) 5\ Ws-.s? V L^ yt Issued AprU 24, 1915. HAWAII AGRICULTURAL EXPERIMENT STATION, J. M. AA^ESTGATE, Agronomist in Charge. Bulletin No. 38. EFFECT OF FERTILIZERS ON THE PHYSICAL PROPERTIES OF HAWAIIAN SOILS. BY WILLIAM McGEORGE, Assistant Chemist. UNDER THE SUPERVISION OP OFFICE OF EXPERIMENT STATIONS, U. S. DEPARTMENT OF AGRICULTURE. WASHINGTON: GOVERNMENT PRINTING OFFICE. 1916. s NOV J»3:£S Issued April 24, 1915. HAWAII AGRICULTURAL EXPERIMENT STATION, J. M. WESTGATE, Agronomist in Charge. Bulletin No. 38. EFFECT OF FERTILIZERS ON THE PHYSICAL PROPERTIES OF HAWAIIAN SOILS. BY WILLIAM McGEORGE, Assistant Chemist. UNDER THE SUPERVISION OF OFFICE OF EXPERIMENT STATIONS, U. 8. DEPARTMENT OF AGRICULTURE. WASHINGTON: GOVERNMENT PRINTING OmOE. 1915. HAWAII AGEICULTURAL EXPERIMENT STATION, HONOLTJLXJ. [Under the supervision of A. C. True, Director of the Oface of Experiment Stations, United States Department of Agriculture.] Walter H. Evans, Chief of Division of Insular Stations, Office of Experiment Stations. STATION STAFF. J. M. Westgate, Agronomist in Charge. J. Edgar Higgins, Horticulturist. D. T. FuLLAWAY, Entomologist. William McGeorge, Assistant Chemist. Alice R. Thompson, Assistant Chemist. V. S. Holt, Assistant Horticulturist. C. A. Sahr, Assistant in Agronomy. F. A. Clowes, Superintendent of Hawaii Substations, (2) LETTER OF TRANSMITTAL Honolulu, Hawaii, August 1,1914. Sir: I have the honor to submit herewith and recommend for pubHcation as Bulletin No. 38 of the Hawaii Agricultural Experiment Station, a paper dealing with the Effect of Fertilizers on the Physical Properties of Hawaiian Soils, by William McGeorge, assistant chemist of this station. The peculiar constitution of Hawaiian soils makes a study of the physical properties of these soils one of unusual impor- tance. In this paper there are described systematic experiments made to determine the effect of various fertihzers upon capillarity, percolation, flocculation, cohesion, apparent specific gravity, vapor pressure, and hygroscopic moisture. Before it is possible to reach a thorough understanding of Hawaiian soils it has been found neces- sary to study them from all standpoints. The paper is a contribu- tion to the knowledge of the physical properties of soils and particu- larly of the effect of fertilizers in other ways than as plant food. Respectfully, E. V. Wilcox, Special Agent in Charge. Dr. A. C. True, Director Office of Experiment Stations, Z7. S. Department of Agriculture, WasMngton, D. C. PubHcation recommended. A. C. True, Director, PubHcation authorized. D. F. Houston, Secretary of Agriculture, (3) CONTENTS. Page, Introduction 7 Soil types 8 Water capacity of soils 9 Specific gravity 9 Capillary movement 9 Effect of various salts in molecular proportions 12 Effect of basicity on capillarity 16 Percolation 17 Flocculation 19 Cohesion 22 Apparent specific gravity 25 Vapor pressure 28 Hygroscopic moisture 29 Sunmiary 30 ILLUSTRATIONS. Page. Fig. 1. Relative capillarity following addition of salts in equal quantity and in molecular proportions 10 2. Effect of salts and fertilizers on capillary rise of moisture 14 3. Relation between cohesion, apparent specific gravity, and moisture content of soils 27 (5) THE EFFECT OF FERTILIZERS ON THE PHYSICAL PROPERTIES OF HAWAIIAN SOILS. INTRODUCTION. It has been the custom in Hawaii since agriculture was first placed upon a commercial basis to stimulate crops with heavy applications of mineral fertiUzers. This procedure has been maintained in spite of the fact that a majority of the soils are naturally well supphed with plant food, in some instances abnormally so. Hawaiian soils are of such a nature that the maintenance of the best possible physical state is imperative. They are derived from the disintegration of basaltic lava, have since been impregnated with coral limestone in many of the lowlands and with large amounts of organic matter in the uplands, where the rainfall is high and the vegetation profuse. Being derived from highly basic rocks, the result- ing soils are highly basic in composition. The silica content varies from 15 to 50 per cent, while the basic constituents, iron and alumi- num, compose the major part of the remainder. The tendency of these metals to form hydrates or sihcates, their influence upon the mechanical structure, drainage, cUmatic conditions, temperature, aeration, and above all the effect upon the moisture supply, are mat- ters which demand the careful consideration of agriculturists in these islands. Soil moisture is a prime factor in successful plant growth. It not only influences the physical condition but also acts as a vehicle for the transmission of plant food from the soil to the plant. Since all mineral and many organic substances are more or less soluble in water and since all dissolved material is known to affect the physical properties of the solvent, it may be concluded that the properties of soil moisture and also the physical condition of the soil are partially dependent upon the composition of the soil solution. These physical properties include capillarity, percolation, floccula- tion, cohesion, apparent specific gravity, vapor pressure, and hygro- scopic moisture. Although the laboratory determinations of these properties of a soil and of the effect of salts thereon have little direct practical value, since the soil in such cases is not in a natural state, and other conditions are abnormal, they may be useful for (7) 8 studying the relationship of different soil types and the effect of fertilizers upon the physical factors influencing plant growth. From previous work in this laboratory and the experiences of prac- tical farmers, physical factors appear to play a large part in the fertility of Hawaiian soils. The effects of heat and volatile antisep- tics, the action of lime, the high absorptive power for fertilizers, the difficulties in drainage, high cost of tillage, and the pecuUar biological effects that ensue, seem to be very largely explainable on a physical basis and to be referable in part to coUoids. In view of the above facts, this station has devoted considerable time to investigations upon the physical properties of soils and the function of fertilizers, other than as a source of plant food, with special reference to the movement of soil moisture. It is quite gen- erally conceded that no simple explanation of the influence of fertili- zers upon the soil or the plant is possible. SOIL TYPES. As in previous investigations upon soils in this laboratory, those of widely differing chemical and physical characteristics were chosen. The following tables show the physical composition and properties of these types. In Table I wiU be found the mechanical composition as determined by sedimentation according to Hall.^ Table I. — Mechanical analyses of the soils. Soil No. Moisture. Volatile matter. Fine gravel. Coarse sand. Fine sand. Silt. Fine silt. Clay. 428 Per cent. 13.80 12.45 10.41 3.58 7.98 12.26 7.60 Per cent. 25.65 28.83 17.64 13.90 17.81 20.44 13.96 Per cent. 11.89 1.70 3.40 Per cent. 28.26 7.52 5.29 Per cent. 13.63 14.29 29.66 5.76 31.26 31.48 8.07 Per cent. 4.64 11.75 10.30 10.34 13.39 19.10 9.35 Per cent. 1.53 17.49 15.75 37.97 17.79 11.93 24.90 Per cent. 0.60 448 5.97 516 7.55 530 0.49 .81 1.50 .36 1 1.15 27.96 542 10.96 573 . 3.29 574 34.87 Of the above soil No. 428 is a dark-colored, highly organic, sandy soil from Glenwood, Olaa, Hawaii. Soil No. 448 is a yellow silty sand from Hilo, Hawaii. Soil No. 516 is a sample of manganiferous soil from the Wahiawa district, Oahu. It has a chocolate-bro^vn color, silty texture, and maintains an excellent mechanical condition. Soils Nos. 530 and 574 are samples of red-clay soils, the former of a light and the latter of a dark red color. Soil No. 542 is a titaniferous soil of grayish red color and silty texture. Its physical condition is very similar to that of soil No. 516. Soil No. 573 is a ''dust" soil from the island of Hawaii. It is a dark-colored, highly organic silt, and very productive. 1 The Soil. London, 1908, 2. ed., p. 51. WATER CAPACITY OF SOILS. Table II contains data illustrating the water capacities of these soils. Columns 4 and 5 show the percentage of water by weight and volume, respectively, required to saturate the soil, and columns 6 and 7 show the percentage by weight and volume required to saturate and fill interstitial spaces. Table II. — Water capacity of the soils . Sou No. Weight of soil. Volume of soil. Percentage of wa- ter to saturate. Percentage of water to saturate and fill spaces. Weight. Volume. Weight. Volume. 428 Gm. 150 150 150 150 150 150 150 Cc. 177 168 176 132 160 189 161 Per cent. 61.1 60.2 66.3 43.7 61.7 76.3 50.3 Per cent. 51.8 53.7 56.5 49.6 57.8 60.7 46.9 Per cent. 64.8 64.3 71.6 48.3 63.7 76.3 62.6 Per cent. 55.1 448 57 5 516 61.0 530 54.7 542 59.7 573 60.7 574 58.3 SPECIFIC GRAVITY. In Table III are given the specific gravities, both real and apparent, as well as the comparative volume occupied by 10 grams of these soils, excluding intei-stitial spaces,' as determined upon the air-dry soils. Table III. — Specific gravity and volume of the soils. Soil No. Real specific gravity. Apparent specific gravity. Volume occupied. Soil No. Real specific gravity. Apparent specific gravity. Volume occupied. 428 2.4825 2.5264 2.8351 2.9438 0.8474 .8929 .8522 1.1363 Cc. 4.03 3.96 3.53 3.40 542 2. 8784 2.4454 2.9087 0.9375 .7936 .9316 Cc. 3.48 44S 573- . . 4 09 516. 574 3.46 530 The foregoing data, while of more or less empirical nature, indicate the variation in physical properties of Hawaiian types of soil. The clays show the highest specific gravity, both real and apparent, the clay silts and silts next, while the sandy soils show the lowest. The opposite relation exists with regard to the volume and water capacity. CAPILLARY MOVEMENT. Upon the capillary movement of water more than upon any other physical factor is the plant dependent for successful growth. The functions of capillary water are many and involve the transmission of plant food from the soil to the plant, sustenance of the enormous evaporation during the heat of the day, and the like. By means of 81436°— Bull. 38—15 2 10 this property water tends to distribute itself in all directions through- out the soil. There may be properly considered to be three kinds of water present in soils — capillary, gravitation, and hygroscopic moisture. The capillary water is that which wiU not drain away but is held around the soil particles in the form of a moisture film; that is to say, there is an equilibrium between the forces of gravity and surface tension. However, capillary action is itself dependent upon and abso- lutely governed by such subfactors as density or gravity, viscosity, surface tension of the soil solution, and the size and composition (both organic and inorganic) of the soil particles. Fig. 1.— Relative capillarity following addition of salts in equal quantity and in molecular proportions. When an object is removed from an immersion in water it retains a thin film upon its surface through the property of surface tension. In the same manner the soil particles are surrounded by a film or elastic membrane of water under a high pressure, the thickness vary- ing within certain limits with the moisture content of the soil. As water is lost at the surface by evaporation or around the roots by absorption, there is a movement of water in the direction of in- creased tension, thereby tending to maintain an equal distribution of water. On the other hand, viscosity, acting in an opposite manner from surface tension, tends to retard the movement of water. Water in soils is never pure, and all dissolved substances affect the degree of surface tension and viscosity of the solvent. Practically 11 all inorganic salts increase the surface tension of water while organic substances decrease it. On the other hand, the opposite relation exists with regard to viscosity. A series of experiments made at the Maryland Experiment Station ^ upon the surface tension of soil extracts show it to be considerably less than pure water. These and many other facts indicate the complexity of the application of theo- retic principles to soils. The capillary power of a soil is generally measured by the height to which water will rise in a soil column, although it may take place in all directions in soils and varies greatly according to the mechanical composition. The present study of capillarity was carried out in 1-inch glass tubes. Experiments were made not only with soils but also with silica sand and kaolin to ascertain the relation of these materials to soils. In aU about 40 salts, fertilizer materials, and mixtures were used, including several organic manures. As a means of studying the effect of these salts, comparisons were made both when added in amounts proportional to their molecular weights and when added in equal amounts. Also measurements were made upon the variation in height to which the water would rise as affected by amounts of the salt varying from 0.06 to 6.66 per cent. In all cases the salts were added in small amounts. Before presenting the data obtained in these experiments it is of interest to know the relative capillary activity of the soils in ques- tion. This is shown in Table IV, in which the figures were obtained by allowing the soil column to stand in about one-half inch of water for 78 days. Table IV. — Capillary rise of water in the soils. Soil No. 6 hours. 24 hours. 48 hours. 4 days. 5 days. 6 days. 7 days. 9 days. 11 days. 19 days. 32 days. 43 days. 78 days. 44S Cm. 26.8 22.4 31.6 39. S 36.8 41.0 33.7 Cm. 35.5 29.1 38.1 51.3 47.8 56.6 40.1 Cm. 40.9 34.4 42.9 57.3 Cm. 46.0 40.0 47.3 62.7 Cm. 47.8 41.6 49.0 64.6 64.0 79.0 45.1 Cm. 49.1 42.5 50.4 65.9 65.0 81.6 45.1 Cm. 50.3 43.8 51.5 67.8 66.8 83.5 45.1 Cm. 52.1 45.2 53.8 70.6 69.2 88.4 45.1 Cm. 54.0 46.7 55.6 72.8 71.2 91.6 Cm. 60.4 51.0 58.4 79.3 76.4 100.8 Cm. 67.0 55.3 62.1 86.6 84.8 109.8 50.5 Cm. 70.8 ■"64.' 8' 89.8 88.7 1115.6 Cm. 82.5 530 71.5 428 76.5 542 100.0 516 55.2 62.0 64.2 69.9 44. 45. 1 100.0 573 574 63.0 1 Top of column. The highly organic silty soil showed the greatest capillary rise, the clay soils least, while the sandy soils were intermediate. In filling the tubes it is necessary to exercise considerable care in order to obtain a uniform mixture. This is made possible by pro- jecting a long mre with a loop on the end into the tube and withdraw- ing it with a rotary motion. 1 U. S. Dept. Agr., Weather Bur. Bui. 4 (1892), p. 16. 12 EFFECT OF VARIOUS SALTS IN MOLECULAR PROPORTIONS. In studying the relations of the properties of salts of widely varying molecular composition it is imperative that a definite procedure be established. With this idea in mind the following curve (fig. 1, p. 10) is presented to show the difference in results obtained when the salts were added in equal amounts and in molecular proportions to soil No. 530. The former rate was 0.5 gram of basic oxid per 100 grams of soil and the latter as follows : Calcium carbonate, with a molecular weight of approximately 100, was chosen as a standard and added at the rate of 0.1 per cent, the other salts in greater or less amounts in proportion to their molecular weights. From these cm*ves it appears that as a whole the general property of a given salt is to affect the soil similarly whether added in equal weights or in molecular proportions. This is at least true of the capillary activity. The curves throughout are very similar and indi- cate that an increase in the concentration of the salt results in a diminished capillary activity. This variation in activity as affected by increasing the concentra- tion of the salt suggested a further study of the phenomenon, the results of which are given in Table V. Three types of soil and silica sand were used and the amount of salt used varied from none, to 10 grams per 150 grams of soil. Table V.- -Capillary rise as affected by increase in percentage cf salt added. Percentage of salt added. Salts. Soil No. 530. Sou No. 448. 0.00 0.06 0.16 0.33 0.66 3.33 6.66 0.00 0.06 0.16 0.33 0.66 3.33 6.66 Cm. Cm. Cm. Cm. Cm. Cm. Cm. Cm. Cm. Cm. Cm. Cm. Cm. Cm. CaO 37.3 37.3 34.1 34.0 33.2 33.0 32.2 31.4 33.5 31.5 18.7 28.0 12.5 26.2 30 30 35.0 35.9 36.5 37.5 36.0 36.0 37.0 36.0 34.0 34.2 29.0 NaNOs 33.0 (NH4)2S04.... 37.3 31.9 31.8 28.6 25.5 21.1 21.4 30 37.6 35.3 36.0 34.2 31.0 29.8 CaSoi. 34.0 29.8 29.3 29.6 28.5 29 8 33.4 30 34.2 34.0 34.4 33.2 34.9 36.0 CaH(P04) 37.3 35.5 37.8 37.2 38.8 32.7 23.3 30 34.6 35.4 34.9 37.0 38.3 38.8 CaCOs 37.3 35. 4 33.0 31.8 33.8 35.3 38.0 30 33.3 32.7 32.2 34.1 35.9 38.5 K3PO4 34.0 31.0 29.0 25.9 26.0 21.7 13.9 30 34.9 33.6 33.0 31.4 31.1 24.0 KCl 34.0 28.7 27.8 29.0 28.0 26.5 26.0 30 34.2 35.0 34.4 35.0 36.3 34.6 MgS04 37.3 32.5 30.6 27.9 27.2 21.9 21.7 30 33.1 38.0 34.8 35.8 31.0 27.6 NH4CI 37.3 31.8 30.1 30.5 29.2 26.4 28.2 30 34.2 34.2 33.2 35.8 34.5 32.3 NajCOa 37.3 28.4 24.3 21.4 18.7 9.4 6.9 30 32.6 33.1 30.1 29.5 17.4 14.8 K2SO4 37.3 32.3 31.8 30.0 28.2 26.0 25.8 30 36.4 36.9 36.4 34.8 31.5 31.5 Salts. Sou No. 428. SUica sand. CaO 29.0 29.0 37.0 38.0 36.1 38.0 38.0 .37.4 40.0 38.0 33.2 35.5 26.2 34.8 9 8 9.8 7.3 10.9 6.4 12.4 6.9 13.7 7.0 13.7 20.0 12.1 28.8 NaNOa 11.4 Slbr::::: 29.0 37.9 34.9 34.6 35.0 33.5 29.4 9.8 12.2 12.6 12.6 12.6 12.3 12.0 29.0 37.0 38.0 .35.3 38.4 38.8 41.2 10.2 12.1 16.7 13.0 14.5 22.2 34.2 CaH(P04) 29.0 38.6 36.5 39.0 40.0 38.2 34.1 10.2 13.9 16.8 15.3 14.4 14.5 18.4 CaCOa........ 29.0 37.7 38.0 35.2 36.3 38.9 37.6 10.2 16.6 14.9 18.3 20.5 27.4 32.6 K8PO4 29.0 38.8 37.9 37.0 36.5 33.1 24.2 10.2 12.4 14.6 16.6 15.7 11.6 12.2 KCl 28.0 36.8 36.8 36.6 36.9 37.1 35.2 9.8 10.9 13.5 11.9 12.4 14.1 16.1 MgS04 30.7 43.2 42.9 42.1 42.8 40.0 36.1 9.8 13.9 16.5 13.8 13.7 13.7 13.7 NH-2i l(NH4)-9-li l(NH4)-6-10i 4(NH4)-ll-10i 4.5(NH4)-23-0i 2(N03)-14-22 2(N03)-12-22 Check Time in hours Weight per 200 gm. soil. Gm. 0.35 .15 .20 .42 .28 .11 .27 .20 .24 .70 .28 .12 .72 .21 .17 .11 .26 .16 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 Soil No. 488. Cm. 45.9 46.6 48.8 42.1 44.8 46.6 46.3 47.6 45.2 47.5 47.2 44.3 45.4 46.5 45.9 45.0 45.5 48.9 40.0 47.8 45.4 46.5 46.8 45.7 44.6 47.6 47.1 47.4 47.9 47.9 47.1 47.7 46.8 47.5 47.6 46.3 47.7 47.6 46.8 35.5 24 Soil No. 530. Cm. 32.7 33.4 33.0 31.1 30.2 35.0 33.4 34.8 36.9 34.9 31.9 31.3 30.5 35.2 29.7 27.2 34.2 32.8 31.0 34.0 33.9 37.4 37.0 35.7 31.4 34.8 34.0 36.8 32.8 35.4 35.0 34.5 32.8 34.3 34.3 33.5 35.5 33.3 33.0 39.2 Soil No. 573. Cm. 55.0 55.3 55. 7 54.2 54.9 55.5 56.2 56.0 55.5 55.1 54.2 55.8 54.0 54.5 54.7 54.4 53.7 54.7 55.2 54.1 54.9 55.2 54.4 54.9 54.8 54.2 55.5 54.5 55.4 54.6 55.9 56.6 56.9 55.8 55.9 54.6 54.6 56.0 55.9 55.0 24 Soil No. 516. Cm. 37.1 39.0 36.6 34.1 36.5 36.9 38.3 35.8 34.5 36.2 36.5 37.1 36.4 36.4 30.0 34.7 38.0 37.8 37.6 38.5 34.5 35.2 .35.4 33.6 33.4 35.7 36.2 34.9 36.0 35.1 37.5 37.3 37.1 37.5 36.1 35.9 37.0 35.0 35.8 40.4 8 Soil No. 542. Cm. 39.0 39.1 39.8 36.7 38.3 40.7 41.4 42.9 40.4 40.3 40.4 41.7 38.7 41.3 36.9 40.0 40.4 41.7 42.1 42.6 40.2 40.5 42.3 42.0 37.7 41.7 42.5 42.9 41.8 40.7 42.4 42.3 40.7 42.0 41.2 40.9 40.9 41.9 40.8 44.9 Soil No. 428. Cm. 36.4 35.1 36.8 34.7 33.5 37.8 38.3 37.6 39.2 37.0 37.0 37.0 38.6 37.1 39.6 36.2 40.3 39.1 37.0 38.1 39.5 40.0 40.4 39.9 31.4 40.3 40.7 39.0 37.4 39.8 41.1 41.5 38.3 39.6 37.5 28.0 21 Kao- lin. Cm. 23.6 25.4 24.8 23.3 23.6 26.1 21.9 25.2 25.6 24.7 25.0 26.5 22.1 23.6 20.2 21.2 22.8 25.7 24.1 25.9 26.0 25.7 26.3 26.6 27.4 21.6 26.4 26.7 26.5 24.6 24.7 26.8 25.5 24.6 26.6 23.9 26.4 24.5 28.0 26 Sand. Cm. 15.4 16.2 14.6 17.5 17.0 10.8 18.9 12.8 15.9 14.5 17.5 23.0 21.1 16.7 20.3 18.7 14.0 13.9 16.5 14.4 15.8 15.8 11.8 15.0 9.5 11.7 14.2 15.8 12.1 14.2 14.8 15.3 14.1 12.2 12.9 13.9 13.3 12.8 14.2 12.9 24 1 Fertilizer mixtures supplying nitrogen (in ammonium sulphate), phosphoric acid, and potash in the order and proportions (percentages) given. 2 Fertilizer mixtures similar to those referred to in footnote 1 except that nitrogen was supplied in sodium nitrate. The accompanying curve (fig. 2) illustrates the effect of the fertilizers on capillarity more clearly than the table. Effect on silica sand. — Here it is possible to explain most of the results by applying the knowTi effects of salts upon the capillary activity of their solvents. Inorganic salts increase, and organic sub- stances retard, the rise of water. The principal exceptions to this rule are calcium oxid and slag. Effect on soils Nos. 428 and 448. — With these soils the rate of rise in moisture is quite rapid at first, decreasing to a very slow rise in a short time. All organic and inorganic substances added produced an increase in rate of capillarity within the time limits of the experi- ment. These soils have a coarse texture, contain a high percentage of organic matter and of hygroscopic and combined water, and hence indicate the relation of these properties to the capillary activity of soil extracts. 16 Effect on soil No. 673. — This soil, of all the types examined, pos- sessed the greatest capillary activity and was least affected by the addition of salts. In fact, it may be assumed that the capillary activity of this soil is unaffected by the addition of outside agents as the results agree with the check within the limits of experimental error. In view of the above facts, a series of experiments was car- ried out in which the amount of salt added was doubled, the results [of which agreed very closely with those reported above. Effect on soils Nos. 516 and 5Jf2. — These soils are of very similar mechanical composition but belong to different chemical types. The data presented indicate that the clay tends to dominate the capillary activity of this type of soil in that the addition of aU substances diminished capillarity. Effect on soil No. 630. — The physical character of this type of soil is such that chemical agents would be expected to materially affect its texture. This theory is borne out in the diminished capillary activity noted in every instance. Effect on Icaolin. — The diminished capillarity observed in the study of the effect of salts on kaolin shows a direct relationship between this substance and clay soils. Any attempt at classification of the above results according to theoretic considerations only indicates the complexity of applying any one theory to soils. More than one factor evidently enters into play to which it is necessary to give due consideration. That fer- tilizers do markedly affect capillarity is clearly shown. In using mixed fertilizers there is little variation in rise as related to variation in mixture. Those in which nitrates are used show decrease in water rise as compared with those containing ammonium sulphate. EFFECT OF BASICITY ON CAPILLARITY. Organic manures, as compared with the salts, retard the rise in the sandy soils and decrease the rise in others. As a rule, magnesium salts affect capillarity less than the calcium salts, potassium salts less than ammonium, sodium less than potassium, and the monobasic salts in most instances less than the dibasic. Among the monobasic salts, carbonates and phosphates show the lowest, sulphates next, nitrates next, and chlorids the highest water line. With the phosphates the rise of moisture seems to depend upon the acidity or basicity of the salt. This may be observed by comparing data in Table VII with those in Table VI under the phosphates of lime. Here also the relation between acidity and rise of moisture is similar to that observed in the case of the potash salt. 17 Table VII. — Showing effect of basicity on capillarity. K,HP04. KaPO*... Weight added. Gm. U.42 1.92 1.75 Height. Cm. 32.6 30.3 23.3 KHjPOi KjHPO^ IC3PO4.. Weight added. Gm. 0. Height. Cm. 32.6 30.9 29.0 1 Equivalent to 0.5 per cent K2O. The easily hydrolyzable salts, phosphates and carbonates of the alkalis, show the lowest water table. How^ever, they are much more active in organic soils due to their solvent action or chemical reaction with the organic matter present. These hydrolyzable salts also cause a swelling of the clay particles which, in Hawaiian soils, are partly composed of iron and aluminum hydrates and are conducive to the colloidal state, thus closing the pores, increasing friction, and lowering the rates of moisture rise. PERCOLATION. All moisture which passes below the surface, in excess of that held through capillary action or surface tension, is subject to the laws of gravity. The rate of movement, however, is dependent upon various factors, such as the size and composition of the soil particles, height of soil surface above water table, surface tension, and viscosity of the soil solution. Percolation is quite generally held to be most rapid in soils in which capillary activity is greatest, decreasing with height of column, and is faster in wet soils than in dry soils. Clay, of course, offers the greatest physical resistance to the passage of water and the resistance varies with the degree of aggregation of the clay particles. The rate with which water w^ill pass downward, then, depends upon the physical state of the soil and this in turn varies with the arrange- ment of the soil particles. Both of these properties, however, are affected by the nature of the soil solution, percolation decreasing with increase in concentration. For studying the effect of fertilizers upon this property of soils 1-inch glass tubes containing soil columns of about 30 centimeters were fitted up as in studying the capillary activity. These tubes were connected with a constant supply reservoir which maintained a 1-inch head of water in the tubes, and the water passing through the soil was measured at intervals. The totals of these measure- ments are given in Table VIII. 18 Table VIII. — Percolation as a by salts and fertilizers . Salts and fertilizers. Soil Soil Soil No. No. No. 530. 573. 428. Soil No. 516. Time of experiment. 6 7 2f days. days. days. Cc. Cc. Cc. 1,757 1,316 9,212 3,543 1,265 5,300 2,602 1,546 4,044 3,824 1,555 2,985 940 1,939 2,134 661 1,564 2,516 1,902 1,484 2,795 2,008 1,504 2,166 4 days. Salts and fertilizers. Soil Soil Soil No. No. No. 530. 573. 428. Soil No. 516. Time of experiment. days. 7 2f days. days. Cc. Cc. 1,413 2,349 1,628 1,253 3,545 1,787 3,601 1,189 3,445 1,452 5,0.38 1,530 6,612 4 days. Potassium sulphate Potassium chlorid Potassium phosphate Calcium oxid Calcium sulphate Calcium carbonate Calcium phosphate Magnesium oxid Cc. 2,312 4,123 3,077 4,851 5,635 4,569 5,489 5,093 Magnesium sulphate. Blank Sodium phosphate. . . Sodium carbonate . . . Sodium nitrate Ammonium chlorid.. Ammonium sulphate a. 1,838 8 1,765 2,021 4,481 4,162 488 Co. 3,404 7,086 3,293 2,633 1,649 3,355 2,987 Superphosphate . Phosphate rock Cottonseed meal Blood Blood and acid phos- phate Blank Blood and potassium sulphate 7 days. Cc. 1,688 4,784 1,990 2,432 1,892 1,476 5,865 4,753 7 days. Cc. 1,940 1,588 1,675 1,443 1,456 1,567 1,710 1,423 40 hours. Cc. 2,568 3,256 2,546 1,924 4,027 2,012 11,936 3,526 3 days. Cc. 1,581 3,632 3,902 4,659 3,656 3,324 2 7,226 1,715 Potassium sulphate and acid phosphate 2(NH4)-14-2 3 1(NH4)-^13 , 4(NH4)-11-10 3 4.5(NH4)-23-0 3 2(N03)-14-24 , Acid phosphate 7 7 40 days. days. hours. Cc. Cc. Cc. 3,491 1,321 2,693 2,305 1,618 1,945 3,685 1,636 3,852 2,763 1,331 1,365 3,198 1,381 2,076 3,745 1,480 2,044 3,209 1,649 1,308 3 days. Cc. 4,631 4,060 2,847 3,239 4,826 5,006 2,456 1 Stopped after 2f days. 2 Stopped after 2 days. ' Fertilizer m.ixtures supplying nitrogen (in ammonium sulphate), phosphoric acid, and potash in the order and proportions (percentages) given. 4 Fertilizer mixture similar to those referred to in footnote 3 except that nitrogen was supplied in sodium nitrate. A glance at this table clearly indicates the complexity of the study of the passage of water through soils. It is quite generally conceded that those soils in which capillary activity is greatest offer the least resistance to the passage of water. Soil No. 573 fails to lend sup- port to this theory, as does also No. 516. In these soils the capillary activity is greatest, while they offer much greater resistance to the passage of water than the sandy soils. Even the heavy clay soil offers less resistance than No. 573. As a whole the calcium salts cause less resistance than magnesium salts, ammonium less than potassium, chlorids less than sulphates in clay soils, but the sulphates least in organic soils. Mixtures in which sodium nitrate is used cause less resistance to flow of water than where ammonium salts were used. Soil No. 530. — All the salts and fertilizers added to this type retarded the percolation of water. There is a sHght relation between the degree of resistance and the flocculating power of the salts. As a rule the most deflocculated samples were among those that offered the greatest resistance and vice versa, as was found by exa minin g 19 the soils at the completion of the experiments. The organic manures resist the flow also but when mixed with mineral fertilizers the resistance is less. The data taken from daily observations, not in- dicated in the table, show that the passage of water in practically all the tubes decreased steadily from day to day after the salts became diffused throughout the soil and the clay began to swell. This applies only to soils Nos. 530 and 573. Soil No. 573. — Percolation through this soil was very slow and regular. Like capillary activity, salts had very little effect upon it. The amount of water passing through the tubes was less on the last day of the experiment than on the first day. There is practically no clay present in this soil, so that the action of the salts is probably upon the organic matter. This soil is the only one in which any salt increased the rate of flow. These salts were calcium sulphate, sodium carbonate, and superphosphate. Sodium nitrate strongly retarded percolation. Soil No. Jf.28. — In this instance it was not possible with the equip- ment available to maintain a constant head of water, due to the large volume which would percolate during the night. All the materials used at first decreased the rate of percolation but the daily rate increased steadily, for which reason the series was not carried out so completely as in the two previous soils. This increase was probably due to a washing out of the substances added. The action of sodium carbonate and sodium nitrate was very similar and unhke the effects in the above organic soil. The organic manures resist percolation quite strongly. The calcium and magnesium salts offer the greatest resistance, sodium salts next, ammonium salts next, and potash salts the least. Soil No. 516. — This soil, owing to its mechanical condition and low organic content, offers little resistance to percolation of water. The effect of adding any agent is to decrease the flow. However, the results obtained indicate that some soils are capable of restoring their equihbrium, as is shown by the fact that the depression of the first day became less as diffusion became more complete. The dibasic salts offer less resistance to percolation than the monobasic. FLOCCXJLATION. The r6le of flocculation in soils is one of considerable importance in a study of soils such as occur in these islands. The red clay is a type possessing an unusual tenacity and requires judicious handling to prevent puddling and to maintain the colloidal clay in the best physical condition. Various investigators, recognizing that a solu- ble salt wiU bring about the flocculation of the suspended material in a turbid liquid, have studied the relation of this to the improve- 20 ment of the texture in heavy soils. Indeed this is said to be the primary function of hme, which is one of the most universally applied soil amendments. The above studies indicate that the maintenance of a crumb structure is seriously menaced by the presence of even a trace of certain compounds. Hence flocculating or deflocculating agents alter the soil structure. The latter not only destroy the compound aggregates but also bring about a diffusion or swelling of the coUoidal clay. This results in a closing of the pore spaces, shutting out the air, development of acid conditions, and menacing the whole cycle of normal soil transformations. Since it is conceded that the best physical state, known as a crumb structure, is due to flocculation of the smaller grains into aggregates, the conclusion is obvious that the study of conditions conducive to the formation of a colloidal state and the relation of salts to this state may be of considerable local application. As a means of studying this property of Hawaiian clay a sample of highly puddled soil was chosen, one in which the clay would remain in suspension for weeks. A stock suspension of this soil sufficient for all experiments was prepared, so that a suspension of known concentration would be available. The degree of floccula- tion, to a certain extent, depends upon the relation of the amount of clay in suspension to the strength of the flocculating agent. Normal solutions were prepared of all the salts used in previous experiments, except the slightly soluble ones, in which case saturated solutions were made. By a series of preliminary experiments those salts having a negative or deflocculating effect were eliminated. These include potassium and sodium phosphates and carbonates. Secondly, salts causing a flocculation of the clay but not sufficiently soluble to form a normal solution were eliminated. These include the oxid, carbonate and sulphate of calcium and the oxid and car- bonate of magnesium. The comparisons were made in glass cylinders of 400 cubic centi- meters capacity in which were placed 2 cubic centimeters of normal salt solution, 10 cubic centimeters of clay suspension, and 188 cubic centimeters of water, making a total of 200 cubic centimeters. This mixture was shaken and allowed to settle. In the case of the stronger flocculants this proved too great a concentration, hence the experi- ments were repeated with a much weaker solution. The results are given in Table IX. 21 Table IX. — Showing relative rate of Jlocadation by acids and their salts. Salt or acid. H,S04..-. Al2(S04)3. CaS04.... MgS04.... K^04.... NasS04... (NH4)sS0 HCl CaClj MgCl, KCl Time required for flocculation. T^normal solution. Hours. 1 1 9 4 105 120 147 1 1 3 165 ^J^normal solution. Hours. Salt or acid. NaCl NH4CI HNO3 Ca(N0,)2.. Mk(N03)2.. KNO3 NaNOs.... NH4N03... H3PO4 CaHP04..- CH3COOH Time required for flocculation. T^normal solution. Hours. 120 103 1 1 2J 48 150 72 li li 15 ^i^normal solution. Hours. 2 72 144 192 I These results agree to a certain extent with the findings of other investigators. They indicate a relationship between the valency of the salt and its flocculating power. The most active salt is aluminum sulphate, a trivalent salt and one which is highly hydrolyzed. The divalent calcium and magnesium salts of nitric, hydrochloric, and sulphuric acids are next, while the monovalent salts of sodium, potassium, and ammonium are least active. The acids are stronger than any of their divalent salts but the trivalent salt, aluminum sul- phate, is stronger than any of the acids. Nitric acid is the strongest, hydrochloric second, and sulphuric third. Likewise the nitrates and chlorids are stronger than the sulphates. This indicates that the degree of flocculation is related both to the acidity and the basicity. Phosphoric and acetic acids cause much less flocculation than the other acids. Thus far no investigator has been able to satisfactorily explain this phenomenon, although various theories have been advanced/ The explanation is probably to be found within the realm of collofd chemistry. Hall and Morrison suggest that some physicist must first arrive at a satisfactory explanation of Brownian motion. They* have recently suggested the possible presence of free alkali derived from the partial hydrolysis of the suspended material and that floc- culation ensues when these are neutralized. Hilgard assumed a chem- ical hydration of the fine particles of clay when in suspension which when lime was added became dehydrated, causing a flocculation of the soil particles. Regarding the composition of soil colloids Uttle is known. While many authors have assumed them to be inorganic it is probable that being characteristic of the states of matter from which they are formed and not of any particular substance, they may be partly organic. Free^ suggests that organic colloids may coat the soil particles with thin films. 1 Jour. Agr. Sci., 2 (1907), No. 3, pp. 244-256. a Jour. Franklin Inst., 169 (1910), No. 6, pp. 421-438; 170 (1910), No. 1, pp. 46-57. 22 There is practically no doubt that colloids exist in Hawaiian soils. The physical properties indicate such to be the case. A chemical analysis of the clay shows it to be primarily a silicate of iron and aluminum with a probabihty of the hydrates being present also. Noncoagulable clay, by analysis, shows a higher percentage of iron and silica and less alumina than the coagulable clay. This indicates that part of the iron exists in the form of ferric hydrate. While the chemical composition may affect the nature of the colloids and the degree of flocculation the phenomenon itself is physical. The relation is between the composition and nature of the colloidal film surrounding the clay particle and its degree of surface tension. The effects of any added acid or salt is to alter the nature of the film probably through penetration or chemical action, thereby increas- ing or decreasing the surface tension, depending on the nature of the added substance, and increasing or decreasing the degree of floccula- tion. Some authors maintain that the salt or acid actually replaces bases within the colloid, thereby altering its composition, while others maintain that it only alters the film and by washing upon a filter with water the clay wiU revert to its original colloidal state. This latter contention seems to apply best to the conditions found to exist in Hawaiian soils. At all events the flocculation of Hawaiian clay is influenced as follows : (1) Most acids and neutral salts, especially electrolytes, increase the degree of flocculation. (2) Highly dissociated acids are the strongest coagulants, and the less dissociated acids act more or less in proportion to their degree of ionization. (3) Electrolytes of greater valency possess a greater degree of flocculation than those of lesser valency. (4) Most highly dissociated alkaUs are strongest deflocculants, as are also the alkah salts of weak acids, such as phosphoric. (5) Ammonium hydroxid is an exception, being only slightly ionized, but at the same time it is the strongest deflocculant. (6) The degree of flocculation depends upon the strength or valency of the anion as well as of that of the cation. COHESION. The film of moisture around soil particles imparts to them cohesion by which the particles are bound together. As the moisture content decreases surface tension of the film increases and the particles are drawn together. Hence, in a clay soil where shrinkage is greatest, there results the formation of cracks. There is a definite moistm-e content at which tenacity of the soil particles is at a minimum, the texture is best for culture, and the whole environment is most con- ducive to the best plant growth. The factors bringing about such 23 conditions depend upon the mechanical composition, the organic matter present, and the presence or absence of certain soluble sub- stances. As a means of measuring the effect of salts upon this cohesive property of Hawaiian soils, the procedure described by the Bureau of Soils was used.^ In this procedure a mechanical shaker having a screening apparatus and operated by a motor is used to insure a uniform packing of the soil. By means of this apparatus a cup was filled with the soil and by means of a penetratmg apparatus the weight necessary to cause a steel cone-shaped needle to penetrate a fixed depth was determined. While the results so obtained are not directly comparable with results obtained by other investigators, due to slight differences which may result from the construction of the apparatus, they are comparable among themselves. In Table X are shown the salts used in the experiments and the penetration with varying moisture contents, 1,500 grams of soil being used in making the determinations. After each penetration water was added, as shown in the table, weU mixed with the soil and allowed to stand one hour before repeating the penetration test. Five penetrations were made on each cup of soil and the average taken. The cup was refilled for further penetrations, repeating these some three or more times at each moisture content if there was any undue variation. Water was added up to the point at which it was impos- sible to work the soil. Salts were added at the rate of 15 grams per 1,500 grams soil, or 1 per cent of salt. Portions were taken from each cup for moisture determination. Table X. — Effect of salts on cohesion in soils under various percentages of moisture. SOIL NO. 573. Check. Potassium Calcium Superphos- AmmoTiium Sodium Sodium sulphate. oxid. phate. sulphate. nitrate. carbonate. H • U • ii • ^^ • ^"3 4J CO 00 ■g C3rd 5| 11 '=''3 05 iD 5S is 51 1.1 11 o § ^^ o § n^^ og g^ o^ g^ o^ g^ o^ g^ Z^ g^ ^ Ph s ^ ^ (^ ^ (^ !^ ^ 1^ Ph § ^ Per ct. Gm. Perci. Gm. Perct. Gm. Perd. Gm. Perct. Gm. Perrt. Gm. Per cf . (?TO. 10.95 38.5 36.65 15.14 14.19 35.95 12.33 39.4 13.09 35.90 11.49 35.3 10.19 37.15 12.18 35.6 18.54 36.3 17.77 37.15 15.96 38.3 16.51 35.9 15.01 39.8 13.98 39.30 15.62 36.9 21.26 38.45 21.16 35.0 19.12 38.35 19.11 36.2 18.22 37.45 17.46 36.5 18.79 35.65 24.13 37.5 23.67 33.3 21.71 40.05 22.46 39.55 21.86 39.5 20.90 36.95 21.25 37.45 26.71 33.9 26.11 32.55 24.77 35.60 24.95 34 22.21 37.45 22.79 36.1 23.37 37.05 SOIL NO. 428. 11.23 37.85 35.40 31.6 31.75 30.3 28.4 15.11 12.97 16.42 19.10 21.98 24.96 34.6 35.95 35.0 33.1 31.15 14.44 18.39 20.00 22.75 26.62 35.1 32.95 34.5 31.25 30.5 13.14 17.30 19.50 22.42 25.34 36.6 35.25 34.4 33.05 32.75 14.18 18.08 20.76 23.25 25.87 37.05 17.96 33.95 18.50 34.7 21.90 . 35.25 25.33 32.1 Soil No. 573; given in Table X, is the silty organic soil. The weight required for penetration increased at first and then decreased with increase in moisture content, reaching a minimum at 25 per cent, which is apparently the optimum moisture content for this type of soil. The effect of the addition of salts is to increase the weight nec- essary for penetration at the optimum moisture content, that is, salts increase the cohesion of the soil particles. This is especially true of lime. The cohesion apparently does not vary with change in moisture content in the presence of sodium carbonate. Soil No. 530. — Table X shows the relation between moisture con- tent and cohesion for the predominating red type of clay soil. Nine- teen per cent moisture represents the point above which it is impossi- ble to work with this type, due to the fact that the soil will not pass through the screen used in the apparatus. The figures obtained show an optimum moisture content of about 1 per cent if conclusions are to be drawn from the theories advanced by previous investigators. However, 10 per cent is rather low for this type of soil and it is prob- able that the optimum point for plant growth is above the range of the experiment. The cohesion of this soil decreases at first, then increases up to 16 per cent moisture, followed by a second decrease. The remarkable effect of sodium carbonate (and this would apply in more or less degree to all deflocculating agents) is clearly shown in the table. Regarding the effect of other salts, little can be concluded from the data at hand. That they do affect cohesion there is no doubt, but it is impossible to definitely classify these effects. 25 Soil No. 516. — Here again it was not possible to determine exactly the optimum point, but 26 per cent water is probably very close to this stage. The effect of increasing the moisture content is to decrease cohesion to a certain point, followed by an increase, then descending as the optimum moisture content is approached. Potassium sulphate decreases cohesion while the rest of the salts apparently increase this property in varying degree. Soil No. 428. — The effect of varying the moisture content of this soil differs from that observed with the other soils in that the de- crease in cohesion is regular and rapid as a result of increasing the moisture content up to the optimum point. The effect of salts is to increase the cohesion of this type of soil. This fact was found to be true in every instance. Owing to a lack of sufficient soil, ammonium sulphate and sodium nitrate were not used. Throughout this work care was exercised to subject each cup of soil to the same procedure. Penetrations were made at equal distances from the center of the cup, the weight was allowed to fall through equal heights, and similar methods used throughout. Even with these precautions it was difficult to obtain closely agreeing results from a clay soil, but very concordant results were obtained from the other types. APPARENT SPECIFIC GRAVITY. Closely related to cohesion and bearing directly on the swelling of soils on wetting and shrinking and cracking upon drying is the appar- ent specific gravity, the relation between the weight of a soil and the volume it occupies. This property has been supposed to be at a minimum at the optimum moisture content of the soil. Like all physical properties it is subject to modification and is more or less affected by the same factors that affect the cohesive power. The same apparatus used for penetration experiments was used for the determination of the apparent specific gravity. The data were obtained by dividing the weight of the soil in the container by its volume. The results are given in Table XI. Table XI. — Effect of salts and moisture content on apparent specific gravity. SOIL NO. 573. Check. Potassium Calcium Superphos- Ammonium Sodium Sodium sulphate. oxid. phate. sulphate. nitrate. carbonate. Ap- Ap- Ap- Ap- Ap. 1 Ap- Ap. Moist- parent Moist- parent Moist- parent Moist- parent Moist- parent Moist- parent Moist- parent ure SRf- ure spe- ure spe- ure spe- ure spe- ure spe- ure sp^e- con- cific con- cific con- cific con- cific con- cific con- cific con- ciflc tent. grav- tent. grav- tent. grav- tent. grav- tent. grav- tent. grav- tent. grav- ity. ity. ity. ity. ity. ity. ity. Perct. Perct. Perct. Perct. Perct. Perct. Perct. 13.43 0. 7017 16.67 .6745 io-ei 6.6807 is. 85 0.6842 15.55 0.6868 15.43 0.6587 14.55 0.6693 16.94 0.6605 20.65 . 6359 19. 14 .6305 17.62 .6605 17.60 .6544 17.87 .6394! 18.05 .6394 19.26 .6412 23.14 .6000 21.94 .5021 20. 4C .6254 21.22 .6193 20.54 .6061 21.10 .5894 21.70 .6079 26.11 24.72 .5438 22.96 .5842 23.02 .5710 23.15 .5674 23.78 .5587 24.07 .5649 26 Table XI. — Effect of salts and moisture content on apparent specific gravity — Contd. SOIL NO. 530. Check. Potassium Calcium Superphos- Ammonium Sodium Sodium sulphate. oxid. phate. sulphate. nitrate. carbonate. Ap- Ap- Ap- Ap- Ap- Ap- Ap- Moist- parent Moist- parent Moist- parent Moist- parent Moist- parent Moist- parent Moist- parent ure spe- ure spe- ure spe- ure spe- ure spe- ure spe- ure spe- con- cific con- cific con- cific con- cific con- cific con- cific con- cific tent. grav- tent. grav- tent. grav- tent. grav- tent. grav- tent. grav- tent. grav- ity. ity. ity. ity. ity. ity. ity. Perct. Perct. Perct. Perct. Perct. Perct. Perct. 3.66 1.0284 1.0596 3.90 7.14 1. 0368 1.0737 6.78 8.35 1.0745 7.47 1.0622 7.47 1.0500 8.56 1.0517 7.47 1.0087 9.98 1.0886 11.42 1.0973 11.61 1.1135 10.50 1.0851 11.69 1.0956 12.52 1. 1052 11.02 1.0517 12.66 1.1096 14.84 1. 1289 14.50 1.1509 14.44 1. 1079 15.02 1.1043 14.87 1. 1079 13.88 1.0394 16. 4C 1. 1017 16.86 1.1114 17. IC 1. 107C 17.42 1.0912 17.33 1.0605 18.00 1.0693 16.76 1.0079 19.30 .9640 21.68 .9745 21.60 .9096 19.86 .9447 20.20 .8789 20.31 .9184 SOIL NO. 516. 10.95 15.14 18.54 21.26 24.13 0.8035 .8973 .8219 .7403 14.19 17.77 21.16 23.67 0.9026 .9219 .8140 .7517 12.33 15. 19.12 21.71 .9351 .8886 .8035 13.09 16.51 19.11 22.46 26.71 .6675 26.11 .6833 24.77 .7210 24.95 .7140 22.21 .7543 22.79 .7552 0.8859 .9158 .8570 .7781 11.49 15.01 18.22 21.86 .9245 .8982 .8219 10.19 13.98 17.46 20.90 0.8684 .9236 .9166 .8307 12.18 15.62 18.79 21.25 23.37 0.8587 .9035 .8833 .8324 .7912 SOIL NO. 428. 11.23 15.11 17.96 18.50 21.90 25.33 0.7394 .7464 .7175 .6737 .6403 12.97 16.42 19.10 21.98 24.86 0.7403 .7342 .7079 .6780 .6377 14.44 18.39 20.00 22.75 26.62 0. 7517 .7245 .7079 13.14 17.30 19.50 22.42 25.34 0.7245 .7166 .7008 .6701 .6623 14.18 18.08 20.76 23.25 25.89 0.8386 .7657 .7298 .6929 .6666 In soil No. 573 specific gravity decreases regularly with increase in moisture content up to the optimum both in the untreated soil and in those treated with salts. The effect of salts is not striking but in some cases increases, in others decreases, the apparent specific gravity. In soil No. 530 the results are very similar to those obtained in the cohesion tests. This type shows an increase in specific gravity up to a maximum, above which point it decreases to what has been assumed to be the optimum point. Calcium oxid, superphosphate, ammonium sulphate, sodium nitrate, and sodium carbonate decrease the specific gravity, while potassium sulphate slightly increases it, differing con- siderably from their action on the previous soil. With soil No. 516 the results are similar to those with No. 530 in that the apparent specific gravity increases with increase in moisture content to 18.5 per cent, beyond which it decreases to the optimum point. All salts affect this soil in the same manner, resulting in an increase in apparent specific gravity, sodium carbonate having the greatest effect. Soil No. 428 shows the same relation between apparent specific gravity and moisture content as soils Nos. 516 and 530. The effect 27 of salts is very similar and also tends to increase this property to a slight extent. The two physical properties of soils known as cohesion and ap- parent specific gravity are more or less dependent upon and governed by the same factors, and it is shown in the table that the effect of varying moisture contents and addition of salts is similar. Both reach a minimum at the point known as that of optimum moisture content, which is conceded to be the stage most favorable to plant growth. Hence these properties are of special significance in soil investigations. /O /3 20 MO/STUP£: CONTENT Fig. 3.— Relation between cohesion, apparent specific gravity, and moisture content of soils. In classifying the Hawaiian types of soil according to these prop- erties, the clay soil possesses the highest cohesive properties, the man- ganese silt next, the sandy soil third, while the lowest and hence the most easily cultivated is the silty soil, No. 573. The same relation applies also to the apparent specific gravity and is true not only at the optimum moisture content but also on the air-dry soils. The curve shown in figure 3 well illustrates the relationship existing between these two properties of Hawaiian soils. At the lower moisture contents the curves diverge considerably while above a given point they again foUow similar lines. 28 VAPOR PRESSURE. Comparatively little work has been done on the effect of soluble salts on vapor pressure of soils, i. e., the rate of evaporation. Theo- retically salts should increase the surface tension of solvents and thereby lower their vapor pressure and hence increase the water- retaining capacity of the soil. The study made of this property indi-. cates that this theory apparently apphes to Hawaiian soils. As a means of measuring this property of soils ordinary weighing bottles were used. Twenty gram lots of soil with which the various salts had been mixed were placed in these bottles, 12 cubic centime- ters, or 60 per cent, of water was added to each and then allowed to stand one week in the open air. Weighings were made at this stage, following which all the samples were placed in the same desiccator with calcium chlorid and weighings were again made after one week. The results are given in Table XII. Table XII. — Effect of salts on vapor pressure. Salts and fertilizers. Sodium carbonate Sodium nitrate Magnesium oxid Calcium carbonate Calcium phosphate Calcium oxid Potassiimi phosphate Potassivun sulphate Ammoniimi sulphate Potassimn chlorid Superphosphate Acid phosphate Cottonseed meal Blood Acid phosphate and blood . . Potassium sulphate and acid phosphate 2(NH4)-14-2i l(NH4)-9-li 2 (N03)-14-2 1 Check Amoimt added. Gm. 0.10 .09 .04 .10 .23 .06 .21 .17 .13 .08 .10 .10 .10 .10 .10 .10 .10 .10 .10 Soil No. 530. Water retained In air. Perct. 65.3 71.2 64.2 66.2 52.8 58.6 51.4 62.7 73.9 50.2 63.3 77.0 32.5 42.6 44.0 58.2 36.5 37.8 59.6 48.0 In des- iccator. Perct. 46.0 55.5 43.6 42.7 21.0 29.7 26.9 42.9 25.6 10.2 10.8 41.3 24.6 Soil No. 573. Water retained. In air. Perct. 39.4 50.0 45.7 41.0 42.7 44.8 41.3 48.1 41.7 45.8 44.7 45.8 41.5 36.1 31.2 40.3 39.3 35.7 45.3 36.3 In des- iccator, Perct. 20.9 33.2 26 21 15 24, 19 33 26, 21, 27, 28.5 9.7 7.2 4.3 16.2 8.3 9.6 25.6 15.3 Soil No. 428, Water retained. In air. Perct. 39.2 44.1 44.4 44.3 43.0 39.8 42.4 39.5 43.0 40.2 42.8 41.3 43.3 48.7 41.4 49.2 41.2 38.7 57.2 37.0 In des- iccator. Per ct. 31.9 38.8 38.2 35.3 30.7 31.5 25.4 31.2 31.7 40.5 25.7 15.3 23.1 30.3 19.3 17.5 22.8 17.8 Soil No. 516. Water retained. In air. Per ct. Perct. 43.7 29.5 46.8 31.4 42.9 24.4 44.9 26.9 45.7 44.8 35.9 37.8 35.6 44.8 38.4 37.3 43.6 43.8 40.7 44.1 40.6 37.2 42.4 35.4 In des- iccator. 28.3 15.2 17.5 17.4 33.8 25.6 22.7 27.4 29.9 22.5 20.4 20.9 28.6 22.1 1 Fertilizer mixtures containing nitrogen, phosphoric acid, and potash in the order and proportions (percentages) indicated; nitrogen from ammonium sulphate in the first two, from sodium nitrate in the third. These figures indicate that the effect of salts upon vapor pressure in soils is one of considerable importance. Salts act upon Hawaiian soils more or less according to theory. The major part of them increase the water-holding power in all four soils. Organic substances in- creased evaporation in soils Nos. 530 and 573, but had the opposite effect upon Nos. 428 and 516. These results also show that the form of nitrogen used in mixed fertilizers bears a definite relation to the vapor pressure. Those in which sodium nitrate was used show a 29 much greater capacity for holding moisture. In fact, sodium nitrate itself has the most striking effect on all types of soils. There is no apparent classification of the results according to the changes in sur- face tension which should theoretically result through the addition of the salts. HYGROSCOPIC MOISTURE. When a soil has been dried in the air and then is exposed to a moist atmosphere it will reabsorb moisture. The amount which it is able to take up depends upon several factors, such as mechanical composi- tion, presence or absence of organic matter and its state of decay, temperature of the air, and presence of colloidal clay and ferric and aluminum hydrates. This form of moisture, while it is not in itself able to support normal plant growth, may materially assist in sustaining vegetation during drought. Some investigators claim it to be of absolutely no service to plants. Hawaiian soils, owing to their high humus and ferric hydrate con- tent, possess a very high hygroscopic coefficient. Of the series used in this study the sandy soil was lowest, as would be expected, but even in this case the hygroscopic moisture is high in comparison with normal sandy soils, due to its high organic content. Table XIII shows the comparative moisture-absorbing power of the types studied. From these data we are led to conclude that, due to the abnormal physical properties of Hawaiian soils, the size of particles is not the primary factor in determining its hygroscopic properties, although surface exposed is an important factor. The data presented in this table were obtained by exposing a very thin layer of soil in a saturated atmosphere for 144 hours, after which the total moisture was determined. Soil No. 428 having the least exposed surface, the highest percentage of organic matter, and the highest moisture content in the air-dry soil, has the least hygroscopic power. The clay soil. No. 530, has the most exposed surface, the largest percentage of ferric hydrate, the lowest moisture content in air-dry form, the least organic matter content, and next to the lowest absorbing power. But soils Nos. 516 and 573, a" manganese silt of high iron content and an organic silt, respectively, soils very dissimi- lar in chemical composition and physical properties, have the highest absorbing power, with the balance in favor of the manganese soil. Table Xlll .—Percentage of hygroscopic moisture absorbed in 144 hours. Sou No. Hygro- scopic moisture. 530 573 428 516 Per cent. 19.32 21.59 15. .')6 24. W 1 1 30 Table XIV shows the effect of salts upon the hygroscopic power of soils. That this property should be affected by the addition of fer- tilizers was to be expected, since most salts possess this property themselves to a greater or less extent, and it is only natural that they should impart it to soils. These experiments were conducted by exposing a thin layer of soil to a saturated atmosphere in two large containers. Conditions were made as nearly similar as possible, but blank samples of soil were exposed in each as checks. Salts were well mixed with a bulk of soil at the rate of 0.5 per cent of salt, and weighings made from this bulk. Samples were exposed for 48 hours and total moisture determined in the air bath at 105° C. Table XIV. — Effect of salts and fertilizers on hygroscojpic moisture. Salts and fertilizers. Soil Sou Soil Soil No. 530. No. 573. No. 428. No. 516. Per cent. Per cent. Per cent. Per cent. 3.17 10.84 9.87 5.54 17.21 19.35 14.80 21.40 16.78 19.80 14.05 21.30 16.76 17.88 13.90 19.90 17.08 18.70 15.53 21.15 17.28 24.80 17.45 22.20 17.64 24.20 15.80 21.30 16.35 18.90 14.55 20.70 16.26 20.81 13.70 21.20 16.90 20.20 14.70 21.50 16.56 22.00 15.08 20.50 17.07 20.30 13.80 21.40 16.50 18.50 14.35 21.80 18.95 32.00 22.02 30.00 16.35 18.58 14.58 21.10 Soil No. 542. Original moisture content of soils Potassium sulphate Calcium oxid Acid phosphate Ammonium sulphate Sodium nitrate Sodium carbonate Check Calcium sulphate 2 (NH4)-14-2 1 2(N03)-14-2i Superphosphate Potassium sulphate and acid phosphate. Sodium chlorid Check Per cent. 4.12 18.45 16.10 16.42 20.30 20.10 17.95 16.11 17.30 16.80 17.06 16.70 16.60 22.50 16.18 1 Fertilizer mixtures containing nitrogen, phosphoric acid, and potash in the order and proportions (percentages) named; nitrogen from ammonium sulphate in the first, from sodium nitrate in the second. The effect of adding salts upon the ferruginous clay is to increase the hygroscopic power in every instance, except where calcium sul- phate is added, in which case there is very little variation. As a matter of fact the general tendency of all the salts on the different types is to increase this property of soils, and in cases where there is a decrease it is almost negligible. Sodium chlorid, being itself a hygroscopic salt, imparts the highest absorbing power to the soil, while the lowest is effected by addition of acid phosphate. SUMMARY. The foregoing pages contain data obtained from an extensive study of the physical properties of Hawaiian soils and the effect of fertilizers upon these properties. It is evident that agents whioh influence the mechanical condition are many and complex. It has also been clearly demonstrated that the addition of salts or fertilizing materials affects the structure of the soil. It is impossible to predict in all cases the degree to which any one or all physical properties will be influenced by the addition of a fer- 31 tilizer, since this depends primarily upon the mechanical composition of the soil, the nature of the organic matter, and probably upon cer- tain factors which are at present unknown. However, within certain limits, the effect of adding a larger application of a salt only magnifies that of a smaller application. Tliis suggests that the measurement of the physical effect may be just as accurately, and possibly more accurately, determined than the chemical effect. The measurement of a normal application of fertilizer through a chemical analysis of the soil is practically impossible. Capillarity is diminished in clay soils by the addition of salts but increased in sandy soils. Also this property is more active in silts than in sandy or clay soils, being slowest in the latter. The percolation of water is most rapid in sandy soils and slowest in types the particles of which are most likely to swell. Fertilizers considerably increase the resistance to percolation. The theory that soils of greater capillary activity offer the least resistance to perco- lation of water does not apply to Hawaiian soils. Salts increase or diminish the size of the soil aggregates. This is of no small importance in the use of fertilizers. The cohesion of the soil particles in most instances is increased by the addition of salts. This is also true of the apparent specific gravity. However, there are too many exceptions to make any definite statement. The hygroscopic moisture is increased by the addition of salts, with but very few exceptions. The vapor pressure is lowered in most instances, but can not be explained from a consideration of the surface tension of the added salts. Acknowledgments are due and thanks hereby extended to Dr. W. P. Kelley for valuable suggestions and for interest shown throughout this investigation. ADDITIONAL COPIES OF THIS PUBLICATION MAY BE PROCURED FROM THE SUPERINTENDEXT OF DOCUMENTS GOVERNMENT PRINTING OFFICE WASHINGTON, D. C. AT 5 CENTS PER COPY V UNIVERSITY OF FLORIDA 3 1262 08929 1016