affect capillary capac- ity by its influence on the angle interstices and the closeness of the contacts. Evidently, therefore, it may alter both the rate and the height of capillary rise. The loosening of a clay soil or the compacting of a sandy soil will lessen the effective film friction, while at the same time it may strengthen the capillary pull resulting in a faster and a higher capillary flow of water. What may be the best structural condition of any soil in which this result is realized to its highest degree can not be predicted exactly. In general, however, this point is approached when the soil is in the best physical condition for crop growth. Tillage operations, tile drainage, and the addi- tion of lime and organic matter operate toward this result by their granulating tendencies; while rolling, by compacting a THE FORMS OF SOIL- WATER 175 too loose .surface, may accomplish the same effect but by an opposite process. At certain seasons of the year capillarity should be im- peded near the surface, as it continually carries valuable water upward to be lost by evaporation. Tliis movement may be checked somewhat by producing on the soil surface, by appropriate tillage, a layer of dry, loose soil. This layer, called a soil-mulch, resists wetting because of its dryness, while at the same time it affords but little surface and few angle inter- stices for effective capillary pull. Moisture also moves very slowly from a moist, cool soil to a dry, warm one.^ Thus it is that a farmer, in order to meet immediate or future plant needs, may alter and control capillary movement by careful attention to physical conditions, especially those at the sur- face where evaporation is always active. 93. Gravitational water and its movement. — As soon as the capillary capacity of a soil column is satisfied, further addition of moisture will cause the appearance of free water in the air spaces. By the attraction of gravity, this water moves forward through the soil at a rate varying with con- ditions. In general, the flow is governed by four factors — pressure, temperature, texture, and structure. An under- standing of the operation of these forces is important, since the rapid elimination of free water from the soil is necessary for normal plant growth. It is very evident that any pressure exerted on a water column will alter the rate of flow. Under normal conditions pressure may arise from two sources, atmospheric pressure and the weight of the water column. Changes in barometric pressure are communicated to gravitational water through a movement of the soil-air. As the mercury column rises more air is forced into the soil and the pressure on the soil-water ^ Bouyoueos, G. J., Effect of Temperature on Movement of Water Vapor and Capillary Moisture in Soils; Jour. Agr. Ees., Vol. V, No. 4, pp. 141-172, Oct., 1915. 176 NATURE AND PROPERTIES OF SOILS increases. The weight of tlie free water column may also have some influence. Although King^ and Welitschkowsky- have shown that definite relationships exist between the move- ment of gravity water and both atmospheric pressure and weight of water column, the practical field importance of these factors are rather slight. A rise in temperature of the soil not only varies the relative amounts of capillary and free water present, but at the same time it increases the fluidity and thus facilitates percolation. The expansion of the soil-air also tends to increase such movement. On the other hand the swelling of hydrogels which may be present tends to impede percolation to such an extent that the movement of free water through a heavy soil is often markedly checked by temperature rise. Of much more practical importance than either pressure or temperature in the flow of gravity water is the texture and the structure of the soil. In working with sands of varying grades, Welitschkowsky,^ WoUny,* and others have shown that the flow of water varies with the size of particle, or texture. King ^ has demonstrated that in general the rate of flow through such is directly proportional to the square of the diameter of the particles. By the use of the effective mean ' King, F. H., Principles and Conditions of the Movements of Ground Water; U. S. Geol. Survey, 19th Ann. Dep^., Part II, pp. 67-206; 1897- 1898. King, F. H., The Soil, p. 180, New York, 1906. *Welitschkowsky, D. von., Experimentelle untersuchungen iiber die Permeabilitdt des Bodens fiir Wasser; Archiv. f. Hygiene, Band II, Seite 499-512. 1884. WoUny, E., Untersuchungen iiber die Permeabilitdt des Bodens fiir Wasser; Forsch. a. d. Gebiete d. Agr.-Physik, Band 14, Seite 1-28, 1891. " Welitschkowsky, D. von., Experimentelle untersuchungen iiber die Permeabilitdt des Bodens fiir Wasser; Archiv. f. Hygiene, Band II, Seite 499-512, 1884. * WoUny, E., Untersuchunger iiber den Einfluss der StruMur des Bodens auf dessen Feuchtighetis — und Temperaturverhaltnisse ; Forsch. a. d. Gebiete d. Agr.-Physik, Band 5, Seite 167, 1882. "King, F. H., Principles and Conditions of the Movements of Ground Water; U. S. Geol. Survey, 19th Ann. Rep., Part II, pp. 222-224, 1897- 1898. THE FORMS OF SOIL-WATER 177 diameter of a sand sample he was able to calculate a theo- retical flow which compared very closely to observed percola- tions. In sandy soils low in organic matter this law holds in a very general way, but in clays it fails entirely. For example, if such a law was in force a sand having a diameter of .5 millimeter would exhibit a flow 10,000 times greater than that through a clay loam with a diameter, say, of .005 millimeter; whereas the actual ratio, as observed experimen- tally by King, was less than 200, Such a discrepancy is to be expected as it is impossible accurately to apply mathematics to soils carrying any appreciable amount of colloidal matter. Evidently, therefore, while it can be stated as a general thesis that the flow of gravity water varies with the texture, being much more rapid through a coarse than through a fine soil, no law can be deduced for soils, since structure exerts such a modifying influence. The percolation in a heavj' soil takes place largely through lines of seepage, which are really large channels developed by various agencies. If in the drainage of average soil, the farmer depended on the movement of water through the individual pore spaces, the soil would never be in condition for crop growth. These lines of seepage are developed by the ordinary forces that function in the production of soil granulation, as freezing and thawing, wetting and drying, lime, organic matter, roots, and tillage operations. 94. Determination of the quantity of free water that a soil v^ill hold. — While there is no particular advantage in finding the quantity of gravitational water that a soil will hold, since a normal soil should never remain saturated for any length of time, it is nevertheless of interest to know by what means such data may be obtained. One method is to saturate a soil column of known weight, and then, by exposing it to percolation, measure the amount of water that is lost. The gravitational water can then be expressed in terms of dry soil. 178 NATURE AND PROPERTIES OF SOILS As valuable a figure may be obtained by calculation, pro- viding the specific gravity and volume weight of the soil is known together with its percentage of moisture based on dry weight when it is capillarily satisfied. The following formu- Iffi ^ may be used : Tvol. wt. 1. Percentage pore space = 100 iOO] Lsp. gr. 2. Percentage free water = % Pore Space _ ^^ ^^^^^ ^^ (based on dry weight ^^^- ^^- maximum of soil) capillarity Suppose, for example, that a sand with a specific gravity of 2.6 and a volume weight of 1.56 contains 20 per cent, of water when at its maximum retentive power. Its pore space would be 40 per cent. If this pore space were filled with water, the soil would contain 25.6 per cent, based on the dry weight of the soil (per cent, pore space -=- vol. wt.). If the total capac- ity of the soil for water is 25.6 per cent, and the hygroscopic plus the capillary capacity is 20 per cent., the free Avater must be 5.6 per cent.^ 95. Importance of the study of the flow and composi- tion of drainage water. — A clear understanding of the factors governing the flow of gravitational water is of special importance in tile drainage operations, particularly regarding the depth of and interval between tile drains. Since percola- tion is so slow in a heavy soil it is evident that the tile must be near the surface in order to secure efficient drainage. In a sand the depth may be increased, because of the slight re- ^ Percentage of pore space represents the percentage of water by volume that would occupy such a space. Percentage of water by volume divided by volume weight gives percentage of water based on dry weight of soil. Conversely, multiplying percentage of moisture calculated on dry weight of soil by volume weight will give percentage of water by volume. The air space in a soil at any particular moisture content may be cal- culated as follows: Percentage of air space = % pore space — (%H20 X Vol. Wt.) ^ Below will be found some generalized moisture data on two distinct THE FORMS OP' SOIL-WATER 179 sistance offered to water movement. The depths for laying tile in a heavy soil range from one and a half to two and a half feet, while in a sand the tile may often be placed as deep as four feet below the surface. It is evident also that the less deep a tile drain is laid the less distance on either side it will be effective in removing the water; consequently on a clay soil the laterals must be relatively close as compared to the interval generally recommended for a sandy soil. A rational understanding of the movements of gravitation water is clearly necessary in the installation of tile drains not only that the system may be efficient, but also that a minimum effective cost may be realized.^ The water lost from the soil by drainage is of especial in- terest in plant production because of the large amounts of nutrient elements carried away each year. Such loss is par- ticularly important in regard to the lime and nitrogen.- The equivalent of approximately 500 pounds of sodium nitrate and 1000 pounds of calcium carbonate have been known to leach from an acre of bare soil every year under humid con- ditions. classes of soils. As usual, all of the moisture data is expressed as per- centage based on absolutely dry soil. Sandy Clayey Soil Soil Specific gravity 2.67 2.65 Volume weight 1.60 1.20 Pore space 40.0% 54.8% Hygro. coefficient 1.0% 10.0% Optimum moisture (average) 10.0% 30.0% Maximum field capacity 17.0% 44.0% Air space at hygro. coefficient 38.4% 42.8% Air space at opt. moisture 24.0% 18,7% Air space at max. field capacity 12.8% 1.9% Possible free water 8,0% 1.6% See Kopecky, J., Die physilcalischen Eigenschaften des Boden; Internal Mitt f. Bodenkunde, Bd. IV, Heft 2-3, Seite 138-180. 1914. ^ For a more complete discussion of tile drains, see Chap. X, para- graph 110. 'Lyon, T, L., and Bizzell, J, A,, Lysimeter Experiments; Cornell Univ, Agr. Exp. Sta., Memoir 12, June, 1918. 180 NATURE AND PROPERTIES OF SOILS Two methods of procedure are available for the study of drainage problems — the use of an efficient system of tile drains, and the construction of lysimeters. For the first method an area should be chosen where the tile drain receives only the water from the area in question and where the drain- age is efficient. A study of the amounts of flow throughout a term of years will yield much valuable data concerning the factors already discussed. An analysis of the drainage water will throw light on the ordinary losses of plant nutrients from a normal soil under a known cropping system. The advantage of such a method of attack lies not only in the fact that a large area of undisturbed soil is considered, but also in the opportunity to study practical field treatments in relation to the movement and composition of drainage water. The lysimeter method, however, has been the usual mode of approaching such problems. In this method a small block of soil is used, being entirely isolated by appropriate means from the soil surrounding it. Effective and thorough drainage is provided. The advantages of this method are that the varia- tions in a large field are avoided, the work of carrying on the study is not so great as in a large field, and the experiment is more easily controlled. One of the best-known sets of lysi- meters is that at the Rothamsted Experiment Station^ in Eng- land. Here blocks of soil one one-thousandth of an acre in surface area were isolated by means of trenches and tunnels, and, supported in the meantime by perforated iron plates, were permanently separated from the surrounding soil by masonry. The blocks of soil were twenty, forty, and sixty inches in depth, respectively. Facilities for catching the drain- age were provided under each lysimeter. The advantages of such a method of construction lies in the fact that the struc- tural condition of the soil is undisturbed and consequently the data are immediately trustworthy. *Lawes, J. B., Gilbert, J. H., and Waring^ton, R., On the Amount and Composition of the Bain and Drainage Waters Collected at Bothar.i- sted; Jour. Roy. Agr. Soc, Ser. II, Vol. 17, pp. 269-271, 1881. THE FORMS OF SOIL-WATER 181 At Cornell University^ a series of cement tanks sunk in the ground have been constructed. Each tank is about four feet and two inches square and about four feet deep. A slop- ing bottom is provided, with a drainage channel opening into <^i^ '^^^//A^-r<^^^^<^///d0'^ -o A O Fig. 33. — Cross section of the lysimeter tanks at Cornell University, Ithaca, New York. Each tank is one of a series, one tmiHel serving the two rows. Dimensions are given in feet and inches. Soils under investigation (a), outlet (p), can for catching drainage water (c) and sky-light (w). a tunnel beneath and at one side. As the tanks are arranged in two parallel rows, one tunnel suffices for both. (See Fig. 33.) The sides of the tanks are treated with asphaltum in ^ Lyon, T. L,, Tanks 'for Soil Investigation at Cornell University; Science, N. Ser., Vol. 29, No. 746, pp. 621-623, 1909. There are other types of lysimeters. See, for example, Mooers, C. A., and Maclntire, W. H., Two Equipments for Investigation of Soil Leach- ings: I. A Pit Equipment. II. A Hillside Equipment; Tenn. Agr. Exp. Sta., Bui. Ill, 1915. Maclntire, W. H., and Mooers, C. A., A Pitless Lysimeter Equip- ment; Soil Sci., Vol. XI, No. 3, pp. 207-209, Mar., 1921. 182 NATURE AND PROPERTIES OF SOILS order to prevent solution. The soil must of course be placed in the tanks, this causing a disturbance of its structural con- dition. As a consequence, data as to rate of flow and com- position of the drainage water are rather unreliable for the first few years. Such an experiment must necessarily be of considerable duration. 96. Thermal movement of water. — Little has been said as yet regarding this mode of water movement, the vapor flow, which is not peculiar to one form of soil-water but affects them all. It is at once apparent that the movement of water- vapor can be of little importance within the soil itself, since it depends so largely on the diffusion and convection of the soil-air. While the soil-air is no doubt practically always saturated with water-vapor, the loss of moisture by this means is slight. Buckingham ^ has shown that, while sand allows such a movement to the greatest degree, the loss through any appreciable depth of layer is almost negligible. The question of the thermal movement of water at the soil surface, however, is vital in farming operations. At this point the moisture is exposed to sun and wind, and drying goes on rapidly, the free, capillary, and a part of the hygroscopic water vaporizing in the order named. If the loss of the moisture in the surface layer of soil was the only consideration, the problem would not be serious; but the movable water of the whole soil sec- tion must be considered also. As the films at the surface be- come thin, a capillary movement begins, and if the evapora- tion is not too rapid a considerable loss of water may occur in a short time. The moisture thus lost is that of most value to plants. The evaporation from the bare soil in the Rotham- sted lysimeters^ averaged about seventeen inches a year, with ^ Buckingham, E., Studies on the Movement of Soil Moisture; U. S. Dept. Agr. Bur. Soils, Bui. 38, pp. 9-18, 1907. See also Bouyoucos, G. J., Effect of Tem^perature on Movement of Water Vapor and Capillary Moisture in Soils; Jour. Agr. Ecs., Vol. V, No. 4, pp. 141-172, Oct., 1915. =" Warington, R., Physical Properties of the Soil, p, 109; Clarendon Press, Oxford, 1900. THE FORMS OF SOIL-WATER 183 a rainfall ranging from twenty-two to forty -two inches. This means that from one-third to one-half of the effective rainfall was entirely lost as thermal water. The necessity of checking such a loss becomes apparent, especially in regions where rainfall is slight or drought periods are likely to occur. As no country is free from one or the other of such con- tingencies, the great prominence that methods of moisture conservation hold in systems of soil management is under- standable. While means of checking losses by leaching and run-off are advocated, effective retardation of surface evapora- tion is always emphasized. CHAPTER IX THE WATER OF THE SOIL IN ITS RELATION TO PLANTS Water begins its service to plants by promoting the proc- esses of soil weathering, which results in the simplification of compounds for plant utilization. It also functions more di- rectly in plant development in maintaining the turgidity of the cells, in carrying materials, regulating temperature and in furnishing a supply of hydrogen and oxygen for the plant. These direct and indirect functions of water in relation to plant growth may be considered from a number of different viewpoints. 97. Functions of water to plants. — Water acts as a solvent and as a medium for the transfer of nutrients from the soil to the plant. This transfer relationship is rather complex, since most nutrient materials penetrate the cell-walls of the absorbing surfaces of the roots in an ionic condition. As a nutrient water becomes a part of the cell contents with- out change or is broken down into its elements and utilized in the production of new compounds. In addition, water by maintaining turgidity, in equalizing the temperature by evap- oration from the leaves, and in facilitating quick shifts of nutrients and food from one part of the plant to another, acts as a carrier during assimilation and while synthetic and metabolic processes are going on. Soil-moisture, therefore, in proper amounts, becomes one of the controlling factors in crop growth and must be looked to before the maximum utilization of the nutrient elements can be expected. The amount of water held within the plant 184 WATER OF SOIL IN ITS RELATION TO PLANTS 185 is not large, however, in comparison with the amount lost by transpiration, although green plants contain from 60 to 90 per cent, of moisture. Because of the readiness with which moisture passes from plants into the atmosphere, large quantities must be taken 16 Q Z :> o Q. U. O tf) Q Z > y Yi y "^1 Y ' UNAVA1LABLE\ available SUPERFLUOUS AVAILABLE UNDER CERTAIN CONDITIONS Fig. 35. — Diagram showing the various forms of water that may be present in the soil and their relations to higher plants. able for normal plant use. However, when free water begins to appear, a condition detrimental to growth is established, conditions becoming more adverse as the saturation point is approached. This free water is designated as the superfluous water and its presence generates conditions unfavorable to plants. The bad effects of free water on the plant arise largely from the poor aeration that results from its presence.^ Not only are the roots deprived of their oxygen, but toxic materials tend to accumulate. Favorable bacterial activities, such as the production of ammonia and nitrates, are much re- * Alway, F. J., Studies on the Eelation of the Non-available Water of the Soil to the Hygroscopic Coefficient ; Nebr. Agr. Exp. Sta., Res. BuL 3, 1913. ' It must be kept in mind that in a clayey soil the superfluous water may include some of the upper capillary moisture. 200 NATURE AND PROPERTIES OF SOILS tarded also. The various forms of water in the soil and their availability to the plant are illustrated diagrammatically in Fig. 35, page 199. This diagram may be evaluated in a general way as below, using the sandy and clayey soils for which full physical data have already been given in Chapter VIII. (See footnote on page 179.) Table XXXVIII THE EVALUATION OF FIG. 35 FOB A SANDY AND CLAYEY SOIL, RESPECTIVELY. Hygroscopic coefficient , Wilting point Maximum field capacity Unavailable water Available water Superfluous water Sandy Soil Clayey Soil 1.00 10.00 1.47 14.70 17.00 44.00 1.47 14.70 15.53 29.30 8.00 1.60 106. Optimum moisture for plant growth. — It is very evident that there must be some moisture condition of a soil which is best for plant development. This is usually desig- nated as the optimum content. It is not to be assumed, how- ever, that the total range of the available soil-water repre- sents this condition. Nor is this optimum water content in any particular soil to be designated by a definite percentage. In reality the moisture in a soil may undergo considerable fluctuation and yet allow the plant to develop normally.^ This ie because the physical condition of the soil changes with varying water content and the plant is able to accommodate *Wollny, E., Untersuchung iiber den Einfiuss der Wachsthumsfaktoren auf des Produktionsvermoqen der Kulturj)flanzen ; Forsch. a. d. Gebiete d. Agri.-Physik., Band 20,'Seite 53-109, 1897. Mayer, A., fiber den Einfiuss Jcleinerer oder grosser er Mangen von Wasser auf die EntwicTcelung einiger Kulturpflanzen ; Jour. f. Landw., Band 46, Seite 167-184, 1898. WATER OF SOIL IN ITS RELATION TO PLANTS 201 itself to such a fluctuation without a disturbance in its normal development. Granulation has considerable influence on the range of optimum moisture conditions, since the better the granulation, the better able is the soil to accommodate itself to changes in water content without a disturbance of normal growth. In moisture conservation-^and control, a granular soil is one of the first improvements to be aimed at. Drainage, liming, addition of organic matter, and tillage, by leading up to such a condition, increase the effectiveness and economy of soil moisture utilization. Many of the ordinary farming operations have to do with the maintenance of an optimum moisture condition in the soil. During periods of excessive rainfall, especially during the growing season, conditions should be such as to allow the pres- ence of free water in the soil for the briefest time possible. This means adequate under-drainage and satisfactory arrange- ments whereby the run-off may be removed with but little damage. Moisture control also demands conservation meth- ods of more or less intensity in arid and semi-arid regions sup- plemented by irrigation, whereby the soil-moisture may never drop much below the point of lento-capillarity. By such ar- rangments the optimum moisture conditions, so essential to normal and uninterrupted crop growth, are maintained. CHAPTER X TEE CONTROL OF SOIL-MOISTUBE In the discussion of the water requirements of plants it was apparent that for a normal yield of any crop, the amount used by the plant alone was very great, varying from five to ten acre-inches according to conditions. Were this the only loss of water, the question of raising crops with given amounts of rainfall would be a simple one. Three further sources of water loss, however, are usually operating in the soil and tend- ing to lower the water that would go toward transpiration, a loss absolutely necessary for proper growth. The various ways by which water finds an exit from a soil are : (1) tran- spiration, (2) run-off over the surface, (3) percolation, and (4) evaporation. The diagram (Fig. 36) makes clear their relationships. It is immediately obvious that, as the losses by run-off, leaching, and evaporation increase, the amount of water left for crop utilization decreases. Some control of soil-water is, therefore, necessary both in an arid and a humid region. Under arid and semi-arid conditions, where run-off and per- colation are not of such great importance except where irriga- tion is practiced, loss by evaporation is of especial consequence, as it competes directly with the plant. Under humid condi- tions, losses by percolation and run-off seem to merit the greater attention, because of the loss of nutrients with the former and the erosion damage from the latter. The influence of evaporation, however, is not to be under-estimated or ne- glected. Control of moisture is, therefore, necessary in all regions. This control consists in so adjusting run-off, leach- 202 THE CONTROL OF SOIL-MOISTURE 203 mg, and evaporation as to maintain optimum moisture condi- tions in the soil at all times. Such control should result in a proper and economical utilization of soil-water by the plant. 107. Run-off losses. — In regions of heavy rainfall or in areas where the land is sloping or rather impervious to water, a considerable amount of moisture received as rain is likely to be lost by running away over the surface. Under TR(C^NSP/RflT/ON. EVAPORATION. >V/ ? / Fig. 36. — Diagram illustrating the various ways by which water may be lost from a soil. such conditions two considerations are important: (1) the loss of water that might otherwise be of use to plants; and (2) the erosion that usually occurs when much water escapes in this manner. Of the two, the latter is generally the more impor- tant. The amount of run-off varies with the rainfall and its distribution, the slope, the character of the soil, and the vege- tative covering. In some regions loss by run-off may rise as high as 50 per cent, of the rainfall, while in arid sections it is of course very low, unless the rainfall is of the torrential type as in the arid Southwest. 204 NATURE AND PROPERTIES OF SOILS The quantity of water entering a soil is determined almost entirely by the physical condition of the soil. If it is loose and open, the water enters readily and little is lost over the surface as run-ofif. If, on the other hand, the soil is com- pact, impervious and hard, most of the rainfall runs away, and not only is there a serious loss of water, but considerable erosion may also result. The first step in checking run-off losses, therefore, is strictly physical in nature. Good tillage and plenty of organic matter by encouraging granulation have much to do with the proper entrance of water into the soil as well as with its economic utilization therein. 108. Erosion by water and its control.^ — While every one is familiar with the importance of water in the forma- tion of alluvial and marine soils, the concurrent destructive action that is going on in the uplands is generally overlooked. This is due to the fact that erosion is often considered as more or less uncontrollable, an ill that can not be avoided. In "Wisconsin, for example, 50 per cent, of the tillable land is subject to erosion of economic importance.^ Even in as level a state as Illinois, 17 per cent, of the area is detrimentally eroded.^ The waste by erosion is as great in other states, even those of an arid climate. Davis * has estimated that 870 million tons of suspended material are carried each year into the ocean by the streams of the United States. Since this is only a very small fraction of the soil brought down from the * Davis, K. O. E., Economic Waste from Soil Erosion; U. S. Dept. Agr., Year Book for 1913, pp. 207-220. Eamser, C. E., Terracing Farm Lands; U. S. Dept. Agr., Farmers' Bui. 997, 1918. Eastman, E. E., and Glass, J. S., Soil Erosion in Iowa; la. Agr. Exp. Sta., Bui. 183, Jan., 1919. Fisher, M. L., The Washed Lands of Indiana; Ind. Agr. Exp. Sta., Circ. 90, Feb., 1919. ^Wliitson, A. R., and Dunnewald, T. J., Keep Our Hillsides from Washing; Wis. Agr. Exp. Sta., Bui. 272, Aug., 1916. * Hosier, J. G., and Gustaf son, A. F., Soil Physics and Management, p. 358, Philadelphia, 1917. ••Davis, E. O. E., Economic Waste from Soil Erosion; U. S. Dept. Agr., Year Book for 1913, p. 213, THE CONTROL OF SOIL-MOISTURE 205 uplands by running water, erosion is no insignificant factor in soil management considerations. Two types of erosion are generally recognized, sheet and gully. In the former, soil is removed more or less uniformly from every part of the slope. Gullying occurs where the vol- ume of water is concentrated, resulting in the formation of ravines by undermining and downward cutting. Both types of erosion are serious. A number of different methods for the effective prevention and control of erosion may be utilized. Anything that will increase the absorptive capacity of the soil, such as deep plow- ing, surface tillage, and increase of organic matter, will lessen the run-off over the surface. On steep slopes, however, such influence is of little importance, since during heavy rainfall absorption is too slow to lessen materially the surface losses. In cultivating corn and similar crops, it is important that the last cultivation be across the slope rather than with it. On long slopes subject to erosion, the fields may be laid out in long narrow strips across the incline, alternating the tilled crops, such as corn and potatoes, with hay and grain. The grassed areas tend to check the surface flow of water. Where the slopes are subject to very serious erosion, they should either be reforested or kept in permanent pasture, guarding always against incipient gullying. About the only effective means of controlling sheet erosion is by terracing of some kind. Strong prejudice exists in many communities against terraces, since they usually waste land, are often unsightly and are a serious obstacle to harvesting machinery. The Mangum terrace ^ however, is worthy of es- pecial attention, since it obviates the really serious objections to the ordinary terrace while maintaining the desired water control. The Mangum terrace is generally a broad bank of earth with gently sloping sides, contouring the field at a * First constructed by P. H. Mangum of Wake County, North Caro- lina. 206 NATURE AND PROPERTIES OF SOILS grade from 10 to 12 inches to the 100 feet. It is usually formed by back-furrowing and scraping. The interval be- tween the embankment depends on the slope. Since the terrace is low and broad, it may be cropped without difficulty and offers no obstacle to cultivating and harvesting machinery. It wastes no land, and eliminates breeding places for insects. Small gullies, while at first insignificant, soon enlarge into deep unsightly ravines. While they may be plowed-in or otherwise filled up, such a procedure is generally a waste of time, since the gullies form again with the next heavy wash. A number of different methods are in use for the control of gullying, depending on conditions. Staking is a very common procedure, the size of the stakes increasing with the magni- tude of the gully. The stakes are usually interwoven with brush, although stone, straw, and other material may be utilized. If brush or other loose material is used, it should be staked to the ground or held down by stone or dirt. Other- wise, the water will run beneath the fill and no benefit will result. Dams of earth, concrete, or stone are often installed with success. They must be supplemented by a tile-drain outlet, however, with an elbow just above the dam. The dam checks the water until it rises to the level of the elbow outlet and is then carried away through the tile. Most of the sedi- ment is deposited above the dam and the gully is slowly filled. 109. Percolation losses and their control. — When at any time the amount of rainfall entering a soil becomes greater than its water-holding capacity, losses by percolation will result. The losses will depend largely on the amount and distribution of the rainfall and the capability of the soil to hold moisture. The objectionable features of excessive per- colation are two: (1) the actual loss of water, and (2) the leaching-out of salts that may function as nutrients to plants. The results from the Rothamsted lysimeter ^ from 1871- * Hall, A. D., The Bool^ of the RotJiamsted Experimejits, p. 22, New York, 1917. THE CONTROL OF SOIL-MOISTURE 207 1913 on a bare clay loam three feet deep are interesting as to the light they afford regarding actual drainage losses in humid regions : Table XXXIX PERCOLATION THROUGH A SIXTY-INCH COLUMN OF BARE CLAY LOAM. ROTHAMSTED EXPERIMENT STATION, ANNUAL AVERAGE OF 42 YEARS. Periods EAINFAIjL Inches Drainage Inches Percentage OF Eainfall AS Drainage Dec.-Feb Mar.-May June-Aug 6.77 5.96 7.83 8.29 5.58 2.11 1.82 4.50 82.4 35.4 23.2 Sept.-Nov 54.2 Mean Total 28.85 14.01 48.8 It appears from these figures that the drainage loss is much lower in summer than winter, the ratio being about one to three. It is also to be noted that about 50 per cent, of the rainfall in such a climate as England is lost by percolation through a bare soil. This compares fairly well with Wollny's ^ summary on eighteen soils in England, Switzerland, and Ger- many. These soils, most of which were bare, f^howed a loss of over 41 per cent, of the rainfall by drainage. Recent results,- due to variable conditions, are by no means in agreement, ranging from a low to a very high percentage loss of the rainfall. It seems fair to assume, however, that, as soils are handled in humid regions, over half of the rain- fall is lost by percolation and run-off combined. Percolation seems to be influenced, not only by the amount * Wollny, E., Untersuchungen uber die Sickerwassermengen in verscMe- denen Bodenarten; Forsch. a. d. Gebiete d. Agri.-Physik., Band 11, Seite 1-68, 1888. ' For excellent review of literature see Lyon, T. L., and Bizzell, J. A., Lysimeter Experiments ; Cornell Agr. Exp. Sta., Memoir 12, June, 1918. 208 NATURE AND PROPERTIES OF SOILS of rainfall and its distribution, but also by evaporation, the character of the soil, and the presence of a crop. As the rain- fall increases, percolation increases, being much greater in New York, for example, than in Utah. Evaporation has a marked influence, reducing drainage losses to a considerable degree. The drainage through sandy soils is generally larger than through clayey soils under strictly humid conditions and where run-off is a factor. When evaporation is high, sandy soils have been known to percolate very much less than those of a heavier nature.^ Field crops, in that they utilize a large amount of moisture, have always been found to reduce per- colation losses. The loss of moisture by percolation is the least objectionable feature of the phenomenon, since it is often necessary, espe- cially during the spring and summer, to rid the soil very quickly of superfluous water. The loss of nutrient salts is more vital, since the materials so carried away might be used by plants. The loss of nitrogen, calcium, and potassium from a bare clay loam at Cornell University ^ over a period of ten years averaged, respectively, 69, 398, and 72 pounds an acre annually. This is equivalent to an acre loss of 419 pounds of sodium nitrate, 995 pounds of calcium carbonate and 137 pounds of potassium chloride every year, which is a larger amount of nutrient material than is removed by an average crop. Control of percolation is exerted, not so much to save water, as to conserve nutrients. As water enters a soil it moves downward and is continually changing into the capillary state. If the absorptive capacity of the soil is high, little of the rain- fall may appear as drainage. The presence of organic matter and the influence of good tillage will do much toward check- ing drainage losses. Once the absorptive capacity of the soil * Fraps, G. S., Losses of Moisture and Plant Food by Percolation; Tex. Agr. Exp. Sta., Bui. 171, 1914. * Unpublished data. Cornell Agr. Exp. Sta., Ithaca, N. Y. THE CONTROL OP SOIL-MOISTURE 209 is reached, however, the drainage should be as rapid and com- plete as possible in order to insure good sanitation. The main- tenance of a high absorptive capacity for available water and the facilitation of rapid drainage are the secrets of rational percolation control. Fig. 37. — Influence of drainage on the ground water and the extent of the root zone. In this connection it is well to remember that drainage losses are profoundly affected by cropping. The following data from the Cornell Experiment Station are especially interesting in this regard. The data for the Dunkirk and Volusia soils are for ten and fifteen years respectively: Table XL AVERAGE ANNUAL LOSS OP WATER BY PERCOLATION FROM BARE AND CROPPED SOILS. CORNELL LYSIMETER TANKS. Conditions Eainfall Inches Percolation Inches Eainfall as Percentage op Drainage Dunkirk clay loam : Bare 32.41 32.41 32.97 32.97 24.92 18.70 27.13 20.62 76.8 Cropped 57.7 Volusia silt loam: Bare 82.3 Cropped 62.5 210 NATURE AND PROPERTIES OF SOILS Table XLI average annual loss of nutrients by percolation from bare and cropped soils. cornell lysimeter tanks. Conditions Annual Loss in Pounds an Acee NITROGEN CALCIUM POTASSIUM Dunkirk clay loam : Bare 69.0 7.3 2.5 51.8 10.2 397.9 247.1 259.9 341.4 356.4 72.0 Rotation Grass Volusia silt loam: Bare 57.3 61.7 84.5 Cropped 73.2 The influence of the crop on percolation is obvious, the loss of water by drainage being markedly decreased. The saving of nutrient is also very marked, especially as regards the nitrogen. The loss of nitrogen is only about one-seventh as much from the soils under a rotation, as where the land was bare, while the saving of calcium and potassium is con- siderable. The importance of catch- and cover-crops in eco- nomical soil management need not be emphasized further. 110. Drainage.^ — While percolation, especially in hu- mid regions, causes the loss of a large proportion of the rainfall received and carries away in addition many tons of ^Klippart, J. H., Principles and Practice of Land Drainage; Cin- cinnati, 1894. Miles, M., Land Drainage; New York, 1897, Faure, L., Drainage et Assainissement Agricole des Terres; Paris, 1903. Elliott, C. G., Drainage of Farm Lands; U. S. Dept. Agr., Farmers' Bui. 187, 1904. King, F. H., Irrigation and Drainage, Revised Edition; Part IT, New York, 1909. ' Warren, G. M., Tidal Marshes and their Beclamation; U. S. Dept. Agr., Office Exp. Sta., Bui. 240, 1911. Woodward, S. M., Land Drainage by Means of Pumps; U. S. Dept. Agr., Office Exp. Sta., Bui. 243, 1911. Elliott, C. G., Engineering for Land Drainage; New Yorkj 1912. Parsons, J. L., Land Drainage; Chicago, 1915. THE CONTROL OF SOIL-MOISTURE 211 soluble material, it is generally wise to facilitate the rapidity of its action while checking, if possible, its magnitude. The encouragement of the rate of percolate is spoken of as land drainage, which is the process of removing the excess or superfluous water from the soil as rapidly as possible. Excess water, by interfering with aeration, sets up unsanitary conditions within the soil. By draining the land many favor- able reactions are promoted. Granulation is encouraged, heaving is checked, while the root zone and water capacity of the soil are markedly increased. By facilitating aeration, favorable chemical and biological changes are encouraged, thus increasing the nutrients available for plants. The sum- total of good drainage is an increase of crop production to such an extent as to meet the investment costs and pay a hand- some profit besides. While the drainage of swamps and the reclamation of over- flow areas are urgent, the drainage of lands already under crop is more important. Practical farm drainage is para- mount in almost every community, even in arid regions where irrigation must be practiced. Tw^o types of drainage are feasible — open and closed. Ditch drainage is the usual type of the first group. Ditches have the advantage of large ca- pacity and are able to carry water at a low grade. On the other hand, they waste land, are ineffective and inconvenient, encourage erosion and demand a yearly up-keep expenditure. Wherever possible under-drains should be used. Jeffery, J. A., Text-Book, of Land Drainage; New York, 1916. Fippin, E. O., Drainage in Neiu York; Cornell Agr. Exp. Sta., Bui. 254, 1908. Brown, C. F., Farm Drainage; Utah Agr. Exp. Sta., Bui. 123, 1913. Lynde, H. M., Farm Drainage in North Carolina; N. C. Agr. Exp. Sta., Bui. 234, 191.5. Yarnell, D. L., Trenching Machinery Used for the Construction of Trenches for Tile Drains; U. S. Dept. Agr., Farmers' Bui. 698, 1915. Leidigh, A. H,, and Gee, E. C, Tile Drainage; Tex. Agr. Exp. Sta., Bui. 188, 1916. Hart, E. A., The Drainage of Irrigated Farms; V. S. Dept. Agr., Farmers' Bui. 805, 1917. 212 NATURE AND PROPERTIES OF SOILS 111. Tile drains are the only reliable means of under- drainage under all conditions. While stone drains ^ are of value in certain cases, they must always be short and are likely to clog. Besides, their drainage is slow and in- efficient. On silty soil they do not long remain in service. The operation of the tile drain is simple. The tile, generally about twelve inches long with a diameter varying with the water to be carried, are laid end to end in strings, on the bottom of a trench of sufficient slope, a carefully protected outlet being provided. The tile are then covered with earth, straw or surface soil often being placed directly around the tile to facilitate the entrance of the water. The superfluous water enters the tile through the joints, mostly from the sides. As a consequence, the tops of the joints may be cov- ered with paper, cloth or even cemented in order to prevent the entrance of silt or quick-sand. The function of a tile drain system is twofold: (1) to collect the superfluous water and (2) to discharge it quickly from the land. Where the land possesses considerable natural drainage, the tile are laid along the depressions. This is spoken of as -the natural system of drainage in that the tile facilitate the quick removal of the water from the places of natural accumulation. Where the land is level or gently rolling, it often needs uni- form drainage. A regular system must then be installed. This may be either of the fishbone or gridiron style, or a modification or combination of the two, natural drainage being taken advantage of where possible. Where springs or seep- age spots occur, cut-off systems must be devised. (See Figs. 38 and 39.) 'Stone drains are built by arranging stone in a properly located and graded trench in such a manner as to provide a continuous channel or throat from the upper end of the drain to the lower. One of the safest modes of construction from the standpoint of clogging is to place flat stone on edge in the trench with their faces parallel to the walls of the ditch. The spaces between the stone provide for the movement of the drainage water. THE CONTROL OF SOIL-MOISTURE 213 Every regular system consists of two parts, the laterals and the main drain. The laterals are usually constructed of three 3- or 4-inch tile, seldom smaller. These laterals should always enter the main at an angle of about 45 degrees. This causes a joining of the currents with no loss of impetus and Hfgh Ground ^^ \ Fig. 38. — Natural (1) and interception (2) systems for laying tile drains. allows the more rapidly moving lateral streams to speed up the flow in the main drain. The size of the main depends on the rainfall, the area drained, and the slope. It, of course, must be larger near the outlet than at any other point. The following practical table from Elliott ^ indicates the influence ^Elliott, C. G., Engineering for Land Drainage; p. 108, New York, 1912. 214 NATURE AND PROPERTIES OF SOILS of area and slope on the size of the main near the outlet of any system: Table XLI grades to a hundred feet in decimals of a foot with ap- proximate equivalents in inches, Grades to a Hundred Feet IN Decimals of a Foot with Diameter Approximate Equivalents in Inches OF Tile (in Inches) y-2 inch 1 iuch 2 inches 3 inches 6 inches 9 inches 0.04 0.08 0.16 0.25 0.50 0.75 Acres Acres Acres Acres Acres Acres 5 17.3 19.1 22.1 25.1 32.0 37.7 6 27.3 29.9 34.8 39.6 50.5 59.4 7 39.9 44.1 51.1 58.0 74.5 87.1 8 55.7 61.4 71.2 80.9 103.3 121.4 9 74.7 82.2 95.3 108.4 138.1 162.6 10 96.9 106.7 123.9 140.6 179.2 211.1 12 152.2 167.7 194.6 221.1 281.8 331.8 The grade necessary for the satisfactory operation of a tile drain system varies with the system itself and the portion under consideration. The grade of the main drain may be very low, especially if the laterals deliver their water with a high velocity. In general, the grade will vary from 4 to 20 inches to the hundred feet, 8 inches being more or less ideal. The depth of the tiles beneath the surface and the distance between laterals will vary with the soil. With sandy soils the tile may be placed as deep as 3 or 4 feet. With clayey soils the depth must be shallower, ranging from 15 to 30 inches, while the interval is reduced as the soil becomes finer in texture. On a clayey soil the distance between the strings is sometimes as low as 35 feet although 50 to 70 feet is com- moner. The maintenance cost of a tile drain system is low, the only especial attention needful being at the outlet. The outlet THE CONTROL OF SOIL-MOISTURE 215 should be well protected, so that the end tiles may not be loosened and the whole system endangered by clogging with sediment. It is well to embed the end tile in a masonry or concrete wall. The last eiglit or ten feet of tile may even be replaced by a galvanized iron pipe or with sewer tile, thus ?i ^■"-^c^ ?0 5: 5 y y ><; 2Si2^ I*) / 3 y N ^ 10 J r y^ / a 7^ -«£:--' TJ^. f£ /N HOi/ RS M 6 10 N /O Fig. 42. — Curves showing the temperature variations of different colored soils at a four inch depth compared with air temperature. Munich, June 23, 1876. region which tends to bring a unit surface more nearly normal to the sun's rays will increase its absorbed energy and raise its average seasonal temperature. In the north temperate zone this is of course a southerly inclination. The diagram (Fig. 43) illustrating conditions on the 42d parallel at noon on June 21 makes clear this relationship. It is seen that in this case a southerly slope of 20° received the greatest amount of heat to a unit area with the level soil 230 NATURE AND PROPERTIES OF SOILS next and the northerly slope last. The amount of heat for a given area is in the order of 106, 100, and 81, respectively. Fig. 43. — Diagram showing the distribution of a given amount of radiant energy on different slopes on June 21, at the 42nd parallel north. These generalizations have been established by the work of a number of investigators.^ Wollny- found near Munich that the temperature of soutli- * King, F. H., Physics of Agriculture, p. 218. Madison, Wis., 1910. - Wollny, E., Untersuchungen iiber den Emfluss der Exposition auf die SOIL HEAT 231 ward slopes varied with the time of year. For example, the southeasterly inclination was warmest in the early season, the southerlj^ slope during mid-season and the southwesterly slope in the fall. Such a relationship is of course governed entirely by local climatic conditions, especially cloudiness, and might not be true of any other place. A southeasterly slope is gen- erally preferred by gardeners. Orchardists also pay strict attention to the aspect as it is often a factor in sun-scald and certain plant diseases. 120. Rise of temperature and the factors involved. — The rise of temperature of a layer of soil following a given absorption, depends (1) on the specific heat of the soil, (2) on the rate at which the heat moves to other parts of the soil mass, and (3) on the losses of heat to the atmosphere. It is evident that in a study of the influence of insolation on soil temperature, specific heat should receive the first attention. 121. Specific heat and soil temperature. — The specific heat of any material may be defined as its thermal capacity compared with that of water. It is expressed as a ratio to the quantity of heat required to raise the temperature of a given amount of a certain substance 1° C. to the quantity needed to change an equal amount of water from 15° to 16° C. The specific heat figure for soil generally refers to the heat capacity of the dry substance. Under normal conditions, soils contain variable amounts of pore spaces and consequently have different weights to the cubic foot. A specific heat figure based on weight, therefore, does not give a true idea of the relative heat capacities of two soils. The expression of spe- cific heat by volume seems a more rational basis of compari- son.^ The specific heat of the soil is important because of the relation it has to the warming up of soil in the spring, the Erwdrmung des Bodens; Forsch. a. d. Gebiete d. Agr.-Physik., Band I, Seite 263-294, 1878. This publication contains a number of other papers on this subject by Wolhiy. * Weight specific heat of a substance may be expressed by the number of calories required to raise the temperature of one gram, 1° C. Volume 232 NATURE AND PROPERTIES OF SOILS rate of cooling in autumn, drainage influences, and like phe- nomena. Specific heat data from different investigators do not show the agreement that might be expected.^ This is probably due (1) to inaccuracies in the naming of the soils used, (2) to difference in methods, and (3) to difficulties in technique. Everything considered, the following table from Ulrich - dis- plays in a suitable way the important specific heat phases: Table XLVII volume of specific heat of soil Soils Sand Clay.. Organic matter. Weight Volume 1.52 1.04 .37 Volume Specific Heat .2901 .2333 .1639 It is evident that specific heat is partially governed by the organic matter of the soil and partially by texture and struc- specific heat is the number of calories necessary to raise the temperature of one cubic centimeter of the substance one degree. In the case of soil, weight specific heat may be changed to volume specific heat by multiplying it by the volume weight, since volume weight is the weight in grams of one cubic centimeter of dry soil. ^ The following weight specific heats from Lang,* Patten t and Bou- youcos t are interesting: Lang Patten Bouyoucos Coarse sand 198 Sand 185 Sand 193 Limestone soil. . . .249 Sandy loam .183 Gravel 204 Organic soil 257 Loam 191 Clay 206 Garden soil 276 Loam 194 Loam 215 Peat 477 Clay 210 Peat 252 * Lang, C, JJher Wiirme Capacitcit der Bodenconstituenten ; Forsch. a. d. Gebiete d. Agr.-Phys., Band I, Seite 109-147, 1878. t Patten, H. E., Heat Transference in Soils; XJ. S. Dept. Agr., Bur. Soils, Bui. 59, p. 34, 1909. $ Bouyoucos, G. J., An Investigation of Soil Temperature ; Mich. Agr. Exp. Sta., Tech. Bui. 17, p. 12, 1913. ^Ulrich, R., Untersuchungen iiber WdrmeTcapazitdt der Bodenhonsti- tuenten; Forsch. a. d. Gebiete d. Agr.-Phys., Band 17, Seite 1-31, 1894, SOIL HEAT 233 tnre. Organic matter will lighten and loosen a soil, and lower the volume weight. Moreover, its heat capacity is low. The effect of such an addition is to lower the specific heat figure. It is apparent also that the finer the texture of the soil, the lower the specific heat. That is due not to a difference in chemical composition but to a lowered volume weight. Any practice, therefore, that tends to vary volume weight will in a like manner vary specific heat. The farmer may encourage the warming of his soil by deep and efficient plowing. By increasing its organic content, he may create a tendency in the same direction. One other factor, more important than those already men- tioned, yet remains to be discussed. This is water, so univer- sally present in soils and so important in natural soil phe- nomena. As the specific heat of water is several times greater than that of the soil constituents, any addition of it must raise the thermal capacity of the mass. The following data from Ulrich^ show that moisture rather than texture and organic matter is the controlling factor in normal soil : Table XL VIII THE EFFECT OF MOISTURE ON VOLUME SPECIFIC HEAT OF SOIL (moisture expressed as a percentage of the total water capacity) Dry Soil 10%- Water 20% Water 40% Water 60% Water 80% Water 100% Water Sand .291 .330 .294 .242 .368 .355 .320 .444 .478 .476 .520 .600 .632 .597 .723 .788 .675 Clay Organic matter .233 .164 .845 .945 The overwhelming influence of moisture is at once evident from these data. Fine texture, because of its high water capacity, usually accentuates the dominance of moisture. Organic matter functions in the same way. While an organic ^ Ulrich, R., Untersuchungen iiher die W drmekapazitat der BodenJconsti- tuenten; Forsch. a. d. Gebiete d. Agr.-Phys., Baud 17, Seite 27, 1894. 234 NATURE AND PROPERTIES OF SOILS soil of low volume weight may warm up easily when dry, its high water content usually markedly retards its temperature change. A muck soil is usually the last to freeze in winter and, conversely, the last to thaw in spring. The advantage of drainage is evident as a wet soil is of necessity colder in the spring than one that is well drained. This at least par- tially accounts for the fact that a sandy soil is usually an early one and is, therefore, of particular value in trucking. 122. Heat movements in soil. — While volume weight, or- ganic matter, and moisture seem largely to control the degree to which a soil will become heated when exposed to insolation, it is evident that there must be some mode of energy transfer whereby such phenomena may be facilitated. Heat movement is necessary in order that the lower layers of the soil may become warm enough for proper biological functionings. Energy transmission both downward and laterally is abso- lutely essential and deserves as much attention as the factors influencing insolation absorption. Two methods of heat transfer function in a normal soil — conduction and convection. These modes of energy move- ment are extremely difficult to analyze, due to the impossi- bility of controlling one while studying the other. 123. Conduction of heat in soil. — While radiation has to do with the oscillatory transfer of energy conduction relates to the molecular transmission of heat through any material. When one part of a substance is heated, the movement of its molecules is stimulated. These molecules strike their neighbors with increased force, thus quickening their motion. These in turn accelerate others until the energy applied at one point becomes apparent at another. Solids as a class are better con- ductors than liquids, while liquids in general are superior to gases in this respect. It must be remembered in studying the conductivity of heat through soil, that we are dealing with a heterogeneous mixtvire of mineral and organic matter con- taining varying amounts of air and water. The movement SOIL HEAT 235 of soil heat involves not only the question of conduction througli solids but through liquids and gases as well. More- over, transfer resistance, which occurs at the boundary of two substances in contact, has much to do with the rate of trans- mission. In addition, the air and water of the soil are capable of considerable movement which makes conductivity studies extremely difficult due to convection currents. The heat conductivity of soil is aifected by a number of factors which may or may not lend themselves to field con- trol. Important among these are texture, structure, organic matter, and moisture. The influence of the first is clearly shown by the following comparative data obtained by Bou- youcos,^ with field soils: Table XLIX relative conductivity as measured by the time required for a thermometer 7 inches from the source of heat to indicate a rise in temperature Soil Relative Rate OF Conductivity Sand. Loam , Clay. Peat. 100 150 143 362 These results are comparative only in a qualitative way. Quantitative determinations are so beset by error that only few investigators have made any consistent attempt along this line. Patten's results^ expressed as metric K ^ (the heat con- * Bouyoucos, G. .T., An Investiqation of Soil Temperature; Mich. Agr. Exp. Sta., Tech. Bui. 17, p. 20, 1913. =" Patten, H. E., Heat Transfer in Soils; U. S, Dept. Agr., Bur. Soils, Bui. 59, p. 26-28, 1909. ^ The conductivity of a substance is measured by the number of gram- calories of heat transmitted in 1 second through a cube with 1 centi- meter edges, when the opposite faces differ in temperature by 1°C. The calories of heat transmitted (H) will be proportional to the area of the 236 NATURE AND PROPERTIES OF SOILS ductivity coefficient in C.G.S. units) shows the same general comparisons as already presented: Table L conductivity coefficients of different dry soils Soils Coarse quartz Leonardtown loam Podunk fine sandy loam. Hagerstown loam Galveston clay Muck K .000917 .000882 .000792 .000699 .000577 .000349 It is evident, in general, that the finer the texture of the soil, the lower is the conductivity. This cannot be construed as indicating that the conductivity coefficients of sand and clay particles are particularly different. The variance ob- served is adequately explained by the great number of trans- fers necessary in a fine-textured soil. It is also evident that the addition of organic matter will lower conductivity. Humus itself has a low conductivity coefficient and would markedly affect the transfer resistance by changing the struc- ture of the soil. Compacting a soil should accelerate heat transfer due to a more intimate contact of the soil grains and a consequent diminution of transfer interference. Tillage, on the contrary, must impede not only the movement of heat downward in the soil but from the subsoil into the furrow slice. The greatest single factor to be considered in heat conduc- tivity is the moisture content of the soil. The curve (Fig. 44) faces (A) and to the differences in temperature of the faces (f — t"), while it will be inversely proportional to the thickness (d) of the cube. K is a constant, depending on the material studied. SOIL HEAT 237 for fine sandy loam, constructed from Patten's data/ illus- trates its effect and indicates how heavily it must override the factors already mentioned: , 00$00 C X -/- I / >. / bw / i^ / k> J ^ ^^ o ^^^ ^ ^^ ^^''^ s> ^ ^ y PERCEJ\/'j '" OF MC iJSTUJRE IN 30/L ,00400 .00300 .00200 .00100 ^ /O I^ 20 25 Fig. 44. — Conductivity curve for Podunk fine sandy loam, showing the influence of moisture content upon the rate of heat transfer. The curve apparently flattens out at a high moisture content indicating that good conductivity may be obtained at optimum moisture. At first glance it appears peculiar that the heat movement through a soil, the mineral constituents of which possess a conductivity coefficient of about .01066, should be accelerated by the addition of a liquid possessing a value for K of about .00149. The explanation lies in the lowering of the transfer ^ Patten. H. E., Heat Transfer in Soils; U. S. Dept. Agr., Bur. Soils, Bull. 59, p. 27, 1909. 238 NATURE AND PROPERTIES OF SOILS resistance. Heat passes from soil to water about 150 times easier than from soil to air. As the water increases, the air decreases and the rate of conductivity is raised. When suf- ficient water is present to join all of the soil particles, further additions will have little effect on character of heat movement. Moisture, optimum for crop growth, amply provides for heat transfer. The slow warming up of the lower subsoil must not be taken as an indication of lower conductivity. It is due rather to a lessened heat supply. As a matter of fact, the rate of heat transmission has been shown to be more rapid in the subsoil, due to a greater compaction and to the presence of more water. This brief discussion of conductivity shows the vital im- portance of such a phenomenon to plants in that the necessary heat is carried broadcast through the soil. While conduc- tivity is affected to a certain extent by texture, structure, and organic matter, moisture is the dominant factor. Under nat- ural conditions, it is necessary to maintain a medium amount of water in the soil. This moisture condition, fortunately, supports almost maximum heat conduction. Good tilth and increased organic matter probably exert their greatest in- fluence on this type of heat transfer by their influence on soil moisture. 124. Convection transfer of heat. — Convection, the third manner by which energy may be conveyed, is a heat transfer by means of currents in liquids or gases. It functions by an actual and obvious movement of matter. In the soil absorp- tion tends to heat the air as well as the solid substance. This produces currents due to the expansion and rise of the warmed gases. It is obvious that such heat movement must always be lateral or upward, never downward. Such convection exerts its greatest influence in equalizing the temperature of the soil, overcoming the effects of unequal conduction and uneven ab- sorption due to vegetation or stone. Air currents as they escape into the upper air carry considerable heat away from SOIL HEAT 239 the soil. Such a loss is of little moment, however, compared to that continually occurring through conduction and radiation. Some heat is carried downward into the soil by percolat- ing water. This is a true convection activity. The impor- tance of such a heat transfer is only conjectural. As percola- tion is generally intermittent in a soil, it is probable that it does not modify to any extent the influence exerted by con- duction. 125. Effect of organic matter on soil temperature. — Plants entrap a considerable amount of radiant energy from the sun, part of which is utilized during the growth period. The remainder exists as latent energy in the tissue. If any amount of plant remains are incorporated in the soil and de- cay proceeds, this heat is liberated. Thus a heat transfer is similar in a way to convection, except that, in this case, the transfer is by the movement of a solid and the energy is latent. To what extent the decay of organic matter is effective in bringing about any important modification of field soil, it is difficult to say. In greenhouses and hotbeds perceptible in- creases are obtained by the use of fresh manure. In the field, however, where the absorption and loss of heat are very large and where the organic matter makes up but a small portion of the soil mass, it is doubtful whether any important heat increase occurs. Georgeson,^ in Japan during the first twenty days after an application of eighty tons of manure to the acre, obtained an increase of only 3.4° F. over a soil un- treated. Wagner ^ found an average increase of 1° F. from the use of twenty tons of barnyard manure to the acre. Bou- youcos ^ has obtained the latest data on the subject. Under ' Georgeson, C. C, Influence of Manure on Soil Temperature; Agri. Sci., Vol. 1, pp. 2.5-.52, 1887. ' Wagner, F., Pher den Einfluss der Bungung mit Organischen Sub- stance auf die Bodentemperatur ; Forsch. a. d. Gebiete d. Agr.-Phys., Band V, Seite 373-405, 1882. * Bouyoucos, G. J., An Investigation of Soil Temperature ; Mich. Agr. Exp. Sta., Tech. Bui. 17, pp. 180-190, 1913. 240 NATURE AND PROPERTIES OF SOILS carefully controlled conditions, he found that unless excessive amounts of manure were added no appreciable effects were observed. Such results indicate that the heat of decay and fermentation has little practical effect in modifying the tem- perature of field soils. AVithout doubt there are certain local- ized influences, but how important they may be is beyond our present knowledge. As far as heat relations are con- cerned, it seems that organic matter exerts its greatest effects through a darkening of the color and an increase in the mois- ture capacity of the soil, 126. Loss of heat — conduction, radiation, and evapora- tion. — Although small amounts of heat may be carried from the soil by percolating water, the only important loss is into the atmosphere above. This loss occurs in three ways, con- duction, radiation, and evaporation. The loss due to evapora- tion is easily the least important of the three. Conduction and radiation have much to do with climatic control, since the atmosphere receives its energy in large degree from the earth rather than directly ■ from the sun. Conduction from soil to air and vice versa can be modified but to a slight extent by man, a fortunate provision of nature. Terrestrial bodies are continually radiating energy waves into the atmosphere, the change of temperature depending on whether the receipt of such oscillations exceeds or falls short of the loss. In the case of the soil, there is a very great dis- sipation of energy in this way, radiation with conduction being important climatic controls. The rapid changes in air temperature are often directly due to these phenomena. These energy waves of terrestrial origin are very long,^ being within the infra-red group and consequently make no impression on the eye. They are often spoken of as the dark rays. Their energy capacity is higher than that of shorter oscillations. The trapping of heat in a greenhouse * Terrestrial bodies at ordinary temperatures give out waves varying in length from .000270 to .001500 cm. The warmer the body, the shorter the wave length. SOIL HEAT 241 is partially due to the tendency of the objects within the house to give off these long rays, which do not pass through the glass with the facility possessed by the shorter vibrations by means of which a large proportion of the energy was intro- duced. The texture, structure, and color of the soil have little in- fluence on radiation. Moisture tends to hasten it a trifle, since water is a better radiator than soil. Mulches, as they are loose and dry, may check radiation slightly. Artificial coverings, shelters, and clouds seem to exert the greatest effect. It is often feasible to protect plants from frost by interfering with radiation and conduction. Clouds by shutting in heat, may in some cases prevent a frost that would otherwise occur, due to the rapid cooling. Snow likewise has a protecting effect and may often prevent the soil underneath from freez- ing. While man may influence radiation locally, it is evident that the total energy loss can be checked but little. The effect of evaporation on the temperature of the soil is especially noticeable because of its rapid action. This vapor- ization of water is caused by an increased molecular activity and requires the expenditure of a certain amount of heat,^ which results in a cooling effect on the water remaining and consequently on the soil and air with which it is in contact. It requires 267.9 kilogram calories to evaporate one pound of water at 50° F. This is sufficient to lower the temperature of a cubic foot of saturated clay about 20° F., providing that all of the energy of evaporation comes from the soil and its water. The low temperature of a wet soil is due partially to evapo- ration and partially to high specific heat. King ^ found during * It requires 536.6 gram-ealories to evaporate one gram of water at 100°C., while 596.7 calories are necessary if evaporation takes place at 0°C. The calories (C) required to vaporize one gram of water at any temperature (t) may be calculated by the formula: C = 596.73 — .601 t *King, F. H., Physics of Agriculture, p. 20; Madison, Wia., 1910. 242 NATURE AND PROPERTIES OF SOILS April that an undrained soil in Wisconsin ranged from 2.5°F. to 12.5° F, lower than one of the same type well drained. Parks ^ reports data of the same order from England. Drained and undrained soil held in trays at Urbana, Illinois,- showed maximum differences of 13.7° F., 9.0° F., and 6.2° F. at depths of 1, 2, and 4 inches, respectively. The differences were greatest in the day. Wollny considers that the depres- sion of temperature due to evaporation is roughly propor- tioned to the moisture present. Texture, structure, and or- ganic matter influence the cooling action of evaporation, since they exert such a marked effect on water capacity and capil- lary movement. The practical importance of evaporation study lies in the fact that it can be controlled to such a marked extent in the field. Such is not true of radiation and conduction. Windbreaks and shelters have been shown by King ^ to reduce evaporation over short distances as much as 25 per cent. This means a conservation of soil energy for the time being. Thorough under-drainage not only checks evaporation losses but lowers the specific heat of the soil, retards its radiation and facilitates convection. This means a faster warming up, especially of the root zone. Optimum moisture encourages optimum heat conditions as well as other favorable phenomena. Drainage, tillage, and organic matter are the dominant factors in this moisture control. 127. Soil temperature and its variations. — The tempera- ture of the soil at any time depends on the ratio of the energy absorbed and the heat being lost. The constant change in this coordination is reflected in the seasonal, monthly, and daily soil temperatures. The following data * are representa- * Parks, J., On the Influence of Water on the Temperature of Soils; Jour. Roy. Agr. Soc. Eng., Vol. 5, pp. 119-146, 1845. ^Hosier, J. G., and Gustafson, A. F., Soil Physics and Management; p. 302; Philadelphia, 1917. 'King, F. H., The Soil, p. 189; New York, 1906. *Swezey, G. D., Soil Temperatures of Lincoln, NebrasTca; Nebr. Agr. Exp. Sta., 16th Ann. Rep., pp. 95-102, 1903. SOIL HEAT 243 tive of soil temperatures in temperate climates with moderate rainfall : Table LI AVERAGE TEMPERATURE READINGS TAKEN AT LINCOLN, NEBRASKA, 1890-1902. DEGREES FAHRENHEIT 1 3 6 12 24 36 Season Air Inch Inches Inches Inches Inches Inches Deep Deep Deep Deep Deep Deep Summer. . . 25.9 28.8 28.8 29.5 32.2 36.3 39.1 Autumn . . . 49.9 54.8 53.6 51.6 48.5 45.7 44.3 Spring. . . . 73.8 83.0 80.9 79.1 73.8 69.0 66.2 Winter. . . . 53.9 56.4 57.6 57.1 57.5 59.3 60.3 It is apparent that the seasonal variations of temperature are considerable even at the lower depths. The surface layers vary more or less in accord with the air temperature and, therefore, exhibit a greater fluctuation than the subsoil. In general, the surface soil is warmer in spring and summer than the lower layers but cooler in fall and winter. The soil, on the average, is warmer than the air in winter. This occurs because the air responds more quickly to a change in solar insolation than the soil. The curves showing the monthly march of soil temperature at Lincoln, Nebraska (Fig. 45), reveal the lag of the tempera- ture change in the subsoil due to slow heat penetration. It is also noticeable that the monthly range in temperature change in the surface soil is higher than that of the air. The abso- lute range is, of course, greater for the air. It must be kept in mind that changes in soil temperature are gradual, while the air may vary many degrees in an hour. The daily and hourly temperature of the air and soil in the temperate zone may show considerable agreement or marked divergence according to whether the weather control is cyclonic or solar. With solar control and a clear sky the air temperature rises from morning to a maximum at about 244 NATURE AND PROPERTIES OF SOILS two o'clock. It then falls rapidly. The soil, however, does not reach its maximum temperature until later in the after- noon, due to the usual soil lag. This retardation is greater and the temperature change less as the depth increases.^ The substratum of a soil shows little daily, or even monthly, varia- tion and is affected, if at all, by seasonal changes only. The JAN. FE8R. MAR. APR. MAV JUNE JULY AUG. SEPT. OCT, NOV^ DEC. Fig, 45. — Curves showing the average monthly temperature readings at various soil depths. Average of twelve years, Lincohi, Nebraska. curves in Fig. 46, comparing soil and air temperatures at Munich^ on a bright day in May, substantiates some of the statements above : 128. Control of soil temperature. — The most important factor in the control of soil heat is obviously moisture. Good ^ The following lavrs hold in a general way : 1. The lag of the temperature wave is proportional to the depth. 2. The diurnal amplitude of the temperature oscillation decreases in geometric progression as the depth increases in arithmetic progression. If the temperature variation at the surface was 24°F and at 6 inches deep 12°F, according to this law the diurnal variation at 12 inches would be 6°F and at 18 inches 3°F. ^Wollny, E., Untersuchungen iiber den Einfluss der PflanzendecTce und der BescliatUinq auf die Physilalischen Eigenschaften des Bodcn; Forsch. a. d. Gebiete d. Agr.-Physik., Band VI, Seite 197-256, 1885. SOIL HEAT 245 drainage, and a proper structural development, sufficient or- ganic matter and deep and careful plowing, favor optimum moisture conditions. Such moisture regulation means a low- ered specific heat, rapid conductivity, and good convection. The increase of soil organic matter may act directly in heat control by darkening the color and thus increasing absorp- S5 60 75 70 65 60 55 50 / ^ \ IW A \ V ^ / 'h \ N }? s / / \ ^^ / / \ / -^ "^ / T/M£ IN MR s. Af S JO /V Fig. 46. — Curves showing the hourly temperature of a bare soil at a depth of four inches and of the air just above the soil in Ger- many, May 26. (Data from Wolhiy.) tion. A soil-mulch, being dry, not only may check evapora- tion but at the same time may lower radiation. Any method of handling the land which tends to benefit its physical condition, better its tilth and control its moisture, tends at the same time towards a proper heat control. The whole question may be summarized by saying that, if a farmer adopts a proper system of moisture control and at the same time employs methods that continually encourage a better 246 NATURE AND PROPERTIES OF SOILS physical condition of the soil, the problem of the control of soil heat will be solved automatically. The farmer will then have brought about the best conditions for heat absorption and will have facilitated conduction and convection, retarding at the same time losses by evaporation and radiation. CHAPTER XII SOIL AIR The soil is a porous mass of material of which only about one-half is solid matter. The pore space that results is occu- pied by water and by air in a constantly varying proportion. When a soil is in good condition for crop growth, the air space rarely makes up more than from 20 to 25 per cent, of its volume. The texture of the soil and the amount of mois- ture are obviously the main controls. The individual air spaces of the soil are more or less continuous and seem to maintain a fairly complete communication between the vari- ous horizons. The better the granulation of the soil and the greater the number of cracks and burrows, the easier and quicker is this communication. The air of the soil is either directly in contact with the roots and the soil bacteria or separated from them by only a thin layer of moisture or col- loidal material. The air of the soil is not merely a continuation of the atmo- spheric air into the interstitial spaces. As it is enclosed by the soil complexes and by the soil-moisture movement does not take place readily. Hence it is greatly influenced by its local surroundings. This leads to important differences between the atmospheric air and the soil air, the character of the latter depending on a variety of conditions in which the physical, chemical and biological properties of the soil play a large part. 129. Composition of soil air. — The air of the soil differs from that of the outside atmosphere in that it contains more" water-vapor, a much larger proportion of carbon dioxide, a 247 248 NATURE AND PROPERTIES OF SOILS correspondingly smaller amount of oxygen, and slightly larger quantities of other gases, including ammonia, methane, hydro- gen sulfide, and the like, formed by the decomposition of organic matter. The percentage of nitrogen is practically the same in all cases. The following average data quoted from three different sources show the comparative compositions as far as the carbon dioxide, oxygen, and nitrogen are con- cerned. All other gases are included with the nitrogen fig- ures. Table LIT AVERAGE COMPOSITION OF SOIL AIR AND ATMOSPHERIC AIR Percentage by Volume CO, • 0. N. Soil Air Germany ^ Iowa ^ .20 .20 .25 .03 20.60 20.40 20.65 20.97 79.20 79.40 England * Atmospheric Air England * 79.20 79.0 Russell and Appleyard,^ in their study of the soil atmo- sphere, found that there are really two types of soil air. The first one occupies the portion of the pore space not taken * Atmosphere air carries about .93 per eent. of argon, with very small amounts of other inert gases such as krypton, xenon, helium and neon. These gases are of course present in the soil. "Lau, E., Beitrdge zur Kenntnis der Zusammensetsung der im Acker- hoden bepidlichen Luft; Inaug. Diss., Eostock, 1906. ^Jodidi, S. L., and Wells, A. A., Influence of Various Factors on Decomposition of Soil Organic Matter; la. Agr. Exp. Sta., Res. Bui. No. 3, Oct. 1911. * Russell, E. J., and Appleyard, A., The Atmosphere of the Soil: Its Composition and the Causes of Variation; .Tour. Agr. Sci., Vol. VII, Part 1, pp. 1-48, 1915. = Russell, E. J., and Appleyard, A., The Atmosphere of the Soil: Its Composition and the Causes of Variation; Jour. Agr. Sci., Vol VII, Part 1, pp. 1-48, 1915. SOIL AIR 249 up by water, is free to move from place to place and is satu- rated or nearly saturated with water-vapor. It is the soil atmosphere most commonly referred to and its composition is set forth in the above tabulation. After this air was drawn off Russell and Appleyard found that still more air could be removed by applying suction. This air at first carried considerable oxygen but by continuing the suction almost pure carbon dioxide was obtained. The amount of gas removed by lowering the pressure varied directly with the moisture content of the soil and consequently it may be considered as air largely absorbed by the moisture of the soil complexes. Two types of atmosphere, therefore, exist in the soil. One, the ordinary soil air, is comparatively rich in oxygen. The other, absorbed by the soil moisture, is very low in oxygen but very high in carbon dioxide. Obviously they insensibly merge. The biological significance of these atmospheric types is very important. Their simultaneous presence admits of both aerobic and anaerobic biological activity. For example, rapid nitrate formation might be progressing but no accumu- lation would be evident, due to just as rapid a synthetic activ- ity of the anaerobic forms.^ It must not be assumed from the data above quoted that the composition of the soil air is at all constant or that it is approximately the same in every soil. The soil is dynamic in nearly every phase and is nowhere more changeable than in its atmospheric composition. This variability will of course be more marked and more important in the air which occupies the interstitial spaces, although the absorbed air will show some fluctuation. The compositions of the air of several soils, as determined by Boussingault and Lewy^ are quoted in the following table : * Gainey, P. L., Beal and Apparent Nitrifying Power of Soils; Science, N. S., Vol. 39, pp. 35-37, 1914. Doryland, C. .T. T., Influence of Energy Material upon the Relation of Soil Microorganisms to Soluble Plant Food; N. Dak. Agr. Exp. Sta., Bui. 116, pp. 818-399, 1916. * Johnson, S. W., How Crops Feed, p. 219; New York, 1891. 250 NATURE AND PROPERTIES OF SOILS Table LIII Character of Soil Percentage by Volume COa 0, N, Sandy subsoil of forest Loamy subsoil of forest Surface soil of forest Clay soil .24 .79 .87 .66 .74 1.54 3.64 19.66 19.61 19.99 19.02 18.80 16.45 79.55 79.52 79.35 Soil one year after manuring Soil freshly manured Vegetable mold compost 80.24 79.66 79.91 The differences in the composition of the atmosphere of different soils and the variability noticeable within the same soil are due primarily to two factors: (1) the production of carbon dioxide, and (2) oxidation. These will be discussed in the above mentioned order. 130. The carbon dioxide of the soil air. — The presence of carbon dioxide in soils may be due in small part to in- filtration from the atmospheric air, there being a tendency for the carbon dioxide, which is heavier than nitrogen and oxygen, to settle out. It may also have a purely chemical origin. The latter source is much more probable. The ab- sorption of the bases of carbonates or bicarbonates would obviously release carbon dioxide. This probably does not take place, however, to any great extent in a natural soil. When ground limestone is added, such a reaction does occur.^ Car- bon dioxide in appreciable amounts might for a short time thus be liberated through chemical reaction. The addition ^Maclntire, W. H., The Carbonation of Burnt Lime in Soils; Soil Sci., Vol. VII, No. 5, pp. 325-446, 1919. See also. The Non-existence of Magnesium Carbonate in Humid Soils; Tenn. Agr. Exp. Sta., Bui. 107, 1914. SOIL AIR 251 of lime has been shown by several investigators to increase the carbon dioxide production/ There is now no doubt tbat biological activities are largely 4.5 1 ; ' ■ — " N ! 4.0 1 i/ > ^.5 ^ N^ / 1 \ K ^0 ]/ \ r y \! \ ^ ?F) r 1 A \ s. 1 / Second .535 .120 .261 .203 .260 .200 .340 .057 .101 .083 .082 .077 .195 .063 Third .160 Fourth .120 Fifth .178 Sixth .123 It will be noticed that the dissolved matter, both organic and inorganic, fell off markedly after the first extraction. Later extractions were doubtless supplied largely from the substances held by absorption, which gradually diffused into the water extract as the tendency to maintain equilibrium of the solution overcame the absorptive action. "With the re- moval of the absorbed substances the equilibrium between the absorption and solution surfaces and the surrounding so- lution is disturbed, diffusion and solution are increased, and more material gradually passes from the soil into the solution. In this way, a more or less uniform and continuous extraction is mantained. In spite of the obvious defects of the water extraction method the work of Hoagland, Burd and Stewart - seems to indicate that such data, if obtained over an extended period, 'Schulze, F., rber den Phosphorsaure-Gehalt des Wasser-Aussugs der Acker erde; Landw. Vers. Stat., Band 6, Seite 409-412, 1864. ^ Burd, J. S.J Water Extractions of Soils as a Criteria of their Crop- CHEMICAL ANALYSIS OF SOILS 323 are a good comparative measure of the concentration and composition of the soil solution (see par. 145). They also con- sider water extractions as criteria of the crop-producing power of a soil so studied. The practical value of such a method as a means of estimating fertility is, however, somewhat ques- tionable, since much time and labor are required to make the necessary extractions and analyses before conclusions at all reliable may be drawn. 171. Fertility evaluation by means of chemical analyses. — The important part that chemistry plays in soil investiga- tion and research should not be overlooked. Nor can a satis- factory presentation of soil phenomena, whether with a tech- nical or an applied bearing, be made without the use of some chemistry. Chemistry, in fact, is the fundamental science that is most utilized in soil study. In spite of these relationships, the value of chemistry in the direct solution of practical fertility problems is neither abso- lute nor final. The objections already raised to the digestion of the soil, either with concentrated or dilute acids, shows the inadequacy of these methods so far as practical problems are concerned. Of all the chemical analyses discussed those that have to do with the determination of organic carbon, total nitrogen, total calcium and phosphoric acid are of outstanding value. Or- ganic matter is such an important soil constituent that a knowledge of its amount cannot fail to throw much light on the physical and chemical condition of the soil. Much of the soil nitrogen is carried by the organic matter and becomes available in much larger proportion than do the mineral nutrients. An analysis for total nitrogen is, therefore, a Producing Power; Jour. Agr. Res., Vol. XII, No. 6, pp. 297-309, 1918. Stewart, G. R., Effect of Season and Crop Groicth in Modifying the Soil Extract; Jour. Agr. Res., Vol. XII, No. 6, pp. 311-368, 1918. Hoagland, D. R., The Freezing Point Method as an Index of Varia- tions in the Soil Solution Due to Seaso7i and Crop; Jour. Agr. Res., Vol. XII, No. 6, pp. 369-395, 1918. 324 NATURE AND PROPERTIES OF SOILS fairly reliable guide in some cases to the fertility of the soil under specific consideration. Although the relationship of organic matter and nitrogen to soil fertility is so close that certain generalized tables ^ may be cited for the interpretation of chemical data, no close cor- relation is possible, especially where soils of markedly different character are compared. So many other factors may enter that practically no opinion can be formed regarding the prod- uctivity of a soil unless other and more detailed data are available. An interesting example of where the nitrogen content fails to indicate the relative fertility of two soils is found in certain unpublished data from the Cornell Agricultural Experiment Station. Two soils are being studied in the lysimeter tanks — Dunkirk silty clay loam and Volusia silt loam. In Table LXXIII is given the nitrogen and calcium content of these soils and the pounds of nitrogen removed to the acre by maize, oats, and barley, respectively, for the years 1915, 1916, and 1917. The treatment and handling of the soils compared has been the same. While the nitrogen, phosphoric acid and potash contents of these soils are about the same, a marked difference is noted in their productivity. This may be due, at least partially, to the calcium content, which is rather high in the Dunkirk, especially in the subsoil. In comparing soils over wide areas ^ The following tentative classification of soils on the basis of their percentages of organic matter and nitrogen is offered for generalized field use: Description Percentage of Organic Matter Percentage of Nitrogen Low Medium . . . High .... Very high .0- 3.0 3.0- 6.0 6.0-10.0 above 10.0 .00- .10 .10- .25 .25- .40 above .40 CHEMICAL ANALYSIS OF SOILS 325 Table LXXIII the percentages of nitrogen and calcium in the dunkirk silty clay loam and the volusia silt loam and the nitrogen removed by certain crops. cornell lysi meter tanks. Soils CaO % N % Pounds of N Removed PER Acre MAIZE 1915 OATS 1916 BARLEY 1917 Dunkirk silty clay loam. . . First foot .340 .280 .490 1.530 .230 .165 .260 .365 .134 .062 .064 .054 .145 .052 .059 .050 53.6 28.3 62.3 21.7 44.0 Second foot Third foot Fourth foot Volusia silt loam 18.8 First foot Second foot Third foot Fourth foot and in a general way there is often some correlation between the amount of calcium present and the productivity. In humid regions soils high in lime are usually fertile. AVithin certain limits, therefore, calcium becomes significant in fer- tility studies.^ Some idea concerning the relative value of the various chem- ical methods, especially those dealing with potash, lime, phos- phoric acid, and magnesia, may perhaps be obtained by com- paring actual data. Burd - has analyzed a number of soils, ^Shedd, O. M., A Proposed Method for the Estimation of Total Calcium in Soils and the Significance of this Element in Soil Fertility; Soil Sci., Vol. X, No. 1, pp. 1-14, 1920. * Burd, J. S., Chemical Criteria, Crop Production and Physical Classi- fication in Two Soil Classes; Soil Sci., Vol. V, No. 6, pp. 405-419, 1918. 326 NATURE AND PROPERTIES OF SOILS some good, some poor, by several different methods. Repre- sentative figures are given below : Table LXXIV chemical composition of a good and a poor soil as indicated by several different methods Conditions Percentage of K,0 CaO MgO P.O. Bulk analysis Productive silt loam 1.98 1.85 1.05 .89 .039 .039 p.p.m. 57 52 1.48 1.50 1.43 1.48 .452 .422 p.p.m. 127 45 2.66 3.57 2.46 3.32 .220 .144 p.p.m. 40 23 .23 Unproductive silt loam Concentrated HCl digestion Productive silt loam .21 .22 Unproductive silt loam One per cent, citric acid Productive silt loam .20 .101 Unproductive silt loam Water extract Productive silt loam .072 p.p.m. 12 Unproductive silt loam 5 A comparison of the figures from the good and poor soil seems to indicate no differences large enough to warrant opin- ions regarding their relative fertility, except in the case of the water extracts. These latter figures, however, are seasonal averages and required as long a time to procure as was neces- sary to grow a crop. Such fertility measurement is not as practicable as actually using the crop as an indicator. 172. Resume. — The conclusion that chemical analyses are of but little direct practical value as a guide to soil prod- uctivity is unavoidable. In spite of the great importance of chemistry in research and teaching, it fails to indicate either the permanent or the immediate fertility of the land. No chemical method is capable of showing substantial and con- stant differences between soils producing within 20 per cent. CHEMICAL ANALYSIS OF SOILS 327 of each other. Even if an analysis should show the nutrients, which would be available over a term of years, it would still be inadequate, since available nutrients are only one of a great number of factors which govern productivity.^ This produc- tivity equation may be indicated as follows : Productivity := Texture X structure X organic matter X moisture X available nutrients X soil reaction X weather X plant disease X care of farmer, etc., etc. The factors of this equation are variables, their importance in determining productivity depending on many things. An accurate knowledge of the available soil nutrients, even if procurable, would aid but little in solving such an equation. The solution of individual or community fertility problems is best accomplished by the aid of experienced and technically trained men, who understand the scientific principles under- lying the common field procedures and who also are in touch with the experiences of farmers over wide and diverse areas. Such men may advise not only in regard to the crops that should be grown but also as to their rotation, management, and fertilization from seeding until harvest. These men may also institute such cooperative experiments and tests as will best throw, light on fertility problems untouched by practical experience. * The samples sent to a chemical laboratory by farmers are gen- erally improperly taken and consequently are not representative. It would be unwise to analyze such soils even if the methods were capable of showing all that could be wished for. CHAPTER XVII 'ALKALI SOILS ^ It has already been shown that soils are acted on by a ^eat variety of weathering agents which gradually render soluble a portion of the most susceptible constituents. This soluble material becomes a part of the soil solution and may come in contact with the roots of any crop growing on the land. In humid regions, where a large quantity of water percolates through the soil, this soluble matter has little opportunity to accumulate.^ In arid regions, however, where loss by drainage is slight, these salts may often collect in large amounts. Dur- ing periods of dry weather they are carried upward by the capillary rise of the soil-water, while during periods of rain- fall they may move downward again in proportion to the leach- ing action. At one time the lower soil may contain consid- erably more soluble salt than the upper; at another time the condition may be reversed, in which case the solution in con- tact with roots may contain so much soluble matter that vege- tation is injured or destroyed. This excess of soluble salts usually has a marked alkaline reaction, but in any case it pro- duces what is termed an alkali soil. Large areas of land in every continent carry soluble salts to such an extent that alkali injury is either actual or poten- * For a complete and satisfactory treatise on alkali see Harris, F. S., Soil Alkali, New York, 1920. ^ Peat soils in humid regions may sometimes contain high concentra- tions of salts, commonly non-toxic, and lower concentrations of ex- tremely toxic salts. Conner, S. D., Excess Soluble Salts in Humid Soils; Jour. Amer. Soc. Agron., Vol. 9, No. 6, pp. 297-301, 19 17. 328 ALKALI SOILS 329 tial. It is estimated that 13 per cent, of the irrigated land of the United States contains sufficient soluble salts seriously to interfere with crop growth. This. alone amounts to nine mil- lion acres and does not include the millions of acres not under the ditch that are affected to a marked degree by alkali. Sim- ilar figures are available from other continents and, since alkali conditions can be alleviated and controlled to a certain extent, the importance of the subject becomes apparent. Entirely aside from the economic aspects, alkali is of great interest scientifically, offering a research field of such range and complexity as to involve many sciences. A greater por- tion of the practical information regarding alkali and its con- trol has arisen from the purely scientific interest that has been directed towards this peculiar soil condition. 173. Composition of alkali. — It has been emphasized pre- viously that the solution of a normal humid-region soil is of such dilution as to be largely ionic in character except in periods of low moisture content. In a soil affected with alkali it is obvious that the molecular state is dominant and that certain salts may exist and function as definite entities. Thus the following bases may be expected to be present — sodium, potassium, magnesium, calcium, and sometimes am- monium. The common acid radicals are chlorides, sulphates, carbonates, bicarbonates, phosphates, and nitrates. The salts that are present and their proportion not only in the soil solu- tion but as a precipitant will vary with conditions. The following table indicates not only the salts that may be present but the composition of the alkali as reported by a number of different investigators. (See table LXXV, p. 330.) 174. White and black alkali. — Sulfates and chlorides of the alkalies, when concentrated on the surface of the soil, produce a white incrustation, which is very common in alkali regions during a dry period as a result of the evaporation of moisture. Incrustations of this character are called white alkali. 330 NATURE AND PROPERTIES OF SOILS Table LXXV comparison of alkali expressed in percentage of the dif- ferent salts present. >^ California (Tulare) Exp. Sta.^* Yakima,^ Wash. 12-24 Inches PiLLiNGS, Mont.' Yuma, Ariz.' Salt Crust Surface 10 Inches Crust 0-72 Inches KCl K^SO, K.CO3 Na^SO^.... NaN03 Na2C03.... NaCl Na.HPO^... MgSO, MgCl, CaCL NaHCOg... CaSO, Ca(HC03), Mg(HC03); (NHJ3CO3 1.6 33.1 6.6 12.7 17.3 21.5 3.9 25.3 19.8 32.6 14.7 2.2 1.4 5.6 9.7 13.8 36.7 1.9 16.5 15.7 1.6 85.6 .5 8.9 .6 2.7 21.4 35.1 7.3 4.0 22.0 10.0 4.0 81.1 7.7 .2 .3 6.6 22.0 13.7 6.9 4.0 21.0 32.2 Carbonates of the alkalies, particularly sodium carbonate, dissolve organic matter from the soil, thus giving a dark color to the solution and to the incrustation. For this reason, alkali containing large quantities of these salts is called tlack alkali. Black or brown alkali may also be produced by calcium chlo- ride or by an excess of sodium nitrate. Black alkali is much more destructive to vegetation than is the white. A quantity of the latter which would not seriously ^Headden, W. P., The Fixation of Nitrogen; Colo. Agr. Exp, Sta., Bui. 155, p. 10, 1910. ' Hilgard, E. W., Soils, p. 442, New York, 1906. 'Dorsey, C. W., Alkali Soils of the United States; U. S. Dept. Agr., Bur. Soils, Bui. 35, 1906. ALKALI SOILS 331 interfere with the growth of most crops might completely pre- vent the development of useful plants if the alkali were black. 175. Origin of alkali. — While the presence of alkali and its influence on plants has been known for centuries, it is only within recent years that its probable mode of origin has been understood. The soluble salts have undoubtedly come from the materials which have formed the soils, the reactions being as complex as the ordinary transformations which take place in soil formation. Some soils have been laid down as deltas in arms of the ocean. If these bodies of water later are cut off from the sea and gradually dry up under arid conditions, an alkali soil will be left. In a similar way saline lakes may disappear and soils heavily charged with alkali will result. The commonest mode of origin for alkali soil is through ordinarj^ weathering under conditions of aridity. Almost any rock will give rise to soils rich in alkali salts if leaching is not a feature in the weathering processes. In western United States the origin of much of the soil affected to the greatest degree with alkali is associated with strata originally carrying much soluble material. When such rock forms soil, the alkali arises not only from the decomposition of the minerals of which the rock is composed, but is greatly reinforced by the soluble salts already present. The Cretaceous and Tertiary beds in Utah, Colorado, and Wyoming are of this character, having been laid down in brackish water. They naturally give rise to soils high in alkali.^ One fact that is often overlooked in practice is that the amount of alkali in the surface layers of soil may be greatly in- creased by improper handling. Rapid evaporation after rain or irrigation will carry the soluble salts toward the surface and deposit them near to or in the root zone. Again, over-irriga- ^ Stewart, R., and Peterson, W., Origin of Alkali; Jour. Agr. Res., Vol. X, No. 7, pp. 331-353, 1917. See also, Breazeale, J. F., Forma- tion of Black Alkali in Calcareous Soils; Jour. Agr. Res., Vol. X, No. 11, pp. 541-589, 1917. 332 NATURE AND PROPERTIES OF SOILS tion may produce leaching into lower lands, an alkali condition generally resulting if the areas so affected remain water-logged for a long time. Very often alkali is localized in small areas called alkali spots. These vary in size from a few square yards to several acres. In years of good rainfall these areas may be pro- ductive, but in dry years they are often quite sterile. Their origin is generally due to seepage, the ground water being near enough the surface to allow a concentration of salts by capillarity, especially in dry seasons. A very peculiar type of alkali spot occurs in the Grand Valley of Colorado and elsewhere, the predominant salt being the nitrate, which does not usually occur in large amounts as alkali. Two theories have been advanced to account for the presence of the nitrate salts. One hypothesis ^ is that the surrounding shales are comparatively rich in nitrates and that the alkali accumulation is a leaching and seepage process. The other theory is biological in nature.^ Such soils are capable of rapid nitrogen fixation by means of their bacterial flora. The idea is advanced that the nitrogen is fixed from the air very rapidly in these spots and later oxidized to the nitrate form. Whatever the origin of the soluble salts the fact re- mains that such spots are quite destructive, spreading very rapidly until whole orchards are wiped out. Water used for irrigation is very often heavily charged with alkali, especially where any amount of the water previously applied to the soil finds its way back into the streams. At Canon City, Colorado, the Arkansas River is very pure. At a point 120 miles below the soluble salts have been known ^ Stewart, E., and Peterson, W., The Nitric Nitrogen Content of the Country Eock ; Utah Agr. Exp. Sta., Bui. 134, 1914. Also, Further Studies of the Nitric Nitrogen Content of the Country Bock; Utah Agr. Exp. Sta., Bui. 150, 1917. " Headden, W. P., The Fixation of Nitrogen in Colorado Soils; Colo. Agr. Exp. Sta., Bui. 186, 1913. Sackett, W. G., and Isham, R. M., Origin of the Niter Spots in Certain Western Soils; Science, N. S., Vol. 42, pp. 452-453, 1915. ALKALI SOILS 333 to reach a concentration of 2200 parts per million. The quan- tity of soluble salts that may be present in irrigation water before it is unfit for use depends on certain conditions. This amount will vary with the crop, the rainfall, the soil, the composition of the alkali, and a number of other factors. ^ ^ ^ I Fig. 54. — Diagram showing the amount and composition of alkali salts at various depths in a soil at Tulare, California. (After Hilgard.) .45 A .35 \ V V; \ .Jo ^ C (i .£^ .£0 ./3 / /?Z-/ ■1 ^ /^ V .05 / (S ^^C ^ U-- ^- pounds of ammoniacal and II/2 pounds of nitrate nitrogen fall on every acre of soil yearly in rainwater. Assuming that all of this nitrogen passes into the soil, an average gain to the acre of 6 pounds of nitrogen may be expected. It is interesting at this point to compare such a gain with the annual loss of nitrogen from the soil. The removal of nitrogen from the Cornell lysimeter soils (see par. 163), through drainage and cropping combined, amounted to 69.0, 77.8 and 56.9 pounds yearly to the acre, respectively, for a bare soil, one carrying a standard rotation, and one continu- ously in grass. While a gain of 6 pounds to the acre yearly seems rather insignificant in comparison to these figures, such an addition is of considerable importance over a period of years, and has had much to do with the accumulation of the nitrogen of our arable soils. Such a gain is equivalent in a practical way to the addition of about 40 pounds of commer- cial sodium nitrate to the acre yearly. 237. Acquisition of nitrogen by free-fixing organisms. — While it has long been known that the soil contains a great variety of organisms, it is only in recent years that it has been in the Rainwater Collected at Uithuiser-Meeden, Gronigen; Jour. Agr. Sci., Vol. IV, pp. 260-269, 1912. ^ Juritz, C. F., Chemical Composition of Bain in the Union of South Africa; S. Africa Jour. Sci., Vol. 10, pp. 170-193, 1914. 'Shutt, F. T., and Dorrance, E., The Nitrogen Compounds of Bain and Snow ; Proc. and Trans. Boy. Soc. Canada, Vol. XI, No. 3, pp. 63-71, 1917. 'Wilson, B. D., Nitrogen in the Rainwater at Ithaca, New York; Soil Sci., Vol. XI, No. 2, pp. 101-110, 1921. SOIL ORGANISMS 431 definitely shown that certain of these organisms have the power of utilizing atmospheric nitrogen, which later becomes a part of the nitrogenous matter of the soil. Boussingault ^ in 1858 suggested the possibility of such a phenomenon, but it was not until 1883 that Berthelot ^ began experiments by which he demonstrated that bare soils appreciably increase in nitrogen on exposure under such conditions. Winograd- ski ^ in 1894 was the first, however, to isolate an organism capable of affecting such a transformation. This bacterium was an anaerobic, rod-shaped organism producing spores and a boat-shaped mass (Clostridium) ; hence the name, Clos- iridium pastorianum. It is very widely distributed in soils. The most important organism fixing nitrogen independently in the soil was discovered by Beijerinck * in 1901. This or- ganism was an aerobic bacillus to which he gave the name Azotobacter. It was at first thought that this bacillus could not fix nitrogen unless certain other organisms, such as Granu- lobacter, Radiobacter and Aerobaeter, were also present. Lip- man ^ has shown this idea to be erroneous, although the effi- ciency of Azotobacter is much higher in mixed than in pure cultures. A number of different species of Azotobacter have been studied, the A. chraococcum apparently being the most widespread. Clostridium pastorianum and Azotobacter are by no means the only soil organisms capable of fixing nitrogen. Among ^ See Voorhees, E. B., and Lipman, J. G., A Eeview of Investigations in Soil Bacteriology ; U. S. Dept. Agr., Office of Exp. Sta., Bui. 194, 1907. ^ Berthelot, M., Becherches nouvelles sur les microorganisms fixateurs de I'azote; Compt. Eend. Acad. Sci. Paris, Tome 115, pp. 569-574 and 842-849, 1892-93. ' Winogradsky, S., Sur I'assim-ilation de I'azote gaseux de I' atmosphere par les microbes; Compt. Rend. Acad. Sci. Paris, Tome 118, pp. 353-355, 1894. ■•Beijerinck, M. W., Vher Oligonitrophile Milroben; Centrbl. Bakt., II, Bd. 7, S. 561-582, 1901. ' Lipman, J. G., Experiments on the Transformation and Fixation of Nitrogen by Bacteria; N. J. Agr. Exp. Sta., 24th Ann. Rep., pp. 217-285, 1903. 432 NATURE AND PROPERTIES OF SOILS bacteria, B. mesentericus, B. pneumonice, B. radiohacter, B. amylohacter, B. prodigiosKS, B. asterosponis, and B. lactis viscusus have certain capacities in this direction. Duggar and Davis ^ have shown that certain filamentous fungi, such as Phoma hetoe, Aspergillus niger, Pencillium digitatum, and others have the ability of utilizing atmospheric nitrogen. The power of fixing nitrogen is, therefore, possessed by a large number of different organisms, yet from the data now at hand the Azotobacter gi^oup seems to be of the greatest economic importance. The nitrogen fixed enters the nitrogen cycle when the organisms die, undergoing decay, ammonifica- tion and nitrification, thus becoming available to higher plants. 238. Conditions for azofication and the amount of nitro- gen fixed.- — The term azofication relates to the fixation of nitrogen by the Azotobacter group,, although it may be used loosely in reference to all free-fixing activities. The soil con- ditions favorable to this phenomenon are those which are opti- mum for higher plants. This is especially true regarding aeration, temperature, and moisture relations. The process is encouraged by the application of lime when soils are acid and seem to require considerable phosphorus. This element is probably utilized in building up proteins within the bodies of the organisms. Potassium, sulfur, iron, and magnesium seem also to be essential to the phenomenon. The Azotobac- ter themselves are influenced by catalytic agents such as manganese. Since considerable energy is required for nitrogen fixation the presence of organic matter in the soil becomes very im- portant in this regard. Almost any non-toxic organic ma- terial may serve as a source of energy, even cellulose being very effective. Farm manure seems especially to encourage 1 Duggar, B. M., and Davis, A. K., Studies in the Physiology of the Fungi; Ann. Mo. Bot. Garden, Vol. 33, pp. 413-437, 1916. ^ A very excellent review of literature and discussion of Azofication : Greaves, J. E., Azofication; Soil Sei., Vol. VI^ No. 3, pp. 163-217, 1918. SOIL ORGANISMS 433 nitrogen fixation. The maintenance of a fair supply of soil organic matter is, tliercfore, as important as the regulation of the temperature, the oxygen, the moisture, and the reac- tion of the soil. While the presence of nitrates in small amounts seems to stimulate azofication, large quantities of nitric nitrogen tend to lessen nitrogen fixation. The amount of nitrogen fixed in the soil by organisms func- tioning independently of higher plants is, as might be ex- pected, a variable quantity. Hall ^ considers it to be on the average about 25 pounds yearly to the acre. Greaves - 25 pounds, Lohnis ^ 36 pounds, and Lipman * from 15 to 40 pounds. As a basis for calculation 25 pounds is perhaps a conservative and reasonable figure. A comparison of this figure with the 6 pounds of nitrogen brought down yearly in rain-water, indicates that the free-fixing organisms are four or five times more important than rainfall as a source of nitrogen, 239. Bacillus radicicola and its relationship to the host plant. — It has long been recognized by farmers that certain crops, as clover, alfalfa, peas, beans, and some others, im- prove the soil, making it possible to grow larger crops of cereals after these plants have occupied the land. Within the last century the benefit has been traced to the fixation of nitrogen through the agency of bacteria contained in nod- ules on the roots. The specific plants so affecting the soil were found to be, with a few exceptions, those belonging to the family of legumes. It has furthermore been demonstrated that the host plant is generally able to appropriate some of the nitrogen so fixed and thus benefit by the relationship. The phenomenon was fully explained in 1886 by Hellreigel ^ Hall, A. D., On the Accumulation of Fertility hy Land Alloiced to Bun Wild; Jour. Agr. Sci., Vol. I, pp. 241-249, 1905. =" Greaves, J. E., Azofication; Soil Sci., Vol. VI, No. 3, pp. 163-217, 1918. ^Lohnis, F., and Westermann, F., Vher Sticlstoff fixierende Balterien, IV. Centrbl. f. Bakt., II, Bd. 22, S. 234-254, 1909. * Lipman, J. G., Marshall's Microbiology, p. 343, 1917. 434 NATURE AND PROPERTIES OF SOILS and Wilfarth. The organisms, of which there are a number of strains, are called Bacillus radicicola. The organisms living in the root nodules take free nitrogen from the air in the soil, and the host plant secures it in some form from the bacteria or their products. The presence of a certain species of bacteria is necessary for the formation of tubercles. Leguminous plants grown in cultures or in soil not containing the necessary bacteria do not form nodules and do not utilize atmospheric nitrogen, the result being that the crop produced is less in amount and the percentage of nitrogen in the crop is lower than if nodules were formed. The nodules are not normally a part of leguminous plants, but are evidently caused by an irritation of the root sur- face, much as a gall is caused to develop on a leaf or a branch of a tree by an insect. In a culture containing the proper bacteria the prick of a needle on the root surface will cause a nodule to form in the course of a few days. The entrance of the organism is effected through a root-hair which it pene- trates, and it may be seen as a filament extending the entire length of the hair and into the cortex cells of the root, where the growth of the tubercle starts. Even where the causative bacteria occur in cultures or in the soil, a leguminous plant may not secure any atmospheric nitrogen, or perhaps only a small quantity, if there is an abundant supply of readily available combined nitrogen on which the plant may draw. The bacteria have the ability to utilize combined as well as uncombined nitrogen, and prefer to have it in the former condition. On soils rich in nitrogen, legumes may, therefore, add little or no nitrogen to the soil, if the above ground portion of the crop is not plowed under; while in properly inoculated soils deficient in nitrogen an important gain of nitrogen may result. While B. radicicola is considered the organism common to all leguminous plants, it is now known that the organisms from one species of legume are not equally well adapted to SOIL ORGANISMS 435 the production of tubercles on other leguminous species. Cer- tain cross inoculations are, however, veiy successful. The organisms seem to be interchangeable within the clovers, the vetches and the bean family. The organisms from sweet clover and burr clover will ino(!ulate alfalfa, while the bac- teria may be transferred from vetch to field pea or from cow- pea to velvet bean. It has been shown by several investigators that bacteria from the nodules of legumes are able to fix atmospheric nitro- gen even when not associated with leguminous plants. There would seem to be no doubt, therefore, that the fixation of nitrogen in the tubercles of legumes is accomplished directly by this organism, not by the plant itself nor through any com- bination of the plant and the organism. The relationship is, therefore, parasitical rather than strictly symbiotic, although the host plant benefits from the relation. The part played by the plant is doubtless to furnish the carbohydrates which are required in considerable quantities by all nitrogen-fixing organisms and which the legumes are able to supply in large amounts. The utilization of large quantities of carbohydrates by the nitrogen-fixing bacteria in the tubercles may also ac- count for the small proportion of non-nitrogenous organic matter in the plants. How the plant absorbs this nitrogen after it has been secured by the bacteria is not well understood nor is it known in exactly what form the nitrogen is at first fixed, although amino and amide nitrogen very soon appear.'^ Early in the growth of the tubercle, a mucilaginous substance is produced, which permeates the tissues of the plant in the form of long slender threads containing the bacteria. These threads de- velop by branching or budding, and form what have been called Y and T forms, known as bacteroids, which are peculiar to these bacteria. The threads finally disappear, and the ^Strowd, W. H., The Forms of Nitrogen in Soybean Nodules; Soil Sci., Vol. XI, No. 2, pp. 123-130, 1921. 436 NATURE AND PROPERTIES OF SOILS bacteria diffuse themselves more or less through the tissues of the root. What part the bacteroids play in the transfer of nitrogen is not known. It has been suggested that in this form the nitrogen is absorbed by the tissues of the plant. It seems quite likely that the nitrogen compounds produced within the bacterial cells are diffused through the cell-wall and absorbed by the plant. 240. The practical importance of B. radicicola. — The nitrogen fixed by the nodule organisms may go in three di- rections in the soil. It may be absorbed by the host plant, the latter benefiting greatly by the association. This rela- tionship has already been discussed. Secondly, the nitrogen may pass in some way into the soil itself and benefit a crop associated with the legume. Thirdly, the nodules may decay, when the legume dies or is turned under, the nitrogen be- coming available to the succeeding crop. The relationship between associated legumes and non- legumes has been particularly studied by Lyon and Bizzell ^ and by Lipman.- It has been quite definitely proven that the non-legume may be greatly benefited by the association under some conditions. This accounts for the practice of growing timothy with clover, which has been common for centuries. Just how the transfer of nitrogen is facilitated yet remains to be shown. The beneficial influences of such legumes as clover, vetch, and alfalfa on the succeeding crops has long been taken ad- vantage of in practical agriculture. Until recently the stimu- lation has been ascribed to an actual increase of nitrogen in the soil, due to the growth of the legume and the activity of its nodule organisms. This will not always account for the phenomenon, since it has been shown by a number of investi- ^ Lyon, T. L., and Bizzell, J. A., Availability of Soil Nitrogen in Relation to the Basicity of the Soil and to the Growth of Legumes; Jour. Ind. and Eng. Chem., Vol. 2, No. 7, pp. 313-315, 1910. ^ Lipman, J. G., The Associative Growth of Legumes and Non-Legumes. N. .J. Agr. Exp. Sta., Bui. 253, 1912, SOIL ORGANISMS 437 gators that the continuous growing of legumes, the tops being removed as forage, does not always increase the nitrogen con- tent of the soil to any greater extent than does a non-legumi- nous crop. The results of Swanson ^ are particularly striking in this respect. This investigator sampled a number of fields in Kansas that had grown alfalfa continuously for twenty or thirty years, at the same time obtaining soil from contiguous native sod. In most cases the alfalfa soil was lower in nitrogen than the sod. Lyon and Bizzell ^ found practically the same content of nitrogen in contiguous alfalfa and timothy soils after the crops had been growing six years. The maize crop following the alfalfa was nevertheless much greater than that after the timothy. Since the soil on which a legume has been growing generally has a rather high nitrify- ing capacity,^ the explanation seems to lie in the ready avail- ability of the nitrogen in the soil which bore the legume, rather than to the presence of an especially large amount. The amount of nitrogen fixed by the nodule organisms of a leguminous crop is very uncertain. If the soil is acid, if it contains alkali salts above a certain amount, or if nitrates develop rapidly, nitrogen fixation is markedly retarded. Much also depends on the virulence of the organisms, the character of the legume, the presence of organic matter, and other im- portant conditions. Hopkins * estimates that about one-third ^Swanson, C. O., TJie Effect of Prolonged Growing of Alfalfa on the Nitrogen Content of the Soil; Jour. Amer. Soc. Agron., Vol. 9, No. 7, pp,.'^ 305-314, 1917. Swanson, C. O., and Latshaw, W. L., Effect of Alfalfa on the Fer- tility Elements of the Soil in Comimrison with Grain Crops; Soil Sci., Vol. VIII, No. 1, pp. 1-39, 1919. ^Lyon, T. L., and Bizzell, J. A., Experiments Concerning the Top- dressing of Timothy and Alfalfa; Cornell Agr. Exp. Sta., Bui. 339, np. 136-139, 1913. ^Lyon, T. L., Bizzell, J. A., and Wilson, B. D., The Formation of Nitrates in a Soil Following the Growth of Bed Clover and of Timothy; Soil Sci., Vol. IX, No. 1, pp. .53-64, 1920. * Hopkins, C. G., Soil Fertility and Permanent Agriculture, p. 223, Boston, 1910. 438 NATURE AND PROPERTIES OF SOILS of the nitrogen of a normal inoculated legitme comes from the soil and two-thirds from the air. He also assumes that one-third of the nitrogen of the plant exists in the roots. Al- though both of these assumptions are questionable, they sug- gest the reason why the removal of the tops of legumes as forage allows no accumulation of nitrogen in the soil. According to Hopkins, the nitrogen in the tops of legumes is a rough measure in general of the nitrogen fixed. On such an assumption, the growth of red clover should facilitate the fixation of about 40 pounds of nitrogen for every ton of air- dry material. On the same basis, the figure should be about 50 pounds for alfalfa, 43 pounds for cowpeas, and 53 pounds for soybeans. These figures, even though they are obviously incorrect, give some idea of the importance of B. radicicola in nitrogen fixation. The growth of an average leguminous crop under proper conditions probably is accompanied by a fixation of 80 to 100 pounds of nitrogen. Of the three nat- ural methods by which atmospheric nitrogen may be fixed by the soil that facilitated by the nodule organisms seems at first thought to be considerably the most important. It must be remembered, however, that with an average rotation a legume occupies the land but one or two years in three to six. Moreover, the gain of nitrogen in a fertile soil is but slight unless the crop is turned under as a green-manure. Unless so used the chief advantages of growing a legumi- nous crop lie in the increase of soil organic matter, the ready and favorable decay of the roots and stubble, and the opportunity of growing a high protein crop without ma- terially depleting the soil nitrogen. 241. Soil inoculation for legumes. — Although the inocu- lation of the soil with free-fixing organisms has not proven of value, since such organisms are always present and suffi- ciently active if soil conditions are favorable, the inoculation with nodule bacteria is of considerable practical importance. Such organism may never have been present in a soil or may SOIL ORGANISMS 439 have disappeared because of unfavorable conditions. If leg- umes, especially of certain types, are to be grown most suc- cessfully, the specific strains of B. radicicola for that crop must be present. Two general methods of inoculation are available: (1) the use of soil from fields where the particular legume in ques- tion is growing or has grown successfully; and (2) the utiliz- ation of artificial cultures of some form. Bacillus radicicola is found in the soil as well as in the plant nodules. As a matter of fact, this bacterium will live in the soil for long periods, even if the host plant is not grown. Whether it fixes nitrogen to any extent under such conditions is a question. At least the organism does not lose its virulence. Such soil may be spread on the land to be inoculated at the rate of 300 to 500 pounds to the acre. It should be applied in the evening or on a cloudy day and harrowed in as soon as pos- sible, as the organisms are injured by direct sunlight. The soil carrying the organism may also be mixed after air-drying with the seed, the latter having been moistened with a dilute glue solution.^ Enough of the dry earth sticks to the seed to carry the organisms into the soil. The advantage of this method is that the bacteria are in contact with the seed and the plants become infected very soon after the seeds germinate. The main objection to the soil method of inocu- lation lies in the possibility of spreading plant diseases and undesirable weeds. * Dissolve ordinary furniture glue in boiling water, two handf uls of glue to every gallon of water used, and allow the solution to cool. Put the seed in a wash-tub, and then sprinkle enough of the solution on the seed to moisten but not to wet it (one quart to a bushel is sufficient), and stir the mixture thoroughly until all the seeds are moistened. Dry the inoculating soil in the shade, preferably in the barn or base- ment, and pulverize it thoroughly into a dust. Scatter this dust over the moistened seed, using from one-half to one gallon of dirt for each bushel of seed, mixing thoroughly until the seed no longer stick together. The seed is then ready to sow. See Vrooman, C, Grain Farming in the Corn Belt with Live Stock as Side Line; Farmers' Bui., No. 704, 1916. 440 NATURE AND PROPERTIES OF SOILS Within recent years a number of cultures for soil inocula- tion have been offered to the public. The first of these util- ized absorbent cotton to transmit the bacteria in a dry state from the pure culture in the laboratory to the user of the cul- ture, who was to prepare therefrom another culture to be used for inoculating the soil. Careful investigation of this method showed that its weakness lay in drying the cultures on the ab- sorbent cotton, which frequently resulted in the death of the organisms. More recently liquid cultures have been placed on the market in this country, and these have, in the main, proved to be more successful, notably those sent out by the United States Department of Agriculture. Another very successful culture medium, now being used by the Department of Plant Physiology at Cornel University, is steamed soil. A soil, favorable to the development of nodule organisms and usually a sandy loam, is sterilized by steaming. It is then brought up to optimum moisture and later inoculated with a number of different strains of B. radicicola. After incubation for several days at a favorable temperature, the soil cultures are ready for distribution. The soil is sent out in small air-tight cans by parcel post. The advantage of such a culture is that the organisms are viru- lent and there is no danger from plant diseases or undesir- able weeds. When a culture of this sort is received it may be used in a number of different ways. It may be mixed with field soil at the rate of 1 pound to 300 of the latter. This 300 pounds of inoculated soil may then be spread on a acre of land in the usual way. The culture may also be disposed of by the glue method or it may be suspended in water and the extract sprinkled on the seed and dried in the shade. In either case, the seed should be sown as soon as possible. 242. Resume. — The biological phases of the soil are so nu- merous and far-reaching that it is obviously impossible in summarizing their practical relationships to do more than call SOIL ORGANISMS 441 attention to certain significant facts. In the first place, the soil fauna and flora, especially the latter, are exceedingly complex. Tlie number of plant forms are so numerous that tlie discussion already presented serves as little more than an introduction. In the second place, the transformations facili- tated by soil organisms involve all of the normal constituents of the soil, both organic and inorganic. Moreover, biological activities determine to a large degree the efficacy of every addition, natural or artificial, made to the land. While the cycles generally recognized are apparently clear cut, the transformations themselves are actually involved in intrica- cies, which man will probably never entirely unravel. A third pliase of outstanding importance is the relationship of the biological activities of the soil to the nitrogen prob- lem. Not only are the complex nitrogenous compounds of the soil readily made available to higher plants by soil organisms, but means are provided whereby considerable nitrogen, in- ert as it is, may be wrested from the atmosphere and forced into activity within the soil. It is not impossible that in cer- tain favored cases 150 pounds of nitrogen to the acre may be yearly added to the soil by such processes. This phase alone is worthy of the most careful practical study. Obvi- ously no_system of soil management can be wholly successful unless full advantage is taken of this and other biological possibilities of the land. CHAPTER XXII COMMERCIAL FERTILIZER MATERIALS^ While the use of animal excrement on cultivated soils was practiced as far back as systematic agriculture can definitely be traced, the earliest record of the use of mineral salts for in- creasing the yield of crops was published in 1669 by Sir Kenelm Digby.^ He says: "By the help of plain salt petre, diluted in water, and mingled with some other fit earthly substance, that may familiarize it a little with the corn into which I endeavored to introduce it, I have made the barrenest ground far outgo the richest in giving a prodigiously plentiful harvest." His dissertation does not however, show any true conception of the reason for the increase in the crop through the use of this fertilizer. In fact, the lack of any real knowl- edge at that time of the composition of the plant would have made this impossible. In 1804, de Saussure,^ a Frenchman, called attention, for the first time to the significance of the ash ingredients of plants not only showing that these mineral materials were ^ The following general references may prove helpful : Hall, A. D., Fertilisers and Manure; New York, 1921. Halligan, J. E., Soil Fertility and Fertilisers; Easton, Pa., 1912. Van Slyke, L. L., Fertilizers and Crops; New York, 1912. Fraps, G. S., Principles of Agricultural Chemistry; Easton, Pa., 1912. Collins, S, H., Chemical Fertilisers and Parasiticides; New York, 1920. ^ Digby, Kenelm, A Discourse Concerning the Vegetation of Plants; London, 1669. ^ Saussure, Theodore de, Eecherclies Chimiques sur la Vegetation; Paris, 1804. 442 COMMERCIAL FERTILIZER MATERIALS 443 obtained from the soil but pointing out that they were ab- solutely essential for plant growth. Liebig/ in Germany, at about the middle of the nineteenth century, emphasized still more strongly the importance of minerals to plants, refuting the theory, at that time current, tliat plants obtained all of their carbon from the soil organic matter. While he showed the importance of potash and phosphoric acid in manures, he failed to appreciate the value of nitrogenous materials, hold- ing that the soil received sufficient ammonia in rain-water. The true conception of the necessity of supplying nitrogen in some form was definitely established in an experimental way in 1857 by Lawes, Gilbert and Pugh - of the Rothamsted Experiment Station, England. The extreme care used by these investigators caused them to sterilize the soil with which they were working. They thus failed to discover the utiliza- tion of free atmospheric nitrogen by legumes. This phe- nomenon, so important in practical agriculture, was explained by Hellriegel and Wilforth in 1886. Between 1840 and 1850 Sir John Lawes placed the manu- facture of superphosphates on a commercial basis by treating bones and coprolites with sulfuric acid. At about this time the importation into Europe of Peruvian guano and sodium nitrate began. The commercial fertilizers industry, which has now attained such importance in practical agriculture, may be considered as dating from this period. 243. Commercial fertilizers. — Although the commercial fertilizer industry is but little more than seventy years old, the sale of fertilizers in this country at the present time amounts to millions of dollars annually. Animal refuse and ^Liebig, J. Justus von. Principles of Agricultural Chemistry with Special Beference to the Late Besearches Made in England; London, 1855. Also, Chemistry in Its Applications to Agriculture and Physiology ; New York, 1856. * Lawes, J. B., Gilbert, J. H., and Pugh, E., On the Sources of the Nitrogen of Vegetation, with Special Beference to the Question Whether Plants Assimilate Free or Uncombined Nitrogen; Rothamsted Memoirs, Vol. 1, No. 1, 1862. 444 NATURE AND PROPERTIES OF SOILS phosphates are exported, while sodium nitrate and potash salts are imported in large amounts. Fifty per cent, of the fertilizers sold in the United States are applied in the south Atlantic states within three or four hundred miles of the seaboard. Nearly one-half of the remainder is purchased by the New England and middle Atlantic states. West of the Mississippi River, the use of fertilizers, especially those car- rying phosphoric acid, is increasing rapidly. The primary function of a commercial fertilizer is to supply plant nutrients to the soil in such a form that the plant may be directly influenced by such an application. The secondary influences of fertilizers may be beneficial or detrimental. The exact nature of the secondary influences depends on the par- ticular fertilizer applied and especially on the type of soil and the crop management in vogue. Prepared fertilizers, as found on the market, are usually composed of a number of ingredients. Since these ingredi- ents are the carriers of the nutrient constituents, and since it is on their composition and solubility that the value of a fertilizer depends, a knowledge of the properties of these materials is not only of interest to every one who uses fer- tilizers but is also a valuable aid in their purchase. FERTILIZERS USED FOR THEIR NITROGEN Nitrogen is usually the most expensive constituent of ma- nure and is of great importance, since it is very likely to be deficient in soils. A commercial fertilizer may have its nitro- gen in the form of soluble inorganic salts or in organic com- bination. On the form depends to a certain extent the agri- cultural value of the nitrogen, as the soluble inorganic salts are very readily available to the plant, while the organic forms must pass through the various biological processes before the plant can use the nitrogen so contained. Only the best- known fertilizer carriers need receive particular attention here. COMMERCIAL FERTILIZER MATERIALS 445 244. Dried blood and tankage.^ — Both of these fertilizers are packing-house products. The former is obtained by dry- ing the blood from the slaughtering pens. It comes on the market as a homogeneous blackish to dark greyish material, often slightly moist and with a characteristic odor. Its con- tent of ammonia (NHg) ranges from 10 to 16 per cent., de- pending on the grade of the fertilizer. It often contains traces of phosphoric acid (PgOg).^ Tankage is a mixture of various refuse materials from the slaughter-houses, such as blood, hair, scraps of meat, and hide and bone. It is generally steam-cooked and part of the gela- tin and fat removed. It is variable in composition, carrying from 5 to 10 per cent, of NH3 and from 3 to 8 per cent, of P2O5. The phosphoric acid is contained in the bone and is in the form of tricalcium phosphate [Ca3(P04)2]. Tankage is easily distinguished from blood meal by its heterogeneous character. When added to a soil, both blood and tankage undergo rapid decomposition, ammonification, and finally nitrification. Such fertilizers are, therefore, very effective in the late spring and summer. For early application, however, a material such as sodium nitrate is much better, since a biological transfor- mation is unnecessary in order that it may be immediately utilized by the plants. 245. Other organic nitrogenous fertilizers. — Below will be found the composition of a number of other organic ma- terials that have been or are still used as fertilizers. Only two need explanation. Guano consists of the excrement and car- casses of sea fowls, the composition depending on the climate and position in which it is found. Guano from an arid region contains ammonia, phosphoric acid, and potash. Under humid conditions only the phosphoric acid remains in any *Fry, W. H., Identification of Commercial Fertiliser Materials; U. S. Dept. Agr., Bui. 97, 1914. ^ The composition of commercial fertilizers is commonly expressed in terms of ammonia (NH3), phosphoric acid (P2O5), and potash (K^O). 446 NATURE AND PROPERTIES OF SOILS amount. Typical guano carries uric acid, urates, and am- monium salts. The phosphorus occurs as calcium, potas- sium, and ammonium phosphates. The potash is found in the chloride, sulfate and phosphate forms. While guano was once a very important fertilizer, the deposits are very nearly exhausted and but little now appears on the market. Process fertilizers are obtained by treating organic trade wastes and refuse with acid or with steam under pressure. Hydrolysis of the proteins occurs with the formation of pro- teoses, peptones, and simple amino acids. The water soluble nitrogen of such materials has been shown by Lathrop of the United States Bureau of Soils to be as readily available as that of dried blood or tankage. Table XCV Fertilizer NH3 P2O. K,0 Guano Process goods 10-14 1- 3 10-13 8-11 ,8^12 10-16 8h-10 4- 6 5e- 7 6- 7 10-12 6- 7 1- 2 1-2 1- 11/2 2-5 Hoof meal Fish scrap Leather meal Wool and hair waste Cottonseed meal Linseed meal 2-3 1-2 Castor pomace 1-11/2 These compounds vary greatly in their values as fertilizers. Guano, process goods, and fish scrap when in the soil decom- pose rapidly and are as effective ordinarily as blood or tank- age. Untreated leather meal and wool and hair waste decay very slowly and are of little value as fertilizing materials. 246. Utilization of nitrogenous organic compounds by plants. — One of the early beliefs in regard to plant nutrition was that organic matter as such is directly absorbed by higher plants. This opinion was afterwards entirely replaced by the COMMERCIAL FERTILIZER MATERIALS 447 mineral theory propounded by Liebig ; and still later the dis- covery of the nitrifying process almost disposed completely of the belief that organic matter is used directly by higher plants. It is quite certain, however, that some organic nitrog- enous compounds furnished suitable material for some higher plants without undergoing bacterial change and producing a nitrate form of nitrogen. The following compounds have been shown by Hutchinson and Miller ^ to be readily assimilated by peas : acetamide, urea, barbituric acid, and alloxan. Formamide, glycerine, cyanuric acid, oxamine, peptone, and sodium aspartate were assimilated but less easily. Creatinine has been shown by Skinner - to be used directly by plants as a source of nitro- gen. Histidine, arginine, and creatine have also been found in soils and it has been demonstrated that they have a direct influence on wheat seedlings. These and numerous other investigations of this subject show that amine as well as amide nitrogen is assimilated by at least some agricultural plants, but to what extent most of these compounds may successfully replace the inorganic forms of nitrogen, such as the nitrates, has not been definitely established. Certain organic nitrogenous fertilizers^as, for example, dried blood — have a high commercial value, the nitrogen in this form selling for more a pound than the nitro- gen in any of the inorganic salts. Many crops, especially cer- tain vegetables, are most successfully grown only when supplied with organic nitrogenous material. Some ni- trate nitrogen is always present under natural soil condi- tions, so that crops are never limited to organic nitrogen alone ; and it may be that the latter form of nitrogen is most useful when it supplements the nitrate form. ^Hutchinson, H. B., and Miller, N. H. J., The Direct Assimilation of Inorganic and Oraanic Forms of Nitrogen by Higher Plants; Centrlb. f. Bakt., II, Bantl 30, Seite 513-547, 1911. * Skinner, J. J., III. Effects of Creatinine on Plant Growth; U. S. Dept. Agr., Bur. Soils, Bui. 83, pp. 33-44, 1911. 448 NATURE AND PROPERTIES OF SOILS 247. Sodium nitrate (NaNO. -\-)} — Sodium nitrate is mined in Chile, occurring as a crude salt (caliche) in the semiarid regions along the coast. It is found near the sur- face under an over burden of varying thickness. The cal- iche contains, besides sodium nitrate, such salts as NaCl, K2SO4, Na.SO,, and MgSO^ besides traces of Na.COg, K0CO3, and boron. The refined salt, which is shipped to this country, carries from 2 to 3 per cent, of NaCl and KNO3. Its am- monium content is generally rated at about 18 per cent. The fertilizer appears on the market in clouded crystals of a yellowish cast, extremely soluble in water and quite de- liquescent. The fertilizer is generally alkaline to litmus. In the soil it diffuses rapidly and is immediately avail- able to plants. For this reason it. is extremely valuable early in the spring before nitrification is active. The long-continued use of sodium nitrate will tend to pro- duce an alkaline residue of sodium carbonate in the soil." This is due to the absorptive power of the soil for sodium and the ease with which the nitrate ions are lost in drainage. The plant, by using large amounts of nitrates, intensifies this se- lective absorption. The origin of the caliche deposits is problematical. The theory has been advanced that the origin is due to the de- composition of great deposits of seaweed on an uplifted con- tinental shelf. Another hypothesis would have the deposits originate from wind-carried guano dust. As rational a the- ory as any is proposed by Singewald and Miller,^ who believe the nitrates were leached from the Andes Mountains and ^ Fertilizer materials are never pure salts. The plus after the formula indicates the presence of impurities. 2 Hall, A. D., TJie Effect of the Long Continued Use of Sodium Nitrate on the Constitution of the Soil; Trans. Chem. Soc. (London), Vol. 85, pp. 950-971, 1904. Also, Brown, B. E., Concerning Some Effects of Long- Continued Use of Sodium Nitrate and Ammonium Sulfate on the Soil; Ann. Eep. Pa. State Coll., 1908-1909, pp. 85-104. ^Singewald, J. N., and Miller, B. L., Genesis of the Chilean Nitrate Deposits; Econ. Geol., Vol. II, pp. 103-113, 1916. COMMERCIAL FERTILIZER MATERIALS 449 carried by ground water to their present location. The con- centration of the salts is considered by these authors as due to surface evaporation and consequent upward capillary movement of the highly charged ground water. 248. Ammonium sulfate ((NHJ.SO^ +).— This fertil- izer is a by-product from coke ovens and from the distilla- tion of coal in gas manufacture.^ About one-fifth of the nitrogen of the coal is thus driven off as ammonia, which is caught in special washing devices. The mother liquid is then distilled, the NH3 being driven into sulfuric acid. The prod- uct is later concentrated and the salt crystallized out. An- other and simpler process provides for a direct union of the gas and the acid, thus eliminating the washers. This fertilizer usually carries about 25 per cent, of am- monia. It usually has a greyish or greenish color due to coal-tar products. This commercial ammonium sulfate is very soluble in water and has a characteristic taste. When heated, it readily breal« up, giving off ammonia gas. It is very acid to litmus paper, due to the union of a weak base with a strong acid radical. The ammonia is very strongly absorbed by the soil and also is used to a greater extent by the plant than are the sulfate ions. It thus leaves in the soil an acid residue - which should be alleviated by lime if the soil is not already supplied with plenty of active calcium and magnesium. In a warm soil the ammonia is quickly nitrified to the nitrate form. This transformation is general- ^ By-Product Colce and Gas Plants; The Koppers Company, Pitts- burgh. Sulfate of Ammonia. Its Source, Production and Use; The Barrett Company, New York. ^ Hall, A. D., and Gimingham, C. T., The Interaction of Ammonium Salts and the Constitution of the Soil; Jour. Chem. Soc. (London), Vol. J)l, pt. 1, p. 677, 1907. White, J. W., The Besults of Long Continued Use of Ammonium Sulfate Upon a Besidual Limestone Soil of the Eagerstown Series; Ann. Rep. Pa. State Coll., 1912-1913, pp. .55-104. Ruprecht, R. W., and Morse, F. W., The Effect of Sulfate of Ammonia on Soil; Mass. Agr. Exp. Sta., Bui. 165, 1915. 450 NATURE AND PROPERTIES OF SOILS ly so rapid as to make this fertilizer almost as quickly effec- tive as sodium nitrate. "While the nitrogen of ammonium salts is quickly changed to the nitrate combination in a well-drained soil, some plants seem to prefer ammoniacal nitrogen to the nitrate form. Kell- ner ^ in 1884 and later Kelley - demonstrated that rice plants growing on lowland soils use ammoniacal nitrogen rather than other forms. On upland soils, however, it is presumable that rice plants utilize nitrate nitrogen, which would indi- cate that some plants, at least, may adapt themselves to the use of a more abundant form of nitrogen. Hutchinson and Miller ^ found that peas obtained nitrogen from ammonium salts as readily as from sodium nitrate, but that wheat plants, although able to obtain nitrogen directly from ammonium salts, grew much better in a solution con- taining nitrates. One feature brought out by the numerous experiments with ammonium salts is the difference between plants of various kinds in respect to their ability to absorb nitrogen in this form. 249. The artificial fixation of nitrogen.* — The vast store of atmospheric nitrogen, chemically uncombined and very inert, will furnish an inexhaustible supply for plants when it can with reasonable economy be combined in some manner to give a product that can be commercially transported and that will, when placed in the soil, become available without liberating substances toxic to plants. The importance of the ' Kellner, 0., AgrikulturchemiscJie Studien iiber die Beislcultur ; Landw. Vers. Stat., Band 30, Seite 18-41, 1884. ' Kelley, W. P., The Assimilation of Nitrogen by Bice; Haw. Agr. Exp. Sta., Bui. 24, pp. 5-20, 1911. ^ Hutchinson, H. B., and Miller, N. H. J., The Direct Assimilation of Inorganic and Organic Forms of Nitrogen by Higher Plants; Centrlb. f. Bakt., II, Band 30, Seite .513-547, 1911. ^Norton, T. H., Utilization of Atmospheric Nitrogen; U. S. Dept. of Comm. and Labor, Special Agents Ser., No. 52, 1912. Knox, J., Fixation of Atmospheric Nitrogen; New York. Slosson, E. E., Creative Chemistry, Chaps. II and III; New York, 1920. COMMERCIAL FERTILIZER MATERIALS 451 nitrogen supply for agriculture may be appreciated when it is considered that nitrates are being carried otl in the drain- age water of all cultivated lands at a surprisingly rapid rate. A Dunkirk silty clay loam at Cornell University/ carrying a rotation of maize, oats, wheat, and hay, lost in crop and drainage water in a period of ten years over 77 pounds to the acre of nitrogen annually. This is equivalent to 520 pounds of commercial sodium nitrate or to about 380 pounds of com- mercial ammonium sulfate. The exhaustion of the supply of nitrogen in most soils may be accomplished within one or two generations, unless a re- newal of the supply is brought about in some way. Natural processes provide for an annual accretion through the wash- ing-down of ammonia and nitrates by rain-water from the atmosphere, and through the fixation of free atmospheric nitrogen by bacteria. Farm practice of the present day re- quires the application of nitrogen in some form of manure, and, as the end of the commercial supply of combined nitro- gen is easily in sight, there is urgent need of discovering a new source. The world war has given great impetus to the study of the artificial fixation of nitrogen and a number of compounds thus produced are on the market or will appear shortl3^ 250. Calcium cyanimid (CaCN. +).2— The manufacture of this fertilizer begins with calcium carbide (CaCg) which is produced by heating lime and coke together. CaO + 3C = CaC, + CO This impure carbide is then powdered and heated elec- trically in special ovens. At the proper temperature nitro- gen gas is passed through the carbide with the following re- sult : CaCa + N, = CaCN^ -f C *For complete data, see par. 163, this text. *Pranke, E. J., Cyanamid; Easton, Pa., 1913. 452 NATURE AND PROPERTIES OF SOILS The product is a black dry crystalline powder of rather light weight, containing about 20 per cent, of NH3. It is very impure as shown by the following analysis: CaCN^ 45.9 C 13.1 CaCO, 4.0 Fe.Og and AI.O3 1.9 CaS ^. 1.7 SiO. 1.6 CagPg 1 MgO 1 Ca(OH)., 26.6 ILO 3 Its odor and the presence of carbon are characteristic. It is intensively alkaline to litmus. In the soil it undergoes a number of very complex changes, urea ultimately being pro- duced. Toxic compounds are present as the reactions pro- ceed. It should, therefore, be placed in the soil some time be- fore the crop is seeded. The carbon seems to aid in the trans- formation as a catalytic agent. The urea quickly breaks down biologically to ammonia : CON A + 2H,0 = (NHJ2 CO3 This ammonia is then oxidized to the nitrate form. 251. Basic calcium nitrate (Ca(N03)2+).— This fertil- izer, like calcium cyanimid, is produced by the artificial fixa- tion of nitrogen. Air is passed through an electric arc of high temperature. Under such conditions a part of the oxygen and the nitrogen are forced together forming nitric oxide. This gas is then oxidized in suitable chambers to the peroxide, which is passed into water, producing nitric acid. The nitric oxide which also results is led back to the oxidizing chambers. The reactions are as follows : N, 4- 0, = 2N0 2N0 + 0. = 2NO2 3N0. + H,0 = 2HN0, -f NO The nitric acid is passed into lime-water, giving calcium nitrate. This fertilizer contains from 13 to 16 per cent, of ammonia and is intensely alkaline to litmus. Due to its high COMMERCIAL FERTILIZER MATERIALS 453 deliquescence, it must either be treated in some way, which raises the cost of manufacture, or must be shipped in sealed casks. It is very soluble in water and is immediately available to plants. It leaves no harmful residue in the soil. 252. Other methods of nitrogen fixation. — Calcium ni- trate, because of its cost, cannot compete either with sodium nitrate or ammonium sulfate and is not manufactured in this country. Calcium cyanamid is produced only in amounts sufficient to satisfy the demands of mixed fertilizer manu- facture. Its dry character makes it valuable in such com- pounding. At the present time a number of more efficient methods of artificially fixing nitrogen are known. The Haber process proved extremely successful in Germany, especially when supplemented by the Oswald method of converting ammonia into nitric acid. In the Haber method a mixture of nitrogen and hydrogen are placed under pressure and moderately heated in the presence of a catalyst. A good yield of ammonia results. N, + 3Ho = 2NH3 In the Oswald method this ammonia is passed over a cata- lytic agent in the presence of oxygen. NH3 + 2O0 = HNO3 + H2O The advantage of producing both ammonia and nitric acid is obvious, as ammoniun nitrate (NH4NO3), ammonium phos- phate ((NH^)3P0J, and potassium nitrate (KNO.) may be produced at one plant. During the war Professor Bucher of Brown University per- fected a simple and inexpensive method of producing sodium cyanide synthetically. Producers gas, formed by passing air over hot coal, is forced through a heated revolving drum con- taining soda ash, iron, and coke. The reaction is as follows: NagCOa -f 4C + N, = 2NaCN + SCO 454 NATURE AND PROPERTIES OF SOILS Ammonia may be produced very easily from the sodium cyanide and used as such or changed to nitric acid by the Oswald method. 253. Relative availability of nitrogen fertilizers.^ — It is very difficult to rank nitrogenous fertilizers on the basis of their rate of availability, since the conditions within the soil so markedly influence the transformations, especially those of a biological nature. Dried blood and ammonium sulfate, for example, will give almost as quick results in a warm, well aerated soil, as far as higher plants are concerned, as sodium nitrate. In general, however, the nitrate fertilizers should be rated as most readily available, followed in order by ammo- nium salts, dried blood, tankage, and similar materials. Such substances as wool, hair, and untreated leather waste should rank last. FERTILIZERS USED FOR THEIR PHOSPHORUS Phosphorus is generally present in nature in combination with calcium, iron, or aluminum. Some phosphates carry or- ganic matter and when thus associated are generally consid- ered to decompose more readily when added to the soil. 254. Bone phosphate (Ca3(P04)2+). — ^Bones were for- merly applied to the soil in the raw condition, either ground or unground. Most bone as now sold is merely steamed or boiled to remove the fat and nitrogenous matter, which is used in other ways. Bone-meal comes on the market as a dusty powder of characteristic odor. It contains about 27 per cent, of phosphoric acid as tricalcium phosphate. Tankage, which has already been spoken of as a nitrogenous fertilizer, con- tains from 3 to 8 per cent, of phosphoric acid, largely in the form of tricalcium phosphate. All bone phosphates are slow- acting manures, and should be used in a finely ground form and for the permanent benefit of the soil rather than as an ^ Thome, C. E., Carriers of Nitrogen in Fertilizers; Soil Sci., Vol. IX, No. 6, pp. 487-494, 1920. COMMERCIAL FERTILIZER MATERIALS 455 immediate source of phosphorus. In the soil, water charged with carbon dioxide sh)wly converts the insoluble tricalcium phosphate into the soluble mono-calcium form : Ca3(POj2 + 4C0, + 411^0 = CaH,(P0j2 + 2CaH2(C03)2 255. Rock phosphate^ (Ca3(Po4)a+). — There are many natural deposits of mineral phosphates in different parts of the world, some of the most important of which are in North America. The phosphorus in all of these is in the form of tricalcium phosphate, but the materials associated with it vary greatly. Rock phosphate may occur in nature as soft phosphate, pebble phosphate, boulder phosphate, and hard rock phosphate. South Carolina phosphate contains from 26 to 28 per cent, of phosphoric acid and a very small amount of iron and aluminum. As these latter substances interfere with the man- ufacture of acid phosphate from rock, their presence is veiy undesirable, rock containing more than from 3 to 6 per cent, being unsuitable for that purpose. Florida phosphates exist in the form of soft phosphate, pebble phosphate, and boulder phosphate. Such phosphate contains from 18 to 30 per cent, of phosphoric acid, and be- cause of its being softer than most of these rocks it is often applied to the land without being first converted into a soluble form. The other two forms, pebble phosphate and boulder phosphate, are highly variable in composition, ranging from 20 to 40 per cent, in phosphoric acid content. Tennessee phosphate, which is now very important, contains from 25 to 35 per cent, of phosphoric acid. Rock phosphate, or floats as it is often called, appears on the market as a heavy finely ground powder of light gray color. It generally carried about 27 per cent, of phosphoric acid as Ca3(,P04)„. A typical analysis is as follows: * Waggaman, W. H., and Fry, W. H., Phosphate Bock and Methods Proposed for Its Utilisation as a Fertilizer; U. S. Dept. Agr., Bui. 312, 1915. 456 NATURE AND PROPERTIES OF SOILS Moisture, organic matter, etc 5.06 Ca3(P0J.. 77.76 FePO^ and AlPO, 1.50 CaCOg 4.43 MgCO. 50 CaF. 4- CaCl 6.11 FeS 77 FcoOg and ALO3 3.87 Rock phosphate undergoes the same change in the soil as bone-meal but generally much more slowly, unless the soil is very high in organic matter. Mixing the rock with manure seems to hasten its availability to plants. 256. Acid phosphate ^ (CaH4(P04)o+). — Acid phosphate is a dry material of a browning gray color, partially soluble in water, and has a characteristic acrid odor. It is intensely acid to litmus, as it contains certain acid salts. It carries from 14 to 16 per cent, of available P2O5 and small amounts of insoluble P2O5. It is made by treating raw rock with sul- furic acid under the proper conditions.^ CaaCPOJ^ + 2H2SO4 = CaH,(POJo + 2Ca SO4 (insoluble) (water soluble) The acid is never added in amounts capable of quite com- pleting this reaction. Some di-calcium phosphate [CagHg (FOi)o], spoken of as citrate soluble or reverted phosphoric acid, is thus produced. CaaiPOJ. + H2SO, = Ca,Ho(POJ. + CaSO^ (insoluble) (reverted) Acid phosphate consists mostly of gypsum and mono-cal- cium phosphate with some di-calcium phosphate and impuri- * Chemically, three forms of phosphoric acid are recognized by the fertilizer industry: (1) insoluble (Ca.,(PCf4)2), (2) reverted or citrate soluble (Ca.H^CPO,),), and (3) water soluble (CaH.CPO,)^. The water soluble and citrate soluble phosphates are rated as available to plants. The insoluble form is considered as tinavailahle. ^Waggaman, W. H., The Manufacture of Acid Phosphate; U. S. Dept. Agr., Bui. 144, 1914. COMMERCIAL FERTILIZER MATERIALS 457 ties. The water soluble and reverted phosphoric acid are both rated as available. The phosphates of acid ])hospliate wlicn added to the soil quickly revert to an insoluble form : CaH,(PO,).> + 2CaH3(C03),=:Ca3(POJ, + 4CO., + 4H,0 Ca2H3(PO,)", + CaH,(C03),=:Ca3(POj2 + 2C02 + 2H,0 Plenty of active calcium should be present when acid phos- phate is used to insure this reaction instead of the formation of the very insoluble ferric phosphate (FeP04) and alumiiium phosphate (AIPO4). Acid phosphate does not seem to make the soil acid.^ In fact, it is considered by some investigators to decrease the acidity by rendering aluminum and iron in- soluble. 257. Basic slag ((CaO)s.Po05.Si02+).— Iron or steel con- taining over 2 per cent, of phosphorus is too brittle to be useful and, as a consequence, ores of this character were little used until methods of removing this phosphoric acid were discovered. The use of wood in smelting provided a basic ash, thus removing phosphorus from the pig iron. With coal, how- ever, the slag is acid and the phosphorus remains with the ore. In the open-hearth method of smelting the furnaces are lined with a specially prepared dolomitic limestone. Lime is later added as the smelting proceeds. The calcium of the slag unites with the phosphorus of the iron, thus reducing the percentage of that element in the steel. The most prob- able formula for the phosphorus compound in basic slag is (CaO)5.Po05.Si02. Basic slag contains a large amount of iron and calcium hydroxide. Below is a typical analysis : CaO 45.0 ALO3 1.7 MgO 6.2 SiO. 6.9 FeO -f Fe.O,, 17.6 P,0, 18.1 MnO 3.5 Other constituents... 1.0 * Conner, S. D., Acid Soils and the Effect of Acid Phosphate and Other Fertilizers Upon Them; Jour. Ind. and Eng. Cliem., Vol. 8, No. 1, pp. 35-40, 1916. 458 NATURE AND PROPERTIES OF SOILS Basic slag comes on the market as a heavy dark gray pow- der, extremely alkaline to litmus, and contains from 14 to 20 per cent, of P2O5. The phosphorus of basic slag is almost all soluble in citric acid and, therefore, is rated as available phosphoric acid. It does not revert in the soil as does acid phosphate, but is immediately attacked by carbon dioxide and rendered rather quickly available. A possible reaction is as below : (CaO)5.P205-Si02 + 8CO2 + 6H2O = CaH4(POj2 + 4CaH_.(C03)2 + Si02 258. Relative availability of phosphate fertilizers. — Acid phosphate carries most of its phosphoric acid in a water- soluble form and although the phosphates revert to the tri- ealcium form immediately when added to the soil, they are rather readily available to plants. This is due to the charac- ter of the freshly precipitated salt and the surface exposed for solution activities. To insure a good distribution in the soil of the phosphoric acid and a rapid influence on crops, acid phosphate should be well mixed with the soil. Basic slag, since its phosphoric acid is largely citrate sol- uble, is generally considered as next to acid phosphate in availability. Steamed bone-meal usually gives better results than raw rock phosphate and rates third, with rock phos- phate fourth in availability. The degree of fineness makes a great difference in the availability of the less soluble phos- phate fertilizers, especially the ground bone and raw rock phosphate. The latter material should be ground fine enough to pass through a sieve having at least one hundred meshes to the inch. 259. Raw rock phosphate versus acid phosphate. — Con- siderable discussion as well as controversy has of late arisen regarding the relative merits of acid phosphate and raw rock phosphate not only when applied on the basis of equal amounts of phosphoric acid but also when compared on the basis of COMMERCIAL FERTILIZER MATERIALS 459 equal money values. If rock phosphate could be made to equal or nearly equal the availability of acid phosphate, ob- vious advantages would accrue, since raw rock costs much less than acid phosphate and carries about twice as much phos- phoric acid. The availability of the phosphorus of raw rock phosphate varies considerably with conditions. At least four major in- fluences have been recognized: (1) the character of the crop grown, (2) reaction of the soil, (3) the character of accom- panying salts, and (4) the decomposition of organic matter. It is to be expected that the various kinds of plants should not be equally influenced by the phosphorus of tri-calcium phosphate. Prianischnikov ^ found that lupines, mustard, peas, buckwheat, and vetch responded to fertilization with raw rock phosphate in the order named, while the cereals did not respond at all. He did not include maize in his ex- periments, but that crop is said to respond well to difficultly soluble phosphates. It is generally considered that those plants which have a long growing season are better able to utilize tri-calcium phosphate than are more rapidlj^ growing plants. A number of investigators have stated, as a result of their experimentation, that the availability of raw rock phosphate is greater in acid soils than in those strongly basic. If acidity of the soil is due to the presence of an actual acid, it is con- ceivable that the availability may be due to the solvent action of the soil acid on the calcium of the tri-calcium phosphate, producing the di-calcium salt which appears to be fairly read- ily available to plants. When, however, soil acidity is due to a lack of certain active bases, the case is different. Gedroiz ^ ' Prianischnikov, D., Bericht uber Verschiedene Versuche mit Rohphos- phaten unter Reduction ; Moscow, 1910. *Gedroiz, K. K., Soils to which Bock Phosphates May Be Applied with Advantage; Jour. Exp. Agron. (Russian), Vol. 12, pp. 529-539, 811-816, 1911. The authors are indebted to Dr. J. Davidson for the translation. 460 NATURE AND PROPERTIES OF SOILS explains this on the basis of the absorptive properties of the so-called acid soil. He regards rock phosphate, not as a chemi- cal compound, but as a solid solution of di-calcium phosphate with lime. According to Gedroiz it is this excessive basicity of the phosphate which is responsible for its unavailability. Absorption of the excess calcium would leave the phosphate in a more readily available condition by forming the di- calcium salt. The presence of certain salts has been found to influence the availability of difficultly soluble phosphates. The subject has been investigated by a large number of experimenters, and it will be possible to summarize their results only in part and very briefly. It has been found, for example, that calcium carbonate decreases the availability of raw rock phosphate and bone-meal. Sodium nitrate reduces the availability of the tri-calcium phosphates, while the ammonium salts increase their availability. Iron and aluminum salts decrease avail- ability. The influence of other salts has not been so well worked out. Prianischnikov,^ as the result of his extended experiments on the subject, holds that salts from which plants absorb acid radicals in larger amounts than they do bases decrease availability, or at least do not affect it, while salts from which plants absorb the bases in the greater quantity have a tendency to render the phosphate more available be- cause of the hydrogen ion concentration. There has been great differences of opinion among investi- gators as to the effect of the decomposition of organic matter on the availability of the phosphorus of tri-calcium phosphate. The contention that the availability is increased probably originated with Stoklasa,^ whose experiments with bone-meal ^ Prianischnikov, D., fj&er den Einfluss von Kohlensduren Kalk auf die WirTcung von VerscJiiedenen Phosphaten; Landw. Vers. Stat., Band 75, Seite 357-376, 1911. *Stoklasa, J., Duchacek, F., and Pitra, J., uier den Einfinss der Bak- terien auf die Knochenzersetsung ; Centrlb. f. Bakt., II, Band 6, Seite 526-535, 554-558, 1900. COMMERCIAL FERTILIZER MATERIALS 461 indicate that the availability is increased by decay. A large number of experiments have been conducted with raw rock phosphate composted with stable manure, among which may be mentioned those by Hartwell and Pember ^ and also by Tottingham and Hoffman,- who, in carefully conducted experi- ments, failed to find that the availability of the raw phos- phate, as indicated by chemical methods, was increased by fermentation with stable manure. Opposing results have also been obtained, however, and the evidence is somewhat con- flicting. With so many factors active in varying the results, espe- cially those from raw rock phosphate, it is not surprising that satisfactory field data where acid phosphate and raw rock are compared are difficult to obtain. Thorne,^ after a critical review of the field experiments where acid phosphate and raw rock were used, comes to the conclusion that, while raw rock phosphate is an excellent fertilizer, acid phosphate is gener- ally superior. He finds that, while raw rock may be used with profit on land materially deficient in phosphorus, acid phosphate has generally proven to be the more effective and the more economical carrier of phosphoric acid for crops. These conclusions, which are corroborated by other in- vestigators,* do not imply that raw rock phosphate is never equal or superior to acid phosphate, nor that raw rock does not have a place as a fertilizer on the average farm. On a * Hartwell, B. L., and Pember, F. R., The Effect of Cow Dung on the Availability of Bocic Phosphate; R. I. Agr. Exp. Sta., Bui. 151, 1912. ' Tottingham, W. E., and Hoffman, C, The Nature of the Changes in Solubility and Availability of Phosphorus in Fermenting Mixtures; Wis. Agr. Exp. Sta., Res. Bui. '29, 1913. 'Thome, C. E., Baw Phosphate BocTc as a Fertiliser ; Ohio Agr. Exp. Sta., Bui. 305, 1916. * Wiancko, A. T., and Conner, S. D., Acid Phosphate versus Baw Bock Phospiiate as Fertilizer; Purdue Univ. Agr. Exp. Sta., Bull. 187, 1916. Brooks, W. P., Phosphates in Massachusetts Agriculture; Mass. Agr. Exp. Sta., Bull. 162, 1915. Waggaman, W. H., and Wagner, C. R., Analysis of Experimental Work with Ground Baiv Bock Phosphate as a Fertilizer; U. S. Pept. Agr., Bui. 699, 1918. 462 NATURE AND PROPERTIES OF SOILS soil rich in organic matter it may be added to advantage. It is especially useful in reinforcing farm manure, seemingly be- ing about as effective under such conditions as is acid phos- phate. Its higher phosphorus content and lower cost a ton gives it an added advantage. The figures from Ohio/ cover- ing a period of fourteen years in a rotation of maize, wheat, and hay may be taken as evidence regarding these points. The manure, reinforced to the ton with 40 pounds of acid phosphate and raw rock phosphate, respectively, was applied to the corn at the rate of eight tons to the acre. Table XCVI a comparison of acid phosphate and raw rock in equal weights when added to the soil with manure. Manxjre Average Annual Increase to the Acre Maize 14 Crops Wheat 14 Crops Hay 11 Crops With raw rock 25.0 bu. 30.6 bu. 12.9 bu. 15.1 bu. 1578 lbs. With acid phosphate 1853 lbs. FERTILIZERS USED FOR THEIR POTASSIUM The production of potassium fertilizers is largely confined to Germany, where there are extensive beds varying from 50 to 150 feet in thickness, lying under an area extending from the Harz Mountains to the Elbe River and known as the Stassfurt deposits. Large deposits of crude potash salts occur in other sections of Germany, and also in France. While small deposits occur in other parts of the world the French and German mines are at present the only ones of any great commercial importance. The World War stimu- lated considerable investigation regarding possible sources of ^Thorne, C. E., et ah, Plans and Summary Tables of the Experiments at the Central Farm; Ohio. Agr. Exp. Sta., Circ. 120, p. 112, 1912. COMMERCIAL FERTILIZER MATERIALS 463 potash, especially in the United States. Kelp, saline brines, deposits in old lake beds, and flue dust yielded considerable potassium. Most of these sources, however, are too expensive to compete with European potash in normal times. 260. Stassfurt salts and their refined equivalents. — The Stassfurt salts contain their potassium eitiier as a chloride or as a sulfate. The chloride has the advantage of being more diffusible in the soil, but in most respects the sulfate is pref- erable. Potassium chloride in large applications has an in- jurious effect on certain crops, among which are tobacco, sugar-beets, and potatoes. On cereals, legumes, and grasses the muriate appears to have no injurious effect. Kainit is the most common of the crude products of the Stassfurt mines and is imported into this country in large amounts. It is generally a greyish vari-colored salt, soluble in water and alkaline to litmus. It carries from 12 to 14 per cent, of KoO, largely as potassium sulfate. Its potash is immediately available to the crop. Below is a typical analysis : K2SO4 21.3 NaCl 34.6 KCl 2.0 CaSO, 1.7 MgSO^ 14.5 Insolul)le 8 MgClo 12.4 H2O 12.7 Silvinit contains its potassium both as a chloride and as a sulfate. It also contains sodium and magnesium chlorides. Potash constitutes about 16 per cent, of the material. Owing to the presence of chlorides, it has the same effect on plants as has kainit. There are a number of other Stassfurt salts, consisting of mixtures of potassium, sodium, and magnesium in the form of chlorides and sulfates. They are not so widely used for fertilizers as are those mentioned above. A great proportion of the crude salts are refined for ex- port purposes, appearing on the market as either the chloride or the sulfate. They usually contain from 48 to 50 per cent. 464 NATURE AND PROPERTIES OF SOILS of potash. The chief impurity is common salt. Some of the potash salts produced in this country carry boron, which is extremely toxic to plants. Such is not generally true of the German and French products. Potassium chloride and potassium sulfate when added to the soil are immediately soluble, being held in the soil solu- tion or absorbed either physically or chemically by the col- loidal complexes. Due to the selective absorption of the soil for the potassium ion and the fact that plants absorb more of this ion than of the acid radical, an acid residue tends to re- sult from the use of such fertilizers. Some means, such as the use of lime, should be employed to counteract this tendency. 261. Other sources of potash.^ — For some time after the use of fertilizers became an important farm practice, wood- ashes were the source of most of the potash. They also con- tain a considerable quantity of lime and a small amount of phosphorus. The product known as unleached wood-ashes contains from 5 to 6 per cent, of potash, 2 per cent, of phos- phoric acid, and 30 per cent, of calcium oxide. Leached wood- ashes contain about 1 per cent, of potash, II/2 per cent, of phos- phoric acid, and from 28 to 29 per cent, of lime in the form of the hydroxide and carbonate. Unleached ashes carry the oxide, hydroxide, and carbonate forms of calcium. Ashes contain the potassium in the form of a carbonate, (K2CO3), which is alkaline in its reaction and in large amounts may be injurious to seeds. Otherwise this form of potash is very de- sirable, since no acid residue is left in the soil by its use. * Young, G. J., Potash Salts and Other Salines in the Great Basin; U. S. Dept. Agr., Bui. 61, 1914. Waggaman, W. H., and Cullen, J. A., The Recovery of Potash from Alunite; U. S. Dept. Agr., Bui. 415, 1916. Hirst, C. T., and Carter, E. G., Some Sources of Potassium; Utah Agr. Exp. Sta., Circ. 22, 1916. Waggaman, W. H., The Production and Fertiliser Value of Citric- Soluble Phosplwric Acid and Potash; U. S. Dept. Agr., Bui. 143, 1914, Ross, W. H., et al., The Recovery of Potash as a By-Product in the Cement Industry; U. S. Dept. Agr., Bui. 572, 1917. COMMERCIAL FERTILIZER MATERIALS 465 Ashes are beneficial to acid soils through the action of both the potassium and calcium salts. Insoluble forms of potassium, existing in many rocks usually in the form of a silicate, are not regarded as having any manurial value. Experiments with finely ground feld- spar have been conducted by a number of investigators, but have, iji the main, offered little encouragement for the suc- cessful use of this material. Leucite and alunite have given but little better results. An insoluble form of potassium is not recognized as of value when a fertilizer is rated on the basis of chemical analysis. During the World War, since the German importation of potash salts ceased, potassium was sought commercially from a number of sources in this country. Alunite, a hydrous sul- fate of aluminum and potassium, has been experimented with to some extent as have also the green-sand marls which carry glauconite. In a number of cases the recovery of potash from flue dust has proven commercially profitable. It is esti- mated that 87,000 tons of potash are lost yearly from cement kilns alone in the United States and Canada. During the war considerable progress was made in harvesting and drying the kelp which grows off the coast of southern California. The kelp was later extracted for its potash. This source of potas- sium is rather expensive, however, when brought into com- petition with European products. Perhaps the most reliable sources of domestic potash are the brines of certain alkali lakes of western United States and from the deposits in old lake beds in the same region.^ The exploitation of such sources will, of course, depend upon the price at which German potash can be laid down in this country. ^ Such salts unless properly prepared are likely to contain borax which is usually toxic when applied at a greater rate than five pounds to the acre, the influence being more intense at low soil moisture. Neller, J. E., and Morse, W. J., Effects upon the Growth of Potatoes, Corn and Beans, Eesultinq from the Addition of Borax to the Fertilizer used; Soil Sci., Vol. XII, 'No. 2, pp. 79-105, 1921. 466 NATURE AND PROPERTIES OF SOILS SULFUR AND SULFATES AS FERTILIZERS ^ The use of these substances as a means of increasing plant growth when applied to soils has recently received much at- tention. While sulfates have been used for centuries as a soil amendment, it is only within the last f cav years that sulfur itself has been applied to soil. The question of the effect of the latter has received considerable study, not only in France and Germany but in this country as well. The influence of both sulfur and sulfates may be a direct nutrient relationship or the action may be that of a soil amendment. Only in case the former influence occurs could these materials be rated as fertilizers. 262. The use of free sulfur. — Boullanger ^ in 1912 added * Another group of fertilizers may be mentioned — the so-called catalytic fertilisers. Such materials are supposed to aid plant growth by accelerat- ing natural soil processes. The catalytic action of any material is very difficult to establish when it is added to the soil, since the soil itself carries many substances of a catalytic nature. Manganese has been most seriously considered as a catalytic fertilizer. Konig, J., Hasenbaumer, J., and Coppenrath, E., Einige Neue Eigen- schaften des Ackerbodens ; Landw. Vers. Stat., Band 63, Seite 471-478, 1905-1906. May, D. W., and Gile, P. L., The Catalase of Soils; Porto Eico Agr. Exp. Sta., Circ. 9, 1909. Sullivan, M. X., and Eeid, F. E., Studies in Soil Catalysis; U. S. Dept. Agr., Bur. Soils, Bui. 86, 1912. Konig, J., Hasenbaumer, J., and Coppenrath, E., Beziehungen zwischen den Eigenschaften des Bodens und der Nahrstoff'aufnahme durch die pflanzen; Landw. Vers. Stat, Band 66, Seite 401-461, 1907. Kelly, M, P., The Influence of Manganese on tJie Growth of Pine- apples'; Jour. Ind. and Eng. Chem., Vol. I, p. 5.3.3, 1909. Sullivan, M. X.^ and Eobinson, W. O., Manganese as a Fertilizer; U. S. Dept. Agr., Bur. Soils, Circ. 75, 1912. Skinner, J. J., and Sullivan, M. X., The Action of Manganese in Soils; U. S. Dept. Agr., Bui. 42, 1914. Skinner, J. J., and Eeid, F. E., The Action of Manganese Under Acid and Neutral Soil Conditions ; U. S. Dept. Agr., Bui. 441, 1916. Bertrand, G., The Action of Chemical Infinitesimals in Agriculture ; Address before 8th Inter. Cong. App. Chem., New York, 1912. Eoss, W. H., The Use of Radioactive Substances as Fertilizers; U. S. Dept. Agr., Bui. 149, 1914. Hopkins, C. G., and Sachs, W. H., Radium as a Fertilizer; 111. Agr. Exp. Sta., Bui. 177, 1915. ^Boullanger, E., Action du soufre en fleur sur la vegetation; Compt. Rend. Acad. Sci. Paris, T. 154, pp. 369-370, 1912. COMMERCIAL FERTILIZER MATERIALS 467 flowers of sulfur to a soil at the rate of 23 parts per million of soil. He obtained increased growth in all treated soils on which carrots, beans, celery, lettuce, sorrel, chicory, potatoes, onions, and spinach were grown, the weights of the crops on the treated soil being from 10 to 40 per cent, greater tlian those on the untreated soil. On soils that had been sterilized before applying sulfur, the effect was less marked, from which he concludes that the beneficial effects were due to the influence of the sulfur on the micro-organisms of the soil. There may be some question, however, whether this conclusion is justi- fiable. Sulfur was found by Boullanger and Dugardin ^ to favor ammonification in soils. Beneficial effects from the use of free sulfur have also been obtained by Demelon,- and by Bernhard,^ while von Feilitzen ^ found it to be ineffective as a fertilizer. In this country, Shedd ^ of Kentucky obtained increases in tobacco yield with sulfur. Perhaps the most marked results with sulfur are reported by Reimer and Tartar ® from Oregon. Alfalfa and clover yields were increased from 50 to 100 per cent. That free sulfur may, under certain conditions, exert a ben- eficial influence on plant growth must be conceded, but that the action is a direct nutritive one remains to be proven. Free sulfur is insoluble and cannot be absorbed as such by plants. It readily undergoes oxidation, however, producing the sulfate, as already explained under sulfofication. As such ^Boullanger, E., and Dugardin, M., Mecanisme de V action fertilisante du soufre; Compt. Eend. Acad. Sei. Paris, T. 155, pp. 327-329, 1912. ^Demelon, A., Sur I'action fertilisante du soufre; Compt. Rend. Acad. Sci. Paris, T. 154, pp. 524-526, 1912. ' Bernhard, A., Versuche iiher dis Wirlcung des Schivefels als Bung im Jahre 1911; Deutsche Landw. Presse., Band 39, S. 275, 1912. *von Feilitzen, H., ijier die Verwendung der Schivefelblute sur Be- kampfung des Eartoffelschorfes und als indirlctes Dungemittel ; Puhling's Landw. Zeit., Band 62, Seite 7, 1913. ° Shedd, O. M., The Relation of Sulfur to Soil Fertility: Kv. Agr. Exp. Sta., Bui. 188, 1914. 'Reimer, F. C, and Tartar, H. V., Sulfur as a Fertilizer for Alfalfa in Southern Oregon; Ore. Agr, Exp. Sta.. BuL 163, 1919. 468 NATURE AND PROPERTIES OF SOILS a reaction tends to encourage soil acidity, injurious influ- ences may easily occur on soils already acid or possessing only small quantities of active calcium and magnesium. If sulfur functions as a fertilizer, it is by a change to the sulfate, in which form it is absorbed by plants. 263. The use of sulfate sulfur. — The experimental evi- dence regarding the direct fertilizer influence of sulfate sulfur is much more difficult to interpret than that regarding flowers of sulfur. Gypsum has been applied to soils for centuries and marked influences on crop growth are of common observa- tion. Whether this stimulation is due to the sulfate or to the base which accompanies it cannot be determined. Even if the sulfate influence could definitely be proved, there would still remain the question as to whether the action was direct or indirect. 264. Relation of sulfur to soil fertility. — The possible deficiency of sulfur in arable soils was first established by Hart and Peterson.^ They point out that crops remove more Table XCVII pounds sulfur trioxide and phosphorus pentoxide removed to the acre by average crops. Crop and Yield to the Acre Pounds to the Acre S03 P20„ 15.7 21.1 14.3 20.7 19.7 19.7 12.0 18.0 64.8 39.9 92.2 33.1 98.0 61.0 11.5 21.5 11.3 12.3 Wheat (30 bu.) Barley (40 bu.) Oats (45 bu.) Corn (30 bu.) Alfalfa (9000 lbs. air dry) Turnips (4657 lbs. air dry) . . . . Cabbage (4800 lbs. air dry) Potatoes (3360 lbs. air dry) Meadow hay (2822 lbs. air dry) * Hart, E. B., and Peterson, W. H., Sulfur Requirements of Farm Crops in Relation to the Soil and Air Supply; Wis. Agr. Exp. Sta., Ees. Bui. 14, 1911. COMMERCIAL FERTILIZER MATERIALS 469 sulfur from the soil than is indicated by the earlier analyses of plant ash, since considerable sulfur was lost by volatization in the former determination. On the basis of their own methods, they present the data given as to the removal of sulfur trioxide and phosphoric acid from the soil by average crops. (See Table XCVII, page 468.) It is to be noted that the amount of sulfur removed by crops is generally about equal to and in some cases much in excess of the phosphoric acid taken from the soil. The fact that soils are generally as low in sulfur as in phosphoric acid lends weight to the argument, that if the latter is a limiting factor in productivity the former should be also. To ascertain whether the supply of sulfur in the soil is really depleted by cropping, Hart and Peterson made parallel determinations of sulfur in five virgin soils and in five soils of the same respective types that had been cropped for sixty years. In each type the cropped soil contained less sulfur than the virgin soil, the average for the former being .053 per cent. SO3 and for the latter .085 per cent. SO3. Considerable sulfur is added to the soil every year in the rain-water, largely in the sulfate form, although near cities appreciable amounts of hydrogen sulfide and sulfur di-oxide are formed. The amount of such sulfur is variable. Miller,^ at the Rothamsted Experiment Station, reports 17.4 pounds of SO3 to the acre, while Crowther and Ruston ^ near Leeds, England, found 161 pounds of SO3 to the acre. Peck ^ found the addition of SO3 to be at the rate of 1 pound to the acre a month at Mt. Vernon, Iowa, while Trieschmann,* over a ^ Miller, N. H. J., TJie Amount of Nitrogen, as Ammonia and as Nitric Acid, and of Chlorine in the Rain-Water Collected at Rotham- sted; Jour. Agr. Sci., Vol. I, pp. 280-303, 1905. * Crowther, C, and Ruston, A. C, The Nature, Distribution and [Effect Upon Vegetation of Atmospheric Impurities In and Near an Industrial Town; Jour. Agr. Sci., Vol. 4, pp. 25-55, 1911. ^Peck, E. L., Nitrogen, Chlorine and Sulfates in Bain and Snoiv; Chem. News., Vol. 116, p. 283, 1917. * Trieschmann, J. E., Nitrogen and other Compounds in Bain and Snow; Chem. News, Vol. 119, p. 49, 1919. 470 NATURE AND PROPERTIES OF SOILS different period at the same place, determined the addition to be less than .2 pound a month. Stewart,^ at the University of Illinois, reports the addition of sulfur as SO3 over a period of seven years as amounting to 9.4 pounds of SO3 monthly to the acre or 113 pounds yearly. The loss of sulfur expressed as SO3 from the Cornell lysi- meters,- due to cropping and drainage combined, amounted, over a period of ten years, to 149.5 pounds from an acre yearly from the rotation tanks. The addition of sulfur in the rain-water at Ithaca amounts to about 65.4 pounds of SO3 each year. It is, therefore, safe to assume that rain-water will not replace the sulfur removed by normal cropping and leaching. It must be remembered, however, that in rational soil management, sulfur is returned to the soil in green- manures, crop residues and farm manures. Commercial fer- tilizers are now very commonly used, especially acid phos- phate, which is about one-half gypsum. At the Ohio Experi- ment Station,^ plats treated with sulfate bearing fertilizers were found over a period of years to contain considerably more sulfur than soils not so fertilized but cropped in a similar manner. In the light of such data it seems that the sulfur problem is not comparable with or as serious as the phosphorus prob- lem of soil fertility. By the careful utilization of the normal residues produced on the farm there seems little reason for sulfur being a limiting factor in soil productivity, especially if fertilizers carrying sulfur are used in connection with a rational system of soil management. ^Stewart, E., Sulfur in Belation to Soil Fertility; 111. Agr. Exp. Sta., Bui. 227, 1920. * Complete data on these lysimeters will be found in par. 163 of this text. ^ Ames, J. W., and Boltz, G. E., Sulfur in delation to Soils and Crops; Ohio Agr. Exp. Sta., Bui. 292, 1916. CHAPTER XXIII THE PRINCIPLES OF FERTILIZER PRACTICE ^ The use of commercial fertilizers has increased so rapidly within the last decade that specific knowledge is needed re- garding the various materials offered for sale in order that the most economical results may be attained. The greater the general knowledge, both practical and theoretical, that a person possesses as to the effects of the different nutrient con- stituents on plant growth, the more rational will be the fer- tilizer use. Fertilizer inspection and control, principles of buying and home-mixing, methods of applicatiop, mixtures for special crops, are a few of the many phases of economical fertilizer practice. The final and vital consideration is re- garding the financial return from fertilizer application. A fertilizer should always pay. As all fertilizers exert, either directly or indirectly, a resid- ual effect, the problem necessarily broadens into a study of the systems of applying them to a series of crops or to a rota- tion, rather than a study of the effects of one particular fer- tilizer application on one particular crop. 265. Influence of nitrogen on plant growth.^ — Of the three elements carried in an ordinary complete fertilizer, *Hall, A. D., Fertilizers and Manures; New York, 1921. Halligan, J. E., Soil Fertility and Fertilizers ; Easton, Pa., 1912. Van Slyke, L. L., Fertilizers and Crops; New York, 1912. Fraps, G. S., Principles of Agricultural Chemistry; Easton, Pa., 1913. * Discussions of the effects of the various elements on plants may be found as follows: Eussell, E. J., Soil Conditions and Plant Growth, Chapter II, pp. 19-50; London, 1912. Also, Hall, A. D., Fertilizers and Manures, Chapters III, IV and VI; New York, 1921. 471 472 NATURE AND PROPERTIES OF SOILS nitrogen ^ seems to have the quickest and most pronounced effect, not only when present in excess of other constituents, but also when moderately used. It tends primarily to encour- age above ground vegetative growth and to impart to the leaves a deep green color, a lack of which is usually due to insufficient nitrogen. It tends in cereals to increase the plumpness of the grain, and with all plants it is a regulator in that it governs to a certain extent the utilization of potash and phosphoric acid. Its application tends to produce succu- lence, a quality particularly desirable in certain crops. In its general effects it is very similar to moisture, especially when supplied in excessive quantities. The peculiarity of nitrogen lies not only in its absolute ne- cessity for plant growth, its stimulation of the vegetative parts, and its close relationship to the general tone and vigor of the crop, but also in the fact that it was not one of the original elements of the earth's crust. During the formation of the soil it slowly and gradually became present, brought down by rains and fixed naturally in the soil through the agency of bacterial action. Now it exists in complex nitrog- enous compounds of the more or less decayed organic matter, and becomes available to plants largely through bacterial activity. It may be stated with certainty that one of the possible limiting factors to crop growth is a lack of water-soluble nitro- gen at critical periods in amounts necessary for normal devel- opment. Since soluble nitrogen may be very readily lost from the soil by leaching, the problem of proper plant nutri- tion becomes a serious one. Not only must the farmer be able so to regulate the addition of nitrogen in fertilizers as to obtain the highest efficiency, but he must understand the control and * For a discussion of nitrogen in relation to crop yield, see Hunt, T. F., ■ The Importance of Nitrogen in the Growth of Plants; Cornell Agr. Exp. Sta., Bui. 247, 1907. THE PRINCIPLES OF FERTILIZER PRACTICE 473 encouragement of the natural fixation as well. Due to the practical possibility of keeping up the nitrogen supply of the soil by the proper use of farm manure, crop residues, green- manures, and the utilization of legumes in the rotation, the quantity of nitrogen purchased in commercial fertilizers should be as small as possible if its use is to be profitable. When so purchased it should function more or less as a crop starter rather than as a source of any large amount of the plants' supply of nutrient. The emphasis i)laced on all phases of the nitrogen problem serves to reveal its great importance in fertility practices. Because of the immediately visible effect from the applica- tion of soluble nitrogen, the average farmer is prone to ascribe too much importance to its influence in proper crop develop- ment. This attitude is unfortunate, since nitrogen is the highest priced constituent of ordinary fertilizers and should usually be purchased to a less extent than potash and espe- cially than phosphoric acid. Moreover, of the three common fertilizer elements, it is the only one which, added in excess, will result in harmful after-effects on the crop. These pos- sible and important detrimental effects of nitrogen may be listed as follows : 1. 7^ may delay maturity by encouraging vegetative growth. This oftentimes endangers the crop to frost, or may cause trees to winter badly, 2. It may weaken the straw and cause lodging in grain. This is due to an extreme lengthening of the internodes, and as the head fills the stem is no longer able to support the in- creased weight, 3. It may lower quality. This is especially noticeable in certain grains and fruits, as barley and peaches. The ship- ping qualities of fruits and vegetables are also impaired. 4. It may decrease resistance to disease. This is probably due to a change in the physiological resistance within the 474 NATURE AND PROPERTIES OF SOILS plant, and also to a thinning of the cell-wall, allowing a more ready infection from without. While certain plants, as the grasses, lettuce, radishes, and the like, depend for their usefulness on plenty of nitrogen, it is generally better to limit the amount of nitrogen for the average crop so that growth may be normal. This results in a better utilization of the nitrogen and in a marked reduction of the fertilizer cost for a unit of crop growth. This is a vital factor in all fertilizer practice, and shows immediately whether nitrogen fertilization is or is not an economic success. 266. Influence of phosphorus on plant growth. — It is difficult to determine exactly the functions of phosphoric acid in the economy of even the simplest plants. Neither cell divi- sion nor the formation of fat and albumen go on to a suffi- cient extent without it. Starch may be produced when it is lacking, but will not change to sugar. As grain does not form without its presence, it very probably is concerned in the pro- duction of nucleoproteid materials. Its close relationship to cell division may account for its presence in seeds in compara- tively large amounts. Phosphoric acid hastens the maturity of the crop by its ripening influences. This effect is especially valuable in wet years and in cold climates where the season is short. The use of acid phosphate is being advocated in the Middle West, espe- cially for maize, as an insurance against frost-injury and a means of avoiding soft corn. Phosphoric acid also encourages root development, especially of lateral and fibrous rootlets. This renders it valuable in such soils as do not encourage root extension and to such crops as naturally have a restricted root development. Phosphoric acid is especially valuable for fall- sown crops, such as wheat. A sturdy root growth is developed which tends to prevent winter injury and prepares the plant for a rapid spring development. Phosphoric acid decreases the ratio of straw to grain in cereals. It also strengthens the straw, thus decreasing the THE PRINCIPLES OF FERTILIZER PRACTICE 475 tendency to lodge, which is likely to occur especially with oats if too much available nitrogen is present. In certain cases, phosphoric acid decidedly improves the quality of the crop. This has been recognized in the handling of pastures in England and France. The effect on vegetables is also marked. Phosphorus is also known to increase the resistance of some plants to disease, due possibly to a more normal cell development. In this respect phosphoric acid counteracts the influence of a heavy nitrogen ration. Excessive quantities of phosphoric acid ordinarily have no bad effect, as phosphorus does not stimulate any part unduly, nor does it lead to a development which is detrimental. The lack of phosphoric acid is not apparent in the color of the plants as in the case of nitrogen, and as a consequence phos- phoric acid starvation may occur without any suspicion there- of being entertained by the farmer. One of the most important phases to be noted from this comparison of the effects of nitrogen and phosphorus is the balancing powers of the latter on the unfavorable influences generated by the presence of an undue quantity of the former. The possible detrimental effects of too much nitrogen have already been noted. This relationship between the phosphorus and nitrogen in plant nutrition is very important in fertilizer practice, since normal fertilizer stimulation generally results in the most economical gains. 267. Effects of pota-ssium on plant growth. — The pres- ence of plenty of available potash in the soil has much to do with the general tone and vigor of the plant. By increasing resistance to certain diseases it tends to counteract the ill effects of too much nitrogen, while in delaying maturity it works against the ripening influences of phosphoric acid. In a general way, it exerts a balancing effect on both nitrogen and phosphate fertilizer materials, and consequently is espe- cially important in a mixed fertilizer, if the potash of the soil is lacking or unavailable. 476 NATURE AND PROPERTIES OF SOILS Potash is essential to starch formation, either in photo- synthesis or in translocation, and is necessary in the develop- ment of chlorophyll. It is important to cereals in grain for- mation, giving plump heavy kernels. As with phosphorus, it may be present in large quantities in the soil and yet exert no harmful effect on the crop. While potassium and sodium are similar in a chemical way, sodium cannot take the place of potash in plant nutrition. Where there is an insufficiency of potash, however, sodium seems in some way, either directly or indirectly, to be useful.^ 268. The element in the "minimum." — In connection with the obvious importance of utilizing, for any particular soil and crop, a fertilizer well balanced as to the three primary elements, two queries naturally arise. These are: (1) What are the proper proportions of nitrogen, phosphoric acid, and potash to apply under given conditions? (2) What would be the effect if any one of these should not be present in suffi- cient quantity as to make it equal in function to the others ? The first query cannot be disposed of until the question of fertilizer mixtures has been considered. The second, how- ever, is not affected by so many factors, and is more clearly a question of the function of the elements concerned and is logically discussed at this point. Any element that exists in relatively small amounts as com- pared with the other important nutrient constituents natur- ally becomes the controlling factor in plant development. Any reduction or increase in this element will cause a corre- sponding reduction or increase in the crop yield. This ele- ment, then, is said to be "in the minimum." In fertilizer practice, ideal conditions would exist if no constituent func- tioned as a decided minimum and the entire influence of each single element was fully utilized. In other words, the fertil- izer would be balanced as to its relationship to normal plant ^Hartwell, B. L., and Damon, S. C, The Value of Sodium when Potassium is Insufficient ; E. I. Agr. Exp. Sta., Bui. 177, 1919. THE PRINCIPLES OF FERTILIZER PRACTICE 477 growth. That such a condition is more or less ideal and is seldom realized is obvious, from the fact that the various fer- tilizer carriers undergo more or less radical changes after being applied to the soil. The composition of the soil itself is also a disturbing factor. Nevertheless, the nearer an ap- proach can be made to such conditions, the greater will be the economy in fertilizer practice. Numerous persons have investigated the question as to what effect an increase of an element in the minimum may have on crop yield, and various ideas have been advanced to explain the effect. The idea of a definite law governing the increase of plant growth according as the element in the minimum is increased, was first suggested by Liebig. Wagner ^ later stated definitely that up to a certain point the increase yield was proportional to the increase in the application. This, however, evidently cannot apply except over a very limited field, since it is a matter of common observation that increased crop yield becomes lower as the lacking element is continu- ously supplied. Mitscherlich ^ has formulated a law which is a logarithmic, rather than a direct, function of the increase in the element occupying the position of the minimum. Mitscherlich 's law may be stated concisely as follows : the increased growth pro- duced by a unit increase of the element in the minimum is proportional to the decrement from the maximum. In other words, the increase is proportional to the difference between the actual yield and the possible yield at which the element ceases to be a limiting factor. Mitscherlich has proposed a definite formula for such a * Wagner, H., Beitrdge zur DungerleJire ; Landw. Jahr., Band 12, Seite 691 ff., 1883. ^ Mitscherlich, E. A., Bas Gesetz des Minimums und das Gesetz des Abnehmen den Bodenertrages ; Landw. Jahr., Band 38, Seite 537-552, 1909. Also, Ein Beitrage zur Erforschung der Ausnutzung des im Minimum Vorliandenen Ndhrstoffes durch die Pflanze; Landw. Jahr., Band 39, Seite 133-156, 1910. 478 NATURE AND PROPERTIES OF SOILS growth curve.^ This formula has been questioned by several investigators,^ who have shown that a number of conditions, such as light, heat, and moisture, tend to disturb the applica- tion of such a law. The fact that crop yield is the summation of so many varying factors seems to argue in favor of no hard and fast rule regarding the increased growth due to the added increments of an element in the minimum. It is enough, in the practical utilization of fertilizers, to remember that in order to obtain the best results from fertilizers a mixture should be used that is approximately balanced so far as the effects of the nutrients are concerned, the crop as well as the chemical constitution of the soil being considered. 269. Fertilizer brands. — In an attempt to meet the de- mands for well-balanced fertilizers suited to various crops and soils, manufacturers have placed on the market a large num- ber of brands of materials containing usually at least two of the important nutrient elements, and nearly always the three ; the former being designated as incomplete fertilizers, while the latter are spoken of as complete. These various brands usually have a significant name,^ which frequently implies the usefulness of the material for some special crop growing on a particular soil. Oftener, however, the brand name bears no relation either to crop or soil. The name should always be ignored in fertilizer purchase, the availability and composi- tion being the important considerations. — — =(a — y)k. Integrating, log (a — y) = c — kx. QX y=r total yield from any number of increments. X z= amount of any particular fertilizer constituent utilized. a = maximum yield and is a constant. k = a constant depending on y and x, variables. ='Pfeiffer, Th., Blanck, E., and Flugel, M., Wasser und Licht als Veffe- tationsfaMoren und ihre Besiehungen sum Gesetze vom Minimum; Landw. Ver. Stat., Band 76. Seite 211-223, 1912. Also, Maze, P., Eecherches sue les Eelations de la Plante avec les Elem-ents Nutritifs der Sol; Compt. Eend., Tome 154, pp. 1711-1714, 1912. 'Potato and Corn Fertilizer, Golden Harvest, Ureka Corn Special, Blood and Bone, Harvest King, Soil Builder and the like. THE PRINCIPLES OF FERTILIZER PRACTICE 479 A brand of fertilizer is usually made up of a number of materials containing the important nutrient ingredients. These materials, already described, are called carriers. The making-up of a commercial fertilizer consists in mixing the various carriers together so that the required percentages of ammonia, potash, and phosphoric acid are obtained, care being taken that no detrimental reaction shall occur and that a physical condition consistent with easy distribution shall be maintained. Brands of fertilizer put out by reputable com- panies carry a large proportion of their nutrients in a readily available form. A fertilizer made up principally of dried blood, tankage, acid phosphate, and kainit or muriate of pot- ash is a good example of the ordinary composition of ready mixed goods. The various brands on the market, besides being complete or incomplete, may be designated as high-grade or low-grade as to availability, or high-grade or low-grade as to amount of plant nutrients carried. In the fertilizer trade the terms generally refer to the latter condition. A low-grade fertilizer in the latter sense is always encumbered with a large amount of inert material, called filler, which adds to the cost of mix- ing, transportation and handling. A low-grade fertilizer is generally more expensive a unit of nutrient obtained than are higher grade goods, and consequently should be avoided. Fertilizer concerns have always found it more profitable to sell ready mixed fertilizers than to deal in the separate car- riers, such as dried blood, muriate of potash, and the like. Of late years, however, it has been possible to buy the separate materials. The conditions during the World War greatly encouraged the application directly to the soil of separate carriers, especially acid phosphate, since potash was almost unobtainable and nitrogen fertilizers were very high in price. The use of phosphoric acid alone is often much more eco- nomical and rational than the use of a complete mixture, since the nitrogen removed from the soil by normal cropping and 480 NATURE AND PROPERTIES OF SOILS drainage may be replaced in other and more practical ways. By maintaining the soil organic matter the natural supply of potash may in a loamy or clayey soil often be so influenced as to render a potash fertilizer unnecessary. At least there may be enough soil potash available so that the use of a com- mercial form will not be profitable. 270. Fertilizer inspection and control. — From the fact that so many opportunities are open for fraud either as to availability or as to the actual quantities of ingredients pres- ent, laws have been necessary for controlling the sale of fer- tilizers. These laws apply not only to the ready mixed goods but to the separate carriers as well. Most states have such laws, the western laws generally being superior to those in force in eastern states, where the fertilizer sale is heavier. This is because the western regulations are more recent and the legislators have had the advantage of the experience gained where fertilizers have long been used. Such laws are a pro- tection not only to the public but to the honest fertilizer com- pany as well, since spurious goods are kept off the market. Certain provisions are more or less common to most fer- tilizer laws. In general, all fertilizers selling for a certain price or over must pay a state license fee or a tonnage tax and print the following data on the bag or on an authorized tag : 1. Number of net pounds of fertilizer to a package. 2. Name, brand, or trade-mark. 3. Name and address of manufacturer. 4. Chemical composition or guarantee. For the enforcement of such laws the states usually pro- vide adequate machinery. The inspection and analyses may be in the hands of the state department of agriculture, of the director of the state agricultural experiment station, of a state chemist, or under the control of any two of these. In any case, a corps of inspectors is provided, the members of which take samples of the fertilizers on the market throughout the state. These samples are analyzed in laboratories provided THE PRINCIPLES OF FERTILIZER PRACTICE 481 for the purpose, in order to ascertain whether the mixture is up to guarantee. The expense of the inspection and control of fertilizers is usually defrayed by the license fee or the ton- nage tax. If the fertilizer falls below the guarantee, — allowing, of course, for the variation permitted by law, — the manufacturer is subject to prosecution in the state courts. A more effective check on fraudulent guarantees, however, is found in pub- licity. The state law usually provides for the publication each year of the guaranteed and found analyses of all brands inspected. Not only has this proved effective in preventing fraud, but it is really a great advantage to the honest manu- facturer, as his guarantees receive an official sanction. The found analysis of most fertilizers is generally above the guarantee. 271. The fertilizer guarantee. — Every fertilizing mate- rial, whether it is a single carrier or a complete ready -to-apply mixture, must carry a guarantee. The exact form is gener- ally determined by the state in which the fertilizer is offered for sale. The content of nitrogen is almost invariably ex- pressed in terms of ammonia (NH3), although the amount of total nitrogen is sometimes required in addition. The phos- phorus is quoted in terms of phosphoric acid (P2O5). In some cases, a bone-phosphate of lime (B. P. L. or Ca3(P04)2) equivalent is included. The guarantee of a simple fertilizer material is easy to interpret, since the name of the material is printed on the bag or tag. When the amount of the nutrient element carried is noted, the availability and general value of the goods is immediately known. If the material is sodium nitrate at 18 per cent, ammonia, it is apparent that the fer- tilizer is high-grade and should give immediate and definite results when properly applied to a growing crop. The interpretation of a complete fertilizer analysis is not as easy, however, since the names of the carriers are seldom included in the guarantees. The simplest form of guarantee 482 NATURE AND PROPERTIES OF SOILS is a mere statement of the percentages of NH3, P2O5 and KgO, as, for example, a 2 — 8 — 2} This, however, is too brief for a guaranteed analysis on goods exposed for sale, as it gives no idea whatsoever regarding the solubility of the materials. As might be expected, there is a wide range in the character of the guarantees required by the various states. For example, some states insist on the statement of the percentage of both nitrogen and ammonia, while others insist only on the percent- age of nitrogen. Some require the soluble, the reverted, and the total phosphoric acid, while others require only the soluble and the reverted. As to potash, in some cases the soluble must be stated, while in other cases the total must be given.- In general, a guarantee should show not only the amount of the various constituents but also their form or availability. The following outline analysis is excellent in this respect : Percentage of NH3 as nitrate. Percentage of P2O5 soluble Percentage of NHg as ammonia. in water. Percentage of NH3 total. Percentage of P2O5 reverted. Percentage of K^O water soluble. Percentage of P2O5 as Percentage of K^O as chloride. insoluble. 272. The buying of mixed goods. — The successful buying of mixed fertilizers on the retail market depends on two things: (1) the selection of a composition suitable to soil and crop with carriers of known value; and (2) the purchase of high-grade goods. The farmer who observes these points will at least have purchased successfully. Whether he obtains a ' In the South, the order is different. An 8-3-2 means 8 per cent, of PA, 3 per cent, of NH, and 2 per cent, of K^O. ' Below is the guarantee of a complete fertilizer : Nitrogen 4.2% Equal to ammonia 5.0 Soluble PA 4.0 Keverted P2O5 2.0 Available PsO^ 6.0 Insoluble P2O5 1.0 Total PA 7.0 Water soluble K,0 3.0 THE PRINCIPLES OF FERTILIZER PRACTICE 483 profit from the use of the fertilizer depends on the interrela- tion of a number of factors more or less variable from season to season. The selection of a suitable fertilizer, as to carriers and com- position, entails, after the need of the crop and soil are de- cided, a careful study of the guarantee. Should the guarantee be such as that just cited, a large amount of information is at hand concerning the forms of the carriers and the availa- bility of the important constituents. This knowledge, prop- erly correlated with the probable needs of the crop and the soil, will determine whether a particular brand should be pur- chased or not. The real question here is not so much the actual quantities of the elements in a ton of the fertilizer, as it is their balance among themselves. The actual pounds of nitrogen, phosphoric acid, or potash applied to the acre can be governed by the rate at which the mixture is added. The purchase of high-grade goods is the second important point to be considered. Data collected from practically every State show that the higher the grade of the fertilizer, both as to availability and as to the percentage of the constituents carried, the greater is the amount of nutrients obtained for every dollar expended. Avoiding the abnormal war prices, the following data from Vermont ^ for 1909 seem representative : Table XCVIII Cost (in Cents) of One Pound of Cents' Worth OF Nutrients Mixed Fertilizer NH, P,0, i K,0 Eeceived fob Every Dollar Expended Low grade 32 26 23 7.6 6.3 5.7 8.5 7.0 6.3 50 Medium grade High grade 60 67 ^ Hills, J. L., Jones, C. H., and Miner, H. L., Commercial Fertilizers; Vt. Agr. Exp. Sta., Bui. 143, pp. 147-149, 1909. 484 NATURE AND PROPERTIES OF SOILS It is always true that the lower the grade of a fertilizer the higher is the proportional cost of placing the goods on the market. In other words, it costs just as much a ton to market a low-grade material as a high-grade one. This accounts for the fact that the nutrients are cheaper a pound in a high- grade mixture, and that the value received for every dollar expended is greater. 273. The purchase of unmixed fertilizers. — There has always been a tendency among fertilizer manufacturers to discourage the purchase by the farmer of the separate car- riers of fertilizer nutrients. When this was possible the fer- tilizer manufacturer was able absolutely to control the mar- ket. By selling only mixed goods the manufacturer could not only realize a profit on the ingredients themselves but a profit on the mixing in addition. In order to escape these costs many farmers have begun the practice of buying the separate carriers, thus avoiding the extra charges. In manjr^ cases, the mixing on the farm costs nothing, as it can be done in winter when the farm work is not pressing. Home-mixing has been greatly encouraged by post-war conditions. In 1920 from ten to twenty dollars a ton was often saved on a high- grade mixture by purchasing the carriers separately. In many instances the fertilizing materials purchased sepa- rately need not be mixed at all, thus effecting a considerable saving in time and labor. Acid phosphate is generally added separately, especially to fall wheat. Bone-meal, basic slag, and raw rock give excellent results when applied with farm manure. Sodium nitrate and ammonium sulfate give good returns as a top dressing on meadows, pastures, and small cereals, especially if phosphates have been added at some other point in the rotation. When farm manure is available, the use of acid phosphate with lime and manure in a legume rotation is generally desirable. Even where little manure is available, the application of sodium nitrate or ammonium sulfate as a top dressing for meadows, with acid phosphate in THE PRINCIPLES OF FERTILIZER PRACTICE 485 its proper place, is feasible. The purchase of expensive ready- mixed fertilizers may thus be avoided without necessitating home-mixing. For vegetable crops, however, especially potatoes, a com- plete fertilizer is generally advisable. Home-mixing is in such cases necessary. Special soils often demand a complete mix- ture. Muck soils generally require both potash and phos- phoric acid, while sandy soils, especially if the organic matter is low, respond to a mixture carrying all three of the fer- tilizer elements. As might be expected, this practice of home-mixing has met with much opposition from manufacturers. In general, it is claimed that the factory goods are more finely ground than those mixed by the farmer, and consequently the ready-mixed goods are not only more uniform but also in better physical condition. Also, the manufacturer is able to treat certain materials with acids, and thus increase their availability. While these reasons are more or less valid, good results may be expected from a fertilizer even though it may not be quite uniform, as the soil tends to equalize this deficiency. More- over, by screening and by using a proper filler, a farmer can obtain a physical condition which will in no way interfere with the drilling of the material. While, obviously, one farm- er alone cannot afford to buy small lots direct from the whole- . sale dealer because of the high freight charges, this objection is being met by organizations of various kinds whereby the single carriers may be purchased in carload lots and shipped directly to the association. It is evident that by purchasing the separate carriers, a farmer is able to obtain pure high-grade material at a reason- able price. Even if the fertilizers are not home-mixed, an educational value enters. The farmer is forced to study the influence of the materials on his crops more closely and is thus placed in a position to make changes that will tend to a higher efficiency of the constituents. The chances are that he will 486 NATURE AND PROPERTIES OF SOILS advantageously alter his fertilizer practice as the rotation progresses and his soil changes in fertility. Such arguments do not always mean, however, that it pays to buy the separate materials. As a matter of fact, in many cases it does not pay, especially where only a small amount of fertilizer is needed and it is impossible to cooperate with other farmers. As a general rule, fertilizers should be bought by the method that will give the greatest value for every dollar expended, providing, of course, that the proper material is purchased. Farmers can often avail themselves of the advan- tage of both systems by asking for bids from various manu- facturers on carload lots of mixed goods having a certain composition. The farmers in this case designate the carriers as well as the formula. All the advantages of machinery mix- ing may thus be gained. 274. How to mix fertilizers.^ — The first step in the buy- ing of the separate fertilizer carriers is to obtain quotations which should state the price a ton, the composition, and the freight rate. With this information, the most desirable car- riers are selected and the amount of each is calculated.^ If ^ Certain materials should not be mixed, especially in large amounts. Thus lime, especially the oxide and hydroxide forms or fertilizers carry- ing lime in considerable amount, should not be mixed with ammonium sulfate and animal manures, since ammonia is likely to be freed. Such materials should be kept away from acid phosphate or the reversion of the latter will occur. Calcium carbonate in small amounts, however, is often mixed with fertilizers carrying acid phosphate. It is not wise to allow moist acid phosphate to lie in contact with sodium nitrate, as nitric acid may be liberated by free sulfuric acid. ^ Below are three satisfactory mixtures : 2-12-0 400 pounds of tankage. 100 pounds of sodium nitrate. 1500 pounds of acid phosphate (16%P205). 2-12-2 320 pounds of tankage. 100 pounds of ammonium sulfate. 1500 pounds of acid phosphate (16%P206). 80 pounds of potassium chloride. 4-10-4 150 pounds of sodium nitrate. 100 pounds of ammonium sulfate. THE PRINCIPLES OF FERTILIZER PRACTICE 487 the materials are to be applied separately, the rate to the acre and the number of acres must be known. If a mixture is to be made, the formula of this mixture must be decided on in addi- tion. The pounds of the various carriers necessary to produce a given amount of a certain mixture can now be calculated. All of this is a matter of good judgement and careful arith- metic.^ With the separate carriers at hand, the mixing, if necessary, is quickly accomplished. All that is needed may be listed as follows: (1) a tight floor, (2) a coarse sand screen, (3) a tamper or grinder, and (4) shovels, a rake, and like tools. Since the pounds of fertilizer are quoted on each bag, weigh- ing is unnecessary in making up a given amount of a mixture having a certain formula. Bags may be divided into half or quartered with sufficient accuracy. The bulkiest material is spread on the floor first and leveled uniformly by raking. The remaining ingredients are then spread in thin layers above the first, in the order of their bulk. Beginning at one side, the material is next shoveled over, care being taken that the shovel reaches the bottom of the pile each time. The pile is then again leveled, and the process is re- peated a sufficient number of times to insure thorough mixing. Sometimes a mixing machine may be used for this operation. For storage and general convenience, the fertilizer may be weighed into sacks of 100 to 150 pounds capacity and put in a 240 pounds of tankage. 100 pounds of dried blood. 1250 pounds of acid phosphate (16%P205). 160 pounds of muriate of potash. ^ A 2-8-2 fertilizer is to be compounded from dried blood containing 12% NH3, acid phosphate carrying 14% P2O5 and kainit containing 12% K2O. In one ton of the mixture there should be 40 pounds of NH3, 160 pounds of P2O5, and 40 pounds of K,0. 40 H- .12 = 333 lbs. of dried blood. 160^.14=1142 lbs. of acid phosphate. 40 — .12= 333 lbs. of kainit. 192 lbs. of filler. 2000 lbs. total. 488 NATURE AND PROPERTIES OF SOILS dry place until needed. Each sack should be labeled, especi- ally if different mixtures are made. A word of caution should be inserted here regarding the concentration of the mixture. Some farmers, in order to les- sen the work of mixing and application in the field, raise the percentage of the elements exceedingly high — a condition very likely to occur when high-grade materials are used. This sometimes is bad practice, in that it may interfere with ger- mination after the fertilizer is applied and may also injure the young plants. Also, it is likely to result in a poor physi- cal condition, which may clog the drill, and in uneven distribu- tion, which will bring about a lowered efficiency of the fertil- izer. The use of sufficient dry finely divided filler will obviate such dangers.^ 275. The choice of a fertilizer. — Two primary considera- tions must be observed in the actual utilization of fertilizers. The first of these has to do with the composition of the fer- tilizer and its suitability to soil and to crop. A careful study should be made not only of the percentages of ammonia, phos- phoric acid, and potash but also the availability of these con- stituents. The second consideration in the rational use of fertilizing materials is in regard to the amounts to be applied. As much care and good judgment are necessary in handling a single carrier as a complete ready-mixed material, especially if the rotation as a whole is considered. It is evident, due to many factors that cannot be controlled, that fertilizer formulae for different crops on particular soils are difficult to determine. In fact, such data can never be more than merely suggestive. Further, the best quantity of a mixture to apply, even though it is perfectly balanced, is a figure that can only be approximated. Probably the largest percentage of the fertilizer waste that occurs annually can ^Sand, dry soil, saw dust, dry muck, and even ground limestone, if in small amounts, may be used as fillers. THE PRINCIPLP^S OF FERTILIZER PRACTICE 489 be charged to this factor. Many farmers make the mistake of applying too much fertilizer. Any information along such lines, however, can only be suggestive, rather than literal, it being understood that the general formula suitable to vari- ous crops, and the quantities ordinarily applied, are subject to wide variations. 276. Fertilizer formulae.^ — In the popular mind, the nu- trition of a plant is considered as similar to and as easy as the proper feeding of an animal. With animals, the food is compounded with the correct balance of nutrients and if other conditions are favorable, normal results should be obtained. The nutrition of a plant is by no means as simple as the proper feeding of an animal. In the first place, the plant receives most of its nutrients from the soil and air and not from the fertilizer, since the latter usually merely supplements the nu- trients already present in the soil. Again, the food for the animal remains balanced as it is utilized. In the case of plants, the fertilizer nutrients undergo great changes on addition to the soil, the soil influencing the availability of the fertilizer as well as the fertilizer influencing the soil in a great number of different ways. Moreover, the question of fertilizer resi- dues, especially those of an acid nature, is always paramount when fertilizers are used over long periods. The proper for- mula for a given crop and a given soil under a probable series of weather conditions is thus more or less of a guess and will always remain so. ^ The following example of fertilizers similarly named but carrying strikingly different guarantees are taken from Bull. 206 of the Vt. Agr. Exp. Sta. Potatoes and Maize Potatoes and Tobacco 4-7-8 2- 6-7 4-8-4 2- 6-4 4-8-0 2-12-0 Vegetables Top Dressings 3- 7-10 7-6-5 4- 8- 4 7-6-2 5-10- 7-6-0 490 NATURE AND PROPERTIES OF SOILS In spite of the intangible nature of the question, certain gen- eral rules seem to govern the compounding and use of fertiliz- ers. In the first place, the ratio of the nutrients removed by the average crop bears no relation to the composition of the fertilizer usually added. This is to be expected because of the complex changes that the fertilizer undergoes in the soil and because the different nutrients influence the plant di- versely. Table XCIX Eatio of the Eatio of the Constituents Constituents Constituents AS They Occur Carried by the in the Average Average Crop Fertilizer Ammonia 4 2 0^2 Phosphoric acid 16-8 Potash 3 (V-2 It is immediately noticeable that the ratios of the ammonia and potash in fertilizers are low. The ammonia ratio is low because of the ready response of plants to nitrogen and the ease with which this constituent is lost from the soil. The potash ratio is likewise small because potassium is a rather expensive constituent and it is generally better if possible to render available by suitable means that which is already in the soil than to buy it commercially. The phosphoric acid is high in comparison with the ammonia and potash because of its complex reversion in the soil and the tendency of much of it to remain unavailable for long periods due to the high absorptive power of the soil. The following data may now be presented. These for- mulae are tentative and suggestive only, being a modification and curtailment of certain analyses standardized for the use of fertilizer manufacturers in the United States. THE PRINCIPLI^IS OF FERTILIZER PRACTICE 491 Table C group i : fodder and staple crops. Wheat (fall) Maize Millet Oats Barley Beans (field) Rye (fall) Buckwheat Peas (field) Soil Without Farm Manure With Farm Manure Sandy soil 2-10-6 2-10-4 J 2-12-2 I 2-12-0 0-12-4 I or Acid 0-12-2 j Phos. Acid Phosphate Loamy soil Clayey soil Table CI GROUP II : TOP DRESSINGS. Soil Timothy, Orchard Sod and Meadows * Wheat, Eye AND Oats for Hay (Spring Dressing)* Pastures* AND Legumes Sandy soil Loamy soil Clayey soil 7-8-6 7-8-3 7-8-0 7-8-3 7-8-0 7-8-0 0-10-8] %;^^^^^ 0-12-4 P^«^: f. ,r, 9 for Basic "■-^^-^J Slag * Note. — Sodium nitrate or ammonium sulfate may be used alone as a top-dressing on all of these crops except legumes. Table CII group iii: vegetables. 1. Extensively — Tomatoes, sweet corn, beets, cab- bage, etc. Sandy soil 3-10-6 Loamv soil 3-10-4 3-10-2 ■10-0 All root-crops should re- ceive at least 2 per cent, of K,0. Clayey soil jg 2. Intensively — Cabbage, let- tuce, celery, asparagus, etc. Sandy soil 4-10-6 Loamy soil 4-10-4 Clayey soil 4-10-2 The ammonia should be re- duced if farm manure is used. 492 NATURE AND PROPERTIES OF SOILS 3. Miscellaneous. rSandy soil 7-6-5 a. Early potatoes * ■< Loamy soil 5-8-5 [clayey soil 4-8-4 rSandy soil 5-8-7 b. Late potatoes * J Loamy soil 4-8-6 [clayey soil 4-8-4 c. General trucking * on sandy soils of Atlantic seaboard 5-8-7 * Note. — Eeduce ammonia if farm manure is used. In this table of suggested formulae, it is noticeable that wberever manure is used, the ammonia is reduced or even eliminated. Ammonia is also unnecessary on leguminous crops. With vegetables, the ammonia is usually high. Top dressings for pastures, meadows, and cereals in the spring should always carry large quantities of readily available nitro- gen. In a mixed fertilizer, the phosphoric acid is generally high, for reasons already explained. Due to the absorptive power of a clay, the mixture applied to such a soil should generally carry more phosphorus than that added to a sandy soil. Pot- ash is usually lower in a fertilizer for clayey soils, due to the possibility of liberating potassium from the soil itself by good soil management. 277. Amounts of fertilizers to apply. — The agricultural value of a fertilizer is necessarily a variable quantity, since, in applying fertilizers, a material subject to change is placed in contact with two wide variables, the soil and the crop. Moreover, soil conditions are constantly changing, thus forc- ing a modification of the fertilizer applied to the same soil bearing the same crop at different times. The factors influ- encing the efficiency of a fertilizer application may be listed as follows : (1) seed, crop, and adaptation of crop, (2) weather conditions, (3) physical condition of the soil, including drain- THE PRINCIPLES OP FERTILIZER PRACTICE 493 age, (4) organic content of the soil, and (5) chemical constitu- tion of the soil and its reaction. Although the conditions affecting fertilizer efficiency have thus been so briefly disposed of, it is evident that they are of vital importance in the economical utilization of fertilizing materials. One point of broader scope stands out particularly in this connection — the necessity of putting a soil in any given climate in the best possible condition for plant growth. This means that drainage, lime, organic matter, and tillage, in the order named, must be raised to their highest perfection in order to realize the best results from fertilizers. Such considerations indicate that the decision as to the amount of a single carrier or of a mixed fertilizer that should be applied will be difficult and probably more indefinite than formula selection. In fact, the amount of a fertilizer applied to the acre is more vital than the actual chemical composition, as far as money returns are concerned. With all the groups considered above, except garden and root-crops, the applications are generally relatively light, rang- ing from 150 to 350 pounds to an acre. Where excessive vege- tative growth is required, as in silage, the rate may be in- creased to 500 pounds. In the top dressings of meadows or grains, the rate varies from 100 to 200 pounds an acre. Very often this dressing is sodium nitrate or ammonium sulfate alone. With garden and root-crops, the amount of fertilizer applied is very large, ranging from 800 to sometimes as high as 2000 pounds. The cropping here is intensive, and the ex- penditure for fertilization may be large and yet yield substan- tial profits. 278. The law of diminishing returns. — It must always be remembered that in fertilizer practice the very high yields obtained under fertilizer stimulation are not always the ones that give the best returns on the money invested. In other words, the law of diminishing returns is a factor in the in- fluence of fertilization on crop yield. After a certain point 494 NATURE AND PROPERTIES OF SOILS is reached, the return for each added increment of fertilizer becomes less and less. It is evident, therefore, that with an excessive application of any mixture, the returns to an in- crement will at last become so small that the increased crop fails entirely to pay for even the fertilizer, not to mention IS • — — ■ / ^ { ■JO sc o IZ dd /bOO zooo Z4C0 POUmS OF FL0AT3 yiPPLIED PEiR. ^CRE 4X10 aoO /200 /600 2000 POUnOS OF FLOATS APPLIED PER /JC/ZEl £400 Fig. 60. — In the upper diagram the heavy line indicates the increase in the yield of maize due to graduated applications of floats. The lower diagram shows how the cost of the fertilizer approaches and finally exceeds the value of the crop as the applications increase such charges as cost of application, harvesting of increased crop, storage, and the like. The application of moderate amounts of fertilizer is to be urged for all soils until the maxi- mum paying quantity that may be applied to any given crop is ascertained by careful experimentation. Over-fertilization probably accounts for the fact that such a large proportion of THE PRINCIPLES OF FERTILIZER PRACTICE 495 the fertilizer sold to farmers each year not only is entirely wasted, but probably in some cases even becomes detrimental to crop yield. The law of diminishing returns may be illustrated by data from the Cornell University Agricultural Experiment ^ Sta- tion. Floats were applied at different rates to plats receiv- ing a uniform dressing of farm manure at the rate of 15 tons to the acre. Table CIII shows the increased yields of maize due to the treatment with the rock phosphate. Pre- war prices were used in the calculations. (See Fig. 60.) Table CIII Pounds of Floats TO THE Acre Maize (bus.) Maize (VALUE) Floats (cost) Difference 200 7.0 8.3 10.2 12.7 $4.62 5.48 6.73 8.38 $ .90 1.80 3.60 10.80 +$3.72 + 3.68 + 3.13 — 2.42 400 800 2400 279. Method and time of applying fertilizers. — Although considerable emphasis has been placed on the selec- tion of the correct fertilizer formulae and on the adequate and economical amounts to use, the method of application must not be lost sight of. A fertilizer is never effective unless uni- formly distributed. It should also be placed in the soil in such a position that it will stimulate the plant to the best advantage. The distribution of the fertilizer by means of machinery is much more satisfactory than is broadcasting by hand, as the former method gives a more uniform distribution. Cereals and other crops are now usually planted with a drill or a planter provided with an attachment for dropping the fertilizer at the same time that the seed is sown, the fertilizer 'Lyon, T. L., Soils arid Fertilizers; p. 216; New York, 1917. 496 NATURE AND PROPERTIES OF SOILS being by this method placed under the surface of the soil. Broadcasting machines are also used, which leave the fer- tilizer uniformly distributed on the surface of the ground, permitting it to be harrowed in sufficiently before the seed is planted, thus preventing injury to the seed by the chemical activity of the fertilizing material. Corn-planters with fertilizer attachments deposit the fer- tilizer beneath the seed, thus avoiding a possible detrimental contact. Grain-drills do not do this, and, where the amount of fertilizer used exceeds 300 or 400 pounds an acre, it is better to apply it before seeding. Grass and other small seeds should be planted only after the fertilizer has been mixed with the soil for several days. For crops to which large quan- tities of fertilizers are to be added, especially potatoes and garden crops, it is desirable to drop only a portion of the fertilizer with the seed, the remainder having been broad- casted by machinery and harrowed in earlier. 280. Systems of fertilization. — During the evolution of fertilizer practice since the middle of the nineteenth century, a number of systems of applying fertilizers have been advo- cated and in many cases actually followed. Perhaps the first plan to be suggested was the single element system. At that time, each crop was supposed to respond largely to one par- ticular element. Thus, nitrogen was supposed to dominate wheat, rye, and oats; phosphoric acid, to dominate maize, turnips, and sorghum; and potash to dominate potatoes, clover, and beans. Present knowledge of plant nutrition and the balancing effects of fertilizer nutrients show this idea to be fallacious. The supplying of abundant minerals as a fertilizer system had its origin from the fact that potash and phosphoric acid are relatively cheap and are rather slowly leached from the soil, while nitrogen is expensive and easily lost in this way. Such a plan, therefore, always provides plenty of potash and phosphoric acid, which are to be balanced each season with THE PRINCIPLES OF FERTILIZER PRACTICE 497 sufficient nitrogen to give paying yields. While this system is not feasible in its entirety at the present time, the prin- ciple involved is worthy of incorporation with more economi- cal plans. A system based on the amount of nutrients removed by crops has received from time to time considerable support. According to this plan, as much plant-food material is added each year as will probably be taken out by the plant, this being determined by chemical analyses of the crop. The system not only overlooks the fact that diverse plants feed differently on the same soil, but that the same crop exhibits marked variability with change of season and change of soil. ]\Ioreover, no allowance is made for losses by leaching, which are known to equal at times the losses due to plant absorption. In trucking or in general farming operations, one crop is often the money crop. Naturally its stimulation by heavy fertilization will pay better than applications to crops that bring less on the market. The general plan in this system is to allow the crops following the money crop to utilize the residuum. When this residual influence works out fa- vorably, the system is likely to be a profitable one; but when the following crops fail to respond, the method becomes wasteful in the extreme. 281. Rational fertilizer practice. — In the selection of a system that will result in an effective utilization of fertilizers, only two of the plans described above need be considered. In any fertilizer, phosphoric acid and usually potash should always be present in amounts sufficient more than to balance the nitrogen, since the activity of nitrogen is so pronounced. Therefore, a scheme that calls for an abundance of minerals is a sound one. This, coupled with the heavy fertilization of the money crop, does not, however, constitute what might be considered a rational system, since the crops that follow may or may not be adequately supplied with nutrients. Not only must the soil, the crop and the fertilizer formula 498 NATURE AND PROPERTIES OF SOILS and amount receive careful study, but the rotation should be considered in addition. This is a fundamental principle not only with the application of commercial fertilizers but with liming and the use of farm manure as well. The care- ful fertilization of the rotation, with special reference to the money crop, is the only rational system that should ordi- narily be employed, since it not only cares for the crop on the land but also looks to those that are to follow. The atten- tion that must necessarily be paid to the fertility of the soil in such a system insures the establishment of a soil manage- ment which will result in an economical use of the plant nutrients, while at the same time the yields will be raised and a continuous productivity will be provided for. CHAPTER XXIV FARM MANURE ^ Of all the by-products of the farm, barnyard manure is probably the most important, since it affords a means where- by the unused portion of the crop may become a part of the soil. Its use not only makes possible a return to the land of a part of the nutrients previously removed by the crop but also permits an actual gain of carbohydrate ma- terials, the elements of which the plant obtains not from the soil but from air and water. This country has already entered an era in which the pre- vention of agricultural waste is becoming necessary and a nearer approach to a self-sustaining system of soil manage- ment more and more essential. For the maintenance of fertil- ity, a careful handling and a wise utilization of all the manure * The following publications will be valuable : Ames, J. W., and Gaither, E. W., Barnyard Manure; Ohio Agr. Exp. Sta., Bui. 246, June 1912. Hart, E. B., Getting the Most Profit from Farm Manure; Wis. Agr. Exp. Sta., Bui. 221, June 1912. Thome, C. E., Farm Manures; New York, 1914. Beavers, J. C, Farm Manures; Purdue Univ. Agr. Exp. Sta., Cire. 49, Mar. 1915. Burdick, R. T., Concerning Farm Manures; Vt. Agr. Exp. Sta., Bui. 206, June 1917. Fippin, E. O., Farm Manure; Cornell Eeading Course for the Farm, Lesson 127, Aug. 1917. Weaver, F. P., Far7yi Manure; Pa. State Coll., Ext. Circ. No. 67, Oct. 1917. Brodie, D. A., Handling Barnyard Manure in Eastern Pennsylvania; U. S. Dept. Agr., Farmers' Bui. 978, July, 1918. Wiancko, A. T., and Jones, S. C, The Value of Manure on Indiana Soils; Purdue Univ. Agr. Exp. Sta., Bui, 222, Sept. 1918. Duley, F. !<., Handling of Farm Manure; Mo. Agr. Exp» Sta., Bui. 166, Sept. 1919. 499 500 NATURE AND PROPERTIES OF SOILS produced on the farm are vital. Obviously an understanding is necessary regarding the character and composition of farm manure, its fermentative and putrefactive changes, its losses in handling and storage, and above all its rational use as an amendment and a fertilizer. This need appeals not only to the wide-awake farmer but to the technical man as well, since in the use of farm manures theory and practice widely over- lap. 282. Composition and general characteristics of farm manures. — The term farm manure may be employed in ref- erence to the refuse from all animals of the farm, although, as a general rule, the bulk of the ordinary manure which ultimately finds its way back to the land is produced by cattle and horses. This arises because these animals consume the greater part of the grain and roughage on the average farm, and because the methods of handling such live-stock make it easier and more practicable to conserve their excreta. Yard manure generally refers to mixed manures. The mixing usually occurs during storage, either for convenience in han- dling or for the purpose of checking losses and facilitating fermentation. Thus, horse and cow manures are commonly mixed, since the too rapid putrefaction and consequent loss of ammonia in the former is checked, while at the same time a more rapid and much more complete decomposition is en- couraged in the latter. Ordinary manure consists of two original components, the solid, or dung, and the urine in about the rate of three to one. As these constituents differ greatly, not only in com- position but also in physical properties, their proportions must appreciably affect the quality of the excreta and its agri- cultural value. Litter added for bedding or for absorptive purposes is almost always an important factor, for while it prevents losses of the soluble constituents, it may at the same time lower the value of the product for a unit amount. While compilations of available data on the composition of FARM MANURE 501 farm manures demand liberal interpretations, they afford considerable light regarding the dififerences to be expected be- tween excrement from various animals. Table CIV THE COMPOSITION OF FRESH MANURE.^ Percentage of H^O NH3 P.0« KjO Solid, 80% Horse j Urine, 20% Whole manure .... 'Solid, 70% Cow purine, 30% Whole manure .... Solid, 67% Sheep j Urine, 33% ^Whole manure [Solid, 60% Swine I Urine, 40% Whole manure 75 90 78 85 92 86 60 85 68 !80 97 87 .66 1.63 .84 .48 1.21 .72 .90 1.63 1.14 .66 .48 .60 .30 Trace .25 .20 Trace .15 .50 .05 .35 .50 .10 .35 .40 1.25 .55 .10 1.35 .45 .45 2.10 1.00 .40 .45 .40 Since the horse does not ruminate its food, the manure is likely to be of an open character. It is also fairly dry, as is that from sheep, the urine in these two manures making up 20 and 33 per cent., respectively, of the whole product. The complete manure from these two animals contains 78 and 68 per cent., respectively, of water — a considerable contrast to the cattle and swine increments. Cattle and swine ma- nures, being very wet, are rather solid and compact. The air, therefore, is likely to be excluded to a large degree and de- composition is relatively slow. They are usually spoken of as cold inert manures as compared with the dry, open, rapidly heating excrements obtained from the horse and the sheep. *Van Slyke, L. L., Fertilizers and Crops, p. 291; New York, 1912. 502 NATURE AND PROPERTIES OF SOILS In every case except that of swine, the urine is much the richer than the dung in ammonia, containing on an average more than twice as much when compared on the percentage basis. The urine is also richer in potash than the solid, aver- aging for the four classes of animals 1.29 per cent, as com- pared to 0.34 per cent, contained in the solid manure. Most of the phosphoric acid, however, is contained in the solid ex- Z&7>?Z. TOTAL TOTOL /^MMON/A PAy03P/V0^/C F>OT^3/-/ o.e% P)C/D c>.s%- S5% 65% 45% /oo% 35% T^AC£ Diy/VG. u^//v£:. PUNG. u/e/r^E. PUNG. lAe/A/fT. Fig. 61. — Diagram showing the distribution of ammonia, phosphoric acid and potash between the dung and urine of average farm manure. crement, only traces being found in the urine except in the case of swine. It is, therefore, evident that the urine, pound for pound, is more valuable insofar as the nutrient elements are concerned. The advantage leans heavily toward the urine also in that the constituents therein contained are im- mediately available ; this cannot be said of the solid manure. 283. Liquid versus solid manure. — While the urine car- ries more nutrients to an equal weight than the dung, it yet remains to be seen whether in the total excreta voided by an animal there are more nutrients in the urine than in the dung. FARM MANURE 503 In general, more solid manure is excreted than liquid, tend- ing to throw the advantage toward the former as a carrier of plant nutrients. The following table, adopted from Van Slyke,^ bears on this point : Table CV distribution of nutrient constituents between the liquid and the solid of whole manure. Animal Percentage OP Total NH3 Percentage OF Total P.O. Percentage OF Total SOLID LIQUID SOLID LIQUID SOLID LIQUID Horse 62 49 52 67 38 51 48 33 100 100 95 88 5 12 56 15 30 57 44 Cow Sheep Swine 85 70 43 Average 57 55 43 45 95 100 5 40 35 60 Average for horse and cow 65 It is seen here that a little more than one-half the am- monia, almost all the phosphoric acid, and about two-fifths of the potash, are found in the solid manure. Nevertheless, this apparent advantage of the solid manure is balanced by the ready availability of the constituents carried by the urine, giving it in total about an equal commercial and agricultural value with the solid excrement. Such figures are suggestive of the care that should be taken of the liquid manure. Its ready loss of ammonia by fermentation and putrefaction, and the ease with which all its valuable constituents may escape by leaching, should make it an object of especial regard in handling. (See Fig. 61.) 284. Poultry manure. — While poultry manure is often produced on the farm in large quantities, it is not included under the term farm manure, which, as generally used, refers 'Van Slyke, L. L., Fertilizers and Crops, p. 295; New York, 1912. 504 NATURE AND PROPERTIES OF SOILS to the excrement of the larger animals. Its general composi- tion is as below, the data being averages from Thorne.^ Table CVI composition of poultry manure. ■ Condition Percentage of H,0 NH3 P.O. K2O Whole manure, fresh Whole manure, air dry 57 7 1.31 2.84 .40 .86 .50 1.08 It is to be seen that poultry manure in the air-dry state, the condition in which it is applied, has over twice the amounts of nutrients carried by the other classes. It should be applied to the soil at at least one-half the rate commonly recommended for ordinary farm manure. Notwithstanding its ease in handling and its great value, poultry manure re- ceives less care and attention than any other produced on the farm. 285. Farm manure — a direct and indirect fertilizer. — Farm manure, when applied to the land, ordinarily fulfills two functions which are usually not so distinctly developed in one material — that of a direct and indirect fertilizer. Mixed farm manure ready to apply to the land contains on the aver- age .6 per cent, of ammonia, .25 per cent, of phosphoric acid and .5 per cent, potash.- It is obviously a low-grade fertilizer ^ Thome, C. E., Farm Manures, p. 90; New York, 1914, Also, Storer, F. H., Agriculture, Vol. I, p. 613; New York, 1910. Vorhees, E. B., Ground Bone and Miscellaneous Samples; N. J. Agr. Exp. Sta., Bui. 84, 1891. Goessman, C. A., Mass. Agr. Exp. Sta., Bui. 37, 1890, and Bui. 63, 1896. *See Analyses, Storer, F. H., Agriculture, pp. 237-248; New York, 1910. Thorne, C. E., Farm Manures, pp. 89-93; New York, 1914. Aikman, C. M., Manure and Manuring, pp. 279-292; Edinburgh and London, 1910. Roberts, I. P., The Fertility of the Land, pp. 159-182; New York, 1904. FARM MANURE 505 both as to the amounts of nutrients carried and as to their availability. Because of the large acre applications of ma- nure commonly made, the fertilizer constituents added in ma- nure are considerable. Ten tons of farm manure, even if only one-half its ammonia, one-sixth of its phosphoric acid and one- half of its potash were readily available, are equal in fertil- izing value to 333 pounds of sodium nitrate, 52 pounds of acid phosphate, and 416 pounds of kainit. This equiva- lent to the addition of 801 pounds of a readily available mix- ture of fertilizer salts. This calculation, however, ignores an equal quantity of nutrients which remain in the soil as a residuum and may be used by succeeding crops. This resi- dual effect of manure is generally a paying one during the period of an ordinary rotation. Farm manure acts as an indirect fertilizer in that it adds to the soil organic matter and thus improves the physical condition of the land. While it may not increase the organic matter of the soil, because of the loss of carbon by exhalation and leaching during the period of crop growth, its use materi- ally influences the rate of reduction. Better aeration, drain- age and bacterial activity ^ of necessity result from such an addition. The influence of manure on the availability of the mineral constituents of the soil is not the least of its indirect actions. The fact that rock phosphate when mixed with manure seems to have a higher availability bespeaks a considerable solvent activity. The tendency of farm manure to alleviate toxic conditions, such as alkali and acid- ity, deserves attention. 286. Outstanding characteristics of farm manure. — As farm manure is essentially^ a fertilizer, whether it is pro- duced on the farm or purchased outright, it is logical to con- trast it with the ready-mixed materials on the market. In * Conn, H. J., and Bright, J. W., Ammonification of Manure in Soil; Jour. Agr. Res., Vol. XVI, No. 12, pp. 313-350, March, 1919. Fulmer, H. L., and Fred, E. B., Nitrogen Assimulating Organisms in Manure; Jour Bact., Vol. II, No. 4, pp. 423-434, 1917. 506 NATURE AND PROPERTIES OF SOILS such a comparison, five characteristics are outstanding: (1) the moist condition of manure, (2) its low grade, (3) its unbalanced nutrient condition, (4) its variability, and (5) its rapid fermentative and putrefactive processes. These characteristics, neither present nor desirable in ordinary fer- tilizers, place farm manure in a class by itself as to its hand- ling, storage, and field utilization. Of the above points, the first three may be disposed of quickly. Average farm manure, whether fresh or well-rotted, contains from 70 to 85 per cent, water. A ton of average mixed manure when applied to the land carries but 12 pounds of ammonia, 5 pounds of phosphoric acid, and 10 pounds of potash to the ton. Approximately one-half, one-sixth, and one-half, respectively, of these constituents are readily avail- able. Farm manure is, therefore, low-grade on two distinct counts. Moreover, its readily available nutrients approximate a ratio of about 6-1-6, a marked contrast to the 2-8-2 often given for the average ready-mixed fertilizers on the market. Obviously, manure is much too low in phosphoric acid for its content of active ammonia and potash. The variability and decomposition of farm manure will be considered separately. 287. Variability of farm manure. — The manure pro- duced on the average farm will obviously vary in its char- acter and composition from time to time. The factors re- sponsible may be listed as follows: (1) class of animal, (2) age, condition, and individuality of animal, (3) food, and (4) the handling and storage which the manure receives be- for it is placed on the soil. The differences in composition due to class of animal have been adequately disposed of in previous paragraphs. In ad- dition, it is obvious that the age and condition of any animal within a class will influence the character of the ex- crement produced. A young animal gaining in bone and muscle will retain large amounts of nutrients, and the manure will be correspondingly poorer in dry matter, nitrogen, lime, FARM MANURE 507 phosphoric acid, and potasli. A fattened animal on a main- tenance ration will return almost all of the nutrient value of the original food. Since the animal will retain oidy a certain quantity of the important food elements, it is only reasonable to assume that the richer the food, the richer will be the corresponding excrement. The following data from Ohio ^ obtained with western lambs substantiate this assumption: Table CVII effect of ration on manurial composition. Percentage of Eation Corn and mix hay Corn, oil meal and hay Corn, oil meal and clover I 2.03 While the factors just disposed of cause some variation in farm manure, the character of the product as it goes on to the land is determined in large degree by the handling. Tight floors and proper bedding hold the liquid manure in contact with the solid and thus maintain the proportion of valuable constituents. A neglect of these two conditions means a grave loss in value. The storage of manure, when it is not taken directly to the field, always results in loss not only of organic matter, but of ammonia and minerals as well. As more than one-half of the ammonia and potash are water-soluble, seri- ous loss is unavoidable. Such losses over-ride other causes of variation. The influence of storage is clearly shown by the following figures from Schutt ^ on mixed horse and cow ^Thorne, C. E., and others. The Maintenance of Fertility; Ohio Agr. Exp. Sta., Bui. 183, 1907. ^Sehntt, M. A., Barnyard Manure; Canadian Dept. Agr., Centr. Exp. Farm, Bui. 31, 1898, 508 NATURE AND PROPERTIES OF SOILS manure. The protected manure was stored in a bin under a shed. The exposed sample was in a similar bin but unpro- tected. Table CVIII loss of constituents from protected and unprotected MANURE. Constituents Percentage Loss at End of Six Months Percentage Loss at End of One Year PROTECTED EXPOSED PROTECTED EXPOSED Loss of organic matter Loss of NH, 58 19 3 65 30 12 29 60 23 4 3 69 40 Loss of PgOr; Loss of K.,0 16 36 288. The fermentation and putrefaction of manure.^ — In the process of digestion, the food of animals becomes more or less decomposed. This condition comes about partly be- cause of the digestive process and partly from the bacterial action that takes place. Of these two influences within the animal, bacterial activities are probably of the greater im- portance as far as the breaking-up of the complicated food- stuffs is concerned. The fresh excrement, then, as it comes from the stable, consists of decayed or partially decayed plant materials, with a certain amount of broken-down animal tissue and mucus. This is more or less intimately mixed with litter and the whole mass is moistened with the liquid excre- ment carrying considerable quantities of soluble nitrogen and potash. This mass of material, ranging from the most com- ^Good general discussions may be found as follows: Lipman, J. G., Bacteria in Relation to Country Life, pp. 303-3.56 ; New York, 1911. Hall, A. D., Manures and Fertilizers, pp. 184-210; New York, 1921. For a technical discussion see Russell, E. J., and Richards, E. H., The Changes Taking Place During the Storage of Farm Manure; Jour. Agr. Sci., Vol. VIII, Part 4, pp. 495-563, Dec, 1917, FARM MANURE 509 plex compounds to the most simple, is teeming with bacteria,^ especially those that function in fermentatio)i and putrefac- tion. The number very often runs into billions to a gram of excrement. In such an environment, it is little wonder that biological changes go on rapidly. These changes may be grouped for convenience of discussion under two heads — aerobic and anaerobic. When manure is first produced, it is likely to be rather loose, and if allowed to dry at once it becomes well aerated. The first bacterial action is, therefore, likely to be rather largely aerobic in nature. Transformations are very rapid and are accompanied by considerable heat, ranging from 100° to 150° F. and sometimes higher. This action falls largely on the simple nitrogenous compounds, although the more complicated nitrogenous and non-nitrogenous constituents are by no means unaffected. Urea is particularly influenced by aerobic activities and quickly disappears from well-aerated manure. CON.H^ + 2H,0 = NHJ2CO3 NHJ.COa = NH3 + CO, + H3O Thus nitrogen may be rapidly lost from manure by allow- ing excessive aerobic decay and decomposition to proceed. This loss, however, is often somewhat checked by the oxidiz- ing influence of nitrifying bacteria, especially in the outer portions of the manure pile. The evolution of carbon dioxide which goes on continuously indicates how extensively the organic matter of the manure is suffering through biological activity. As the manure becomes compacted, especially if it is left moist, oxygen is gradually excluded from the heap and its place is taken by carbon dioxide, which is given off during the progress of any form of bacterial activity. The decay now changes from aerobic to anaerobic, it becomes slower, and ^Murray, T. J., Studii of the Bacteria of Fresh and Decomposing Manure; Va. Agr. Exp. Sta., Bui. 15, Part II, 1917. 510 NATURE AND PROPERTIES OF SOILS the temperature falls to as low as 80° or 90° F. New organ- isms may now function, although many of those active under aerobic conditions may continue to be effective. The prod- ucts become changed to a considerable degree. Carbon diox- ide, of course, continues to be evolved in large amounts, but instead of ammonia being formed, the nitrogenous matter is converted into the usual putrefactive products, such as indol, skatol, and the like. If sufficient reduction occurs, free nitro- gen may escape. The carbonaceous matter is resolved into numerous hydro- carbons, of which methane (CH^) is prominent; and as a by- product of the breaking-down of the proteins, hydrogen sul- fide (HgS) and sulfur dioxide (SO2) are evolved. The com- plex nitrogenous and carbohydrate bodies are attacked with the splitting-off, not only of simpler materials, but often of those more complex. Such compounds may be listed in gen- eral as organic acids and humous bodies. They, of course, ul- timately succumb to simplification. The general changes ^ in any manure pile can readily be recapitulated. First is the aerobic action, with the escape of ammonia and carbon dioxide. Next the manure is wetted, it compacts, and the slow, deep-seated decay sets in with a simplification of some compounds, with the production of acids, and with a gradual formation of humous materials. As the manure becomes alternately wet and dry, the two gen- eral processes may follow each other in rapid succession, the anaerobic bacteria attacking the complex materials, the aerobic affecting both the complex and the simpler com- * The proteid compounds, which are the most important group in farm manures, split up in the soil or compost heap into amino-acids. These amino-acids undergo deaminisation and decarboxylation. The former takes place either under aerobic or anaerobic conditions producing am- monia and a complex acid. The decarboxylation occurs only when oxygen is excluded giving either ammonia and an organic acid as in deaminisa- tion, or carbon dioxide and a complex amine, which may be rather stable. Deaminisation and decarboxylation go on together, the former generally predominating. FARM MANURE 511 pounds. Carbon dioxide is given off continuously during the process. Some gaseous nitrogen as well as ammonia is prob- ably lost because of the rai)id alternations of conditions.^ 289. Effect of decomposition on the value of manure. — Because of the great loss of carbon dioxide and water dur- ing the decay processes, there is considerable change in bulk of the manure. Fresh excrement loses from 20 to 40 per cent, in bulk by partial rotting and 50 per cent, bj^ becoming more thoroughly decomposed. This means that 1000 pounds of fresh manure may be reduced to 800, 600, or 500 pounds, according to the degree of change it has undergone. It is often argued that if the manure is properly stored, this rapid loss of carbon dioxide and water will raise the percentage amounts of the fertilizer elements. The simplify- ing action of the anaerobic fermentation and putrefaction is an additional reason for expecting better results from well- rotted manure when it is compared, ton for ton, with the fresh material. In practice, however, the losses in handling due to leaching and fermentation are so dominant as to place well-rotted manure at a disadvantage except on sandy land or for garden and trucking purposes. At the Ohio Experiment Station,^ yard and stall manure were compared in equal amounts in a three-year rotation of maize, oats, and hay. The 3'ard manure was exposed for some months in the open, while the stall manure came directly from the stable. The increase due to yard manure is taken as 100 in each case. (Table CIX, p. 512.) A change of a biological nature which sometimes takes place in loose and rather dry manure is fire-fanging. Many farmers consider this to be due to actual combustion, as the * Under the alternating ai-robic and anaerobic conditions found in the average manure pile, gaseous nitrogen seems to be lost in considerable amounts. This loss probably occurs through the oxidation of ammonia to nitrites or nitrates with a later reduction of the nitrogen so carried to a free state. - Thorne, C. E., The Maintenance of Fertility ; Ohio Agr. Exp. Sta., Bui. 183, p. 209, 1907. 512 NATURE AND PROPERTIES OF SOILS Table CIX comparative yields prom yard and stall manure, Average Increase to the Acre Manure Corn, 10 Years Wheat, 10 Years Hay, grain STOVER GRAIN STOVER 6 Years Stall 100 72 100 68 100 85 100 87 100 \ ard 54 manure is very light in weight and has every appearance of being burned. This condition, however, is produced by fungi instead of bacteria, and the dry and dusty appearance of the manure is due to the mycelium, which penetrates in all di- rections and uses up the valuable constituents. Manure thus affected is of little value either as a fertilizer or as a soil amendment. 290. Evaluation of farm manure. — For purposes of com- parison, experimentation, and sale, farm manures are often evaluated in a way similar to that used with commercial fer- tilizers. The great difficulty here lies in arriving at prices for the important constituents which are at all comparable with the value of the manure in the field. If the value of the ammonia in manure is arbitrarily placed at 15 cents a pound, phosphoric acid at 5 cents, and potash at 8 cents, certain tentative calculations may be made. While such assumptions do not establish the commercial value either of fresh or stored manure, they are of some use in comparisons and gen- eralizations. The average manure, as it goes on the land, car- ries about 12 pounds of ammonia, 5 pounds of phosphoric acid, and 10 pounds of potash. Using the prices above, such manure is worth commercially about $3.00 a ton. The commercial evaluation must be applied with care be- cause of the many factors tending to vary the composition of P^ARM MANURE 513 the excrement. Litter, particularly, will exert a great influ- ence in this direction. Moreover, this mode of evaluation must never be confused with the much more important figure known as the agricultural value of a manure. The former is based on composition and assumed values of doubtful char- acter. The latter arises from the effect of the manure on crop yield. Obviously, a rational utilization of farm manure, as with any fertilizer, should strive for the highest return to an increment applied. A very good comparison between commercial and agricultural values may be cited from the Ohio experiments ^ with manure. The manure was treated in various ways and applied to maize in a three-year rotation of maize, wheat, and hay. Twenty-six crops were grown. The commercial evaluation is taken as 100 in every case. Table CX commercial and agricultur.\l evaluation of farm manure. Manure Commercial Value Agricultural Value Yard manure, untreated 100 100 100 100 100 152 Yard manure, plus floats 162 Yard manure, plus acid phosphate . . Yard manure, plus kainit Yard manure, plus gypsum 222 192 186 291. Amount of manure produced by farm animals. — A well-fed moderately worked horse will produce daily from 45 to 55 pounds of manure, of which 10 to 12 pounds is urine. A dairy cow, having a greater food capacity, will ex- crete from 70 to 90 pounds during the same period, of which 20 to 30 pounds is liquid. Farm animals, especially sheep and swine, vary so much in size that a thousand pound ^ Thome, C. E,, and others, The Maintenance of Fertility; Ohio Agr. Exp. Sta., Bui. 183, pp. 206-209, 1907. 514 NATURE AND PROPERTIES OF SOILS weight of animal is the only fair and logical basis of calcu- lation. Table CXI MANURE EXCRETED BY VARIOUS FARM ANIMALS TO THE 1000 POUNDS LIVE WEIGHT. Animal Pounds A Day Tons a Year Horse ^ 50 70 40 85 34 23 9.1 Cow 2 12.7 Steer ^ 7.3 Swine * 15.5 Sheep ^ 6.2 Poultry ^ 4.2 It is to be noted that these figures do not include litter, which, in cases of horses and cattle, will range from 15 to 20 per cent, of the weight of the pure excrement. A working horse would be expected to produce from 10 to 11 tons of average manure a year, while a dairy cow on the same basis would produce 14 or 15 tons. Rough calculations as to manurial production from horses and cattle may be made from the food consumed by these animals.'^ It is assumed that 50 per cent, of the dry matter of the food appears in the excrement and that the necessary bedding equals one-half of the dry matter of the excrement. ^Eoberts, I. P., and Wing, H. H., On the Deterioration of Farmyard Manure by Leaching and Fermentation ; Cornell Agr. Exp. Sta., Bui. 13, 1889. Also, Eoberts, I. P., The Production and Care of Farm Manure; Cornell Agr, Exp. Sta., Biil. 27, 1891. Also, Watson, G. C, The Produc- tion of Manure; Cornell Agr. Exp. Sta., Bui. 56, 1893. ^Thorne, C. E., Farm Manures, p. 97; New York, 1914. =• Thome, C. E., and others, The Maintenance of Fertility; Ohio Agr. Exp. Sta., Bui. 183, 1907. * Watson, G. C, The Production of Manure; Cornell Agr. Exp. Sta., Bui. 56, 1893. *Van Slyke, L. L., Fertilisers and Crops, p. 294; New York, 1912. * Hart, E. B., and Tottingham, W. E., General Agricultural Chemistry, p. 125; Madison, Wis., 1913, FARM MANURE 515 Average manure (bedding plus excrement) is about 75 per cent, water. This means that from 100 pounds of mixed food there results 50 pounds of manurial dry matter, 25 pounds of litter, and 225 pounds of water or 300 pounds in all. The weight of the food consumed multiplied by three should give in a rough way the weight of the fresh excrement plus its litter. 292. Loss of crop constituents in the production and handling of manure. — Any system of agriculture, whether it be grain farming, animal husbandry, or some specialized type such as trucking, must ultimately arrange for the addition of certain nutrients to replace those lost in the crop, in drain- age and through biological activity. It is evident, however, that even if all of the crop constituents were returned to the soil, a constant degi'ee of fertility would not be maintained, although the organic matter and possibly the nitrogen, if legumes were included in the rotation, might not greatly de- crease. The large loss of certain nutrients in the drainage water must always be considered in any rational system of soil fertility. Since farm manure lessens or even eliminates the need of a green-manure and at the same time offers a means of lower- ing the fertilizer bill, it is worth while to inquire what pro- portion of the nutrients contained in the crop may be re- turned to the soil in the resulting manure. The losses en- tailed are three: (1) those that occur in the handling and feeding of the crop, (2) those incurred as the food passes through the animal, and (3) those due to the handling and storage of the manure produced. 293. Losses during manurial production. — A certain amount of every crop is lost before it is finally consumed by the animal. Such loss, while important, is usually small on every farm, especially when compared to the nutrients re- tained by the animal. Attention is, therefore, particularly directed towards those losses sustained by the food as it un- 516 NATURE AND PROPERTIES OF SOILS dergoes normal digestion. Some of the data available in this respect are quoted below: Table CXII percentage of original food constituents recovered in fresh manure. Animal Steers, Ohio ^ Steers, Penn.^ Steers, England ^ Milking cows, Illinois'*. Milking cows, Penn. ^ . . Milking cows, England ^ Heifers, England " . . . . Sheep, Ohio '* NH3 P.O, 61.0 86.8 69.4 75.1 95.5 93.0 80.3 73.3 84.6 70.7 71.8 75.0 77.8 78.4 68.0 87.0 K,0 82.4 81.2 98.5 76.0 91.0 90.0 86.4 91.5 As might be expected, the data are quite variable, depend- ing on the age, condition, individuality and class of animal, and the character of the food. As a generalization and for purposes of calculation, it may be considered that three- fourths of the ammonia, four-fifths of the phosphorus, nine- tenths of the potash, and one-half of the organic matter are recovered in the manure.^ This means losses of about 25, 20, ^ Thorne, C. E., Maintenance of Fertility; Ohio Agr. Exp. Sta., Bui. 183, p. 200, 1907. ^Frear, W., Losses of Manure; Pa. Agr. Exp, Sta., Bui. 63; Apr, 1903. ^Hall, A, D., Fertilisers and Manures, p. 180; New York, 1921. * Hopkins, C. G., Soil Fertility and Permanent Agriculture, p, 201, Boston, 1910. ^Sweetser, W. S., The Manurial Value of the Excreta of Milch Cows; Pa. State Coll., Ann. Rep., 1899-1900, j>p. 321-351. "Hall, A. D., Fertilisers and Manures, p. 180; New York, 1921. ^Wood, T. B., Losses in Making and Storing Farm Yard Manure; Jour. Agr. Sci., Vol. II, pp. 207-215, 1907-08. * Thome, C. E., Maintenance of Fertility; Ohio Agr, Exp, Sta., Bui. 183, p. 202, 1907. "See Hopkins, C. G., Soil Fertility and Permanent Agriculture, p. 206; Boston, 1910. Also, Fippin, E. O., Live Stock and the Maintenance of Organic FARM MANURE 517 10 and 50 per cent., respectively, for these constituents. While such losses are necessary and are usually compensated by the animal products, their magnitude must be considered in esti- mating the value of manure in the ordinary rotation. 294. Losses due to handling and storage. — As about one- half of the ammonia and three-fifths of the potash of average farm manure are in a soluble condition, the possibility of loss by leaching is usually great, even though the manure is not exposed to especially heavy rainfall. The loss of phosphorus is also of some consequence. In addition, decomposition, espe- cially that of an aerobic nature, will cause a rapid waste of ammonia, one-half of that present being especially susceptible. Packing and moistening the manure will change the decay from aerobic to anaerobic, thus reducing the waste of am- monia while encouraging the simplification of the manurial constituents. Tight floors in the stables and impervious bot- toms in the manure pit or under the manure pile will con- siderably diminish leaching losses. It is impossible, in quoting figures for waste of manure, to separate the losses due to fermentation and putrefaction from those due to leaching. The two processes go on simul- taneously, the loss from one source being dependent, to a cer- tain extent, on the other. It is only the nitrogen, however, that may be lost hy both decomposition and leaching, the min- erals being wasted only through the latter avenue. While the figures are variable (Table CXIII), it is easilj^ seen that one-half of the ammonia and potash and one-third of the phosphoric acid are readily lost under fairly careful meth- ods of storage. On the average farm where manure very often remains outside for several months, the losses will run much higher, easily amounting to 50 per cent, of the organic mat- ter, 60 per cent, of the ammonia, 40 per cent, of the phos- Matter in the Soil; Jour. Amer. Soc. Agron., Vol. 9, No. 3, pp. 97-105, Mar. 1917. Also Armsby, H. P., and Fries, J. A., Net Energy Values of Feeding Stuffs for Cattle; Jour. Agr. Res., Vol. Ill, pp. 435-491, 1915. 518 NATURE AND PROPERTIES OF SOILS phoric acid, and 65 per cent, of the potash. This means a loss of at least one-half of the nutrient constituents of the ma- nure and considerably over one-half of the fertilizing value, since the elements wasted are those most readily available to plants. Considering the losses which the food sustains during digestion and the waste of the manure in handling and stor- age, it cannot be expected that more than 25 per cent, of the Table CXIII losses from manure through leaching and fermentation. Kind of Manure Horse ^ 183 Horse ^ Horse '^ Cow^ Cow' Steer ■* Days exposed. . . . 183 274 183 77 91 Percentage loss of ammonia 36 60 40 41 31 30 Percentage loss of phosphoric acid 50 47 16 19 19 23 Percentage loss of potash 60 76 34 8 43 58 organic matter, 30 per cent, of the ammonia, 50 per cent, of the phosphoric acid, and 30 per cent, of the potash of the original crop will reach the land.^ Even if leaching losses ^ Roberts, I. P., and Wing, H. H., On the Deterioration of Farmyard Manure by Leaching and Fermentation; Cornell Agr. Exp. Sta., Bui. 13, 1889. ^Schutt, M. A., Barnyard Manure. Canadian Dept. Agr., Centr. Exp. Farms, Bui. 31, 1898. ^ Thome, C. E., Farm Manures, p. 146; New York, 1914. * Thorne, C. E., and others, The Maintenance of Fertility ; Ohio Agr. Exp. Sta., Bui. 183, 1907. ^ Voelcker and Hall have drawn up recommendations for the compen- sation of the out-going English tenant for manure produced on the farm but not realized on. They suggest that he receive pay at fertilizer prices for one-half of the nitrogen, three-fourths of the phosphoric acid, and all of the potash contained in the food consumed during the last year of tenancy. For the second, third, and fourth years previous, the com- pensation value shall be one-half that of the year immediately preced- ing. Voelcker, A., and Hall, A. D., The Valuation of Unexhausted Manures; Jour. Eoy. Agr. Soc. Eng., Vol. 63, pp. 76-114, 1902. FARM MANURE 519 OR.G/R/\/JC ^O^ K^O ^£-r/^//V£D BY^N/MAL Lost //v h/^/^dl /ng /^dd£:d to TH£30/L. Fig. 62. — Diagram showing the proportion of the important constituents of the food retained by the animal, lost in the handling and the storage of the manure and applied to the soil under ordinary conditions. were not important, a self-sustaining system of agriculture could not be established by the use of farm manure alone, as organic matter is the only constituent that would be added to the soil in amounts that approach the magnitude of the loss.' * Fippin, E. O., Live Stock and the Maintenance of Organic Mat- ter in the Soil; Jour. Amer. Soc. Agron., Vol. 9, No. 3, pp. 97-105, Mar. 1917. 520 NATURE AND PROPERTIES OF SOILS 295. Two phases of manurial practice. — A commercial fertilizer, if made properly, may be kept for long periods unimpaired and is always in a condition for instant applica- tion to the soil. The only problem confronting the farmer is the profitable application of such material. Storage is a minor factor. Farm manure, on the other hand, although a true fertilizer, presents, because of its peculiar characteris- tics, serious complications. As it is subject to tremendous losses by leaching, putrefaction, and fermentation, its han- dling and storage, if the latter becomes necessary, is as im- portant as its rational utilization on the land. Manurial prac- tice, therefore, is logically discussed under two headings: (1) handling and storage, and (2) utilization of the manure in the field. 296. Care of manure in the stalls. — Considerable loss to manure occurs in the stable, due to decomposition and leach- ing. Before the urine can be absorbed by the litter, it is likely to decay and leach away in considerable amounts. Therefore, the first care is to the bedding, which should be chosen for its absorptive properties, its cost, and its cleanli- ness. The following table ^ shows the approximate absorptive capacity of some common litters. (Table CXIV, page 521.) The amount of litter to be used is determined by the char- acter of the food. If the food is watery, the bedding should be increased. In general, the litter amounts to about one- fourth of the dry matter of the food consumed. Sheep re- quire about a pound of bedding a head, cattle from eight to ten pounds, and horses from ten to fifteen pounds. No more litter than is necessary to keep the animal clean and to ab- sorb the liquid manure should be used, as the excrement is *Beal, W. H., Barnyard Manure; U. S. Dept. Agr., Farmers' Bui. 192, 1904. W^hisenand, J. W., Water-iiolding Capacities of Bedding Materials for Live Stock, Amounts Required to Bed AninMls, and Amounts of Manure Saved by Their Use; Jour. Agr. Ees., Vol. XIV, No. 4, pp. 187-190, July 19i8. FARM MANURE 521 Table CXIV absorptive power op bedding for water. Material Percentage of Water Eetained Mixed shavings. . . . Mixed sawdust. . . . Fine pine shavings. Muck Wheat straw Oats straw Peat Peat moss 124 160 185 200 210 250 600 1300 thus diluted unnecessarily with material which often does not carry large quantities of available fertilizing ingredients. The next care is that floors should be tight, so that the free liquid cannot drain away but will be held in contact with the absorbing materials. The preserving of manures in stalls with tight floors has been for years a common method of han- dling dung in England. The trampling of the animals, and the continued addition of litter as the manure accumulates, explain the reason for the success of the method. The follow- ing data, from Ohio,^ show the relative recovery of food ele- ments in manure produced on a cement floor and on an earth floor, respectively. The experiment was conducted with steers over a period of six months. Even with a good dirt floor, the leaching losses are considerable. (Table CXV, page 522.) 297. Hauling directly to the field.=^ — Where it is possible ^ Thome, C. E., Maintenance of Fertility; Ohio Agr. Exp. Sta., Bui, 183, p. 199, 1907. * Good discussions of handling farm manure are as follows : Hart, E. B., Getting the Most Profit from Farm Manure; Wis. Agr. Exp. Sta., Bui. 221, 1912. Beal, W. H., Barnyard Manure; U. S. Dept. Agr., Farmers' Bui. 192, 1904. Roberts, I. P., The Fertility of the Land, Chapter IX, pp. 188-213; New York, 1904. 522 NATURE AND PROPERTIES OF SOILS Table CXV recovery of pood elements in manure produced on cement floor; on earth floor. Constituents Percentage Eecovery Cement Floor Earth Floor Ammonia 74.7 77.5 87.8 62.4 Phosphoric acid 78.9 Potash 78.4 Average 80.0 73.2 to haul directly to the field, this practice is to be advised, since opportunities for excessive losses by leaching and fer- mentation are thereby prevented. Manure may even be spread on frozen ground or on the top of snow, provided the land is fairly level and the snow is not too deep. This sys- tem saves time and labor, and when leaching does occur the soluble portions of the manure are carried directly into the soil. The practice of allowing the manure so spread to lie on the surface of the land all winter is sometimes questioned, especially in New England.^ On sandy soils it may some- times be better practice to store the manure until spring. 298. Piles outside. — Very often it is necessary to store manure outside, fully exposed to the weather. When this is the case, certain precautions must be observed. In the first place, the pile should be located on level ground far enough from any building that it receives no extra water in times of storm. The sides of the heap should be steep enough to shed water readily, while the depth of the pile should be such as to allow little leaching even after heavy storms. The earth under the manure may be slightly dished in order to prevent '■ Brooks, W. P., Methods of Applying Mamire; Mass. Agr. Exp. Sta., Bui. 196, Sept. 1920. FARM MANURE 523 loss of excess water. If possible, the soil of tlie depression should be puddled, or, better, lined with cement. The manure should be kept moist in diy weather in order to decrease aerobic action. Each addition of manure should be packed in place, the fresh on and above the older. This allows the gases from the well-rotted dung to pervade the fresher and looser portions, thus quickly establishing the anaerobic conditions so essential to economic and favorable fermentation. Placing fresh manure in small heaps in the field to be spread later, is, in the first place, poor economy of labor. Moreover, it encourages loss by decay, while at the same time the soluble portions of the pile escape into the soil imme- diately underneath. There is thus a poor distribution of the essential elements of the dung, and when the manure is finally spread, an over-feeding of plants at one point and an under- feeding at another results. A low efficiency of the manure is thus realized. This method of handling manure is not to be recommended, 299. Manure pits. — Some farmers, especially if the amount of manure produced is large, find it profitable to con- struct manure pits of concrete. These pits are usually rec- tangular in shape with a shed covering. Often one or even both ends are open to facilitate the removal of the manure. In such a structure, leaching is prevented by the solid bottom while the roof allows a better control of moisture conditions. By keeping the manure carefully spread and well moistened, putrefaction may proceed with a minimum loss of nitrogen. Some European dairymen even go so far as to utilize a cis- tern, into which is shoveled both the liquid and the solid manure. Later when decomposition has proceeded suffi- ciently, the material is pumped out and applied to the land. This method is not to be advocated in this country except under special conditions, owing to the cost of handling. 300. Covered yards. — Another method of storage is by 524 NATURE AND PROPERTIES OF SOILS means of a covered barnyard. Such a yard should have a more or less impervious floor. The manure is spread out in the yard and is kept thoroughly packed as well as damp by the animals. This is a common method of handling the ma- nure in the fattening of steers in the Middle West and pro- duces manure at a minimum loss, providing hogs are not al- lowed to follow the steers. The storage of manure in deep stalls, a favorite method in England, is similar to this system and has been shown to be very economical. It also affords an opportunity for the mixing of the manure from different classes of animals. The desirability of this has already been shown in the case of horse and cow excrements. The advan- tages of trampling, so far as the keeping qualities of manure are concerned, are clearly shown by the following figures taken from the work of Frear : ^ Table CXVI loss of manure in covered sheds. Condition Percentage Loss op NH3 K3O P.0« Covered and tramped 5.7 34.1 5.5 19.8 8.5 Covered and untramped 14.2 Throwing manure in heaps under a shed and allowing hogs to work the mass over, is a desirable practice so far as food utilization is concerned. It interferes, however, with a proper and economical packing of the manure. The question to be decided is whether the added food value of the manure over- balances the extra losses by decomposition incurred by the rooting of the swine. 301. Increased value of protected manure. — From the previous discussion, it is evident that a well-protected and "■ Frear, W., Losses of Manure; Pa. Agr. Exp. Sta., Bui. 63, 1903. FARM MANURE 525 carefully preserved manure will be higher in available plant constituents than one not so handled. Moreover, the agricul- tural value of such manure will be higher. This is shown by actual tests from Ohio.^ Over a period of fourteen years, in a three-years' rotation of maize, wheat, and hay, a stall manure gave a yield 38 per cent, higher than that with a yard manure. Table CXVII increase yields from yard and stall manure. Manure Average Annual Increase to THE Acre Maize 14 Crops Wheat 14 Crops Clover 11 Crops Yard, 8 tons to the rota- tion 18.6 bus. 23.6 bus. 26.8% 9.5 bus. 10.9 bus. 14.7% 801 lbs Stall, 8 tons to the rota- tion 1395 lbs Increase, stall over yard manure 74.1% In New Jersey, fresh manure showed a gain in crop yield 53 per cent, higher than leached manure over the three years immediately following the application. Such figures are worthy of careful consideration. 302. Application of manure.— In the application of ma- nure to the land, the same general principles observed in the use of any fertilizer should be kept in mind. Of these, fine- ness of division and evenness of distribution are of prime im- portance. The efficiency of the manure may be raised con- siderably thereby. Moreover, it is generally better, since the ^Thorne, C. E., and others, PIa7is and Summary Tables of tJie Experi- ments at the Central Farm; Oliio Agr. Exp. Sta., Cire. 120, p. 112, 1912. 526 NATURE AND PROPERTIES OF SOILS supply of manure is usually limited in diversified farming, to decrease the amounts at each spreading and cover a greater acreage. Thus, instead of adding 20 tons to the acre, 10 tons may be applied and twice the area covered. Applications could then be made oftener and a larger and quicker net return realized for each ton of manure. With manure, as with any fertilizer, the yield to the acre is not so important as the crop increase for a given increment of manure added. The influence of rate of application on increased yield to a ton of manure is shown bj' the Ohio ^ experiments over eight- een years in a three-year rotation of wheat, clover and pota- toes, the manure being placed on the wheat. Table CXVIII increased yield to the ton when manure is applied in different amounts. ohio experiment station. Eate Wheat (bus.) Clover (lbs.) Potatoes (bus.) 4 tons to the acre 8 tons to the acre 16 tons to the acre 1.34 .94 .70 177 150 99 3.81 2.79 2.76 Not only is the increased efficiency from the smaller appli- cation apparent, but a greater recovery of the manurial fer- tility in the crops also results. The Ohio experiments show that in the first rotation after the manure is applied, a 25 to 30 per cent, higher recovery may be expected from the 8 tons treatment than from the 16 tons. Evenness of application and fineness of division are greatly facilitated by the use of a manure-spreader. This also makes possible the uniform application of small amounts of manure, ^ Thorne, C. E., and others^ Plans and Summary Tables of the Experi- ments at tJie Central Farm; Ohio Agr. Exp. Sta., Circ. 120, p. 108, 1912. FARM MANURE 527 even as low as 5 or 6 tons to the acre. It is impossible to spread so small an amount by hand and obtain an even dis- tribution. Moreover, a spreader lessens the labor and more than doubles the amount of manure one man can apply a day. When any considerable quantity of manure is to be handled, a manure-spreader will pay for itself in a season or two at the most. Whether manure should be plowed under or not depends largely on the crop on Avhich it is used. On timothy it is spread as a top dressing. Ordinarily, however, it is plowed under. This is particularly necessary if the manure is long, coarse, and not well-rotted. It should not be turned under so deep, however, as to prevent ready decay. If manure is fine and well decomposed, it may be harrowed into the surface soil. The method employed depends on the crop, the soil, and the condition of the manure. The amount to be applied va- ries considerably. Eight tons to the acre would be a light dressing, 15 tons a medium dressing, and 25 tons heavy for an ordinary soil. In trucking land, however, as high as 50 or 100 tons are often used. 303. Reinforcement of manure. — The reinforcement of farm manure is designed to accomplish two things in the han- dling of this product: (1) cheeking loss due to leaching and decomposition, and (2) balancing the manure and rendering its agricultural value higher. Four chemicals may be used in this reinforcement: gypsum (CaSO^), kainit (mostly KoSOJ, acid phosphate (CaH.CPOJ, -f CaSOJ, and floats (raw rock phosphate, Ca3(P04)2). Gypsum and kainit are supposed to react with the ammonia of the manure, changing it to ammonium sulfate, a stable compound. As gypsum is rather insoluble, its action is prob- ably slow. It may be applied either in the stable or on the manure pile, usually at the rate of 100 pounds to the ton. It has no balancing effect. Kainit is soluble and because of its caustic tendencies should not come into contact with the 528 NATURE AND PROPERTIES OF SOILS feet of the animals. It must not be spread on the manure until the stock are out of the way. Since manure is unbal- anced as to phosphorus, the agricultural value of kainit is slight. When applied, it is generally used at the rate of 50 pounds to the ton of manure. Acid phosphate is partially soluble and will not only react readily with the ammonia but will tend to raise the phos- phorus content to the proper point. From 40 to 80 pounds of acid phosphate are generally recommended to a ton of average farm manure. It should not be allowed to come into contact with the feet of farm animals. Raw rock phosphate, or floats, is a very insoluble compound, and consequently reacts but slowly with the soluble constitu- ents of manure. Carrying such a large percentage of phos- phorus, it tends to balance the manure and to raise its agri- cultural value. It is supposed that the intimate relationship between the phosphate and the decaying manure increases the availability of the former to plants when the mixture is added to the soil. The reinforcement is usually at the rate of 75 to 100 pounds to a ton of manure. Experimental data have shown that these various rein- forcements have no particular effect on the nature, function, and number of the bacterial flora. Their conserving influ- ence, if any, when the manure is exposed, might be in check- ing leaching and in preventing loss of ammonia. The follow- ing figures from Ohio experiments ^ show how slight this con- serving effect is. The reinforcement was at the rate of 40 pounds to the ton. (See Table CXIX, page 529.) It is immediately evident that kainit and gypsum do not conserve the manure, and, although acid phosphate and floats show some influence, it is slight and evidently well within the experimental error. The principal benefit from reinforcing manure, if any, must, therefore, be as a balancing agent. The * Thorne, C. E., and others, The Maintenance of Fertility; Ohio Agr. Exp. Sta., Bui. 183, p. 206, 1907. FARM MANURE 529 Table CXIX conserving effect of reinforcing agents on manure exposed for three months. Treatments Ratio Values of a Ton of Manure Percentage Loss IN JANUARY IN APRIL No reinforcement 100 93 102 128 106 64 67 66 93 75 36 With gypsum 38 With kainit 35 With floats 27 With acid phosphate 29 figures from Ohio ^ over a period of fourteen years in a rota- tion of maize, wheat, and hay may be taken as evidence re- garding this point. The manure treated and handled as above was added to the maize at the rate of 8 tons to the acre. It is evident that the principal benefit of reinforcing ma- nure lies in the balancing influence and that acid phosphate and floats are the most desirable agents. It is also evident Table CXX influence of reinforcing on the effectiveness of manure. A.VERAGE Annual Increase to the Acre Ratio Value OF Increase Treatment Corn Wheat Hay Per Ton of 1.4 Crops 14 Crops 11 Crops Manure No reinforcement. . 18.6 bus. 9.5 bus. 801 lbs. 100 With g^-^^sum ...... 23.6 bus. 11.6 bus. 916 lbs. 119 With kainit 2.3.7 bus. 11.3 bus. 1156 lbs. 115 With floats 25.0 bus. 12.9 bus. 1578 lbs. 138 With acid phosphate 30.6 bus. 15.1 bus. 1853 lbs. 161 ^ Thorne, C. E., and others, Plans and Summary Tables of the Experi- ments at the Central Farm; Ohio Agr. Exp. Sta., Circ. 120, p. 112, 1912. 530 NATURE AND PROPERTIES OF SOILS that floats, if added in money values equal to acid phosphate, should be about as satisfactory as a reinforcing material. 304. Lime and manure. — Very often it would be a sav- ing of labor to apply lime and manure to the soil at the same time. This can readily be done with the carbonated forms. Such lime may be mixed with the manure, either in the stable or in the pile, without any danger of detrimental results. The close union of the lime and manure may increase the effective- ness of the former and at the same time promote a better type of decomposition in the latter. If the soil is really in need of calcium, however, a separate application of lime is much bet- ter, as the amount of calcium added with the manure is never large. Caustic compounds of lime such as calcium oxide (CaO) and calcium hydroxide (Ca(0H)2) must be kept from manure. These forms readily react with the ammonium car- bonate coming from the urea, and cause the Tberation of ammonia, which may be readily lost to the air: CON,H, + 2H2O = (NHJXOg (NHJ2CO3 + Ca(0H)2 = CaCOs + 2NH,0H A stable or shed containing manure may be at once deodor- ized by the use of quicklime, but with the loss of much nitro- gen. If the manure is to be worked into the surface soil, the caustic lime may be applied some days before and if it is in thorough contact with the soil, it will change to the carbonate before the manure is added. When the manure is plowed under, the lime is best added after the plowing and thor- oughly harrowed in as the seed-bed is prepared. 305. Manure and composting. — A compost is usually made up of alternate layers of manure and some vegetable matter that is to be decayed. Layers of sod or of soil high in organic matter are often introduced. The manure supplies the decay organisms and starts biological activities. The foundation of such a compost is usually soil, and the pile is preferably capped with earth. The mass should be kept FARM MANURE 531 moist in order to prevent loss of ammonia and to encourage vigorous bacterial action. Acid phosphate or raw rock phos- phate and a potash fertilizer are often added, to balance up the mixture and make it a more effective fertilizer. Lime is also introduced, to react with such organic acids as may tend to interfere with proper decay. Undecayed plant tissue, such as sod, leaves, weeds, grass, sticks, or organic refuse of any kind, may thus be changed slowly to a form which will be valuable in building up the soil and in nourishing plants. Even garbage may be disposed of in such a manner. 306. Residual effects of manure. — No other fertilizer exerts such a marked residual effect as does farm manure. As it is applied in large amounts, its physical and biological influences are of necessity very great and persist for a con- siderable time. As only about one-half the nutrients of farm manure are readily available, the residual effect of its fertiliz- ing elements carry over into succeeding years. Hall ^ pre- sents the following comparative data regarding the recovery of nitrogen from various fertilizers. The crop used was man- golds. The low reco^'ery of the nitrogen from the manure is of especial note. There is no reason to believe that the pot- ash of the manure would be any more readily available and the phosphoric acid would certainly show a lower recovery. Table CXXI recovery of nitrogen in a crop of mangolds. Sodium nitrate. . Ammonium salts. Rape cake Farm manure. . . Rate to THE Acre 550 lbs. 400 lbs. 2000 lbs. 14 tons Yield in Tons 17.95 15.12 20.95 17.44 Percentage Eecovery of Nitrogen 78.1 57.3 70.9 31.6 ^Hall, A. D., Fertilisers and Manures, p. 210; New York, 1921. 532 NATURE AND PROPERTIES OF SOILS The length of time through which the effects of an appli- cation of farm manure may be detected in crop growth is very great. Hall ^ cites data from the Rothamsted Experi- ments in which the effects of eight yearly applications of 14 tons each were apparent forty years after the last treatment. This is an extreme case. Ordinarily, profitable increases may be obtained from manure only from two to five years after the treatment.- The fact remains, nevertheless, that of all fertilizers, farm manure is the most lasting and lends the most stability to the soil. 307. The place of manure in the rotation.'' — With trucking, garden, and greenhouse crops, the applications of large amounts of manure year after year have proven advis- able. As a matter of fact, manure has shown itself, especially if balanced with phosphoric acid, to be the best fertilizer for intensive operations. This is due not only to the nutrients carried by the manure, but to the large amounts of easily decomposed organic matter that are at the same time intro- duced. In a rotation involving the staple crops, such as maize, oats, wheat, hay, and the like, less intensive applications are advisable, not only because of a lack of manure but because the return to a ton of manure applied must be raised as high as possible. On the average farm, there is less than one ton of manure produced to an acre of arable land. Moreover, the return from manure will vary according to its place in the rotation. This has proved to be the case with commercial fertilizers and the fact is becoming more and more apparent with farm manure. In general, meadows and pastures derive more benefit from manure, either residually or directly, than any other crop. ^ Hall, A. D., Fertilisers and Manures, p. 213; New York, 1921. ='Voelcker, A., and Hall, A. D., The Valuation of Unexhausted Manure Obtained by the Consumption of Foods hy Steele; London, 1903. "See Thorne, C. E., Farm Manures, Chaps. XI and XIII, New York, 1914. FARM MANURE 533 The long tests conducted by the Pennsylvania and Ohio ex- periment stations ^ have established this fact. The following data from Illinois - may be cited, comparing the response of maize and oats when manured to the increased yield of clover receiving the same treatment. (See Table CXXII, page 534.) CROP K2O - FOOD LOSSES - 50% NHj- 25 " 26 •• 10 MANURIAL LOSSES OM -50% NHs -60 )- P^Os -40 » KzO -65" .(^W '^^^^^^^^^^^^^'^^^mm^^m^Mm^^?;^^^ PERCENTAGE OF THE CONS- TITUENTS OF CROP ADDED TO SOIL OM - 25% N LEACHING Fig. 63. — Diagram showing the proportion of the harvested crop added to the soil in farm manure under average conditions. It is easy to see that a liberal dressing of manure on the hay and pasture will markedly increase the crop. Neverthe- less, as manure is available in limited amounts on the average farm and as commercial fertilizers will give almost as good returns on hay, it is generally considered judicious, except in * Hunt, T. F., General Fertilizer Experiments ; Ann. Eep. Penn. Agr. Exp. Sta., 1907-1908, pp. 68-93. Thome, C. E., and others, Plaiis and Summary Tables of the Experi- ments at the Central Farm; Ohio Agr. Exp. Sta., Circ. 120, np. 101- 105, 1912. " Hopkins, C. G., Thirty Years of Crop Rotation in Illinois; 111. Agr, Exp. Sta., Bui. 125, p. 337, 1908. 534 NATURE AND PROPERTIES OF SOILS Table CXXII influence of manure on maize, oats, and clover. Treatment Average Percentage Increase Eatio Value of Increase Maize and Oats Clover Maize and Oats Clover Manure alone. . Manure, lime and phosphate 11 30 92 141 100 162 134 206 certain cases, to reserve most of the manure for other crops. The top dressing of meadows is, however, always an allowable practice, especially on new seeding or on hay land that is soon to be plowed for maize. As a food producer, maize has no close rival. Where the climate is favorable, a 75-bushel crop of maize is as easily secured as 40 bushels of wheat or 300 bushels of potatoes to the acre. Moreover, the maize stover may be made more valu- able as roughage than the straw of oats, wheat, or rye. The maize plant must have, however, for its successful growth plenty of available nitrogen. In addition, its response to abundant organic matter indicates the utilization of certain organic compounds. These considerations argue for the use of most of the farm manure on the maize when this crop is important, especially if the supply of manure is limited. Again the maize crop is ready for the manure in the spring and is generally grown on land where the excreta may be distributed during the previous winter and fall. Potatoes are a spring crop and where they are prominent in the rotation may receive liberal applications of manure. If potatoes are the money crop, this should by all means be the practice. Oats, because of the tendency to lodge, gener- ally follow maize or potatoes as a residual feeder, receiving, if necessary, a dressing of commercial fertilizer. If manure is FARM MANURE 535 used on fall wheat, a great loss of manurial value is incurred, due to the necessity of storage during the summer months. Moreover, commercial fertilizers liig-li in phosphorus are so convenient and effective on wheat that the use of manure on this crop is becoming rather uncommon, although manure may be used to advantage as a fall and winter dressing, since it not only stimulates the wheat but is of great value to the new seeding as well. Where cotton and tobacco are the staple crops, they should receive at least a part of the manure pro- duced. The value of manure in orchards should not be over- looked, especially on sandy soils. The up-keep of organic matter, the conservation of moisture, and the nutrients sup- plied are as important here as in any phase of soil manage- ment. 308. Resume. — Barnyard manure, from the standpoint of soil fertility, is the most valuable by-product of the farm. A careful farmer will, therefore, attempt to utilize it in the most economical way. The handling of manure in such a manner that only a minimum waste occurs from the time the manure is voided until it has reached the land is not an easy problem. Manure is so susceptible to the loss of valuable ingredients, both by leaching and by decay, that careful methods must be employed. Tight floors in the stable and covered sheds or manure pits are always advisable. Hauling immediately to the field is the wisest procedure, yet even with the best of care more than 50 per cent, of the fertilizing value is usually lost. The problem of rational manurial utilization is not solved, however, by careful handling and storage alone. Manure must be applied in such a condition, in such amounts and at such a point in the rotation as to realize a reasonable return for every increment applied. The reinforcement of farm manure with phosphoric acid is by no means an unim- portant feature. In fact, all of the principles which are ob- served in the profitable utilization of commercial fertilizers should be adhered to in the use of farm manures. 536 NATURE AND PROPERTIES OF SOILS A permanent system of agriculture evidently cannot be established by merely returning all the manure possible to the land, as approximately only 25 per cent, of the organic matter, 30 per cent, of the ammonia, 50 per cent, of the phos- phoric acid, and 30 per cent, of the potash of the food con- sumed on the farm ever reach the land in the manure. Never- theless, it is certainly worth the while of a farmer to use some care in handling this product and some thought as to its rational utilization in the field. Even if the manure should aid only in the up-keep of organic matter, the effort would be worth while. Reasonable care in the handling of farm manure will save this country thousands of pounds of manurial fertility which are now utterly lost and at the same time increase by thousands of dollars the food production. CHAPTER XXV GREEN-MANURES ^ From time immemorial the turning-iinder of a green-erop to supply organic matter to the soil has been a common agri- cultural practice. Records show that the use of beans, vetches, and lupines for such a purpose was well understood by the Romans, who probably borrowed the practice from nations of greater originality. The art was lost to a great extent dur- ing the Middle Ages, but was revived again as the modern era was approached. At the present time, green-manuring is considered a part of a well-established system of soil man- agement, and is given a place, when possible, in every ra- tional plan for permanent soil improvement. 309. Importance of green-manures. — The plowing under of some succulent rapid-growing crop, such as oats, rye, or clover, tends to bring about three desirable soil conditions; additional organic matter, a betterment of the physical con- dition of the soil, and a rise in the nitrogen content of the land, if the crop is an inoculated legume. If conditions are ^ Penny, C. L., Clover Crops as Green Manures; Del. Agr. Exp. Sta., Bui. 60, 1903. Storer, F. H., Agriculture, pp. 137-175; New York, 1910. Lipman, J. G., Bacteria in Relation to Country Life, Chapter XXIV, pp. 237-263; New York, 1911. Piper, C. v., Leguminous Crops for Green Manuring ; U. S. Dept. Agr., Farmers' Bui. 278, 1907. Spillnian, W. J., Renovation of Worn-out Soils; U. S. Dept. Agr., Farmers ' Bui. 245, 1906. Pieters, A. J., Green Manuring: A Eevieic of the Americayi Experi- ment Statio)i Literature ; Jour. Amer. Soe. Agron., Vol. 9, No. 2, pp. 62-82, Feb. 1917; Vol. 9, No. 3, pp. 109-126, Mar. 1917; Vol. 9, No. 4, pp. 162-190, Apr. 1917. 537 538 NATURE AND PROPERTIES OF SOILS favorable, an increase in crop production slionld result. Where there is a shortage of farm maiiurt", the practice be- comes of special importance since roots and crop residues are usually insufficient to maintain the organic content of the soil. Even where manure is available, a green-manuring crop now and then in the rotation does much towards sus- taining normal production. The effects of turning under green plants are both direct and indirect — direct as to the influence on the succeeding crop, and indirect as to the soil so treated. In the first place, cer- tain ingredients are actually added to the soil by such a procedure. The carbon, oxygen, and hydrogen of plants come largely from the air and water, and the plowing-under of a crop, therefore, increases the store of such constituents in the soil. The compounds that result from crop decay increase the absorptive power of the soil, and promote aeration, drain- age, and granulation — conditions that are extremely impor- tant in successful plant growth. If the crop turned under is a legume and the nodule organisms are active, the store of soil nitrogen is markedly augmented, a point of extreme impor- tance in fertilizer practice. Green-manures may function also as cover-crops, insofar as they take up the extremely soluble plant nutrients and pre- vent them from being lost in the drainage water. The nitrates of the soil are of particular importance in this regard as they are very soluble and are absorbed only slightly by the soil complexes. Besides this, green-manures, especially those with long roots, tend to carry nutrients upward from the subsoil and when the crop is turned under this material is deposited within the root zone. Again, the added organic material acts as a food for soil organisms, and tends to stimulate biological changes to a marked degree. This biological action is espe- cially important in the production of carbon dioxide, am- monia, nitrates, and organic compounds of various kinds, which are necessary in plant nutrition. GREEN-MANURES 539 310. Gain of constituents by green-manuring'. — In an average crop of g:reon-mamire, from five to ten tons of mate- rial are turned under. Of this, from one to two tons are dry matter, and from four to eight tons water. Of this dry matter, a great proportion is carbon, hydrogen, and oxygen. It might seem at first thought that such an addition is pure gain as far as carbon and carbonaceous matter are concerned. Such is not the case. Large amounts of carbon are lost continu- ously in drainage, to say nothing of that removed by crops or that which is respired by the soil as carbon dioxide. It has already been shown, from results obtained with the Cornell lysimeters, that a heavy soil will yearly lose over 250 pounds of carbon, in drainage alone (see par. 220). This is approxi- mately equivalent to a 2-ton application of green-manure. Although the loss of carbonaceous material is considerable, even during the period that the green-manuring crop is being grown, nevertheless the practice offers a rapid as well as a natural means of increasing the soil organic matter. The mineral parts of the turned-under crop came from the soil originally and they are merely turned back to it again and represent no gain. As they return, however, they are in intimate union with organic materials, and are thus readily available as the decay processes go on. Indeed they are prob- ably more readily available than they previously were, when the green-manuring crop acquired them. The amount of nitrogen added to a soil if the green-manure is a legume ^ is an uncertain quantity. Much depends on the virulence of the organisms occupying the nodules. These bac- * Smith, C. D., and Robinson, F. W., Influence of Nodules on the Soots upon the Composition of Soybean and Cowpea; Mich. Agr. Exp. Sta., Bui. 224, 1905. Hopkins, C. G., Alfalfa on Illinois Soil; Til. Agr. Exp. Sta., Bill. 76, 1902. Hopkins, C. G., Nitrogen Bacteria and Legumes; 111. Agr. Exp. Sta., Bui. 94, 1904. Shutt, F. T., The Nitrogen Enrichment of Soils through the Growth of Legumes; Canadian Dept. Agr., Rept. Centr. Exp. Farms, 1905, pp. 127-132. 540 NATURE AND PROPERTIES OF SOILS teria are in turn much influenced by plant and soil conditions, such as amount of organic matter, presence of nitrates, acidity and the like. Hopkins ^ estimates that about one-third of the nitrogen in a normal innoculated legume comes from the soil and two-thirds from the air. He also considers that one-third of the nitrogen exists in the roots. Both of these assumptions are questionable and at best tentative. The amount of nitrogen fixed by legume organisms is extremely variable, probably more so than that assimilated by the azotobacter and allied groups. Again the percentage of the nitrogen held in the roots of legumes is by no means the same for all species. The amount varies within the species with age, degree of maturity and, season. The Delaware in- vestigations - show that the proportion of the total nitrogen of the plant occurring in the roots may be as low as 6 per cent, in case of cowpeas and as high in the roots of alfalfa as 42 per cent. A range from 6 to 28 per cent, of the total nitrogen of crimson clover was noted in the roots under different condi- tions. According to Hopkins, the nitrogen found in the tops of legumes will be a rough measure of the nitrogen fixed by the nodule organisms. When the crop is turned under, this will represent the gain to the soil. If the preceding assumption is correct, red clover turned under would actually add about 50 pounds of nitrogen for every ton of air-dry substance util- ized, alfalfa about 50, cowpeas 43, and soybeans 53 pounds. These figures, even though they may be far from correct, at least give some idea of the possible addition of nitrogen by green-manuring practices, and show how the soil may be en- riched by such management. As in the case of farm manures, the indirect effects of such a procedure on the physical and bacteriological properties of the soil may over-ride the direct ^ Hopkins, C. G., Soil Fertility and Permanent Agriculture, p. 223 ; Boston, 1910. ^ Penny, C. L., The Growth of Crimson Clover; Del. Agr. Exp. Sta., Bui. 67, 1905. GREEN-MANURES 541 influences, lessening the advantage that legumes as green- manures are supposed to have over non-legumes, due to their ability to use atmospheric nitrogen. 311. Green-manures as cover-crops. — When green-ma- nures are seeded in the late summer or early fall, they func- tion as cover-crops and may have rather important influences aside from their effects when turned under. Their greatest influence seems to be on the nitrate content of the soil. Nitri- fication is usually checked,^ a disappearance of nitrates gen- erally following. This reduction in the amount of nitrates probably occurs because of a retardation of nitrification ac- companied by a stimulation of biological utilization of the nitrates. Such an effect is important in conserving the soil nitrogen and is of particular value in orchards,- as it hastens the maturity of the new growth. At Cornell University, green-manures were seeded in July and plowed under in the following spring. Nitrate determinations were made on the soil in July and in October. The figures are five-year aver- ages. (See Table CXXIII, page 542.) 312. The decay of green-manure. — When a green-crop is turned under, the process of its decay is the same as that of any plant tissue that becomes a part of the soil body. The organisms that are active are those common to the soil, to- gether with such bacteria as are carried into the soil on the turned-under crop. The decomposition that results is prob- ably both aerobic and anaerobic in nature, carbon dioxide be- ing given off continuously. When proper decay has occurred, end products should result which can be utilized as nutrients, 1 Wright, E. C, The Influence of Certain Organic MateriaU upon the Transformation of Soil Nitrogen; Amer. Soc. Agron., Vol. 7, pp. 193- 208, 1915. Martin, T. L., The Decomposition of Green Manures at Different Stages of Growth; Thesis for degree of Doctor of Philosophy, Cornell University, 1919. ^ Lyon, T. L., The Formation of Nitrates in Soil Under Grass; Proc. West. N. Y. Hort. Soc, pp. 82-87, Jan., 1915. Lyon, T. L., Eelation of Certain. Cover Crops to the Formation of Nitrates in Soil; Proc West. N. Y. Hort. Soc, pp. 32-34, Jan., 1917. 542 NATURE AND PROPERTIES OF SOILS Table CXXIII effect of various crops on the nitrate nitrogen of the soil during october, 1916-1920.^ Green-Manuring Crop Rye Oats Vetch Peas Rye and vetch Rye and peas. Sod Nitrates in the Soil in October. Eye Taken as 100 Percentage Ee- duction of Nitrates in October Compared WITH July 37 44 57 10 58 58 The intermediate compounds that are formed should yield an organic matter carrying a black pigment, should readily split up into simple compounds, and should be in general beneficial, both directly and indirectly, to plant growth. Plenty of moisture is essential when green-manures are de- caying, not only to hasten the transformation itself but that the normal soil processes may not be interrupted by a lack of water. The caution with which green-manures must be utilized in semi-arid regions arises because of the drying influ- ences of rapid decay and the danger of filling the soil with undecomposed plant residues. Even in humid regions, green- manures may be detrimental if dry weather sets in before a major portion of the decay processes is completed. As plant tissue decays in the soil, there seem to be two general groups of forces at work which produce three distinct stages of organic destruction.^ In the first stage, humus pro- ^ Unpublished data. Dept. Soils. Cornell University. - Martin, T. L., The Decomposition of Green-Manures at Different Stages of Growth; Thesis for degree of Doctor of Philosophy, Cornell University, 1919. GREEN-MANURES 543 duction is dominant and the amount of the humous materials increases. In the second stage, humus production and humus destruction are more or less balanced, while in the third stage humus destruction is in the ascendant. The amount of humus is on the decrease in the latter stage. The length of these stages will vary with the season, with soil conditions,' and with the character of the crop turned under. Obviously, the influence of decomposing green-manure on the chemical and biological activities of the soil will vary as the decay cycle progresses. In general, over one-half of the organic matter of the average green-manure disappears during the first nine months after application. 313. Influence of decaying green-manure. — In the first stage of decay, which should be a rapid one, many complex compounds are generated along with carbon dioxide and other simple products. The complex materials, which result partly from protein decomposition and partly from the breaking down of easily attacked carbohydrates, may be harmful to ordinary crops. Germinating seeds and young plants are especially susceptible, and detrimental influences are some- times noticed immediately after the turning under of a green- manure. Fred ^ found that the germination of oily seeds, such as cotton and soybean, was much reduced. Starchy seeds, such as maize, oats, and wheat, were little affected. The germination of flax, hemp, mustard, and clover was some- what reduced. An actual contact of the seed with the de- caying material was usually necessary for serious damage. The detrimental influence always occurred during the first two or three weeks after the green-crop was turned under. Obviously the more succulent the crop, the shorter will this period be. ^Eussell, E. J., and Appleyard, A., The Influence of Soil Conditions on the Decomposition of Organic Matter in the Soil; Jour. Agr. Sci,, Vol. VIII, Part 3, pp. 385-417, 1917. *Fred, E. B., Eelation of Green Manure to the Failure of Certain Seedlings; Jour. Agr. Ees., Vol. V, No. 25, pp. 1161-1176, Mar., 1916. 544 NATURE AND PROPERTIES OF SOILS Not only do the products of the first stage of decay influ- ence the crop growing on the soil, but they affect the biological activities as well/ Nitrification in particular seems to be in- fluenced, as nitrates do not begin to appear until the process of humification is well advanced. Nitrification, however, is probably not entirely suppressed as it is possible for soil or- ganisms to use up the nitrates as rapidly as they are formed. If 3., IK ^N / III ^*- M z o CD tz u TIME AFTER APPLICATION Fig. 64. — Diagram illustrating the three stages in the decay of a green-manure. I, humus production dominant; II, a balance be- tween humus production and destruction; III, humus destruction dominant. A depression in nitrate accumulation generally occurs in stage I followed by an increase. (After Martin.) As the humus destruction gradually dominates over humus production, the end products of the decay become prominent. The complex proteid decomposition is practically completed and cellulose destruction is slowly progressing. Of the sim- ple nutritive products, the nitrates are of particular impor- tance. In fact, they have been chosen by a number of in- ^ Briscoe, C. F., and Harned, H. H., Bacterial Effects of Green Manures; Miss. Agr. Exp. Sta., Bui. 168, Jan. 1915. Hutchinson, H. B., The Influence of Plant Residues on Nitrification and on Losses of Nitrates in Soil; Jour. Agr. Sci., Vol. IX, Part 1, pp. 92-111, Aug. 1918. GREEN-MANURES 545 vestigators ^ as a measure of humification, since a favorable environment for nitrification probably does not occur until the more rapid decomposition processes are completed. In general, the more rapid the decay of the green-manure, the sooner will nitrification be active again. Besides affecting the bacterial activity of the soil, the de- caying green-crop influences the solubility of the soil min- erals. Jensen - found that the addition of 3 per cent, of green-manure raised the solubility of lime and phosphoric acid 30 to 100 per cent. This was over and above the mineral constituents which came directly from the decomposing green- crop. Magnesium and iron were also markedly influenced. 314. Crops suitable for green-manures. — An ideal green- manuring crop should possess three characteristics: rapid gro"«i;h, abundant and succulent tops, and the ability to grow well on poor soils. The more rapid the growth, the greater the chance of economically using such a crop as a means of soil improvement. The higher the moisture content of the crop, the more rapid the decay and the more quickly are bene- fits obtained. As the need of organic matter is especially urgent on poor land, a hardy crop has great advantages. The crops that may be utilized as green-manures are usually ^Hutchinson, C. M., and Milligan, S., Green-Manuring Experiments, 1912 and 1913. India Agr. Kes. Inst. Bui. 40, Pusa, India, 1914. Mavnard, L. A., The Decomposition of Siceet Clover as a Green- Manure under Greenhouse Conditions ; Cornell Agr. Exp. Sta., Bui. No. 394, 1917. Martin, T. L., The Decomposition of Green-Manures at Different Stages of Growth; Thesis for degree of Doctor of Philosophy, Cornell University, 1919. ^ Jensen, C. A., Effect of Decomposing Organic Matter on, the Solu- bility of Certain Inorganic Constituents of the Soil; .Jour. Agr. Res., Vol. IX, No. 8, pp. 253-268, May 1917. See also, Snyder, H., Humus as a Factor in Soil Fertility; Minn. Agr. Exp. Sta., Bui. 41, 1895; and Production of Humus from Manures; Minn. Agr. Exp. Sta., Bui. 53, 1897. Hopkins, C. G., and Aunier, J. P., Potassium from the Soil; 111. Agr. Exp. Sta., Bui. 182, 1915. Hopkins, C. G., and Whiting, A. L., Soil Bacteriology and Phosphates; 111. Agr. Exp. Sta., Bui. 190, 1916. 546 NATURE AND PROPERTIES OF SOILS grouped under two heads, legumes and non-legumes. Some of the common green-manures are as follows: LEGUMES NON-LEGUMES Annual Biennial Cowpea ' Red clover Rye Soybean White clover Oats Peanut Alsike clover Mustard Vetch Alfalfa Mangels Canada field pea Sweet clover Rape Velvet bean Buckwheat Crimson clover Hairy vetch When other conditions are equal, it is of course always bet- ter to choose a leguminous green-manure in preference to a non-leguminous one, because of the nitrogen that may be added to the soil. However, it is so often difficult to obtain a catch of some of the legumes that it is poor management to turn the stand under until after a number of years. Again, the seed of many legumes is very expensive, almost prohibit- ing their use as green-manures. Among the legumes most commonly grown as green-manures, cowpeas, soybeans, and peanuts may be named. Many of the other legumes do not so fit into the common rotations as to be turned under conven- iently as a green-manure. For the reasons already cited, the non-legumes have, in many cases, proved the more popular and economic as green- manures. Rye and oats are much used because of their rapid, abundant, and succulent growth and because they may be accommodated to almost any rotation. They are hardy and will start in almost any kind of a seed-bed. They are thus extremely valuable on poor soils. Often the value of such a green-manure as oats is greatly increased by sowing peas with it. The advantages of a legume and a non-legume are thus combined. It has already been shown that the nitrate production in a GREEN-MANURES 547 soil may be used as a rough measure of the rate of decay of green-manures. Admitting such a criterion, certain data from Cornell University become particularly interesting. In a five-year continuous test, green-manuring crops were seeded in July and plowed under in the early part of the succeeding Ma.y. The nitrate content of the soil was determined at a number of times during the spring, summer, and fall. A de- crease in nitrates always occurred in the autumn, while an increase began soon after the crops were turned under in the spring. In the following table the rye crop is taken as 100 in both October and July : Table CXXIV relative influence of green-manltres on the accumulation of soil nitrates.^ Green-Manuke Nitrates in July, Soil Fallow Since May 1. Rye Taken as 100 Nitrates in Oct., Soil Under Crop Since July. Rye Taken as 100 Rye Oats Vet.'h Peas Rye and vetch Rye and peas. 100 73 73 83 74 75 It is immediately apparent that the succulent rye and vetch that survive the winter give better results, as far as nitrate production is concerned, than the dry and dead oats and peas. This shows clearly the value of succulence and the necessity of turning under a crop partially matured.^ The advantage of the legumes over the non-legumes is not hard to explain. * Unpublished data. Dept. Soils, Cornell University. ^ Martin, T. L., The Decompvsition of Green Manures at Different Stages of Growth; Thesis for the Degree of Doctor of Philosophy, Cornell University, 1919. 548 NATURE AND PROPERTIES OF SOILS The combination of rye and vetch, both of course in a succu- lent condition, seems especially efficacious. Sod as a green- manure always appears more or less at a disadvantage. 315. The use of green-manures. — The indiscriminate use of green-manures is of course never to be advised, as the soil may be injured thereby and the normal rotation much interfered with. When soils are poor in nitrogen and organic matter, they are very often in poor tilth. This is true whether the texture of the soil be fine or coarse. The turning-under of green-crops must be judicious, however, in order that the soil may not be clogged with undecayed matter. Once or twice in a I'otation is usually enough for such treatments. Proper drainage must always be provided. In regions where the rain- fall is scanty, great caution must be observed in the handling of green-manures. The available moisture that should go to the succeeding crop may be used in the process of decay, and the soil left light and open, due to an excess of undecomposed plant tissue. In western United States, it is still a question whether green-manures have any advantage over summer fallowing. It is generally best to turn under green-crops when their succulence is near the maximum and yet at a time when abundant tops have been produced. This occurs at about the half mature stage. A large quantity of water is carried into the soil when the crop is at this stage, and the draft on the original soil-moisture is less. Again, the succulence encour- ages a rapid and more or less complete decay, with the maxi- mum production of humus and other products. The plowing should be done, if possible, at a season when a plentiful supply of rain occurs. The effectiveness of the manuring is thereby much enhanced. At Cornell University various green-manures were seeded in the summer and plowed under that fall or the next spring. The experiment was continuous for three years, the nitrates being determined in the soil each year in April and in June. The results are as given on the next page. GREEN-MANURES 549 Table CXXV influence of the time of turning-undeb of green-manures on the nitrate accumulation in the soil.^ Crop Rye, fall plowed Rye, spring" plowed .... Oats, fall plowed Oats, spring plowed . . . , Vetch, fall plowed Vetch, spring plowed. . Average, fall plowed. . . Average, spring plowed Paets Per Million of Nitrates In April Just Before the Spring Plowing In June, Soil Fallowed Since Plowing 58 53 57 67 61 36 42 50 79 41 45 67 66 43 48 61 It is apparent that the decay of the green-manuring crop is hastened by fall plowing, as the nitrates in every case are higher in April on land so handled. In June, however, the nitrate accumulation has passed its highest point in the fall- plowed soil, leaving the spring-plowed plats, where the decay was initiated later, in the ascendancy. The table also shows the advantage that a legume has over a non-legume in causing nitrate accumulation. Oats fall-plowed appear about on an equality with rye. Spring plowing, since the oats are then dry and dead, gives the rye a marked advantage. All of the points above noted have a very practical field application. In turning under green-manures, the furrow slice should not be thrown over flat, since the green-crop is then deposited as a continuous layer between the surface soil and the sub- soil. Capillary movement is thus impeded until a more or * Unpublished data, Dept. Soils, Cornell University. 550 NATURE AND PROPERTIES OF SOILS less complete delay has occurred, and the succeeding crop may suffer from lack of moisture. The furrow ordinarily should be turned only partly over, and thrown against and on its neighbor. The green-manure is then distributed evenly from the surface downward to the bottom of the furrow. When decomposition occurs, the resulting materials are evenly mixed with the whole furrow slice. Moreover, this method of plowing does not interfere with the capillary movements of water, and in actual practice is a great aid in drainage and aeration. 316. Green-manure and lime. — The decay of organic matter in the soil is always accompanied by the production of organic acids of various kinds. The greater the succulence of the material, the more rapid is the accumulation of such products. In spite of this, however, the effect of a green- manure is to decrease the acidity rather than increase ^ it and later greatly to stimulate nitrification even if the soil origi- nally was quite acid. The decrease in lime requirement may be due to the liberation of mineral constituents from the de- caying organic matter and to the effect of the decomposition on the inorganic constituents of the soil. The ultimate influence of green-manure on acidity is some- what in doubt. The bulk of the evidence available seems to indicate that decaying organic matter, if it has any effect, ulti- mately tends to decrease rather than increase the lime re- quirement of the soil.^ Nevertheless, plenty of active calcium should be in the soil, since it promotes the decay of the plant tissue added and seems to control to a certain extent the pres- ence of toxic materials. Lime may be added to the green- manure seeding and be turned under with that crop. The ^ White, J. W., Soil Acidity as Influenced by Green Manures; Jour. Agr. Res., Vol. XIII, No. 3, pp. 171-197, April, 1918. " Hill, H. H., A Comparison of Methods for Determining Soil Acidity and a Study of the Effects of Green Manures on Soil Acidity; Va. Poly. Inst., Tech. Bui. 19, April 1919. Ames, J. W., and Schollenberger, C. J., Liming and Lime Eequire- ment of Soils; Ohio Agr. Exp. Sta., Bui. 306, pp. 381-383, Dec. 1916.' GREEN-MANURES 551 amendment would thus be in very close contact with the de- caying vegetable tissue. Ordinarily, however, the application of lime at some point in the rotation is sufficient. Lime, besides its capacity to alleviate toxic residues, tends to hasten organic deeay.^ This is a very important function as the first stage of decomposition, during which soil and plant activities may under certain conditions be detrimentally af- fected, is markedly shortened. Such a promotion is indicated in a green-manuring experiment at Cornell University. The green-manures were seeded in the fall under two treatments, limed and unlimed. The parts per million of nitrates in the soil are given for two dates on the year succeeding, the green- manures having been plowed under either in the fall or early spring. The data are average^ of three years. Table CXXVI influence of lime on the nitrate accumulation in a soil receiving various green-manures." Parts Per Million of Nitrates April June Rye, no lime 66 45 53 45 77 43 65 44 53 Rye, limed 71 Oats, no lime Oats, limed Vetch, no lime 43 50 52 Vetch, limed 63 Average, no lime 49 Average, limed 61 ^ Lemmermann, O., et al., Untersucliunr/ iiher die zerzetzung der Kohlen- stoff Verbindungen Verscheidener Organischen Substanzen im Boden SpezieUe unter dem einfluss der Kalk ; Landw. Jahrb., Bd. 41, S. 216- 257, 1911. ^ Unpublished data. Dept. Soils, Cornell University. 552 NATURE AND PROPERTIES OF SOILS The effect of lime on nitrification is very noticeable in June. In April the no-lime plats are higher in accumulated nitrates, due to the lesser growth of the green-manuring crop. 317. Practical utilization of green-maimres. — Green- manures seem to have their greatest value where a permanent instead of a rotation pasture is used, where a long cycle rota- tion of grain is practiced, or where little or no manure is available. The experimental data bearing on the use of green- manures seems to indicate that such a practice is productive of larger crop yields. The following data from Nappan, Nova Scotia, is from one of the more reliable and conclusive experi- ments. A catch-crop of clover in the grain was turned under for grain the following year. The figures are for 1905, the third year of the test. Table CXXVII yield of wheat, oats and barley in bushels to the acre on the nappan farm in 1905 on plats cropped continuously to grain.^ Treatment Wheat Oats Barley No green-manure Clover catch-crop 34.3 40.0 41.2 55.3 32.7 37.9 The use of a green-manure is often determined by the char- acter of the rotation. Very often it is somewhat of a problem as to when, in an ordinary rotation, a green-manure may be introduced so that it may fit in well with the crops. In a rotation of maize or potatoes, oats, wheat, and two years of hay, a green-manure might be introduced after the corn or potatoes. This would not be a very good practice, however, as a cultivated crop usually should follow a green-manure in order to facilitate decomposition and decay. In such a rota- tion, the plowing-under of the hay stubble is really a form ^ Ottawa Exp. Farms Kept., 1905, p. 284. GREEN-MANURES 553 of green-manuring, there being a considerable accumulation of stubble and aftermath on the soil. When a rotation of this kind is used, it is better either to supply organic matter in other ways, or to alter or break the rotation in such a man- ner as to admit of a more advantageous use of green-crops. Where trucking crops are raised and no very definite rota- tice is adhered to, green-manuring is easier. It is especially facilitated when cover-crops are grown, as in orchards. Soil- ing operations also favor the easy and profitable use of green- manures. In general, it may be said that the organic matter obtained from such a source should be supplemented by farm- yard manures where possible. A better balanced and richer soil organic matter is more likely to result. CHAPTER XXVI THE MAINTENANCE OF SOIL FERTILITY ^ The maintenance of a profitable and continuous soil pro- ductivity is an intricate problem, since many variable factors are involved. Weather conditions, moisture relations, soil or- ganic matter and tilth, plant diseases, soil reaction, and avail- able nutrients are only a few of the influences that function continuously throughout the growing season. No scheme of soil management and crop production is perfect, even though it is fairly profitable. Except in special cases, every system is open to improvement and modification as soil and plant knowledge increases. The sources of knowledge regarding the profitable growing of plants are numerous. Much data have arisen from expe- rience and observation, much are empirical, while some are confessedly conjectural. In spite of the large amount of scientific information available regarding the soil and its plant relationships, practical experience has contributed more towards a profitable and continuous soil productivity. Soil survey classification and mapping have contributed some- thing. Field tests, both practical and technical, have added to such information, while laboratory and greenhouse experi- ments, although often arbitrary and artificial, are by no means unimportant. These latter contributions, however, always need practical confirmation under typical field con- ditions over a period of years. 318. Loss of plant nutrients from the soil. — A consid- eration of the principles governing the rational management ^ Fertility is here used in the sense of continuous productivity. 554 THE MAINTENANCE OF SOIL FERTILITY 555 of a soil is obviously impossible unless some knowledge is at hand regarding: the losses and additions which a soil sustains in the course of a definite rotation. Fortunately, some fairly reliable data have already been presented regarding the re- moval of soil constituents under controlled conditions. The Cornell lysimeter tanks, bearing a rotation of maize, oats, wheat, and two years of hay, offer very satisfactory informa- tion (paragraphs 95 and 163). The losses covering a ten-year period are expressed in pounds to the acre a year. The soil is a Dunkirk silty clay loam. While such figures are probably open to considerable error and obviously would not apply with any degree of accuracy to a light soil, they indicate in a general way the magnitude and order of the losses that may be expected from such a soil under the conditions specified. Table CXXVIII losses from a dunkirk silty clay loam soil expressed in pounds to the acre a year over a ten- year period. rotation: maize, oats, wheat and two years hay. cornell lysimeter tanks. Source of Loss N P.O. K^O CaO SOs Drainage (par. 163) .... Cropping (par. 163) .... Atmosphere (pars. 220 and 233)^ 7.3 70.5 ? trace 43.5 68.7 105.4 345.9 24.3 108.5 41.0 Total 77.8 43.5 174.1 370.2 149.5 The organic carbon in this soil over the ten-year period was reduced at the rate of approximately 1 per cent, a year.- This is equivalent to a reduction in organic matter of about 1200 * The largest loss of carbon is probably to the atmosphere as carbon dioxide. The other avenue of loss is in the drainage water. " Lipman and Blair report a reduction of organic carbon of .74 per cent, a year over a period of ten years on Sassafrass loam in New 556 NATURE AND PROPERTIES OF SOILS pounds each year to the acre-four feet. It is evident, there- fore, that the losses sustained by the average soil fall most heavily on the organic constituents, a condition often ignored in practical soil management. The removal of calcium oxide is also very large, being equivalent to a loss of 661 pounds of calcium carbonate an acre a year. Although losses of sulfur trioxide and phosphoric acid are smaller than that of the potash, they are far more important, since there is very com- monly one hundred times more potash in a soil than of the other two constituents combined. The magnitude of the loss of a soil constituent is never a safe measure of its importance. The removal of nitrogen is equivalent to over 500 pounds of commercial sodium nitrate and consequently is also a loss of no small consideration. 319. Additions of nutrients to the soil. — The figures presented above are based on reliable experimental data. Un- fortunately the information regarding the additions which normally occur to a soil under any particular rotation are by no means so exact. Certain assumptions and estimates, often of questionable validity, must be admitted in order that a complete survey may be possible. Table CXXIX sets forth the additions which the Dunkirk clay loam of the Cornell lysi- meters may reasonably be expected to receive each year when cropped to a five-year rotation of maize, oats, wheat, and two years hay. The data are expressed in pounds to the acre a year. (See Table CXXIX, page 557.) The additions listed above are not the only avenues open for important acquisitions. The crops removed may be fed to animals and the manure returned to the land. Moreover, the utilization of a green-manure is also possible. Below will be found the additions that may reasonably be expected from the Jersey. The rotation was maize, oats, wheat, and two years hay. No lime was added. Lipman, J. G., and Blair, A. W., The Lime Factor in Permanent Soil Improvement. I. Rotations Without Legumes; Soil Sci., Vol. IX, No. 2, p. 87, 1921. THE MAINTENANCE OF SOIL FERTILITY 557 Table CXXIX estimated additions that might occur to a soil under a rotation of maize, oats, wheat, and two years hay. expressed in pounds to the acre a year. Source of Addition N P.O. K^O CaO SO3 Rain-water (pars. 236 and 264) Free fixation by soil organisms (par. 238) Crop-roots and residues ^ 12.5 25.0 65.0 Total 37.5 65.0 use of farm manure and a green-manure on the soil in question. The green-manure is leguminous and is applied once during the five-year rotation. Table CXXX further additions that might be made to the five-year rotation on dunkirk silty clay loam, expressed in pounds to the acre a year. Additions N P2O, K,0 CaO SO3 Organic Matter Farm manure - Leguminous green- manure ^ 21.1 20.0 21.7 31.6 7.3 12.4 1000 600 Total 41.1 21.7 31.6 7.3 12.4 1600 ^ Important because of the additions of organic matter that occurs thereby. ^ It is estimated that of the crops removed and fed or used as bedding, only 30 per cent, of the N, KjO, CaO and SO3, 50 per cent, of the PjOg and 25 per cent, of the organic matter reach the soil as farm manure (par. 294). The crops removed carried about 4000 pounds of organic matter to the acre. ^ The green-manure is estimated as 4000 pounds of air-dry matter carrying 100 pounds of nitrogen, which is considered as fixed from the air. This should yield 3000 pounds of soil organic matter. 558 NATURE AND PROPERTIES OF SOILS 320. The balance sheet. — For convenience of compari- son, the data previously presented are drawn together in a single table and presented below as pounds to the acre an- nually. These figures are considered as relating to the Dun- kirk silty clay loam carrying a five-year rotation of maize, oats, wheat, and two years hay. It must always be remem- bered that such data are specifically applicable to only one soil. Nevertheless the practical deductions that may be drawn are of wider scope. Table CXXXI summary table of losses and additions that might occub to dunkirk silty clay loam under a five-year rota- tion, expressed in pounds to an acre a year. Conditions Reductions when farm manure and green ma- nure are not used ^. . . . Additions from farm ma nure Additions from farm ma nure and green-manure Additions using green- manure N P2O5 KjO CaO SO3 40.3 43.5 174.1 370.2 84.5 21.1 21.7 31.6 7.3 12.4 41.1 21.7 31.6 7.3 12.4 20.0 — — — — Organic Matter 1200 1000 1600 600 It is immediately apparent that when farm manure and green-crops are not utilized, a notable decrease occurs in every constituent cited. Such a system of soil management must reduce the productivity of the soil very quickly and certainly is not a rational scheme of soil and crop adjustment. Nevertheless, it is the condition under which much of the arable land is producing crops today. When farm manure is utilized, even allowing for a large * Obtained by subtracting the natural additions from the rormal losses. THE MAINTENANCE OF SOIL FERTILITY 059 waste in its production and handling, the organic matter is almost maintained and the loss of nitrogen is met to some ex- tent. Under such a system, the addition of nitrogen and of mineral constituents is a problem, although sCme attention should be paid to the soil organic matter. Liming will be necessary ultimately if not immediately, while the addition of phosphoric acid obviously will some day be profitable. If acid phosphate is utilized at a normal rate, the sulfur losses that occur should be very nearly counterbalanced. Potash, especially as the soil under consideration is a clay loam, will no doubt be available for a long period if the organic matter is adequately maintained. The use of a green-manure once in the rotation in addition to the farm manure Avill adequately care for the soil or- ganic matter and reduce the nitrogen problem to a minor position. When animal products are relatively high in price and crop values are low, stock farming will be advisable and a sj^s- tem whereby considerable farm manure will be available may be followed. It has already been indicated that under such conditions the organic matter, and to a lesser degree the nitro- gen content of the soil, may adequately be maintained espe- cially if a green-manure is used once in the rotation. Where grain farming is necessary, reliance must be placed almost wholly on green-manures for the upkeep of the soil organic matter, especial care being given to the full utilization of crop residues. According to the data presented in Table CXXXI, such a system, as far as the nitrogen and organic matter are concerned, could be made about as satisfactory as where farm manure is available and has the possibility and advantage of considerable expansion. Grain farming makes necessary, however, a more intensive and careful use of mineral constitu- ents. Liming and commercial fertilizers will, therefore, fig- ure somewhat more prominently in grain-growing than where dairying or stock-raising are practiced. 560 NATURE AND PROPERTIES OF SOILS 321. The maintenance of soil fertility.^ — The practical management of a soil, whereby profitable crops may be grown without materially reducing the fertility of the land rests on five fundamental principles. The basic factors are: (1) drainage, (2) tillage, (3) organic matter, (4) lime, and (5) fertilizers. Obviously, the removal of excess water depends on adequate drainage, while aeration and all of the activities that attend it rests both on drainage and tillage. The upkeep of the soil organic matter by the use of crop roots and resi- dues, by farm manure, and by the turning under of green- crops has already been emphasized as fundamental to con- tinuous productivity. These factors are by no means the whole program of ra- tional soil management. Artificial additions must be made. Of these lime is of vital importance. Calcium and magnesium are lost from the soil in such large amounts that outside sources must be drawn on. Every arable soil will ultimately come to the point where liming will be profitable. Finally, the judicious use of commercial fertilizers must receive attention. The addition of phosphoric acid will probably be the first fertilizer element to be considered seriously, especially in gen- eral farming. Under special conditions of soil and crop, nitro- gen and potash will also be a part of the program. The adap- tation of crops in suitable rotation to climate and soil, with adequate attention to the factors emphasized above, are the prime essentials of a paying system of permanent soil pro- ductivity. ^ Hartwell, B. L., and Damon, S. C, Six Years' Experience in Im- proving a Light Unproductive Soil; Jour. Amer, Soc. Agron., Vol. 13, No. 1, pp. 37-41, 1921. Lipmau, J. G., and Blair, A. W., Tiie Lime Factor in Permanent Soil Improvement. I. notations without Legumes. II. Botations with Legumes; Soil Sci., Vol, IX, No. 2, pp. 83-114, 1921. INDEX OF AUTHORS Aarnio, B., 134. Abbott, J. B., 347. Aberson, J. H., 252. Agee, A., 362. Ageton, C. U., 376. Aikman, C. M., 504. Allen, E. R., 424. Allison, F. E., 387. Alway, F. J., 43, 44, 63, 115, 118, 119, 120, 155, 163, 160, 195, 198, 199, 221. Ames, J. W., 58, 406, 470, 499, 550. Ammon, Georg, 156. Appiani, G., 73. Appleyard, A., 158, 248, 252, 253, 257, 273, 543. Ashby, S. F., 424. Ashley, H. E., 132, 134, 137. Atterburg, A., 68, 140, 141, 187. Averltt, S. D., 118. Aunier, J. P., 545. Bancroft, W. B., 127, 129. Barker, P. B., 221. Barlow, J. T., 358. Barrett Company, The, 449. Bauniann, A., 106. Beattie, J. H., 351, 520, 521. Beaumont, A. B., 134, 157. Beavers, J. C, 499. Bennett, H. H., 38, 50, 63, 118. Bernard, A., 467. Bertholot, M., 431. Bertrand, G., 466. Bishop, E. S., 120. Bizzell, J. A., 179, 207, 251, 252, 284, 422, 43G. 437. Blair, A. W., 307, 354, 356, 381, 556, 560. Blanck, E., 478. Bogue, R. H., 267. BoUey, H. L., 397. Boltz, G. E., 406, 470. Boullanger, E., 466, 467. Bouyoucos, G. J., 153, 154, 155, 159, 160, 171, 175, 182, 196, 223, 228, 232, 235, 239, 259, 277, 280, 281, 286, 287, 291, 356. Bradley, O. E., 380. Breazeale, J. F., 113, 331, 381. Brcnchley, W. E., 284, 287. Briggs, L. J., 67, 113, 156, 168, 171, 172, 188, 190, 195, 196, 197, 277, 278, 380. Bright, J. W., 505. Briscoe, C. F., 544. Brodie, D. A., 499. Bronet, G., 267. Brooks, W. P., 461, 522. Broughton, L. B., 377. Brown, R E., Ill, 422. Brown, C. F., 211, 340. Brown, P. E., 44, 362, 388, 392, 405, 406, 421, 424. Bryan, H., 71. Buckingham, E., 182, 260. Buckman, H. O., 29. Buddin, AV., 414. Bunger, H., 186. Burd, J. S., 280, 322, 325. Burdick, R. T., 499. Burr, W. W., 194, 221. Burton, E. F., 127, 129. Caldwell, J. S., 195. Call, L. E., 220, 221. Cameron, F. K., 113, 141, 283. Carr, R. H., 116, 143. Carrero, P. L., 299. Carter, E. G., 392, 393, 421, 464. Gates, J. S., 221. Chamberlain, T. C, 57. Chase, L. W., 144. Christrnsen, H. R., 354. Clarke, F. W., 4, 13. Clarke, V. L., 155, 198. 561 562 INDEX OF AUTHORS Coffman, W. B., 190. Coleman, D. A., 387, 388. Coleman, L. C, 420. Collins, S. H., 442. Comber, N. M., 360. Conn, H. J., 388, 389, 505. Conn, H. W., 384. Conner, S. D., 295, 328, 347, 349, 351, 457, 461. Cook, R. C, 273. Coppenrath, E., 466. Cox, H. R., 221. Cowles, A. H., 380. Crosby, W. A., 36. Crowther, C, 429, 469. Cullen, J. A., 464. Cummins, A. B., 267. Curry, B. E., 267, 380. Curtis, R. E., 389. Cushman, A. S., 73, 132. Czermak, W., 143, 296. Damon, S. C, 381, 476, 560. Darbishire, F. V., 414. Davidson, G., 109. Davidson, J. B., 144. Davis, A. R., 432. Davis, N. B., 137. Davis, R. O. E., 171, 204. Davis, W. M., 46. Davy, J. B., 340. Demolon, A., 267, 467. Digby, Kenelm, 442. Diller, J. S., 33. Dobeneck, A. P., 156. Dorrance, R. L., 430. Dorsey, C. W., 330, 334, 337, 340. Doryland, C. J. T., 249. Duchacek, F., 460. Dugaidin, M., 467. Duggar, B. M., 432. Duley, F. L., 499. Dupre, H. A., 169. Dyer, Bernard, 318. Eastman, E. E., 204. Ehrenberg, P., 134, 143. Ellett, W. B., 362. Elliott, C. G., 210, 213. Emerson, H. L., 17, 40, 46. Ernest, A., 110, 252. Failyer, G. H., 77, 268, 279, 321. Faiire, L., 210. Feilitzen, H. von, 429, 467. Fellers, C. R., 387. Fippin, E. O., 122, 143, 211, 499, 516, .'519. Fisher, M. L., 204. Fleischer, M., 44. Fletcher, C. C, 71. Floess, R., 134. Floyd, B. F., 363. Flugel, M., 478. Fraps, G. S., 208, 319, 420, 443, 471. Frear, W., 362, 376, 377, 516, 524. Freckmann, W., 186. Fred, E. B., 505, 543. Fry, W. H., 6, 76, 133, 445, 455. Fulmer, H. L., 505. Funchess, M. J., 347. Gaither, E. W., 58, 499. Gainey, P. L., 249, 356, 392, 420, 424. Gallagher, F. E., 141, 143, 273. Gans. R., 265. Gedroiz, K. K., 459. Gee, E. C, 211. Georgeson, C. C, 239. Gerlach, U., 272, 306. Gilbert, J. H., 180, 443. Gile, P. L., 299, 376, 466. Gillespie, L. J., 281, 350, 356. Glass, J. S., 204. Goessman, C. A., 504. Gortner, R. A., 116. Grandeau, L., 115. Greaves, J. E., 392, 393, 420, 421, 422, 432, 433. Gustafson, A. F., 124, 220, 242. Guthrie, F. B., 336. Haberlandt, H., 140, 224. Hall, A. D., 10, 68, 78, 206, 217, 284, 287, 294, 305, 421, 433, 442, 448, 449, 471, 508, 516, 519, 531, 532. Halligan, J. E., 442, 471. Harned, H. H., 544. Harris, F. S., 328, 334, 336, 340. Hart, E. B., 303, 315, 468, 499, 514, 521. Hart, R. A., 211, 340. Barter, L. L., 337. INDEX OP AUTHORS 563 Hartwell, B. L., 347, 349, 354, 381, 461, 476, 560. Hasenbaumer, J., 466. Headden, W. P., 330, 332. Heinrich, R., 197. Hellriegel, H., 189, 191, 192. Helms, R., 336. Hendrick, J., 78. Hibbard, P. L., 342. Hildebrandt, F. M., 267. Hilgard, E. W., 31, 68, 73, 116, 120, 155, 162, 330, 340. Hill, H. H., 550. Hills, J. L., 483. Hiltner, L., 389. Hirst, C. T., 464. Hitchcock, E. B., 421. Hoagland, D. R., 279, 280, 281, 284, 285, 323, 350. Hoffman, C, 461. Hopkins, C. G., 122, 355, 362, 437, 466, 516, 533, 539, 540, 545. Houston, H. A., 116. Howard, L. P., 353, 354. Hubbard, P., 73. Hudelson, R. R., 362. Hudig, J., 429. Humphreys, W. J., 53. Hunt, T. F., 472, 533. Hurst, L. A., 350, 356. Hutchinson, C. M., 545. Hutchinson, H. B., 108, 355, 387, 402, 411, 414, 415, 424, 447, 450, 544. Ingle, Herbert, 100. Isham, R. M., 332. Israelsen, O. W., 92, 93, 163. Jaffrey, J. A., 211. Jensen, C. A., 545. Jodidi, S. L., 106, 248. Joffe, J. S., 351, 356. Johnson, H. W., 405. Johnson, S. W., 249. Jones, C. H., 355, 483. Jones, S. C, 499. Juritz, C. F., 430. Karraker, P. E., 163, 171, 358. Kaserer, H., 416. Kearney, T. H., 337. Keen, B. A., 151. Kellerman, K. F., 424. Kellner, O., 450. Kellogg, E. H., 405. Kellogg, J. W., 365, 379. Kelley, N. P., 107, 267, 347, 415, 421. 450. Kelly, M. P., 466. Kiesselbach, T. A., 188. King, F. H., 94, 145, 163, 172, 176, 189, 210, 230, 241, 242, 282, 284, 422. Kinnison, C. S., 140. Klippart, J. H., 210. Knox, J., 450. Knox, W. H., 355. Knudson, L., 402. Koch, G. P., 388. Konig, J., 466. Kopecky, J., 179. Kopeloff, N., 377, 387, 388, 413. Koppers Company, The, 449. Kratzman, E., 347. Krusekopf, H. H., 362. Krzymowski, R., 187. Lang, C, 228, 232. Lapham, M. H., 171. Lathrop, E. C, 107, 410. Latshaw, W. L., 437. Lau, E., 248. Lawes, J. B., 180, 189, 443. Leather, J. W., 190. Leidigh, A. H., 211. Lemmermann, O., 551. Liebig, J. Justus von, 443. Lipman, C. B., 32, 158, 278, 376. Lipman, J. G., 307, 342, 381, 384, 403, 406, 431, 433, 436, 508, 537, 556, 560. Loew, 0., 376, 415. Lohnis, F., 433. Loughridge, R. H., 78, 122, 156, 198, 337. Lugner, I., 429. Lyon, T. L., 179, 181, 207, 251, 252, 284, 297, 422, 436, 437, 495, 541, 542. Lynde, C. J., 169. Lynde, H. M., 211. 564 INDEX OF AUTHORS Maclntire, W. H., 181, 250, 345, 370, 371, 422. MacLennan, K., 355. Martin, J. C, 280, 285. Martin, L. M., 46. Martin, T. L., 541, 545, 547. Martin, W. H., 351. Marchal, E., 335, 413. Marshall, C. E., 384. Massey, A. B., 109. May, D. W., 466. Mayer, A., 200. Maynard, L. A., 545. Maze, P., 402, 478. McBeth, I. G., 267, 389. McBride, F. W., 116. McCall, A. G., 267, 278. McCaughey, W. G., 6, 36, 76, McCool, M. M., 362. McDole, G. R., 118, 166. McGeorge, W. T., 78, 79, 273. McLane, J. W., 168, 277. McLean, H. C, 388. McMlllar, P. R., 380. Merrill, G. P., 3, 17, 32, 36, 38, 265. Middleton, H. E., 133. Miles, M., 210. Millar, C. E., 362. Miller, B. L., 448. Miller, E. C, 190, 194. Miller, M. F., 362. Miller, N. H. J., 10, 108, 402, 411, 415, 447, 450, 469. Milligan, S., 545. Miner, H. L., 483. Minges, G. A., 421. Mirasol, J. J., 347, 349. Mitscherlich, E. A., 134, 140, 186, 284, 477. Miyake, K., 347. Molisch, H., 296. Montgomery, E. G., 188, 190, 191, 192. Mooers, C. A., 181, 362. Moore, C. J., 133. Morgan, J. F., 278, 281, 282. Morrow, C. A., 99, 105. Morse, F. W., 122, 267, 380, 449. Morse, W. J., 465. Mosier, J. G., 124, 204, 219, 220, 242. Murray, T. J., 427, 509. Mulder, T. J., 105. Neller, J. R., 256, 465. Niklas, H., 127. Norton, T. H., 450. Ogg, W. J., 78. Oliver Plow Book, 144. Olsen, C, 351. Osborne, T. B., 68, 70. Osugi, S., 346, 349. Owen, W. L., 256. Pantanelli, E., 284. Parker, E. G., 267, 269, 271. Parker, F. W., 278. Parks, J., 242. Parsons, L. J., 210. Patten, H. E., 154, 232, 235, 237, 263, 273. Patterson, J. W., 420. Peake, W. A., 113. Peck, E. L., 469. Pember, F. R., 347, 349, 354, 381, 461. Penny, C. L., 537, 540. Peters, E., 266, 271. Peterson, W. H., 303, 315, 331, 332, 468. Pettit, J. H., 355. Pfeiffer, Th., 478. Pick, H., 134. Pickel, G. M., 44. Pieters, A. J., 537. Piper, C. v., 537. Pirsson, L. V., 3, 46. Pitman, D. W., 336. Pitra, J., 460. Plummer, J. K., 5, 6, 76, 251, 256, 356, 373, 392, 420. Potter, R. S., 251, 315. Pranke, E. J., 451. Prescott, J. A., 263. Prianischnikov, D., 459, 460. Prince, A. L., 354, 356. Puchner, H., 78, 141. Pugh, E., 443. Rahn, Otto, 392. Ramann, E., 127, 259. Ramser, C. E., 204. Ramsower, H. C, 145. Rather, J. B., 113. Ravin, P., 402. Reed, H. S., 109, 297. INDEX OF AUTHORS 565 Reid, V. R., 347, 466. Reimer, F. C, 407. Rice, F. E., 346, 349. Richards, E. H., 429. Richmond, T. E., 406. Roberts, I. P., 504, 514, 519, 521. Robbins, W. J., 109. Robbins, W. W., 386. Robinson, C. S., 44. Robinson, F. W., 539. Robinson, G. W., 78. Robinson, W. O., 5, 13, 14, 86, 41, 52, 63, 77, 118, 466. Rodewald, H., 134. Ross, W. H., 464, 466. Rost, C. O., 63, 118, 119, 315. Ruprecht, R. W., 347, 449. Russell, E. J., 9, 68, 78, 158, 248, 252, 253, 257, 273, 387, 414, 424, 429, 471, 543. Russell, I. C, 46. Ruston, A. G., 429, 469. Sachs, J., 296. Sachs, W. H., 466. Sackett, W. G., 331, 412. Salisbury, R. D., 52, 57. Salter, R. M., 116. Saussure, Theodore de, 442. Schantz, H. L., 156, 188, 190, 195, 196, 197. Schoilenberger, C. J., 315, 354. Schone, E., 73. Sehreiner, 0., 105, 107, 108, 109, 111, 268, 279, 297, 321. Schulze, F., 322. Schutt, M. A., 507, 519. Seelhorst, C, von, 186, 187, 192. Sewell, M. C, 144, 220, 221. Sharp, L. T., 281, 350. Shaw, C. F., 92. Shedd, 0. M., 325, 406, 467. Sherman, J. M., 387. Shorey, F. C, 76, 105, 107, 362. Shutt, F. T., 430, 539. Singewald, J. N., 448. Skinner, J. J., 108, 109, 347, 351, 447, 466. Slosson, E. E., 450. Smalley, H. R., 44, 347. Smith, Alfred, 89. Smith, C. D., 539. Smith, O. C, 116. Snyder, H., 122, 284, 317, 545. Snyder, R. S., 251, 313, 315. Spillman, W. J., 537. Spurway, C. H., 287. Stephenson, R. E., 353, 354. Stevenson, W. H., 44. Stewart, C. F., 45. Stewart, G. R., 279, 280, 284, 323. Stewart, R., 331, 332, 376, 381, 404, 420, 422, 470. Stoddard, C. W., 100. Stoklasa, J., 110, 252, 255, 256, 460. Storer, F. H., 504, 537. StiJnner, K., 3,v9. Stremme, H., 134. Strowd, W. H., 435. Sullivan, E. C, 267, 270. Sullivan, M. X., 107, 466. Swanson, C. O., 437. Sweetser, W. S., 511. Swezey, G. D., 242, 260. Tacke, Br., 355. Tartar, H. V., 467. Tarr, R. S., 46. Taylor, W. W., 127. Tempany, H. A., 134. Temple, J. C, 421. Thatcher, R. W., 100, 122, 127. Thomas, W., 376, 377. Thompson, H. C, 44. Thorne, C. E., 375, 382, 454, 461, 462, 499, 504, 507, 511, 513, 514, 516, 519, 521, 525, 526, 528, 529, 532, 533. Tottingham, W. E., 461, 514. Triesehmann, J. E., 469. Tmka, R., 92. True, R. H., 300, 346, 348. Truog, E., 348, 359. Turpin, H. W., 252. Ulrich, R., 232, 233. Underwood, T. M., 284, 287. Vageler, P., 134. Van Bemmelen, J. M., 36, 106, 132, 265, 266, 270. Van Slyke, L. L., 442, 471, 501, 503, 514. Van Suchtelen, F. H. H., 278. Veitch, F. P., 317, 355. 566 INDEX OF AUTHORS Voelcker, A., 519, 532. Von Englen, O. D., 58. Voorhees, E. B., 431, 504. Vrooman, C, 439. Waggaman, W. H., 263, 455, 456, 461, 464. Wagner, F., 239. Wagner, H., 477. Waksman, S. A., 387, 388, 389, 392, 413. Walker, S. S., 51, 118. Walters, E. H., 107. Warington, R., 113, 132, 144, 180, 182, 265, 303, 426. Warner, H. W., 406. Warren, G. M., 210. Watson, G. C, 514. Way, J. T., 132, 264. Waynick, D. D., 113. Weaver, F. P., 499. Weir, W. W., 362. Welitschkowsky, D., von, 176. Wells, A. A., 248. Wills, C. F., 116. Westerman, F., 433. Whitbeck, R. H., 58. Whitney, M., 81, 83, 85, 89. Whisenand, J. W., 520. White, J. W., 351, 353, 365, 377, 421, 449, 550. Whiting, A. L., 545. Whitson, A. R., 44, 204, 362, 422. Wiancko, A. T., 365, 381, 461, 499. Widtsoe, J. A., 172, 186, 187, 190, 192. Wiegner, G., 127, 265, 270. Wiley, H. W., 73, 112, 114, 115, 314. Williams, C. B., 49, 52, 118. Williams, H. F., 5. Wilson, B. D., 11, 374, 404, 430, 437. Wilson, G. W., 388. Wilson, J. K., 297. Wing, H. H., 514. Winogradsky, S., 431. Wolkoff, M. I., 131. Wollny, E., 110, 163, 171, 176, 189, 200, 228, 230, 245. Wood, T. B., 516. Woodward, S. M., 210. Wright, R. C, 541. Wyatt, F. A., 363, 376, 381. Wyckoff, M. I., 267. Yarnell, D. L., 211. Yoder, P. A., 73. Young, G. J., 464. Young, H. J., 221. Zzigmondy, R., 127. INDEX OP SUBJECT MATTER Ability of plants to grow on poor soils, 299. Abrasion defined, 18. Absorption by litter, 521. Absorption, by soils explained, 263. capacity of soils to retain nitrates, 321. due to soil colloids, 26.'j. effect of on soil acidity, 352. efl^^ect of texture on, 267. importance of in soils, 273. of litter in stable, 521. selective by soils, nature of, 269. selective by soils, types of, 269. Absorption by soils, capacity for, 266. causes of, 264. defined, 263. importance of, 273. influence of time on, 269. law of, 269. relation to acidity, 274. relation to the soil solution, 276. selective, 269. types of, 263. Absorption of solar insolation, as influ- enced by atmosphere, 226. as influenced by color, 228. as influenced by slope, 229. as influenced by soil, 226. Absorptive capacity of different crops, 301. Acid phosphate, 456. changes in soil, 457. character, 456. compared with rock phosphate, 458. composition, 456. manufacture, 456. reinforcement of manure with, 528. Acidity, as influenced by absorption, 274. development of by hydrolysis, 348. production of by selective absorp- tion, 270. soil, nature of, 345. Acids, production of by plant roots, 296. Actinomyces in soils, character of, 389. Actinomyces in soils, importance of, 389. number of, 289. Addition of nutrients to soil, 556. Additions to and losses from soil under various types of farming, 558. Adobe, inportance of, 64. origin of, 64. wind formation of, 21. iEolian soils, adobe, 64. loess, 61. sand dunes, 64. volcanic dust, 65. ASration of soil, effect on nitrification, 418. importance in soil, 256. influence on bacteria in soils, 393. Agglutination of colloids, 131. Agricultural lime, defined, 363. forms of, 363. Agricultural value of farm manure, 513. Air of the soil, carbon dioxide of, 250. composition of, 247, 248. composition data, 248, 250. effect of oxidation on, 254. general characteristics of, 247. importance of oxygen in, 256. practical modification of, 261. movement of, 258. types of, 249. volume of, 257. Alkali, black, 329. composition of, 329. conditions affecting influence of, 338. control of, 343. control of by means of gj-psum, 342. effect of concentration of on crops, 337. effect on crops, 334. effect on soil organisms, 335. eradication of, 341. eradication of by means of drainage, 341. influence on nitrification, 421. in river water, 332. in irrigation water, 334. 567 568 INDEX OF SUBJECT MATTER Alkali, origin of, 331. resistance of crops to, data on, 338. rise of as influenced by irrigation, 339. white, 329. Alkali lands, handling of, 340. Alkali salts, listed, 330. Alkali soils, defined, 328. importance of, 328. Alkali spots, nature of, 332. Alkali tolerance by plants, factors of, 336. Alkali vegetation, 340. Alluvial fans, 47. Alluvial soils, chemical composition, 49. classified, 46. deltas, 47. fans, 47. flood plain, 47. importance of, 49. origin of, 46. Aluminum, hydrolysis of in soil, 348. relation of to soil acidity, 347. relation to the reversion of acid phosphate, 457. Alunite as a fertilizer, 465. Amino acids defined, 106. in farm manure, 510. Amides defined, 106. Ammonia in rain water, data on, 429. Ammonification, conditions for, 414. influence of protozoa on, 387. nature of, 412. organisms of, 413. products of, 413. reactions of, 414. Ammonifying efficiency of soil, determi- nation of, 414. Ammonium salts, utilization by higher plants, 415, 450. Ammonium sulfate, changes in soil, 449. character of, 449. composition of, 449. source of, 449. Amounts of fertilizer to apply, 492. Amounts of lime to apply, 368. Analysis of plant tissue, method of, 102. Analysis of soil, bulk, 311, 314. carbon in, 113, 114. extraction, dilute acids, 317. extraction, strong acids, 316. extraction, with water, 319. Analysis of soil, humus, 115. lime requirement of, 355. minerological, 76. nitrogen in, 311. organic matter of, 115. value of, 323, 326. Apatite in soil, 6. Application of farm manure, amounts, 526. evenness, 526. incorporation in soil, 526. Arid soils, biological activity in, 32. chemical analysis of, 31. humus content of, 120. Assimilation of nitrates by soil organ- isms, 426. importance of, 428. Available water in soil, 198. Availability of nitrogen fertilizers, 454. of phosphate fertilizers, 458. Azofication, amount of nitrogen fixed, 433. energy for, 432. organisms of, 432. Azotohacter ehroococcum in soil, 431. B. Radicicola, amount of nitrogen fixed by, 437. availability of nitrogen fixed by, 437. function of, 434. importance of, 436. inoculation of the soil, methods of, 439. nature of organism, 435. nodules of, 434. relation to host plant, 435. strains of, 434. Bacteria, decomposition of organic mat- ter by, 103. increase of in frozen soil, 394. influence on aeration in soils, 393. injurious to higher plants, 396. method of counting in soil, 392. multiplication of, 391. production of carbon dioxide by, 252. relation of to alkali, 332. relation to liming, 395. relation of moisture to, 393. relation to organic matter in soil, 394. relation to soil acidity, 395. relation to soil temperature, 394. INDEX OF SUBJECT MATTER 569 Bacteria, shape of, 391. seasonal flora, 394. spore formation by, 391. Bacteria in soils, character of, 390. determination of numbers of, 392. factors affecting growtli of, 393. influence of green manures on, 544. numbers of, 392. position in soil, 391. production of enzjones by, 390. size of, 391. Bacterial activity, measured by carbon dioxide produced, 25C. Bacterial growth, conditions affecting, 393. Bases, substitution of in soils, 271. those used to correct soil acidity, 362. toxic nature of in acid soils, 346. Basic exchange, 270. influence on drainage water, 305. Basic slag, changes in soil, 458. character of, 458. composition of, 457. source of, 457. Beaker method of mechanical soil analy- sis, 69. Biological cycles of the soil, importance of, 398. names of, 399. nature of, 398. Biological effects of lime on soil, 371. Bog lime, nature of, 45. Bomb method for determining soil or- ganic matter, 114. Bone phosphate, changes in soil, 455. character, 454. composition, 454. source, 454. Brands of fertilizers, 478. Bromberg soil tank.s, data from, 306. Brownian movement, explained, 128. Bucher method of fixing nitrogen, 453. Bulk analysis of soils, carbon and nitro- gen, 311. mineral constituents, 314. Burned lime, 364. Calcium, amount in soils, 13. forms of in soil, 11. importance of in fertility evalua- tions, 324. Calcium, in soil minerals, 6. lack of in relation to soil acidity, 348. loss of from soil, 307, 370, 555. of di-silicate as an amendment, 380. relation to reversion of acid phos- phate, 457. use of as lime, 363. Calcium eyananiid, change in soil, 452. character of, 452. composition of, 452. manufacture of, 451. Calcium and magnesium ratio in soils, 375. Calcium in gypsum as an amendment, 379. Calcium losses, Bromberg lysimeters, 30G. from Cornell soils, 307. Calcium nitrate, character of, 452. composition of, 452. manufacture of, 452. Capillary-absorbed water, defined, 196. Capillary capacity of soils, factors affect- ing, 163. Capillary film, thickness of and effect on capillary movement, 171. Capillary movement of soil water, data on rate, 174. effect of structure on, 174. effect of texture on, 173. factors affecting, 170. explained, 168. influence of film thickness, 171. relation to soil mulch, 175. role in supplying plants with water, 193. Capillary pull of soils, data on, 169. determination of, 168. Capillary water of soil, amounts in soil columns, 165. colloidal control of, 159. defined, 159. determination of amount, 161. kinds of, 159. position of inter film, 160. surface tension control, 159. Carbide method of fixing nitrogen, 451. Carbon, cycle of in soil, 399. determination of in soil, 113. gain of by green manures, 539. in Cornell drainage water, 402. in organic matter, 113. 570 INDEX OF SUBJECT MATTER Carbon, loss from the soil, data of, 402. loss of from soil, 555. use of organic carbon by higher plants, 402. Carbon cycle of the soil, loss of carbon from, 400. nature of, 399. organisms of, 399. products of, 400. Carbon dioxide, a measure of bacterial activity, 256. from decaying manure, 509. from lime, 369. function in soil, 255. in atmospheric air, data, 110. in soil air, data, 110. influence on nitrification, 256, 419. relation to mineral cycle of soil, 408. of soil air, 250. of soil air, influence of farm manure on, 254. of soil air, influence of organic mat- ter on, 253. production of, 110. produced by bacteria in soil, 252. produced by plant roots, 252, 295. source of in soil air, 251, 400. Carbonated lime, 3G5. Carbonation, influence in soil formation, 26. Carbonized materials in soil, importance of, 112. nature of. 111. Castor pomace, composition of, 446. Catalytic fertilizers, 466. Catalyst defined, 103, 135. Cell sap, nature of in relation to plant absorption, 300. Centrifugal mechanical analysis of soils, 71. Character of soil particles, 69. Chemical absorption by soils, 263. Chemical analysis, alluvial and upland soils, 49. arid and humid soils, 31. bulk and extraction methods, 311. by digestion with strong acids, 316. by wateT extraction, 319. glacial soils, 57. granite soil, 33. importance in fertility evaluation, 323. Chemical analysis, limestone soil, 33. loess soils, 63. marine soils, 52. of alkaline river water, 334. of Cornell soils, 325. of good and poor Ohio soils, 326. of Minnesota soils, 316. of Minnesota and Maryland soils, 317. of soil, popular conception of, 311. of soil separates, 78, 79. peat and muck, 44. residual soils, 41, 52, 57. resume as to value of, 326. value as shown by actual data, 326. with weak acids, 317. Chemical composition of soils, compared to lithosphere, 13. Chemical composition of soil separates, 78, 79. Chemical effects of lime on soil, 371. Chromic acid method for determination of soil organic matter, 113. Chile salt petre, source and character of, 448. Classification of methods of mechanical analysis, 72. Classification of soils, geological, 38. for soil survey, 85. Classification of soil particles, Bureau of Soils, 67. Classification of soil particles other than Bureau of Soils, 68. Climate, effect on transpiration, 191. relation of to soil formation, 30. Clostridium pastorianutn in soil, 431. Coastal plain soils, chemical composition of, 52. Cohesion, cause of in soils, 136. defined, 136. Colloidal materials, properties of, 130. in soils, 265. Colloidal matter, absorptive power for water, 153. Colloidal matter, influence on soil prop- erties, 135. Colloidal matter in soils, influence on structure, 137. estimation of, 134. generation of, 132. resume of, 138. Colloidal particles, size of, 128. INDEX OF SUBJECT MATTER 571 Colloidal state, defined, 127. defined briefly, 75. electrical condition of, 131. examples of, 130. phases of, 129. practical importance of, 135. relation of to granulation, 142. Colloids and crystalloids, 129. Color of soil, compounds of, 36, 37. influence on absorption of insulation, 228. nature of, 36. Colluvial soils, origin and nature, 45. Commercial fertilizer, amounts to apply, 492. development of use, 442. used for their nitrogen, 444. used for their phosphorus, 454. used for their potash, 462. used in United States, 444. Commercial value of fami manure, 512. Composition of average soil, 12. of cow manure, 501. of drainage water, 304. of farm manure, average, 504. of horse manure, 501. of muck and peat, 44. of plant tissue, 100. of sheep manure, 501. of swine manure, 501. Composts, use of manure in, 530. Composts of sulfur, 406. Conductivity of heat, measurement of, 235. formula for, 236. Conductivity coefficients of various soils, 236. Conductivity of various soils, 235. Conduction, loss of heat from soil by, 240. Conduction of heat in soils, factors af- fecting, 235. nature of, 234. Constituents of soil, organic and inor- ganic, 2. Control of alkali in soils, 343. of evaporation, 218. of soil air, 261. of soil temperature, 244. Conservation of soil moisture, 219. Conversion factors for lime, 367. Convection of heat in soil, 238. Correction of soil acidity, bases useful for, 363. Corrosion defined, 18. Cotton and tobacco, influence of manure on, 535. Cotton seed meal, composition of, 446. Cover crops, influence on nitrates of soil, 541. Crop resistance to alkali, generalized table of, 338. in pounds per acre, 338. Crop residues, to maintain organic mat- ter, 124. Crop rotation, relation of to green ma- nuring, 552. Crops, absorptive capacity of, 301. amounts of fertilizers for, 493. bacteria injurious to, 390. detrimental influence of nitrogen on, 473. effect of calcium and magnesium ratio on, 376. effect of concentration of salts on, 337. effect of on conservation of plant nutriants, 308. fertilizer formulae for, 491. for green manures, 546. fungi injurious to, 396. influence of green manures on, 547. influence of manure on, 532. influence of nitrogen on, 472. influence of phosphorus on, 474. influence of potassium on, 475. injurious effect of soil organisms on, 396. quantities of nutrients removed by, 303. removal of nutrients by, 555. removal of sulfur by, 404. removal of sulfur and prohphorus by, 468. response to lime, 372. systems of fertilizing, 496. Crushers, action of, 148. Cultivation, implements for, 147. importance of, 219. Cultivators, action of, 147. Cumulose soils, agricultural importance, 43. location of, 42. origin, 42. 572 INDEX OF SUBJECT MATTER Decay and decomposition defined, 103, 410. and putrefaction, in nitrogen cycle, 410. and putrefaction, organisms of, 411. effect on soil temperature, 239. of farm manure, importance of, 511. of green manure, influence on lime and phosphorus, 545. of green manure, influence on nitrate accumulation, 543. of green manure, influence on nitri- fication, 544. of green manure, stages of, 542. of organic matter in soil, products of, 110. Decomposition, defined, 17. of organic matter in soil, 103. Delta soils, 47. Denitrification, use of term, 426. Deoxidation, influence in soil formation, 24. Deposition, its relation to soil formation, 16. relation to lime requirement, 356. Depression of the freezing point, a method of studying soil solu- tions, 280. Determination of soil humus, 115. Determination of soil organic matter, bomb method, 114. chromic acid method, 113. loss on ignition, 112. Di-calcium silicate as a soil amendment, 380. Diffusion, differential into plants, 292. of nutrients into plants, 291. of soil air, 260. Diminishing returns, law of, 493. Disc plow, influence on soil, 146. Disintegration, defined, 17. Dissociation, defined, 270. Drainage and evaporation at Rotham- sted, 217. Drainage, importance of, 210. influence of, 210. loss of sulfur by, 404. nutrient losses from .soil, 555. qualitative composition of water of, 304. quantitative composition of water of, 304. Drainage, use of in eradication of alkali, 341. usual type of, 212. Drainage water, carbon in, 402. composition data of, 305. composition of, at Bromberg, 272. importance of study, 178. qualitative composition of, 304. quantitative composition of, 304. Dried blood, changes in soil, 445. character, 445. composition, 445. source, 445. Earth worms, importance of, 385. Earth's crust, minerals in, 4. Electric arc method of fixing nitrogen, 452. Electrolyte, defined, 130. effect on colloids, 131. Element in the minimum, 476. Energy necessary for evaporation of water, 241. Energy, wave length of, 225. Enzymes, action of, 390. defined, 103, 390. importance in soils, 103. Eradication of alkali from soils, 341. types of, 341. Erosion defined, 18. relation to soil movement, 16. Erosion of soil, control of, 204. types of, 205. Evaluation of farm manure, 512. Evaporation and drainage at Rotham- sted, 217. Evaporation of soil moisture, control of, 218. energy necessary for, 241. influence of on soil heat, 241. loss of soil water by, 216. water influenced by, 182. Exfoliation in soil formation, 21. Exhaustion of soil, discussion of, 309. possibility of, 308. Exosmosis, nature of, 290. Extraction of soils, with concentrated acids, 316. with dilute acids, 317. with water, 319. with water, successive extractions, 322. Exudates, excretion of by plant roots, 296. INDEX OF SUBJECT MATTER 573 Factors influencing rise of soil tempera- ture, 231. Fami manure, a direct and indirect fer- tilizer, 504. agricultural value of, 513. agn"icultural value of protected ma- nure, 525. amounts applied, 527. amount produced by cows, 514. amounts produced by farm animals, 513. amount produced by horses, 514. amount produced by poultry, 514. amount produced by sheep, 514. amount produced by steers, 514. amount produced by swine, 514. average composition of, 504. care of in stalls, 520. characteristics of, 500. commercial evaluation of, 512. composition of from various animals, 501. covered yards for, 523. effect on carbon dioxide of soil air, 254. eflicient application of, 525. evaluation of, 512. factors influencing composition of, 506. fermentation and putrefaction of, 508. fresh and well rotted compared, 511. fresh and yard, crop effects, 512. hauling directly to field, 521. importance of, 499. importance of its decay, 511. importance of protection, 524. importance of tight floors, 521. influence of handling on, 507. influence of tramping on, 524. influence on cotton, 535. influence on maize, 534. influence on meadows, 532. influence on potatoes, 534. influence on tobacco, 535. liquid and solid compared, 502. loss of constituents from, 508, 515. losses during handling and storage, 519. maintenance of .soil organic matter by, 124, 519, 559. modern manurial practice, 520. Farm manure, nutrient lo.sses during pro- duction, 51C. outstanding characteristics of, 505. piles outside, 522. pits for, 523. place in rotation, 532. produced by animals, calculation of, 514. products of decay, 510. reinforcement of, 527. residual effects of, 531. resume of use, 535. use of in sulfur composts, 406. use of lime with, 530. use of litter with, 521. use in composting, 530. variability of, 506. Feldspar as a fertilizer, 465. Fermentation, defined, 103, 410. of farm manure, 508. Fertility, maintenance of as influenced by different types of farming, 558. Fertility of soil, defined, 554. effect on transpiration ratio, 192. possible exhaustion of, 308. Fertility evaluations by means of a chemical analysis, 323. Fertilization, systems of, 496. Fertilizers, advantages of liome mixing, 485. amounts to apply, 492. brands of, 47s. calculations of for lionie-mixing, 487. carrying free sulfur, 467. catalytic, 466. containing nitrogen, 444. containing phosphorus, 454. containing potash, 463. development of their use. 442. early use of, 442. effect of on soil acidity, 353. element in the minimum, 476. factors which determine the choice of, 488. farm manure, 504. formulre, for different soils and crops, 491. formulae, nature of, 489. fomiuhe, theorj- of, 490. function of, 444. guarantees of, 481. how to buy, 483. 574 INDEX OF SUBJECT MATTER Fertilizer, how to home mix, 487. importance of high grade, 483. importance of residues from, 295. inspection and control, 480. interpretation of guarantee, 481. laws of, 480. law of diminishing returns, 493. low grade and high grade, 479. method and time of application of, 495. purchase of unmixed, 484. rational utilization of, 497. systems of applying, 496. use in United States, 444. which should not be mixed, 486. Fertilizer mixtures, those of value, 487. Fertilizer practice, principles of, 471. rational system, 497. Fertilizer residues, cause of, 294. nature of from different salts, 294. Fillers, use of in fertilizers, 488. Fineness of limestone, data as to impor- tance, 377. importance of in liming, 377. influence of on decomposition, 378. Fish scrap, composition of, 446. Fixation of nitrogen artificially, 450. Bucher method, 453. carbide method, 451. electric arc method, 452. Haber method, 453. Fixation of nitrogen by free-living soil organisms, 430. by nodule bacteria, 433. Floats, see rock phosphate, 455. Flocculation, cause of, 131. defined, 130. relation of to granulation, 144. Flood plain soils, 47. Floors, importance in care of manure, 521. Flue dust, a source of potash, 465. Food for plants, defined, 8. Forms of water in soil, diagram of, 199. Forms of soil water, 151. Forms of lime to apply, 367. Formulae of fertilizers for different soils and crops, 491. examples of, 489. theory of, 490. Freezing and thawing, effect on soils, 23. Frost, importance in soil formation, 23. Fungi and algoe, smaller forms in soil, 388. fixation of nitrogen by, 432. injurious to higher plants, 396. in soil, number of, 388. large forms in soil, 386. Germination of seeds, temperature of, 224. Geological classification of soils, 38. resume of, 65. Glacial lakes, origin of, 59. soils of, 58. Glacial soils, chemical composition of, 57. compared with residual, 57, 58. fertility of, 57. general character, 54. importance of, 57. origin, 54. Glaciation, American ice sheet, 53. effect of, 54. influence on agriculture, 58. in North America, 54. Glaciers in soil formation, 18. Grading of ground limestone, 377. Grandeau method, nature of, 312. Granite, cliemical composition of, 33. weathering of, 33. Grass, influence on nitrate accumulation, 427. influence on nitrification, 422. Granulation of soil, as influenced by lime, 143. beneficial effects, 141. defined, 139, 141. forces producing, 143. influence of plowing on, 146. influence of tillage on, 144. production of, 142. Gravity water, amount soil will hold, 177. calculation of, 178. factors affecting movement, 175. importance of study, 178. Green manures, ancient use of, 537. as cover crops, 538, 541. constituents gained by use of, 539. crops for, 545. decay of in soil, 541. general influence of, 538. INDEX OF SUBJECT MATTER 575 Green manures, importance of, 537. influence of decay of, !H3. influence of decay on lime and phos- phorus, 545. influence of decay on nitrate accu- mulation, 544. influence on crops, 552. influence on nitrate reduction, 426. manner of turning under, 549. practical utilization of, 552. relation of to the rotation, 552. relation to humus formation, 543. relative value of different crops for, 547. time for plowing under, 548. to maintain organic matter, 123. use of, 548. use of lime with, 550. Ground limestone, 365. Guano, nature of, 445. Guarantees on fertilizers, statement of, 481. Gullying and its control, 206. Gypsum as a soil amendment, 379. effect of on soils, 379. reinforcement of manure with, 527 use of in alkali control, 342. Haber method of fixing nitrogen, 453. Handling of manure, covered yards, 523. hauling directly to field, 521. influence on composition, 507. manure pits, 523. piles outside, 522. care of in stalls, 520. Hematite, as a soil color, 37. change of to limonite, 27. formation of in soil, 25. source of in soil, 7. Heat, conduction of, 240. conduction of in soil, 234. conductivity of various soils for, 235. convection transfer of, 238. cycle between soil and atmosphere, 225. factors affecting conduction of, 235, 236. evaporation loss by, 241. importance In soil formation, 21. loss of from soil, 240. movement in soil, 234. radiation of, 240. Heat of wetting of soils, amount of, 153. data of, 154. effect of texture on, 154. significance of, 153. High grade fertilizers, importance of, 4f3. Higher plants, influence on nitrification, 422. Home-mixing of fertilizers, calculation of, 487. advantages, 485. good mixtures for, 487. how performed, 486. Hoof meal, composition of, 446. Humid soil, biological activity in, 32. chemical analysis of, 31. humus content of, 120. Humidity, influence of on the hygro- scopic coeflicient, 158. Humus, amount in California soils, 120. amount in Nebraska loess, 120. defined, 115. determination of, 115, 312. formation of from green manures, 543. Hydration, influence of in soil formation, 26. Hydrogen-ion concentration, a measure of soil acidity, 356. method of expression, 350. relation to soil acidity, 346. Hydrolysis, explanation of, 348. production of by enzymes, 390. Hygroscopic capacity of soils, data on, 156. Hygroscopic coeflicient, data as to spe- cific soils, 157. defined, 152. determination of, 154. factors affecting, 157. range of in soils, 158. Hygroscopic water of soils, specific char- acter of, 153. Ice, disintegration of rocks by, 23. glacial, in soil formation, 18. Ice action, glaciation, 53. Ice age, 53. Ignition method for determining soil or- ganic matter, 112. Influence of alkali, condition affecting, 338. 576 INDEX OF SUBJECT MATTEB Inoculation of soil with B. Radicicola, methods of, 439. Insulation, absorbed by earth's atmos- phere, 226. absorbed by soil, 227. absorption of as influenced by color, 228. absorption of as influenced by slope, 229. received by soil, 225. Insoluble phosphoric acid, defined, 456. Inspection and control of fertilizers, 480. Ions, absorption of by soils, 270. differential diffusion of, 293. diffusion of into plants, 291. Ionization, defined, 270. of water, 270. Irrigation, relation to rise of alkali, 339. Irrigation water, alkali content of, 333. Iron, in soil minerals, 7. relation to the reversion of acid phosphate, 457. relation of to soil acidity, 347. Kainit, reinforcement of manure with, 527. Kaolinite, importance in soils, 7. source of in soil, 6. Kelp, a source of potash, 465. Kjeldahl method for determination of nitrogen, 312. Lacustrine soils, character of, 60. glacial lake, 58. importance of, 60. location in U. S., 60. recent lake, 60. Lake salines, a source of potash, 465. Law of diminishing return, 493. Leaching, effect of on soil acidity, 352 loss of lime thereby, 370. use of in alkali eradication, 341. Leather meal, composition of, 446. Legumes, inoculation of, 438. Leguminous crops, amounts of nitrogen fixed by, 438. cross inoculation of, 434. effect on soil nitrogen, 437. nitrogen fixation by, 433. Leucite as a fertilizer, 465. Lime, agricultural terminology of, 364. agricultural use of, 363. Lime, amounts to apply, 368. biological effects in soil, 371. burned, 364. ~ carbonated, 365. cause of crop response to, 372. changes in soil, 369. chemical effects of on soil, 371. composition of as sold in Pennsylva- nia, 366. contact action of, 374. conversion factors of, 367. crop response to, 372. effect of caustic forms on manure, 530. forms of, 363. forms to apply, 367. importance of in soil improvement, 381. influence of green manures on, 545. influence on availability of nutrients, 373. influence on decay of green manures, 651. influence on granulation, 143. influence on nitrification, 373. influence on soil bacteria, 395. influence on sulfofication, 405. losses from Cornell soils, 307, 370. methods of applying, 374. need of determinations, 365. physical effects on soil, 371. problem showing form to buy, 368. proper utilization of, 382. relation of to fertilizer mixtures, 486. relation to reversion of monocalcium phosphate in soil, 457. relation to the use of manure and fertilizers, 382. time to apply, 374. use of manure with, 530. use of with green manure, 551. water slaked, 364. Lime requirement determinations, on soils, 355. types of, 355. Lime requirement of soils, Veitch method, 35G. Limestone, amounts to apply, 368. burning of, 364. changes in soil, 370. chemical composition of, 33. INDEX OF SUBJECT MATTER 577 Limestone, conversion factors, 3C7. fineness of average product, .S78. fineness of for agricultural use, 307. grading of as to fineness, 377. importance of fineness, 377. mechanical composition as sold in Pennsylvania, 379. ratio of calcium and magnesium, 37.5. Liming, amounts of lime to apply, 368. calcium and magnesium ratio of, 375. cause of crop response to, 372. crop response to, 372. forms of lime to apply, 367. importance of in soil improvement, 381. method and time of applying lime, 374. reasons for, 362. Limonite, source of in soil, 7. production of from hematite, 27. weathering of, 33. Limonite group, as soil color, 37. Linseed meal, composition of, 446. Lithosphere, composition of compared to soils, 13. Litmus paper test, criticism of, 360. procedure, 358. use of potassium nitrate with, 358. Litter, absorptive power of, 521. influence on character of manure, 521. Loam, defined, 82. Loess, a wind laid soil, 21. character of, 62. chemical composition of, 63. importance of, 63. location of, 62. minerals of, 62. origin of, 61. Loss of nutrients from soil, 554. types of, 289. Loss of soil heat, by conduction, 240. by evaporation, 240. by radiation, 240. Loss of soil water by run off, 203. Losses during the production and han- dling of manure, 515. Losses from and addition to soils under various types of farming, 558. Lysimeter experiments, at Bromberg, 272. 306. Lysimeter experiments, at Cornell Uni- versity, 307. at Rotliamsted Experiment Farm, 180, 217, 288. Lysimcters, nature of, 180. of Cornell University, 181. of Rothamsted Experiment Station, 180. Macro-organisms of the soil, 384. Maintenance of soil fertility, 554. influence of different types of farm- ing on, 558. program of, 560. Maintenance of soil organic matter, 122. Maize, influence of farm manure on, 534. influence on nitrate accumulation, 428. influence on nitrification, 422. Manganese, relation of to soil acidity, 347. Mangum terrace, 205. Manurial practices, phases of, 520. Marine soils, character of, 51. chemical composition of, 52. importance of, 51. origin of, 50. Marl, origin and nature of, 45. term defined, 45. use of, 45. Maximum retentive power of soil for water, 162. determination of, 162. Maximum water capacity of soils, data on, 166. Meadows, influence of manure on, 532. Mechanical analysis of soils, 67. beaker method, 69. Bureau of Soils method, 71. determination of soil class from, 84. value of, 79. Mechanical analyses of typical soils, 83. of various soils, 81. Methods of applying fertilizers, 495. Methods of studying drainage losses, 180. Micro-organisms of the soil, 386. Micron, magnitude of, 128. Millimicron, magnitude of, 128. Mineral constituents of soils, bulk analy- sis of, 314. extraction of with dilute acids, 317. extraction of with strong acids, 316. extraction of with water, 319. 578 INDEX OF SUBJECT MATTER Mineral cycles in soils, importance of, 408. nature of, 407. organisms of, 408. types of, 407. Minerals in earth's crust, 4. Minerals of the soil, 77. importance of, 6. list of, 5. source of, 4. specific gravity of, 89. Minerological analysis of soils, 76. Minerological character of soils, 77. of soil particles, 75. Minimum, element in the, 476. law of Mitscherlich, 477. Modification of soil air, 261. Moisture of soil, conservation by mulch, 221. conservation, weed control, 220. control, summary of, 221. data for sandy and clayey soils, 179, 200. determination on soil, method of, 161. effect on heat conductivity, 236. effect on movement of soil air, 258. effect on nitrification, 420. effect on specific heat of soils, 233. influence on nitrification, 420. influence on bacteria, 393. Moisture equivalent of soils, defined, 167. for various soils, 168. method of determination, 167. Molecules, absorption of by soil, 269. Moraines, agricultural value, 54. ground, 54. terminal, 54. Movement of soil air, factors affecting, 258. Muck, agricultural value of, 43. capacity for water, 164. character of, 43. chemical analysis of, 44. term defined, 43. Mulch, artificial, 218. soil, use of, 218. Muscovite, change of to kaolinite, 26. present in soils, 5, 77. Nitrates in alkali spots, 332. Nitrates in rain water, data, 429. Nitrates in soils, accumulation, 419. accumulation as influenced by green manure, 544, 549. accumulation, influence of grass on, 427. accumulation, influence of maize on, 428. as a source of nitrogen for higher plants, 415. assimilation of by soil organisms, 426, 428. influence of green manures on, 543. production of. 111. reduction of, 424. Nitrate reduction, cause of, 425. control of, 426. influence of green manures on, 426. influence of straw on, 425. nature of, 425. organisms of, 425. Nitrification in soil, as affected by soil conditions, 418. as influenced by carbon dioxide, 256. efTect of aeration on, 418. effect of alkali on, 421. effect of carbon dioxide on, 419. effect of farm manure on, 418. effect of moisture on, 420. effect of soil acidity on, 421. effect of temperature on, 420. efficiency of, 417. influence of higher plants on, 422. influence of lime on, 373. influence of previous crops on, 423. influence on carbon dioxide produc- tion, 255. nature of, 415. organisms of, 415. products of, 415. reactions, 415. relation to ammonification, 416. relation of to carbon cycle, 408. relation of to mineral cycle, 408. relation to soil fertility, 423. Nitrifying organisms, types of, 415. Nitrites, production of in soils. 111. Nitrobacter in soil, 415. Nitrogen, additions to soil, by free-fixing organisms, 430. additions to soil, in manure, 557. additions to soil, in rain water, 429. additions to soil, modes of, 429. INDEX OP SUBJECT MATTER 579 Nitrogen, additions to soil, nature of, 429. amount fixed by li. Radiricola, 437. amount in ammonium sulfate, 449. amount in calcium cyanamid, 452. amount in calcium nitrate, 452. amount in California soils, 120. amount in dried blood, 445. amount in Nebraska loess, 120. amount in sodium nitrate, 448. amount in soils, 12. amount in soils of United States, 118. amount in tankage, 445. artificial fi.xation of, 450. availability of in fertilizers, 454. contained in rocks, 10. determination of in soils, 312. fixation by B. Radicicola, 433. fixed in soil by azofication, 433. forms of in ■ soil, 10. from B. Radicicola, availability of, 437. gain due to green manures, 539. gain due to natural causes, 429. importance in biological processes, 409. importance of in fertility evaluation, 323. importance of in soils, 409. in farm manure, 501. in liquid and solid manure, 503. in rain water, data on, 429. inert character of, 409. influence on plant growth, 471. losses from Bromberg lysimeters, 306. losses from Cornell soils, 307. losses from decaying manure, 511. losses from farm manure, 519. losses from soil, 555. natural addition to soil, 557. of food recovered in farm manure, 516. organic forms used by plants, 411. possible detrimental influences of, 473. relation of to life, 409. removed by crops from Cornell soils, 325. utilization of organic forms by plants, 447. Nitrogen cycle, addition of nitrosen to soil by free-fixing bacteria, 430. Nitrogen cycle, addition of nitrogen to soil in rain water, 429. ammonification, 412. assimilation of nitrates by soil or- ganisms, 426. complexity of, 409. decay and putrefaction, 410. fixation of nitrogen by Azolohacter, 432. fixation of nitrogen by B. Radici- cola, 433. nitrification, 415. reduction of nitrates, 424. relation of to other cycles, 410. Nitrogenous fertilizers, ammonium sul- fate, 449. calcium cyanimid, 451. calcium nitrate, 452. castor pomace, 446. cotton seed meal, 446. dried blood, 445. flsh scrap, 446. guano, 446. hoof meal, 446. leather meal, 446. linseed meal, 446. process goods, 446. relative availability, 454. sodium nitrate, 448. tankage, 445. utilized by higher plants, 411, 447. wool and hair waste, 446. Nitrosomonas in soils, 415. Nitrous acid, relation to mineral cycle, 408. Nodules on the roots of leguminous plants, nature of, 434. Number of particles in soil, 96. Number of soil particles, calculation of, 96. Nutrient elements used by plants, amounts in soil, 12. defined and explained, 8. listed, 9. primary, 10. source of, 10. Nutrients in soils, addition by leguminous green manures, 557. addition of in farm manure, 557. dift'erential diffusion into plants, 293. difltusion into plants, 291. 580 INDEX OF SUBJECT MATTER Nutrients in soils, direct influence of plants on solubility of, 295. how lost from the soil, 289. influence of lime on solubility of, 373. lost by drainage and cropping, 307. lost by leaching, Cornell data, 210. lost by plant influence, 303. lost during manurial production, cows, 516. lost during manurial production, heifers, 516. lost during manurial production, sheep, 516. lost during manurial production, steers, 516. lost from Cornell soil, 554. lost from soil, relative losses, 308. lost in handling and storage of ma- nure, 517. natural additions of to soil, 556. quantities removed by crops, data of, 303. recovery of in farm manure, 616. solubility as influenced by carbon dioxide, 255. Nutrient losses from and addition to soil under various types of farming, 558. Ocean, soils found in, 50. Ohio results with raw rock phosphate, 462. Optimum soil moisture, influence of structure on, 201. for plant growth, 200. Organic carbon, determination of, 311. use of by higher plants, 402. Organic compounds of soil, character of, 105. classification of, 107. nitrogenous, 106. relation to plants, 108. Organic decay, effect on soil tempera- ture, 239. Organic decomposition, simple products of, 110. Organic matter, amount in Nebraska loess, 120. amount in soil of United^tates, 117. decay of, 103. compounds isolated from, 108. Organic matter, defined, 99. determination of in soils, 112. effect of on soil acidity, 353. effect on capillary capacity of soil, 164. effect on carbon dioxide of soil air, 253. effect on specific heat, 233. influence in soil, 8. influence of soil conditions on decay of, 124. influence on availability of rock phosphate, 460. influence on the soil, 121. in Minnesota soils, 119. maintenance of in soil, 122. Organic matter of soil, effect on heat conductivity, 236. general nature, 7. influence on bacteria, 394. portion alive, 100. sources of, 7, 99. Organic nitrogenous compounds, utiliza- tion by higher plants, 411, 446. Organic nitrogenous fertilizers of secon- dary importance, 445. Organic toxins, elimination of, 109. of soil, 108. Organisms, benefits of in soil, 397. pounds of in soil, 384. Orthoclase, change of to koalinite, 26. importance in soil, 6. Osmosis, defined, 289. how demonstrated, 290. pressure developed by, 290. Osmosis of water into plants, 290. Osmotic pressure, nature of, 290. Oswald method of converting ammonia into nitric acid, 453. Outlets for tile drains, construction of, 214. Oxidation, effect of on composition of soil air, 254. importance of in soil formation, 24. of sulfur in soil, 403. Oxidases, production of by plant roots, 297. Oxygen, importance in soil air, 256. Packers, action of, 148. Partial analysis of soils, 315. digestion with dilute acids, 317. INDEX OF SUBJECT MATTER 581 Partial analysis of soils, digestion with dilute acids, objections, 318. digestion with dilute acids, value, 318. digestion with strong acids, 316. digestion with strong acids, objec- tions, 316. extraction with water, 319. extraction with water, method of, 321. extraction with water, value, 322. of Minnesota soils, 316. of Minnesota and Maryland soils, 317. Partially decomposed matter of soils, 105. Particles of soil, number to a gram, 96. Peat, agricultural value of, 43. capillary capacity of, 164. character of, 43. chemical analj'sis of, 44. term defined, 43. Percolation, control of, 208. Cornell data, 209. efiects of crops on, 210. loss of nutrients by, 208. in arid regions, 208. in humid regions, 208. Rothamsted data, 207. Ph values of acidity explained, 350. Phosphate fertilizers, acid phosphate, 456. basic slag, 457. bone phosphate, 454. relative availability of, 458. rock phosphate, 455. Phosphoric acid, amount of in acid phos- phate, 456. amount of in apatite, 6. amount of in basic slag, 457. amount of in igneous rocks, 6. amount of in manure, 501. amount of in bone, 454. amount of in rock phosphate, 455. amount of in soils, 12. forms of in fertilizers, 456. forms of in soil, 11. influence of green manures on, 545. influence of lime on reversion of, 373. influence on plant growth, 474. in liquid and solid manure, 503. loss of from farm manure, 519. Phosphoric acid, loss of from soil, 555. losses from Cornell soils, 307. of food recovered in farm manure, 516. of soil, 140. organic and inorganic in soils, 315. organic nature of, 11, 314. pounds removed by various crops, 468. relative availability of in fertilizers, 458. Phosphorus, see phosphoric acid. Physical absorption by soils, 263. Physiological character of plants in re- lation to alkali toxicity, 336. Piconometer, for determination of specific gravity, 90. Plankers, action of, 148. Plants, absorptive activity of as deter- mined by certain factors, 299. acquisition of nutrients by, 291. alkali vegetation, 340. capacity of to grow on poor soils, 299. cause of drought resistance by, 195. cause of wilting, 194. detrimental influence of nitrogen on, 473. differential diffusion into, 292. different absorptive capacity for soil nutrients, 301. direct influence of upon soil nu- nutrients, 296. effect of alkali on, 334. effect of calcium and magnesium ratio on, 376. effects of on percolation, 208. factors affecting transpiration from, 188. function of water in, 184. growth of in acid medium, 350. influence of on the soil solution, 284. influence of phosphorus on, 474. influence of potassium on, 475. influence of roots on soil colloids, 297. influence of soil water on, 186. production of acids by, 296. productions of oxidases by, 297. reduction produced bj' roots of, 297. resistance of to alkali, 335. 582 INDEX OF SUBJECT MATTER Plants, response of to lime, 372. soil organisms injurious to, 396. tolerance of to soil acidity, 353. used for green manures, 546. utilization of ammonia by, 415, 450. utilization of organic carbon by, 402. utilization of organic nitrogen by, 411, 447. water requirements of, 187. Plants and animals, relation to soil formation, 23. Plant diseases, control of in soil, 397. Plant food, defined, 8. Plant growth, factors for, 8. influence of nitrogen on, 472. optimum moisture for, 200. temperature for, 224. Plant nutrients, amounts in soil, 12. contained in minerals, 5. defined, 8. derived from air, 9. derived from soil, 9. listed, 9. primary, 10. Plant roots, production of carbon dioxide by, 252. prying effect on rocks, 23. Plant tissue, composition of, 100. method of analysis, 102. Plasmolysis, defined, 290. Plowing, influence of on the soil, 146. Pore space of soils, calculation of, 94, 178. data on, 95. importance of, 95. nature of, 93. Potash, amount in soils, 12. fertilizers carrying, 463. forms of in soil, 11. ill farm manure, 501. influence on plant growth, 475. in liquid and solid manure, 503. in minerals, 6. loss of from soil, 555. loss of from farm manure, 519. losses from Cornell soils, 307. miscellaneous fertilizers of, 464. of food recovered in farm manure, 516. Potash fertilizers, alunite, 465. feldspar, 465. flue dust, 465. Potash fertilizer, kelp, 465. lake salines, 465. leucite, 465. Stassfurt salts, 463. wood ashes, 464. Potassium, see potash. Potassium chloride as a fertilizer, 463. Potassium nitrate, use of in litmus test, 358. Potassium sulfate as a fertilizer, 464. Poultry manure, character and composi- tion of, 503. Potatoes, influence of manure on, 534. Practical soil management, factors of, 560. Precipitation, addition of sulfur by, 469. Pressure, effect on gravity water, 175. effect on movement of soil air, 260. Process fertilizers, nature of, 446. Productivity, as influenced by soil solu- tion, 287. equation of, 327. Protection, influence of on farm manure, 525. Proteid compounds, changes of in de- caying manure, 510. Protozoa, importance of in soil, 387. number of in soil, 387. relation of to ammonification, 387. types of in soil, 387. Puddling of soils, 141. Purchase of commercial fertilizers, 483. Purchase of unmixed fertilizers, 484. Putrefaction, defined, 103, 410. of farm manure, 508. products of, 411. Qualitative composition of drainage water, 304. of the soil solution, 280. Quantitative composition of drainage water, 304. of soil solution, 282. Qualitative tests for soil acidity, 358. compared and criticized, 359. Quantitative tests for soil acidity, nature of, 355. value of, 357. Radiation, loss of heat from soil by, 240. Rain-water, analysis of, 429. sulfur in, 404. INDEX OF SUBJECT MATTER 583 Rational fertilizer practice, 497. Recovery of nutrients in fann manure, 516. Reduction, as aflfected by plant roots, 297. Reduction of nitrates in soil, 424. Reinforcement of farm manure, 527. agricultural value of, 529. balancing influence, 529. conserving effects, 529. Residual influence of manure, 531. Residual soils, age of, 39. analysis of, 33, 41. chemical composition of, 52, 57. compared with glacial soils, 57. colors of, 32, 39. formation of, 38. from specific rocks, 39. location of in U. S., 41. organic content, 41. Residues in soil from differential dif- fusion, 294. Resistance of plants to alkali, generalized table of, 338. Resistance to alkali by various plants, data on, 338. Reversion of mono-calcium phosphate in soil, 457. Reverted jjhosphoric acid, defined, 456. Rock phosphate, as a reinforcement for manure, 528. changes in soil, 456. compared with acid phosphate, 458. composition, 456. composted with manure, 462. influence of organic matter on, 460. Ohio results on, 462. source, 455. use of in sulfur composts, 406. Rocks, igneous, sedimentary and meta- morphic, 4. soil forming, 3. Rodents, macro, soil organisms, 384. Rollers, actions of, 148. Roots of higher plants, a type of macro- organism, 386. production of carbon dioxide by, 295. production of exudates by, 296. Rooting habit of plants, in relation to alkali toxicity, 336. Rotation, farm manure and the, 532. Sampling of soil, method of, 311. Sand dunes, nature of, 64. Season, influence on soil solution, 283. Sedentary soil, explanation of term, 28. Sediment carried into ocean, 46. Selective absorption by soils, nature of, 269. types of, 269. Selection of a commercial fertilizer, fac- tors to consider, 482. Sheet erosion and its control, 205. Size of colloidal particles, 128. Slope, influence on soil temperature, 229. influence upon absorption of solar insolation, 229. Sod, influence on nitrate accumulation, 427. Sodium chloride as a soil amendment, 380. presence of in alkali, 333. Sodium nitrate, changes in soil, 448. character of, 448. composition of, 448. retention of by soils, 321. origin of, 448. source of, 448. Soils, absorption by, 263. absorptive capacity of, 266. acid nature of, 345. acquisition of nitrogen by, 428. addition of sulfur to by precipita- tion, 469. aeolian, 61. alkali, 328. alluvial, 46. amendments used on, 363. ammonification in, 412. amounts of capillary water in, 166. amount of gravity water in, 177. available water of, 198. average composition of, 12. bulk analysis of, 311. capacity of to retain nitrates, 321. capillary capacity for water, 163. capillary movement of water in, 168. capillary water of, 159. cause of acid condition of, 351. changes of lime in, 369. cliemical analysis of water extract from, 320. coUuvial, 45. color of, 36. 584 INDEX OF SUBJECT MATTER Soils, composition of air in, 247. conductivity coefficients of, 236. conditions, effect on nitrification, 418. control of air in, 261. control of alkali in, 343. control of erosion, 205. cumulose, 42. defined, 2. diseases, control of, 397. diseases, nature of, 396. dynamic nature of, 3. eradication of alkali from, 341. erosion of by water, 204. fertility evaluation of by chemical analysis, 323. formation of, 16. forms of water in, 152. functions of water in, 184. general composition of, 2. geological classification of, 38. glacial, 54. granulation of, defined, 139. handling of alkali soils, 340. heat conduction in, 234. heat convection in, 238. hygroscopic water of, 152. importance of absorption by, 273. influence of earth worms on, 385. insolation received by, 225. lacustrine, 58. losses of water from, 202. management, practical factors of, 560. marine, 50. metliod of moisture determination on, 161. moisture data of, 200. movement of air in, 258. movement of gravity water in, 175. mulch on, 218. names in common use, 82. names, origin and meaning, 80. nitrification in, 416. nitrogen content of, 118. partial analysis of, 315. partial analysis with strong acids, 316. partial analysis with weak acids, 317. particles of, 67. plasticity of, 140. Soils, pore space of, 93. practical management of, 560. productivity of as related to soil solution, 287. puddling of, 141. reaction, importance of, 345. reaction, types of, 345. reduction of nitrates in, 424. residual, 38. sampling of, 311. series defined, 86. specific gravity of, 88. specific heat data, 232. sulfofying power of, 405. survey classification of, 85. tests for acidity in, 354. thermal movement of moisture in, 182. the solution of, 275. tilth, defined, 149. toxins of organic nature, 108. type defined. 86. weathering, importance of, 37. weight of, data, 93. wilting coefficient, 197. Soil acidity, active toxic bases, 346. as influenced by absorption, 274. causes of development, 352. causes of harmful effects, 346. expression of by ph values, 350. general nature of, 345. influence of absorption on, 352. influence of fertilizers on, 353. influence of leaching on, 352. influence on bacteria, 395. influence on nitrification, 421. lack of calcium in relation to, 348. lack of nutrients tlieory, 348. lime requirements, determination of, 355. litmus paper test for, 358. present status of question, 349. relation of iron to, 347. relation of manganese to, 347. resume of, 360. tests for, 354. theory, aluminum, 347. theory, hydrogen ion, 346. tolerance of plants to, 353. Truog test for, 358. types of tests, 355. zinc sulfide test for, 358. INDEX OF SUBJECT MATTER 585 Soil air, carbon dioxide of, 250. general characteristics, 247. composition data, 248, 250. control of, 261. general composition of, 247. movement of, 258. resume of, 262. types of, 249. volume of, 257. Soil amendments, forms of lime, 363. organic matter important as, 124. Soil analysis, alluvial and upland, 49. arid and humid soils, 31. determination of organic matter, 112. glacial soils, 57. granite soil, 33. good and poor soils, 326. humus determination of, 115. humus in California soils, 120. humus in Nebraska soils, 120. lime requirement of soil, 355. limestone soil, 33. loess soils, 63. marine soils, 52. mechanical, 67. nitrogen in California soils, 120. nitrogen in Nebraska soils, 120. nitrogen in soils of United States, 118. organic matter in Nebraska loess, 120. organic matter in Minnesota soils, 119. organic matter in soils of United States, 117. peat and muck, 44. residual soils, 41, 52, 57. Soil class, discussion of, 79. determination from a mechanical analysis, 84. practical determination of, 83. Soil colloids, absorption by, 265. as influenced by plant roots, 297. generation of, 132. importance of, 135. influence of, 135. resume of, 138. Soil color, cause of, 36. significance of, 36. Soil erosion and its control, 204. types of, 205. Soil exhaustion, discussion of, 309. possibility of, 3 OS. time for, 309. Soil extraction, a metliod of studying the soil solution, 279. Soil fertility, defined, 554. effect on transpiration, 192. factors involved in maintenance, 554. importance of nitrification to, 423. influence of plants and animals on, 23. maintenance program of, 560. relation of sulfur to, 468. sources of knowledge, 554. Soil formation, forces of, 16. general statement of, 29. glacial action, 18. influence of carbonation, 26. influence of climate, 30. influence of hydration, 26. influence of solution, 27. oxidation and deoxidation, 24. processes classified, 16. special cases of, 32. temperature changes, 21. water action, 17, 19. Soil heat, importance of, 223. influence of on the soil, 224. loss of by conduction, 240. loss of by evaporation, 240. loss of by radiation, 240. transfer of, 238. Soil humus, determination of, 115. Soil minerals, importance of, 6. list of, 5. Soil moisture, conservation of, 219. data of, 179. effect on conductivity of heat, 236. effect on heat capacity, 233. effect on transpiration, 191. importance of amount in plowing, 146. influence of on the soil solution, 286. optimum for efficient tillage, 150. optimum for plants, 200. relation of to granulation, 142. Soil mulcli and moisture conservation, 221. relation of to capillary movement, 175. use of, 218. 586 INDEX OF SUBJECT MATTER Soil organic matter, amount of in Minnesota soils, 119. amount of in soils of United States, 117. general nature, 7. importance of, 121. maintenance of, 122. resume of, 126. source and character of, 99. Soil organisms, and the free-fixation of nitrogen, 431. benefits of, 397. general methods of study, 399. groups of, 384. influence in nitrate assimilation, 426. influence of alkali on, 335. injurious to higher plants, 396. macro-animal forms, 384. macro-plant forms, 385. micro-animal forms, 386. micro-plant forms, 388. resume of, 440. Soil particles, character as determined by size, 69. classification of, 67. minerological character, 75. number of, 95. surface of, 97. Soil separates, chemical and minerological characters, 75. chemical composition of, 78, 79. physical characters of, 73. sizes of, 67. specific gravity of, 89. Soil solution, as studied by aqueous ex- traction, 279. as studied by depression of freezing point, 280. composition data of, 283, 288. concentration data of, 282, 285, 286. general character of, 275. influence of crop on, 284. influence of miscellaneous factors on 286. influence of season on, 283. methods of study, 277. qualitative composition of, 280. quantitative composition of, 282. relation to absorption, 276. relation to productivity, 287. summary of, 288. Soil structure, ideal, 88. nature of, 87. types of, 139. Soil temperature, control of, 244. data of, 243. influence of slope, 229. variations of, 242. Soil water, availability of, 198. diagram of forms, 199. effect on air movement, 258. effect on specific heat of soils, 233. form of molecule, 28. forms of, 151. function to plants, 184. general characteristics of, 152. influence on plants, 18G. loss by evaporation, 216 loss by percolation, 206. loss by percolation at Cornell, 209. loss by percolation at Rothamsted, 207. modes of loss, 202. methods of expressing, 156 run-off losses, 203. summary of control, 221. thermal movement of, 182. Soluble matter carried into ocean, 40. Soluble salts in soil, influence on nitri- fication, 421. Solubility of nutrients as influenced by carbon dioxide, 255. Solution, loss of nutrients because of, 2S. importance of in soil formation, 27. relation of to soil productivity, 28. Specific gravity of minerals, 89. Specific gravity of soils, defined, 88. determination of, 90. Specific gravity of soil separates, 89. Specific heat, data on soils, 232. defined, 231. Specific heat of soil, 231. factors affecting, 232. Stages ill the decay of green manures, 542. Stassfurt salts, chlorides and sulfates, 463. kainit, 463. silvinit, 463. Stone drains, construction of, 212. Straw, influence on nitrate reduction, 425. Streams, soil formation by, 46. INDEX OF SUBJECT MATTER 587 structure of soil, effect on capillary capacity, 164. effect on capillary movement, 174. effect on gravity water, 176. effect on heat conductivity, 236. ideal condition, 88. influence on optimum water, 201. nature of, 87. summary of, 149. types of, 139. Substitutions of bases in soils, 270. Sulfate sulfur as a fertilizer, 468. Sulfofication, effect of lime on, 405. factors influencing, 405. influence on carbon dioxide produc- tion, 255. determination of, 405. reactions of, 403. relation of to mineral cycle, 408. Sulfur, amount added to soil in precipi- tation, 469. amount in soils, 13. as a fertilizer, 467. experiments with as a fertilizer, 467. forms of in soil, 11. how lost from soil, 404. importance of in soil fertility, 470. loss of from Cornell soils, 307, 404. loss of from soil, 5.35. natural addition to soil, 557. oxidation of in soils, 403. possible deficiency in arable soils, 468. pounds removed by various crops, 468. sources of in soils, 403. use in composting, 406. use of as a sulfate, 468. Sulfur composts, 406. Sulfur cycle of soil, losses of sulfur from, 404. sources of sulfur, 403. sulfofication, 403. Sulfurous acid, relation to mineral cycle, 408. Superfluous water, 198. Surface of soil particles, calculation of, 97. importance of, 97. Surface tension, defined, 160. effect on capillarity, 170. Surface tension, force of, 160. relation to capillary movement, 169. Synergism, relation of to plant absorp- tion, 300. relation of to soil acidity, 349. nature of, 349. Systems of applying fertilizers, 496. Tankage changes in soil, 445. character of, 445. composition of, 445. source of, 445. Temperature of soil, control of, 244. data of, 243. effect of change on soil air, 259. effect on capillary capacity, 163. effect on gravity water, 176. importance in soil formation, 21. influence of decay on, 239. influence of slope, 230. Influence on bacteria, 394. influence on hygroscopic coefliclent, 158. influence on nitrification, 420. variations of, 242. Temperatures for crop growth, 224. for germination of seeds, 224. Terracing, 205. Texture of soil, definition of, 66. effect on absorption, 267. effect on capillary capacity, 164. effect on capillary movement, 173. effect on gravity water, 176. effect on heat conduction, 236. effect on specific heat, 232. influence on moisture equivalent, 168. Thermal movement of soil water, na- ture of, 182. relation to evaporation, 182. Tile drains, depth and interval of, 214. effective grade for, 214. functions of, 212. outlets of, 214. size of tile, 213. study of drainage water from, 180. systems, 212. table for determination of size, 214. Tillage, influence on granulation, 144. influence on soil solution, 286. killing of weeds by, 219. Tilth of the soil, defined, 149. 588 INDEX OF SUBJECT MATTER Time, influence on absorption by soils, 269. Time of applying fertilizers, 495. Tolerance of plants to soil acidity, 353. Tramping, influence on farm manure, 524. Transpiration, factors affecting, 188. Transpiration ratio, defined, 187. determination of, 187. of different crops, data, 189. Transported soil, explanation of term, 28. Truog test for soil acidity, 358. Types of farming, influence on the main- tenance of fertility, 558. Urea, ammonification of, 414. decomposition of in manure, 509. production of from calcium cyana- mid, 250. Unavailable water in soil, 198. Unmixed fertilizers, purchase of, 484. use of, 484. Utilization of ammonium in salts by higher plants, 450. of organic compounds by plants, 446. Variability of farm manure, 506. Value of farm manure, agricultural, 513. commercial, 512. Vegetables, fertilizer formulae for, 491. Vegetation, resistant to alkali, 340. Veitch method of determining the lime requirement of soils, 356. procedure, 356. value of, 357. Viscosity, effect on capillarity, 170. Volcanic dust, as soil, 65. Volume of soil air, 257. calculation of, 258. Volume weight, determination of, 91. relation to specific gravity, 94. Volume weight of soils, data, 93. explanation of, 91. Water, alkali in river water, 332. availability of to plants, 198. deposition of sediment by, 46. diagrams of forms in soil, 199. effect on rocks by freezing, 23. erosive effects of, 204. function of to plants, 184. in farm manure, 501. Water, influence on concentration of soil solution, 286. influence on plants, 186. intake of by plants, 289. loss of from soil, 202. loss of from soil by percolation, 206. loss of from soil by evaporation, 216. mechanical action of, 17. methods of expression in soil, 156. movement in soil, 168, 175, 182. movement in soil in relation to plants, 193. production of hydration by in soil, 27. relation of to granulation, 142. required to mature a crop, 193. use of alkali water in irrigation, 333. Water requirements of plants, factors affecting, 188. investigations of, 189. nature of, 187. Water slaked lime, 364. Water soluble phosphoric acid, defined, 456. Weathering, character of in arid regions, 30. character of in humid regions, 30. defined, 16. losses due to, 33. of granite, 33. of limestone, 33. of soil, practical relations of, 37. relation of to alkali, 331. Weeds, killing of, 219. Weight of soils, data of, 93. Wilting, cause of, 194. explanation of, 194. Wilting coefiicient, calculation of, 198. determination of, 196. effect of texture on, 196. explained, 195. for different soils, 197. Wind in soil formation, 19. Wool and hair waste, composition of, 446. Zinc-Bulfide test, criticism of, 360. for soil acidity, 358. Zeolites, not present in soil, 265. '! i I |! liilill'IIIIMii'ii liillmtiilSi! illillililiiil