Soil science is largely a product of the twentieth century because it is only in most recent years that a reasonably satisfactory theory of soil and its formation has been proposed. Justus von Liebig (1840) had explained soil fertility in terms of chemical elements as plant nutrients stored in the soil. These could be absorbed by the plants and replaced by man in the form of chemical fertilizer. There was an important truth in Liebig's theory, but it was not an explanation of soil. In the United States, E. W. Hilgard (1833-1916) anticipated much of the modern theories which are credited to the Russian school of V. V. Dokuchaev (1846-l903), and his followers, N. M. Sibirtzev (1860-l899), and K. D. Glinka (1867-1927). It is fundamental to the concept of soils to distinguish between the parent soil materials derived from the disintegration of geological formations, and soil as a complex body derived from the operation of several soil forming factors. In equation form, soil may be defined in principle as the product of parent materials, climate, organisms, topography, and time. This does not disguise the fact, however, of wide differences in emphasis and interpretation of this or a similar formula. The followers of the Russian school placed climate first and minimized the others in varying degrees of extreme. Some virtually ignored parent materials, and also relegated organisms, vegetation, animals, and microorganisms to a minor role of dependent variables.
Until the early twentieth century European and American soil scientists looked primarily to geology and chemistry as the parent sciences, and their studies reflected that emphasis in soil analysis and classification according to geological origins of the materials, the manner of their distribution (residual, or transported by water, wind, or glacier), and their chemical composition. Under the leadership of Milton Whitney (1860-1927) the United States soil survey work, begun in 1899, emphasized texture (the sizes and proportions of different sizes of soil particles and their combinations in the surface layer). He attributed the adaptability of a soil for a given crop primarily, if not exclusively, to texture.
In California, Hilgard followed the general trend in emphasizing geological origins and the chemical composition of parent materials, but also he struck out on new lines. Going to California in 1875 as Professor of Agriculture and Director of the Agricultural Station, after many years in Mississippi, he grasped quickly the fact of the importance of climate and made the study of soils of arid regions the most conspicuous feature of his long, active California career (1875-1905), climaxing it with his book Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions (1906). As indicated by the title, he treated climate as an independent soil forming factor. He distinguished between soil and subsoil, and although ill-defined, he attributed "the differentiation of soil and subsoil as due partly to the action of organic matter and micro-organisms, partly to physical-chemical causes...," or in more complete form it was the result of vegetation, moisture, access of air (oxygen), temperature, "and the presence of the several organisms which in the course of time take part in the process of soil-formation." One chapter each was devoted to humus, soil organisms, and vegetation. As respects vegetation- soil relations, he missed the most significant point in viewing vegetation as controlled by soil, rather than distinctive types of vegetation, grasses and forests, contributing as soil formers to equally distinctive soil groups- i.e., mutual interaction between soil materials and vegetation.
In describing the physical effects of the percolation of water, he pointed to the fact that the diffused colloidal clay was carried with it to a certain depth into the subsoil and that this clay layer was more retentive of moisture and "plant food substances in solutions" than the surface layer. The chemical effects of the percolation of water he recognized as dissolving and carrying lime down through the soil; in arid climates depositing it in a lime accumulation zone of the subsoil, but in humid climates carrying it off in drainage. His illustrations showed diagrammatically the differences in the resulting soil profiles in arid California climate and in humid climate. The superior fertility of the soils of arid regions was explained similarly by the theory that nutrients were not leached out by rainfall. In estimating the significance of his contribution to soil science and agriculture his associate R. H. Loughridge (1917) concluded:
Among his California activities there stand out prominently his studies on humid and arid soils, in which he was the first to point out their differences in depth and in physical and chemical characteristics; he was the first to explain endurance of drouth by culture crops in sand soils and why sandy soils are among the most productive in the arid region and least so in the humid....
While Professor Hilgard was not the first to make a soil survey or a chemical analysis of the soil, he was the first to interpret the results of analysis in their relation to plant life and productiveness. He was also the first to maintain that the physical properties of the soil are equal in importance to the chemicals in determining the cultural values.
The review of Hilgard's work has been given at some length because of the distortions by the enthusiasts of the newer Russian-American school of soil scientists of the United States Department of Agriculture. Marbut (1935) said that "up to this time [of the Russian influence] no suggestion that the characteristics of soils were due to other agencies [than geologic] had received recognition in the United States." In the departmental Yearbook Soils and Men (1938) Kellogg made some amends. Outside of the Federal governmental publications, Hilgard was given substantial recognition as in the writings of Joffe (1929, 1936), and Jenny (1941), but an adequate evaluation of his whole career is yet to be written - (cf., further discussion in Ch. 14).
The soil work done in Russia, isolated because of the language barrier, was largely unknown outside of that country until the translation into German of Glinka's work in 1914. C. F. Marbut, of the United States department of agriculture, led in popularizing this point of view among soil scientists in the United States. In 1927 he published a translation into English from the German edition of the major portion of Glinka's The Great Soil Groups of the World and their Development. The work of Dokuchaev on the genesis of the Russian chernozem (black earth) was not a study of isolated samples of soil in the laboratory, but a field study of the profile of the soil body for itself. From this came the recognition of three layers or horizons in the typical soil profile representing different stages of physical and chemical weathering of the parent material at different depths- different amounts and conditions of organic elements incorporated- and the degree of development depending upon the time factor operating in soil formation. To the Russian school, climate was the dominant, almost exclusive independent factor in soil formation as vegetation and other biological factors were viewed as a product of climate. Soil was defined in terms of soil-forming processes, therefore, and not in terms of parent materials.
Attempts to apply the Russian system to the soils of the United States met difficulties because of the fundamental differences in environment. In Russia and in most of eastern Europe, climate belts ran in a general east-west direction, dry and hot at the south and wet and cold at the north. In the United States, the regions east of the Mississippi River were of approximately the same humidity both north and south, the temperature providing the major variable. West of the Mississippi River to the Rocky Mountains the surface rose in altitude to the westward and the rainfall diminished; thus there were three important variables, altitude, rainfall, and temperature, the third roughly running at right angles to the first two. The Great Basin interior provided further differences and the Pacific coast the most unusual variety of all. In American hands, the Russian formula, with its exaggerated emphasis on climate, was substantially reconstructed, and soils were defined on the basis of soil characteristics rather than soil forming processes. The first presentation of soil classification on the American plan was by Marbut, in 1922, and his most complete statement was in "Soils of the United States" (1935) for the Atlas of American Agriculture (1936). Later statements of the work of the United States Soil Survey have appeared mostly under the name of C. E. Kellogg, chief of the Division (1936, 1937, 1938, 1941), and have modified Marbut's conclusions.
Preoccupied with the work of soil description and classification, it was not until after 1930 that the new point of view took shape for popular presentation. Wolfanger's book The Major Soil Divisions of the United States (1930) presented the view of regional distribution of soils like regional distribution of natural vegetation and climate. The principal governmental publications that presented the broader outlines were Marbut's, "Soils of the United States" (1935) a section in the monumental Atlas of American Agriculture (1936), Kellogg's Development and Significance of the Great Soil Groups of the United States (1936), and of widest circulation, the departmental Yearbook, Soils and Men (1938). Among the commercial soil textbooks were Lyon and Buckman (Second Edition 1929, Third Edition 1937, and Fourth Edition 1943), Joffe (1936), Millar and Turk (1943), and most original and stimulating of all, Jenny, Factors of Soil Formation (1941). To these should be added the European books of Robinson (1932, 1936), and de Sigmond (1938). Kellogg's popularization, The soils that support us (1941), has a large measure of political and social propaganda intermingled with science.
Most of the new school of soil scientists treated the soil as "a natural body" or an organic entity, an object of nature in equilibrium with environment. This concept carried with it also the idea of age in relation to immature, mature, and degraded soils. The idea of a mature soil implied somewhat the same thing to the pedologist as climate formation did to the botanist. Jenny (1941) was an exception, taking the ground that soil was "a physical system", not a natural body, and as an open system it might be added to or subtracted from and that there was "an element of organization" in every soil.
Although he defined mature soils in terms of equilibrium with environment he warned that assumptions regarding maturity are all inferences, theories, not demonstrated facts. In Buckman's revision (1943) of the Lyon and Buckman book he carried further than others the idea of the biological nature of soil, "a colloidal-biological concept," in which the genesis of soil began with "a biological phenomenon", the operation of microorganisms and higher plants, living and decaying matter, in the soil materials.
A soil profile was a vertical cross-section of soil from the surface to the underlying bed rock. It was divided into layers or horizons and each such horizon was subdivided. The A or top horizon, at its surface, A1 horizon, accumulated organic matter, the lower part, A2, showed the results of the leaching out of certain solutions and fine material, the layer of eluviation. The B horizon was the layer of accumulation (illuviation) of deposits from above. The C horizon was parent material, physically disintegrated, but on which soil forming processes had not yet operated. The D horizon was the bed rock. The solum was the name given to the A and B horizons when considered together as soil. In the soil forming process, climate operated as a factor in dissolving compounds and translocating them and fine material, organic colloids, from one part of the soil to another or in removing them altogether. The problem of lime, emphasized by Hilgard, illustrates the process, when in low-rainfall climates the dissolved lime was deposited in the lower horizon, while in high rainfall climates it might be removed altogether. The zone of clay concentration in a high-rainfall climate was the product of the downward movement of colloids. The leaching out of more soluble materials left the insoluble forms, alumina, and iron, as conspicuous residual material in leached soils.
The biological factor entered into soil forming processes in a conspicuous manner as each type of vegetation possessed different kinds of root systems, fibrous or woody, penetrating to different depths, and living or dead, influenced the soil both physically and chemically. The grasses and the forests as soil formers contributed substantially different characteristics. Grass soils contrasted as distinctly with forest soils as the grass and forest vegetation above ground. The whole range of rodents and insects that burrowed, and at death contributed to the organic fraction of the soil. Most neglected until last was the soil population of microorganisms. The emphasis on the surface vegetation was relatively obvious, but it was the belated emphasis on the second and third aspects that gave particular significance to the Lyon and Buckman (1943) phase, that the genesis of soil begins with "a biochemical phenomenon." Topography contributed its part to the soil-forming process through its influence on moisture distribution in the soil and consequently on the distribution of the biological factor upon and within the soil mass.
It is inconceivable to think of process and dynamics except in terms of duration. Soil formation required relatively long periods of time, but there was no conclusive data of experimental character which would determine the length of time necessary. Jenny (1941) gave examples of the results of formative processes in particular cases of known duration from a few years to 230 years. The net result of such examples was to throw doubt upon the extreme requirement of a thousand or even thousands of years claimed by some soil conservation enthusiasts. It was at this point that Jenny's (1941) idea of soil as an open system rather than a natural body gained significance. The use of the word age in connection with a natural body carried with it an implication of a life cycle of youth, maturity, and degradation that is not subject to experimental proof - it went beyond the facts. Any change in the components in an open system would only initiate another phase in readjustments of unstable equilibrium with environments, not maturity or decadence.
There are many systems for the classification of soils (Robinson (1936), but for the United States, the Federal Soil Survey set the principal model.
Taking the country as a whole, Marbut (1922, 1935) divided the soils into two parts on the basis of characteristics of the solum (A and B horizons taken together) calling them pedocals (ped = soil; cal = calcium) were the soils of the low rainfall regions with a lime accumulation horizon. The pedalfers (ped = soil; al = alumina; fer = iron) were the leached soils, without lime, exposing conspicuously the alumina and iron. The chemical reaction (pH) of the pedocals was neutral to alkaline, that of the pedalfers was acid. The line dividing these great solum groups followed approximately the eastern rim of the watershed of the Red river of the North from the Canadian boundary southward through western Minnesota, turning southwestward at the Iowa line into Nebraska, then southward passing somewhat west of Lincoln, Nebraska, and Wichita, Kansas, then slightly west of south past El Reno, Oklahoma, to south central Texas, slightly west of the 99 meridian, where it made a sweeping eastward curve to the Gulf coast near Corpus Christi. Although it must be shown on a printed map as a line, it was more accurate to think of it as a transitional zone. Except at the northern end, it was not far from the 98 meridian, the line so much discussed in the literature of the plains region as the line dividing the prairie from the low plains. The Great Soil Groups or Provinces (Marbut, 1935) were subdivisions of the solum groups. In the pedalfer area, except the Prairie Peninsula, from north to south they were tundra soils in Canada; podzols near the borders of the two nations associated largely with the northern evergreen forests; gray-brown podzolic soils of the oak-hickory deciduous forests, and red soils and yellow soils of the southern hardwood and evergreen forests regions. All soils formed under forests were of limited or even inferior fertility, a fact which runs counter to popular traditions. The prairie soils constituted a group by themselves in the region of the Prairie Peninsula and southward through the prairie-forest transition country to the Gulf. Intermixed, however, were gray-brown podzolic soils which occupied most of the eroded valleys. In the pedocal area, just west of the pedocal-pedalfer division line, lay the chernozem belt forming a north-south belt from near the Rio Grande into Canada.
Farther west were the dark-brown soils of the plains and the brown soils of the plains-desert transition, and the gray soils of the deserts and the Great Basin. The Pacific coast region had a wide range of soils; from gray-brown podzolic to gray-desert.
The prairie soils were the subject of much discussion among pedologist. It was much like the problem among botanists, of grass in a high rainfall climate that, by all the known rules, should have produced forests. The prairie soils are chernozem-like soils formed under a grass cover in a mid-rainfall region, but they lie in a high rainfall region, and are surrounded by a network of valley soils of the gray-brown podzolic group which theoretically seem to be proper soils for the entire Prairie Peninsula. Marbut (1934, 1935) insisted that they are normal soils, while Joffe 1936), more directly in the Russian tradition, called them degraded chernozems. At any rate, it is agreed that the prairie soils are unique, that no other substantial area of similar soils exist elsewhere in the world. They possessed the remarkable fertility of a grassland soil, were only slightly acid, and received ample rainfall to make them highly productive. As the prairie belt ran north and south from near the Canadian line to the Gulf, the temperature range, produced differences, the southern line of Kansas marking roughly an irregular division line between southern prairie soils and northern prairie soils.
In the classification of soils of the United States into Great Soil Groups, Marbut assigned such designations only to soils which he held were mature. By his standard of maturity, nearly half of the soils were designated as immature. In his maps (1935) for the Atlas of American Agriculture, one was designated for the Great Soil Groups, and on it the immature soils were not shown. A second map indicated the immature soils. The two must be studied together. Usually only the first of Marbut's maps was reproduced in books using his soil maps. The use of both maps is particularly important to the mixed grass prairie-plains where a large part of the soils were designated as immature. Furthermore, some of these were among the most productive of the central grassland. Most pedologist took the view that only in early stages of soil formation did parent materials largely influence its characteristics. In more developed soils, the mark of the parent materially lessened until in mature soils that influence was practically non-existent, climate and vegetation fully determining the soil properties. In the Yearbook of the United States Department of Agriculture, 1938, Soils and Men, a substantial revision of the Marbut classification was presented, introducing in part new terminology as well as reclassification. The southern prairie soils became reddish prairie, but the area was broken up into several types of soils. The chernozems were retained, the northern dark browns were renamed chestnut, but the southern chernozems and southern dark browns were substantially rearranged and the name reddish-chestnut given to the major portion. The brown and desert gray soils underwent the most extensive revision and one that is traceable only by reference to the detailed maps. In dealing with the low-rainfall regions, Shaw (1937) still held to classifications of soils which emphasized parent materials, and Graham (1941, 1943) concluded that in Missouri the parent materials continued to influence plant growth even in mature soils and that soil classification should recognize this fact. Robinson (1936) noted a swing back to greater recognition of parent materials. The whole subject of soil science was too conspicuously in its formative stage that it was important to recall Robinson's (1936) admonition, that all soil classifications were provisional. The problem of minor elements placed a new emphasis on parent materials of soils as well as upon occurrence of mineral nutritional diseases of plants and animals. The importance of the subject was such that the editor of Soil Science sponsored a symposium of twenty-one papers dealing with sixteen minor elements which were published in the July and August issues, 1945, including a map of minor element distribution in the United States which affected plants and animals.
The least known of the aspects of soils in their relations with plants centered around the problems of microbiology and gave special significance to the biochemical concept of soil. The population of the soil included animals, both large and small, and plants, both the roots of higher plants and the plant micro-organisms. Summarizing from Lyon and Buckman (1943), the animal division included macroorganisms living primarily upon the plant materials, rodents (gophers, squirrels, woodchucks, etc.), insects, millipedes, sowbugs (wood lice), mites, slugs, snails, and earthworms; macroorganisms (largely predatory, living on other animals), moles, insects, (many ants, beetles, etc.), mites, centipedes, and spiders; microorganisms, either predatory or parasitic, or living on plant materials nematodes, protozoa (amoeba, ciliates and flagellates) and rotifers. The plants included the roots of higher plants, algae, fungi (including mushrooms, yeasts, and molds), actinomyces, and bacteria.
On the physical side rodents and insects, along with many of the macroorganisms, were constantly engaged in the process of translocation and pulverizing of soil and their channels and galleries contributed to aeration and drainage, constituting a natural tillage. Humus was largely synthesized through the feces of insects, millipedes, sowbugs, mites, slugs, snails; such animals as use plant material thereby initiating the process of soil decomposition which was continued by bacteria and fungi. Their dead bodies in turn contributed further to the organic content of soil. Actinomyces, next to bacteria, were the most numerous of the soil population, and in bulk were greater. Possibly fungi and certain animals were even more important than bacteria. W. Kette (1865), according to Waksman (1932), was the first to recognize the relation of microorganisms to soil fertility. The first great contribution related to bacteria and the nitrogen problem and belong to the decade beginning with 1886 in association with the names of Hellriegel, Wilfarth, Frank, Beijerinck, and Winogradsky. Almost another generation passed before further fundamental advances were made relative to protozoa (E. J. Russell, 1909, 1913), algae, fungi, actinomyces and nematodes. Soil microbiology was little more than a test tube or laboratory science until the 1920s when H. J. Conn and Winogradsky attacked the problem of direct methods of examination of microorganisms in the soil. Sergi Nikolaevitch Winogradsky, rated by Maksman (1925) as one of the two greatest soil microbiologists of the period, was exiled from Russia by the Bolshevist revolution and in 1922 found refuge in France, with the opportunity to continue his work as head of the soil microbiology section of the Pasteur Institute at Paris. In 1925 he presented his direct method which became the most important project in the new methodology. It was his contention (1925, 1928) that with the laboratory method exclusively, based on isolation of the organism from the soil, pure culture, and bacteriological media, no real soil science could be established; and that as an independent science soil microbiology must be carried out under conditions as near nature as possible, "in the soil itself." He cited the example of the Azotobacter, which was active in isolation, but remained "obstinately at rest in the midst of the soil." Waksman (1925, 1932), Romell (1930, 1935), and Jacot (1936) emphasized the same theme through the next decade. The reaction of the soil solution favored bacteria when it was on the alkaline side and fungus on the acid side (Joffe, 1936; Vandecaveye and Katznelson, 1940; Jenny, 1941). From this the conclusion was clear that any combination of conditions which tended to change the pH of the soil was reflected in the composition of the soil population which must adjust itself to fluctuating biochemical environment. These principles had a practical bearing, therefore, upon the problems of the grass-land and the differences in soils formed under a grass cover and under a forest cover. Wolfanger (1930) pointed out that studies had not been made on an extensive or systematic scale with reference to the relations between the great soil groups and microorganisms. Waksman (1932) had a chapter on the microbiology of peat soils (Ch. 26) and forest soils (Ch. 27), but none on the grassland soils. In neither case, however, could these treatments go much beyond a recognition of the problem. Joffe (1936) lamented how little was known of microorganisms as soil formers, their distribution and behavior in the soil profile, but concluded that each climatic zone had its specific population. Jenny (1941) said practically the same thing, emphasizing that all soils were subject alike to this independent biotic factor, to these micro- organisms dispersed through the atmosphere, but for unknown reasons the response of the several soils was different. Investigations of the relation of microorganisms to the different horizons of the same soil led to a variety of conclusions (Waksman 1932). Vandecaveye and Katznelson (1940) found in a majority of soils tested a larger population in the B than in the A horizon. Other investigators sometimes arrived at different results and Newman and Morgan ( 1943) not only found the subsurface population smaller, but less adaptable, introduced plant material being less rapidly and less extensively decomposed. There was rather general agreement that the soil population contributed essentially to the structure of the soil (Waksman 1932, 1936; Jacot 1936; Jenny 1941; Lyon and Buckman 1943) and therefore to the success of tillage and control of soil erosion either by wind or by water. The problem of soil fertility was largely bound up in the problem of microbiology. Russell's (1909, 1913) protozoan theory was based on the assumption that protozoa consumed bacteria, under some circumstances to such an extent as to destroy fertility. Cutler and Crump (1935) assigned a different role to protozoa; that their presence kept the bacteria at the level of highest efficiency. Waksman (1937) doubted both theories because direct examination of soils did not show either the number or volume of protozoa great enough to serve such purposes. It was evident that there existed within the soil populations a complex balancing of forces, because theoretically each could soon multiply sufficiently to overpopulate the soil. Some were balanced against each other, some destroyed others, and some were beneficial in relation to higher plants. Because of antagonistic relationships, it was sometimes impossible to introduce organisms into a soil, or if possible, only by changing the soil environment (Waksman, 1941; Jacot, 1940).
An investigation by White (1941) of prairie and forest soils in Wisconsin offered interesting suggestions to the problem of the grasslands. He found that by introducing into the prairie soil a fungus, a mycorrhizae from red pine roots, the growth of trees was promoted. The subject needs more extensive investigation under a wider range of soils and environments, but if the conclusion should prove of general significance, it would only change the form of the question. Instead of, why the prairies did not grow trees, the question would be, why prairie soils did not grow mycorrhizae? Seifritz (1938) discussed the relationship between soil acidity and plants concluding that plant growth was largely determined by the pH of the soil, but after a particular vegetation was established it in turn influenced the soil acidity. He did not include the problem of microorganisms in his discussion. The same question of grass and trees was approached by Joffe (1936) from a quite different direction. As a result of lysimeter experiments in the podzol zone he found that the soil reaction was acid during the winter, spring, and early summer, but in late summer and fall it became less acid or nearly neutral. His suggestion to the prairie discussion, therefore, was that should this seasonal cycle hold true for the grass region, the naturally alkaline reaction would be intensified in the fall creating conditions definitely unfavorable to the sprouting and growth of tree seeds, but highly favorable to grass seeds. Stone (1944) cautioned about soil reaction studies in relation to plant growth, pointing to defects in the techniques of most older literature, the complexity of interacting factors, and the fact that vegetation influenced the pH of the soil as well as the fact that soil reaction limited species of plants. In trying to sum up any possible conclusions in the matter, the historian is reminded of Livingston's observation of 1917, that instead of solving the problems of twenty-five years ago, they had only succeeded in analyzing them into several component problems, each of which was as difficult as the original problem.
The modern soil scientist makes a distinction between the terms fertility and productivity of soils. Kellogg (1941) said:
soil productivity may be most clearly conceived as being a response to management, yields alone are not adequate guides to soil productivity-the management under which the yields are obtainable must be known as well.... Soil fertility is included in the concept of soil productivity but refer only to the content, and availability of chemical compounds in the soil that influence plant growth.
Jenny (1941) took the ground that productivity was measured in yields per acre and might be stated in a mathematical formula: Productivity equaled the product of climate, plant, man, soil materials, and time; while fertility was associated historically of Leibig's idea of soil and plant nutrition and became therefore a concept of plant nutrients in the soil which might be taken from the soil by plant growth and restored by fertilizers.
There was little difference here between the two statements of soil fertility, but there was substantial difference between the definitions of productivity. The factor of management in Kellogg's definition might be interpreted as equivalent to the factor man in Jenny's and it might be interpreted to include most of the other factors, but in that form it might be too vague to be of much value as a definition. Tyulin (1938) attacked the problem of fertility from a different direction, emphasizing colloids as being associated with soil fertility, particularly the first humate fraction of group 1 colloids. Atkinson, Turner, and Leahey (1944) found similar results and concluded that the relationship was closer than between yields and percentage of nitrogen in the soil. Robinson (1936), an English soil scientist, argued that fertility had meaning only in terms of particular crops, and therefore the only generalized definition could be the capacity to grow satisfactorily the largest number of the crops suitable to a particular climate. He advocated a new attack on the problems of plant nutrition from the standpoint of plant physiology in relation to the properties of the soil. The maintenance of the original fertility or the destruction of the original fertility of the soil were themes of popular agitation among conservationists and historians in dealing with agriculture and often were based, obviously, on misconceptions. To maintain original soil fertility meant the development of a type of agriculture which would combine in domesticated and artificial forms all the factors, biological and physical, which originally produced the soil in the state of nature. Among other things, the forest soils must grow an exact equivalent and a grassland soil an exact equivalent of the original vegetation. Of course, such an idea of exact equivalents was impossible to achieve. Also tillage methods would have to be adopted to duplicate exactly the natural tillage effected by wild animals, especially those burrowing in the soil. Even if these things were possible, they would be undesirable because some soils in the natural state were unfit for the successful growth of the crops which man required and from the beginning it was necessary to add factors which would produce new equilibriums for crop production. No one would want to restore their original fertility for that would render them relatively useless. In the light of experience of man with soils long in cultivation one is tempted to inquire whether the assumption of the soil scientist was sound or only exaggerated that assigned to surface vegetation so important a role as was implied in the use of the terms forest and grassland soils. If vegetation was so largely a determinant in soil formation, should not the removal destroy those assigned characteristics? To what extent did removal of forests or grass destroy or change the microorganisms within the soil peculiar to each of the Great Soil Groups? Was it possible that the biochemical factor associated with microbiology was more of a determinant than the surface vegetation? Jenny (1941),and Robinson (1936) both recognized explicitly that soils changed with the removal of the original natural vegetation. Robinson's experience was primarily with English soils that had been cultivated for centuries and continued fertile and productive. Jenny emphasized that such a change in vegetation was a modification of one of the major factors in the soil formation equation; so-called soil was reduced to the rank of parent materials, and a new stage in the process of soil formation was initiated. Both writers stated the problem and the principles, but neither had the data to chart the course of the soil changes which had taken place in soils long in cultivation or which would take place in new soils just being brought into domestication. Both phases of the problem were vital for the guidance of the historian.
So long as soil science was the work of a relatively few workers, and those dominated by the USDA, there was little opportunity to deviate from the course marked out as official. The extent to which the situation was changing is emphasized by Kelley's (1946) re-statement of "Modern concepts of soil science." From the five concepts discussed, three aspects are indicated here; the emphasis on parent materials, on the ready response of soil to chemical, physical, and biological forces, and on the revised estimate of the clay factor in its bearing on plant nutrition.