UNIVERSITY OF CALIFORNIA 
 
 COLLEGE OF AGRICULTURE 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 BERKELEY, CALIFORNIA 
 
 CIRCULAR 317 
 
 June, 1930 
 
 FERTILIZER PROBLEMS AND ANALYSIS 
 OF SOILS IN CALIFORNIA 
 
 D. R. HOAGLANDi 
 
 Several years ago a circular 2 was published dealing with certain 
 general ideas concerning the chemical relations of plants and soils. 
 Since that time additional investigations have been carried out, and it 
 seems desirable to revise and to expand the previous discussion, 
 especially because of the recent interest shown in soil problems. 
 
 In beginning the discussion, it is useful to emphasize once more 
 the extreme complexity of the conditions which govern the growth of 
 crops. The application of scientific methods to soil problems involves 
 many difficulties not met with in the application of similar methods 
 to industrial processes. Obviously, the latter can be controlled to a 
 far greater degree than can the processes of plant growth as they occur 
 under field conditions. Once the scientific and practical problems of a 
 mechanical or chemical industry have been overcome, any given process 
 may be repeated indefinitely with exactly predictable results. Such an 
 achievement is seldom possible in the field of agriculture. Plants and 
 soils exist in extraordinary variety, and both are subject to the variable 
 and uncontrolled influence of climate. These statements would be true 
 of any part of the world, but they have more than ordinary significance 
 in California, because of its exceptional diversity of crops, soils, and 
 moisture conditions. Notwithstanding the difficulties inherent in soil 
 problems, real progress may be hoped for by persistent research, in 
 field and laboratory, and only by this means. 
 
 1 Professor of Plant Nutrition and Chemist in the Experiment Station. 
 
 2 Hoagland, D. R. Soil analysis and soil and plant interrelations. California 
 Agr. Exp. Sta. Circ. 235:1-8. 1922. 
 
University of California — Experiment Station 
 
 CHEMICAL ELEMENTS OF THE SOIL ESSENTIAL TO PLANTS 
 
 The chief aspect of soil and plant relations to be dealt with in this 
 circular concerns the processes by which crops take from the soil the 
 mineral elements (plant foods) necessary for their growth. 3 The idea 
 that only seven soil elements (potassium, phosphorus, calcium, mag- 
 nesium, nitrogen, iron, and sulfur) are required by plants, is now 
 known to be incorrect. The normal development of crops depends 
 upon the ability of the soil to supply a conceivably large number of 
 chemical elements, although only a few of these are needed by plants 
 in more than minute quantity. It is doubtless true that such elements 
 as manganese, boron, zinc, copper, etc., are present in nearly all soils 
 in suitable amounts and forms, but various unexplained crop con- 
 ditions, and certain field studies, lead us to suspect that this assump- 
 tion may not be universally true. There are few, if any, practical 
 suggestions to be made now along this line, but it is important to con- 
 tinue the investigations on elements not previously considered essential 
 to plant growth. While recognizing. that elements of the type just 
 mentioned may be essential to plant growth in small amounts, it should 
 also be kept in mind that the same elements can become toxic when 
 present in too high a concentration. Boron is an example of this type 
 of element. 
 
 CHEMICAL COMPOSITION OF THE SOIL MOISTURE AND THE 
 FORMATION OF ACIDS IN THE SOIL 
 
 The present theories of plant nutrition are based on the assump- 
 tion that plants can take up mineral elements only after the latter 
 are dissolved in the soil moisture (soil solution). However, it is 
 seldom true that there is present at any one time in the soil moisture 
 sufficient amounts of all mineral elements needed by plants during 
 the whole period of their growth. For example, during the season a 
 crop may remove from the soil many times the amount of phosphate 
 present in the soil moisture at the beginning of the season, even on 
 the basis of the maximum volume of soil in which roots can develop. 
 Availability becomes then partly, or perhaps chiefly, a question of the 
 
 3 These mineral elements are usually referred to as "plant foods," although 
 this term is frequently used in connection with only three elements, namely, 
 potassium, phosphorus, and nitrogen. In an accurate sense, these elements are 
 not foods, but part of the raw material by virtue of which plants synthesize the 
 actual foods. Water and carbonic acid gas are the other raw materials. In 
 the present discussion, the terms potassium and potash may be considered as 
 equivalent; likewise calcium and lime, magnesium and magnesia. 
 
Circ. 317] Fertilizer Problems and Analysis of Soils 3 
 
 rates and concentrations at which essential elements contained in the 
 solid portion of the soil can dissolve in the soil moisture. These rates 
 and concentrations should be adequate to keep pace with the rates of 
 intake by the plant, to the extent that no limitation in growth can 
 occur because of a shortage in any one of the essential elements. A 
 deficiency of one element will limit growth even if all other elements 
 are present in abundance. 
 
 The solution of certain mineral elements necessary to plant growth 
 depends primarily upon the production of acids in the soil, which in 
 turn is dependent upon biological activities, that is, the activities of 
 microorganisms and of root cells. The most important acids produced 
 in this way are nitric, sulfuric, and carbonic. In most California soils 
 these acids are neutralized by basic substances in the soil as fast as 
 they come into existence. The salts thus formed dissolve in the soil 
 moisture and become part of the nutrient medium of plants. To illus- 
 trate, bacterial action may bring about the production of nitrate 
 (derived originally from nitrogen present in the organic matter of the 
 soil) a soluble and available form of nitrogen, and at the same time 
 calcium, magnesium or potassium will go into solution and become 
 available to plants. Since these activities of microorganisms are 
 primarily dependent upon the presence of organic matter in the soil, 
 it is evident that organic matter is an important agent in making 
 immediately available potassium, calcium, and magnesium. The great 
 importance of organic matter in the soil for other reasons, such as the 
 improvement of physical conditions, has been discussed on many 
 occasions and these points need not be elaborated here. 
 
 The second means by which acids are formed in the soil is the 
 excretion of carbonic acid by roots. The general opinion is that no 
 other acid is excreted by roots, but this opinion is not necessarily con- 
 clusive in its application to all plants and to all conditions. In any 
 event, the carbonic acid excretion by roots is considered by many 
 investigators to be of great importance, because of the very intimate 
 contact between fine roots, or root hairs, and colloidal soil particles. 
 The dissolving of minerals, or their components, can take place in the 
 closest possible proximity to the absorbing root surfaces. There is 
 afforded, therefore, an especially favorable opportunity for the plant 
 to have access to the mineral particles of the soil. 
 
University of California — Experiment Station 
 
 THE ABSORPTION OF ESSENTIAL ELEMENTS BY ROOTS 
 
 It also follows that the total area of root surface capable of absorb- 
 ing' mineral elements may determine in part the ability of a plant 
 to obtain from the soil adequate quantities of some of the essential 
 mineral elements. It is not merely a matter of what kind of soil a 
 plant grows in, but also of how much soil. For this reason, as well 
 as because of the water relations involved, emphasis is placed on the 
 importance of maintaining- conditions in the soil favorable for root 
 growth. Among these conditions, suitable aeration and prevention 
 of the formation of impermeable layers are often of extreme import- 
 ance. Irrigation and cultivation practices are definitely related to the 
 mineral nutrition of plants. Toxic substances, organic or inorganic, 
 should be absent from the soil. The growth of roots is not influenced 
 by soil conditions alone, but also by the environment around the top 
 of the plant, where the food necessary for root growth is manufac- 
 tured. Koot growth, and consequently the ability of plants to obtain 
 essential elements from the soil, may be modified by climatic con- 
 ditions, fruit production, plant disease, insect injury, etc. 
 
 The roots of plants do not take up the soil moisture simply as it 
 exists in the soil. Mineral elements may be removed at a faster or 
 slower rate than the water in which these elements are dissolved. 
 The plant should not be likened to a lamp wick sucking up the soil 
 solution. Roots perform their functions normally only as a, result of 
 the activities of healthy living cells. 
 
 Different nutrients may be removed from solution at very different 
 rates. Plants, therefore, have a " selective" action, but this does not 
 mean that they possess the power to select only those substances 
 required for their growth, rejecting all else. On the contrary, injurious 
 substances may often be taken up by plants when such substances 
 are present in the soil, and essential elements can be absorbed some- 
 times in quantities far greater than those needed for plant growth. 
 
 AVAILABILITY OF POTASSIUM AND PHOSPHATE 
 
 Assuming that the production of acids by microorganisms or by 
 plant roots proceeds at a satisfactory rate, is it certain that potassium 
 and phosphate will dissolve in the soil moisture in the amounts and 
 proportions necessary for satisfactory crop growth? This depends 
 upon the nature of the soil minerals. All minerals are soluble and 
 available to some degree, however slight, but certain minerals have 
 
Circ. 317] Fertilizer Problems and Analysis of Soils 5 
 
 a degree of solubility which makes them especially important in 
 determining" whether or not a soil can supply to a plant adequate 
 amounts of potassium or phosphate. 
 
 First giving attention to the question of potassium, it is found 
 that the amount of potassium easily dissolved by fairly dilute acids 
 (such as Y^ per cent nitric acid) usually comprises only a small frac- 
 tion of the total amount of potassium contained in the soil. The 
 supply of this more easily dissolved potassium has an important bear- 
 ing on the concentration of potassium capable of being maintained in 
 the soil moisture, although the relation is not simple, nor always 
 consistent. In general, it may be said that the degree of solubility 
 of potassium is of much more importance than the total percentage 
 of potassium present in the soil, but conceivably, a high percentage 
 may not be without significance in some soils, if it implies a greater 
 number of contacts between root surfaces and potassium minerals. 
 The fineness of division of such minerals is important in this 
 connection. 
 
 Experiments with a considerable number of California, soils, carried 
 on in Berkeley during the past twelve years, show that continuous 
 cropping reduces the supply of available potassium, as determined by 
 methods of the type described in the above paragraph. In some soils 
 the amount of the reduction is of the same magnitude as that of the 
 total potassium removed in the crop. 
 
 If a soil becomes depleted in the easily soluble forms of potassium, 
 plants will have a relatively less favorable medium from which to 
 absorb this element. But it does not necessarily follow that crops will 
 always make unsatisfactory growth when the more soluble types of 
 potassium are exhausted. Some crops may still find it possible to take 
 out of the soil adequate amounts of this element for an indefinite 
 period of time. The mechanism involved is only partly understood, 
 but it may be repeated that solubility is only a relative term; all 
 compounds of potassium are soluble to some degree. If the plant 
 roots have a sufficiently large absorbing area, together with a sufficient 
 activity in absorbing potassium from very dilute solutions, and if the 
 growth cycle of the plant gives adequate time for absorption, a suitable 
 adjustment may take place, even in soils which never contain in their 
 soil moisture more than a slight concentration of potassium. Of 
 course if this concentration falls too low, then it may happen that 
 many useful plants will fail to thrive without proper fertilization of 
 the soil with potassium. This latter condition is not infrequently 
 reported to exist in soils of humid regions, but so far only a few 
 instances of this extreme deficiency have been reported in California. 
 
6 University of California — Experiment Station 
 
 There is the additional very important idea to be considered, that 
 different kinds of plants may differ in respect to the amounts of 
 potassium required for the desired type of growth. Many agricul- 
 turists believe that plants producing large amounts of starch or sugar 
 have a high potassium requirement, although the function of potassium 
 in plant growth is by no means adequately understood as yet. Such 
 plants are considered to give most satisfactory yields on soils contain- 
 ing relatively large amounts of potassium in easily soluble form. These 
 ideas are chiefly based on experience in other parts of the world. It 
 is not yet known whether or not crops of the type referred to would 
 respond to potassium fertilization when grown on the majority of 
 California soils. In a few cases the evidence is positive, and in others 
 negative. At this point, it should be noted that many experiments 
 indicate that it is possible to increase the percentage of potassium 
 present in a plant by increasing the amount of available potassium 
 in the soil through fertilizer additions. The increased amount 
 of potassium contained in the crop, beyond a certain percentage, 
 would be superfluous in. that it would lead to no increase in 
 growth or improvement of quality. Such excess absorption of an 
 element is sometimes termed a "luxury consumption." The amount 
 of potassium required by a crop may vary, depending on climatic 
 conditions. Certain field experiments in England, and certain pre- 
 liminary studies on plants grown under controlled conditions in 
 California, suggest that the potassium requirements of plants may 
 be altered by changes in light or temperature. 
 
 The total amounts of phosphorus present in ordinary soils are far 
 smaller than the total amounts of potassium, but analogous consider- 
 ations in regard to solubility apply. To give an illustration : two 
 California soils were compared, each containing approximately the 
 same total amount of phosphorus. Practically no phosphorus was 
 dissolved from one soil by an acid of a certain strength, while from 
 the other soil, treated in exactly the same way, more than half of the 
 total phosphorus was dissolved. Many, probably most, plants of agri- 
 cultural interest would make very poor growth in the first soil because 
 of lack of available phosphorus. Various problems of this kind are 
 now being actively studied in California and elsewhere. Present 
 knowledge is entirely inadequate, but many instances could be cited 
 in which field tests with phosphate fertilizers would be worth carrying 
 out, especially with annual crops. 
 
 It is hoped that this discussion, although very incomplete, will 
 nevertheless make it clear how complex the question of availability 
 of potassium and phosphate is. Attention must be given, not only to 
 
Circ. 317] Fertilizer Problems and Analysis of Soils 7 
 
 the soil type, but also to the type of crop, moisture, and other con- 
 ditions in the soil favoring or inhibiting root growth, climatic environ- 
 ment, as well as to plant diseases and pests. 
 
 USE OF FERTILIZERS* 
 
 Having sketched a few important general relations existing 
 between crops and soils, certain views concerning fertilization will now 
 be considered. In the first place, it is important to realize that some 
 type of fertilization is required, sooner or later, for most crops under 
 modern agricultural conditions. If we take a general average of 
 experience throughout the world, we find that special concern is felt 
 about maintaining available nitrogen in the soil. Crop responses to 
 nitrogen applications, although not universal, are met with under the 
 greatest possible variety of soil, crop, and climatic conditions. 
 
 Nitrogen is an element which can easily be lost from soils. When 
 it is in the form of nitrate it is subject to leaching. But it is also 
 possible to lose nitrogen in gaseous form. In this connection certain 
 experiments made in Berkeley are of interest. Thirteen soils, from 
 different parts of California, were studied over a period of years with 
 reference to their total content of nitrogen. The soils were placed in 
 a series of large containers, and one lot of each soil was cropped and 
 one kept uncropped. At first the soils were maintained in a moist 
 condition continuously, but without excess moisture or leaching. 
 Under these conditions, in the course of five years, an important pro- 
 portion of the total nitrogen in the soil was lost. Practically the same 
 amounts were lost from the cropped and uncropped soils, which means 
 that the loss of nitrogen was primarily the result of the activities of 
 microorganisms, rather than removal of nitrogen by crops. On the 
 other hand, under other moisture or cultural conditions, it is possible 
 for a soil to gain nitrogen from the air in important amounts as a 
 result of the activities of still other groups of organisms (other than 
 legume organisms). Much further work must be accomplished before 
 the nitrogen economy of the soil is sufficiently understood, but it is 
 evident that losses of nitrogen may be great under some circumstances. 
 This fact may help to explain why nitrogen fertilization is frequently 
 of importance, whether by the growth of legumes or by addition of 
 commercial forms of nitrogen. In connection with all questions 
 relating to nitrogen, it is of the utmost importance to consider the 
 activities of the soil microorganisms and the possible influence of 
 irrigation practice and additions of organic matter on these activities. 
 
8 University of California — Experiment Station 
 
 To understand correctly the fertilization of a soil with phosphate 
 or potassium requires recognition of the fact that these substances 
 react chemically with the soil. The point of interest, as far as crop 
 growth is concerned, is the resultant condition of the soil after 
 fertilizers or other amendments are added. The same fertilizer will 
 produce different effects, in greater or lesser degree, in every different 
 soil. Sometimes reference is made to the use of "balanced fertilizers." 
 The balance which is important is not in the fertilizers, but in the 
 soil after the fertilizer has been added and has reacted with the soil. 
 A fertilizer cannot, in any accurate sense, be compared with a 
 "balanced ration" for an animal. The feeding of animals and the 
 absorbing of mineral elements by plants are totally different in nature. 
 
 Potassium (potash) reacts in soils chiefly with certain colloidal 
 mineral substances, which are in some degree (but only partly) 
 analogous to artificial zeolites such as are used in softening water. In 
 this reaction there occurs an "exchange of bases." 4 For example, 
 some of the potassium added may go out of solution, and calcium 
 (lime) or magnesium (magnesia) enter into solution to take the place 
 of the potassium which is "fixed." Not infrequently, in soils of fairly 
 heavy character, nearly all of the potassium added in an ordinary 
 fertilizer application is fixed in this way, liberating other bases, 
 especially calcium and magnesium. 
 
 The phenomenon referred to above is of great interest in connec- 
 tion with the fertilization of fruit trees, for the reason that in many 
 soils most of the potassium added may be fixed in a surface zone, out 
 of reach of much of the root system. Soils in which availability of 
 potassium is low sometimes have especially high fixing power, although 
 this statement would not usually apply to sandy soils. Therefore, in 
 soils of high fixing power it may be especially difficult to alter the 
 condition of the soil in contact with the roots developed below the 
 surface zone. Of course, it is not impossible that the required changes 
 might be brought about in some instances by using very large amounts 
 of fertilizer, applied at one time, or by using smaller amounts over 
 very long periods of time. In some cases the total amounts required 
 might be far greater than those used in ordinary commercial practice. 
 Interesting experiments are now being carried on by several investi- 
 gators to determine whether some of the difficulties may be overcome 
 
 * This general phenomenon has been studied extensively by the Laboratory 
 of Agricultural Chemistry, at Riverside. Those who are interested in a fuller 
 development of the discussion should consult the publications issued from this 
 Laboratory. Some of these studies are available as bulletins and Hilgardia 
 from the Agricultural Experiment Station, Berkeley, while others have appeared 
 only in scientific journals and usually are available only in libraries. 
 
Circ. 317] Fertilizer Problems and Analysis of Soils 9 
 
 by new methods of applying- fertilizers, should necessity arise for 
 fertilizing deep-rooted plants with potassium or phosphate. 
 
 Although the fixation of potassium may prevent this element 
 from becoming available to roots in the deeper layers of the soil, 
 nevertheless, in many soils the " fixed" potassium can be absorbed by 
 plants to a large extent, when roots develop directly in contact with 
 the soil particles on which the fixation occurs. This has been found 
 to be true of certain California soils of high fixing power for potas- 
 sium, in experiments with plants such as wheat, barley, tomatoes, 
 beets, etc. Presumably the fixation of potassium in such cases is not 
 so firm as to prevent it from dissolving gradually at the zones of 
 contact between roots and soil particles. The plant is an active agent 
 in the process, because of carbonic or other acids which may be 
 excreted by roots, and because of the rapid removal of potassium from 
 the soil moisture by the growing plant. The latter action makes it 
 possible for new supplies of potassium to enter into solution continu- 
 ously as long as a suitable source of potassium remains in the solid 
 portion of the soil. It is clear from all these considerations that the 
 location of root systems is a very important question in potassium 
 fertilization. 
 
 Phosphate, as well as potassium, when added to a soil may undergo 
 chemical change, although the chemical reactions involved are different 
 from those applying to potassium. It is possible for phosphate easily 
 soluble in water before addition to the soil to become much less soluble 
 afterwards. Consequently, in this case, also, it may become very 
 difficult to change the soil condition below the zone in which the 
 phosphate fertilizer is mixed with the soil. 
 
 Just as with potassium, the fixation of phosphate may not prevent 
 plants from absorbing phosphate added in a fertilizer, provided again, 
 that sufficient root development occurs in that portion of the soil to 
 which the phosphate is added. A striking example of this is found in 
 certain California soils of high "fixing" power, in which some crops 
 make hardly any growth because of the insolubility of the phosphate 
 naturally present. Yet the addition of certain soluble phosphates to 
 these particular soils has an extremely beneficial effect on various 
 surface-rooted crops. The manner in which plant roots absorb phos- 
 phate in such cases is only very slightly understood as yet, but no 
 doubt stress can be placed on the intimate contact between roots and 
 soil particles, and on the finely divided nature of the compounds 
 formed when added phosphate is precipitated in the soil. Plant roots 
 coming in contact with such particles, and giving off carbonic acid, 
 may be able to dissolve the necessary amounts of phosphate. Probably 
 
10 University of California — Experiment Station 
 
 in the case of some fertilizers, and with certain methods of application, 
 there may occur direct contacts between roots and particles of ferti- 
 lizer, before the latter undergo extensive change. 
 
 In some soils the particular form of phosphate used may be 
 important. There is evidence that it is possible for a part of the 
 added phosphate to undergo, gradually, such strong fixation that it 
 becomes unavailable to plants, even when root contact takes place. 
 This loss of availability possibly may be more rapid with some forms 
 of phosphate than with others. These remarks do not apply with equal 
 force to all types of soil, and furthermore, the method of applying the 
 phosphate fertilizer is of importance, and modifies any comparisons 
 of different types of phosphate fertilizers. Some investigators believe 
 that potassium, when added to certain soils, also may slowly undergo 
 changes rendering it unavailable to plants, notwithstanding proper 
 contacts between soil and roots. In the California soils investigated, 
 it does not appear that added potassium can become unavailable to 
 nearly the same extent as phosphate does in certain types of soil, 
 including some found in California. 
 
 From the foregoing considerations, it is evident that in the case 
 of surface-rooted crops it is possible to modify, at least for a time, the 
 nutrient condition of a soil with respect to phosphate and potassium 
 by the use of suitable fertilizers, applied in reasonable amounts, 
 although larger quantities may be needed on some soils than is in 
 accord with current practice. But the practical question arises, how 
 generally phosphate and potassium, as well as nitrogen, must be used. 
 Concerning this point no general statement can be made. The kind 
 of soil, its previous agricultural history, the amount of potassium or 
 phosphate already removed by crops, the crop to be grown, and the 
 climate, all enter into the equation. Over any given period of years, 
 many soils may maintain their fertility with the addition of only one 
 or two of the above named elements, for the reason that the remaining 
 elements are still supplied in sufficient abundance by the minerals 
 already present in the soil. Under these circumstances, a " balanced' ' 
 condition for crop growth might be brought about in a soil by the 
 simple addition of nitrogen, and perhaps organic matter, in appro- 
 priate form. 
 
 The remarks which have just been made are in no way inconsistent 
 with recognition of the fact that continuous and intensive cropping in 
 general tends to lower the amount of easily dissolved phosphate or 
 potassium, even in soils of high initial fertility, such as are often found 
 in California. The question which has to be asked for each soil is : Has 
 a critical point been reached, or will it be reached soon, or is a state 
 
Circ. 317] Fertilizer Problems and Analysis of Soils 11 
 
 of depletion still far in the future? The answer to this question will 
 be modified in accordance with the nature of the crop as well as the 
 soil, since, as already indicated, there is a possibility that some crops 
 (including, perhaps, many types of fruit trees) under otherwise 
 favorable conditions, may be able to absorb sufficient potassium or 
 phosphate from slightly soluble compounds, which are present in most 
 soils in amounts which are large relative to crop withdrawals. This 
 is especially true of potassium. 
 
 However these questions may be decided for particular cases, the 
 general statement can be made that serious soil difficulties will arise 
 in course of time under conditions of exhaustive cropping, unless 
 provision is made for additions to the soil, whether by means of cover 
 crops, animal manure, nitrogen fertilizers, or other commercial ferti- 
 lizers, or by some combination of these materials. Both the chemical 
 and physical state of the soil are involved. The maintenance of 
 fertility will not usually be automatic, although certain California 
 soils seem to be initially so fertile that marked effects of exhaustion 
 may be very long delayed. 
 
 COVER CROPS AND ROTATION OF CROPS 
 
 The turning under of cover crops may tend to build up the soil 
 reserve of easily available potassium and phosphate. This action is 
 explained by the gradual accumulation in the growing plants of 
 phosphate and potassium derived from very slightly soluble com- 
 pounds present in the soil. Later, the entire amounts accumulated in 
 the plant tissues, when returned to the soil, may remain in an easily 
 available form for other crops. How important these changes are in 
 California soils is not yet known. It should be remembered in this 
 connection that plants grown on soils containing very small amounts 
 of available potassium or phosphate are likely to have relatively low 
 percentages of these elements present in their tissues. Consequently, 
 there would be this limitation to the possible increase in availability 
 of potassium and phosphate through the use of cover crops. 
 
 In many parts of the world where agricultural experience is of 
 long duration, it has been found that the practice of continuously 
 growing one crop often gives very unfavorable results. In such cases, 
 suitable rotations of crops, including legumes, frequently accompanied 
 by the use of phosphate or other fertilizers, have been worked out 
 through long periods of field experience. The desirability of crop 
 rotation is not necessarily to be ascribed merely to the maintenance 
 of nitrogen content in the soil, or possible differences in the abilities 
 
12 University of California — Experiment Station 
 
 of different crops to utilize relatively insoluble potassium or phos- 
 phate, however important these factors may be. Some investigators 
 emphasize the development of injurious soil microorganisms, plant 
 diseases, or toxicity caused by residues of crops. There are cases in 
 which it has been possible to grow the same crop for many years, when 
 animal manure, or commercial fertilizers, or both, have been applied 
 in suitable amounts, but it appears that in general crop rotation 
 should be strongly emphasized. Recent opinion is to the effect that 
 insufficient attention has been given to crop rotation in California. 
 
 USE OF ANIMAL MANURE 
 
 In considering the matter of potassium or phosphate fertilization, 
 it is essential to recall that considerable quantities of these nutrients, 
 and especially of potassium, are added to the soil when large amounts 
 of animal manure are systematically applied. From earliest times, 
 the observation has been made that the use of animal manure nearly 
 always produces highly favorable effects on the growth of plants. This 
 question has been under careful study at the Rothamstead Experi- 
 mental Station, England, for over seventy years. It has been found 
 that in the case of wheat, farmyard manure applied at the rate of 
 14 tons per acre annually gives higher yields than does artificial 
 fertilizers containing similar amounts of nutrients. From these, and 
 from many other experiments and observations, it appears that the 
 beneficial effects of manure are not always limited to its content of 
 nutrient elements. The possible improvement of the physical condition 
 of the soil is a very important matter. The improvement of the soil 
 as a medium for desirable microorganisms must be given consideration. 
 The use of manure may increase the amount of nitrogen fixed from 
 the air, according to some investigations. Other beneficial effects may 
 be brought about which are not clearly understood at the present time. 
 
 Although the use of manure may largely solve a problem of soil 
 fertility for certain crops or districts, it is obvious that this is not a 
 universal solution of soil problems. Adequate quantities of manure 
 frequently are not available, and furthermore, manure produced on 
 one soil and applied to another, still leaves open a question of ferti- 
 lization of the soil from which the nutrients contained in the manure 
 were withdrawn. It cannot be denied that under many modern agri- 
 cultural conditions, commercial fertilizers of one type or another must 
 play an ultimately indispensable role, subject to such limitations as 
 this circular attempts to describe. 
 
Circ. 317] Fertilizer Problems and Analysis of Soils 13 
 
 FERTILIZATION AND QUALITY OF CROP 
 
 Much discussion has taken place concerning the possible influence 
 of potassium or phosphate fertilization on quality of crop as distinct 
 from yield. It seems evident that the quality of certain types of crops 
 may be influenced by fertilizer applications under some soil and 
 climatic conditions. The improvement or change of quality occurs 
 primarily in soils which are initially very deficient in ability to supply 
 one or more nutrient elements. Many of the reports dealing with the 
 effects of fertilizers on quality of crops are based on experiments 
 carried out under soil and climatic conditions different from those 
 found in most parts of California. 
 
 Some observations have been made indicating that fertilizers 
 applied to deficient soils may alter the rate of growth or time of 
 maturity of various crops. For example, phosphate, when applied 
 to such slightly fertile soils, may accelerate root development and 
 promote earlier tillering and grain formation in cereals. The same 
 fertilizers may hasten the early growth and time of blossoming of 
 tomatoes. With plants of these types, the presence of adequate 
 amounts of available phosphate in the soil during the early stages of 
 plant growth seems to be very important, The addition of potassium 
 to a soil highly deficient in available potassium tends to produce 
 plumper seed in the case of cereals. It is important to recall that none 
 of the effects just mentioned may be observed if the potassium or 
 phosphate already present in the soil is sufficiently available. 
 
 With fruit trees, it is exceedingly difficult to obtain convincing 
 results concerning quality, when both the fertilized and unfertilized 
 trees are reasonably normal. Most reports by investigators are nega- 
 tive or inconclusive. There is evidence, however, that some types of 
 fruit trees can become diseased on certain types of soil as a result of 
 deficiencies (for example, deficiencies of potassium) or toxicities 
 present in the soil. In any attempt to correct such conditions, the 
 statement already made should be borne in mind, to the effect that 
 it may be very difficult and expensive to increase the amounts of 
 available potassium or phosphate in the deeper root zones, at least by 
 the customary methods of fertilizer application. Also, cases have 
 arisen where favorable effects are obtained only by the use of such 
 substances as iron sulfate and copper sulfate, rather than ordinary 
 fertilizers. Problems of this type are now under systematic investi- 
 gations by various Divisions of the Experiment Station. Information 
 of progress made in the districts concerned may be obtained from the 
 
14 University of California — Experiment Station 
 
 appropriate members of the Experiment Station, or from the farm 
 advisor. 
 
 Numerous field observations have been cited by different investi- 
 gators which are believed to show that potassium renders plants more 
 resistant to certain diseases, especially those caused by fungi. The 
 possible relation of fertilization with phosphate or potassium to some 
 types of plant diseases is a subject of great interest and importance. 
 Unfortunately, the investigational work is extremely difficult, and we 
 do not now have any adequate working knowledge. It is not unreason- 
 able to suppose that with some, but by no means all diseases, plants 
 are more likely to suffer serious injury when in a state of malnutrition. 
 A marked deficiency of an essential element leads to an abnormal 
 change in the organic composition of plant tissues. 
 
 ACID AND ALKALINE SOILS 
 
 Many inquiries are made concerning the acidity or alkalinity of 
 soils. All soils must be either neutral, acid, or alkaline. By a neutral 
 reaction is meant one which is exactly the same as that of absolutely 
 pure water, used as a standard. The degrees of acidity or alkalinity 
 are designated by the symbol pH. 5 Markedly acid soils are common 
 in some regions, but they are comparatively rare in California. Soils 
 of this character may sometimes be found in those areas of the state 
 having a very high rainfall. Highly alkaline soils sometimes occur, 
 but a discussion of these soils would make necessary a consideration 
 of ' ' alkali ' ' conditions, which are discussed in other publications. The 
 reaction of a soil is subject to change resulting from the action of 
 substances added to the soil. Thus, sulfate of ammonia may tend to 
 increase acidity, and nitrate of soda to lessen acidity, or to increase 
 alkalinity. Sulfur tends to increase acidity, and lime to decrease it. 
 It is difficult by the use of ordinary fertilizers to bring about appreci- 
 able changes in the reaction of a soil within a limited period of time, 
 unless the soil is of a light or sandy character and has a low resist- 
 ance to change of reaction through the addition of acid or alkaline 
 materials. 
 
 With most plants of agricultural interest, a considerable latitude 
 in soil reaction is consistent with good growth. A slightly acid soil is 
 not necessarily unproductive. For example, certain rather acid peat 
 
 5 With this system, pH 7 means a neutral reaction ; values below 7 indicate 
 acidity, and values above 7, alkalinity. A soil of pH 5 is decidedly acid, one 
 of pH 9 decidedly alkaline. A great many soils in this state have reactions 
 not far from the neutral point. 
 
Circ. 317] Fertilizer Problems and Analysis of Soils 15 
 
 soils, when properly fertilized, are very productive. It is necessary 
 to emphasize the point that the reaction or ' ' pH ' ' of a soil is merely 
 one factor influencing' growth, and that the determination of this 
 value, important as it is at times, seldom or never should be relied 
 on as a guide to understanding soil conditions, without a suitable 
 knowledge of other factors interrelated with the reaction. 
 
 SOIL ANALYSIS 
 
 It is hoped that this presentation of the complexity of soil prob- 
 lems has made it clear that routine chemical analyses cannot often 
 determine the adaptability of soils to crops, or the best method of 
 fertilization. It is true that special investigations on soils, and the 
 understanding of general principles, cannot progress without the use 
 of chemical methods, but really adequate studies are costly. They 
 can be carried out by the Experiment Station only in selected cases, 
 to obtain knowledge of general relations, or to assist in the planning 
 or interpretation of field experiments. The validity of any interpre- 
 tation of chemical data must rest finally on the results of experiments 
 with plants. Even if there were now available assured methods of 
 obtaining and interpreting chemical data on soils in terms of crop 
 growth, there would still remain the question of securing representa- 
 tive samples of soil for examination. The most uniform field in 
 appearance may, in fact, contain numerous soil variations, so that 
 often it is difficult to obtain samples which may be considered to reflect 
 an average condition. In addition, there arises the question of the 
 relative importance of samples of soil taken from different depths, 
 which would vary with the crop, physical character of the soil, 
 irrigation practice, etc. 
 
 If it be desired to consider the examination of soils by the method 
 of water extraction, it should be recalled that the water soluble portion 
 of a soil does not have a constant composition. In fact, the com- 
 position may vary from day to day. The rapid growth of certain 
 crops may bring about a temporary depletion of substances dissolved 
 in the soil water, even with the most fertile soils. Therefore, if 
 methods of this type are to be employed, it is necessary to recognize 
 that different results may be obtained when samples are taken at 
 different times of the year. The investigations of. recent years 
 emphasize the importance of including studies on the solid portion of 
 the soil, in order to understand its ability to continue supplying 
 nutrients to the soil moisture. 
 
16 University of California — Experiment Station 
 
 Occasionally soils are found on which comparatively simple 
 chemical tests may strongly suggest a deficiency, but such soils are 
 often extreme enough in character so that the general nature of the 
 deficiency is already recognized by practical observations. Any pos- 
 sible future development tending toward a more general application 
 of chemical tests to soils must be the result of comprehensive controlled 
 experiments with different crops, as well as of a more critical study 
 of field experience than it has yet been possible to make in most parts 
 of California. 
 
 Soil and plants are altogether too complex to permit of any easy 
 or quick method of determining the best methods of soil treatment. 
 There must be a patient accumulation of knowledge gained in several 
 ways: (a) by continued investigation of basic relations which enter 
 into soil problems everywhere; (6) by further practical observation 
 and experience; (c) by very carefully conducted and long continued 
 local field tests or experiments, accompanied when necessary by special 
 types of chemical and physiological studies. Immediate practical 
 steps to be taken cannot be decided upon without reference to local 
 conditions, and none of the statements contained in this circular 
 should be construed as a specific recommendation for any kind of soil 
 treatment. Those who desire to avail themselves of the services of 
 the College of Agriculture should, in the first instance, consult the 
 farm advisor of their county, or an extension specialist. These repre- 
 sentatives of the institution have available to them the information 
 gained in the investigations of the Experiment Station, to be utilized 
 as may be possible with due reference to local conditions. 
 
 Other circulars, or mimeographed sheets, may be obtained from 
 the office of the county farm advisor, or from the Division of Plant 
 Nutrition, University of California, explaining the type of service 
 which the College of Agriculture is capable of rendering in connection 
 with soil problems. 
 
 17m-6,'30