Soil scientists do not agree among themselves as to the exact place of organisms in the scheme of soil-forming factors. Nikiforoff, Marbut, and others contend that life in general and vegetation in particular are the most important soil formers. "Without plants, no soil can form," writes Joffe in his "Pedology." On the other hand,
Robinson, in his discussion of the soils of Great Britain writes:
Vegetation cannot be accorded the rank of an independent variable, since it is itself closely governed by situation, soil, and climate. And, therefore, whilst the intimate relationship between natural vegetation and soil cannot be overlooked, it must be regarded as mainly a reciprocal contract.
Likewise, in all studies of soil-climate relationships, vegetation is treated as a dependent variable rather than as a soil-forming factor, because significant changes in climate are always accompanied by variations in kind and amount of plant life. In the ensuing sections, we shall attempt to clarify this controversy and to elucidate the exact role of organisms as soil-forming factors.
A. DEPENDENT AND INDEPENDENT NATURE OF ORGANISMS
It is a universally known truism that microorganisms, plants, and many higher animals affect and influence the properties of soil. But such action alone in no way establishes organisms as soil-forming factors. The mere "acting" is neither a sufficient nor essential part of the character of a soil-forming factor. The A horizon of a given soil acts upon the B horizon, and yet obviously it is not a soil-forming factor. If organisms are to be included among the soil formers, they must possess the properties of an independent variable as indicated in the introductory chapter. It must be shown that the factor organisms can be made to change in its essential characteristics, while all other soil-forming factors such as climate, parent material, topography, and time are maintained at any particular constellation. This variation must be attainable under experimental conditions or by appropriate selections in the field.
For the successful comprehension of the controversial and complicated situation, let us resort to an imaginary laboratory experiment. A number of soils, S1 S2, S3, etc., that differ widely in their properties are sterilized and inoculated with a population of microorganisms consisting of a large number of varied species. This group of organisms represents a biological complex and is given the symbol B. After a period of days or weeks, we shall observe that each soil contains a specific type of microbiological population depending on the properties of the soil and the conditions of incubation. These resultant populations or biological complexes in the different soils we shall designate with the symbols b1, b2, b3, etc. The relationships between the initial and the resulting biological complexes may be represented as follows:
The initial biological complex B deserves the status of an independent variable because, obviously, we may add any kind of biological complex to our group of soils, regardless of their
properties. The initial complex B, therefore, assumes the role of a soil- forming factor. An entirely different situation exists for the resultant biological complexes b1, b2, b3. Here both the species of
microorganisms that survive and the amounts of each (yields) are functions of the soils and of their soil-forming factors. These resultant complexes are dependent variables, in other words, they are not soil- forming factors, though, of course, they influence the properties of the soil.
The foregoing conclusions need not be restricted to the microbiological population of the soil. They apply to all types of organisms, microbes, vegetation, animal life, and, to some extent, to man. For, in principle, we may conduct our imaginary laboratory experiment with any group of organisms.
In view of the properties of an independent variable, which is defined as a variable that can be made to vary independently of other variables, we come to realize at once that the quantity or yield of a given group of organisms can never be a soil-forming factor, because it is completely governed by the properties of the soil and the environment. In speaking of plants, animals, etc., as soil formers, we must modify our habitual picture and divorce the quantity aspect from the concept of organisms. We must focus our attention on the quality aspect, i.e., the kind and relative frequency of species.
The Biotic Factor.—This quality aspect of organic life constitutes the essential part of the biotic factor of soil formation.
Accordingly, in the fundamental equation of soil-forming factors s = f (cl, o, r, p, t, . . . ) (4) the letter o refers to the biotic factor rather than the number,
proportion, and yield of the species actually growing on the soil. The latter results from the interaction of the biotic factor and the other soil formers.
The problem that now lies before us centers around the following question: Knowing the biological complex existing on a given soil, how can we evaluate the corresponding biotic factor? Or, to express the same question in terms of our previously mentioned experiment:
Knowing the dependent variables b1, b2, b3, etc., how can we estimate the independent variable or biotic factor B? It will prove profitable to conduct this inquiry separately for each pedologically important group of organisms, namely, microbes (om ), vegetation (ov), animals (oa), and man (oh).
Microorganisms.—Relatively little systematic knowledge is at hand regarding the distribution of microorganisms in various soil types, but what little we know indicates definitely that each soil possesses its own characteristic microbial population. Furthermore, it has been established by countless experiments that changes in the properties of a soil are accompanied by changes in its microbiological constitution. Strongly acid soils, for instance, are void of the nitrogen- fixing bacteria Azotobacter. However, when neutralized, these soils,
as a consequence of natural reinoculation, soon abound in this important group of microorganisms. In nature, the facilities for a wide redistribution of microbes are practically unlimited, largely because of the small size and ease of transportation of the organisms. Air movements, precipitation, and dust storms bring about a continuous reinoculation of soils. We are, probably, not far from the truth by assuming that nearly all soils at some time or another receive samples of most of the soil microorganisms. This belief leads to the
assumption that in the vast majority of soils the same microbiotic factor is operative. Hence, within a large region, the microbiological component of the biotic factor o in Eq. (4) may be taken as constant, being nearly identical for all soils. The approximate composition of the microbiotic factor simply corresponds to the sum of the species of the microorganisms found within the region. According to this viewpoint, the individual microbial populations of each soil within the region are merely the consequence of the great variety of
constellations of the remaining soil-forming factors.
To forestall any misunderstandings, we should like to emphasize that the discard of each specific microbial complex as a separate soil- forming factor does not in any way minimize their practical and scientific significance in the intricate mechanism of physical, chemical, and biological processes. However, this book is a treatise on soil-forming factors and does not undertake to study soil-forming processes.
Vegetation.—In pedological research we may wish to evaluate the specific effect of various plant species or types of vegetation on soil formation, or we may desire to study soil development under conditions of constancy of the vegetational factor. Both problems merit separate consideration.
a. If we wish to contrast the influence of two plant species or plant associations on soil formation, all other soil formers must be kept constant. This postulate follows directly from the specific equation
s = f (vegetation) cl, r, p, t, . . . .
In the field it must be shown that climate, topography, parent material, and time are similar for the two types of vegetation to be compared; in other words, the two species or groups of species must appear in the role of independent variables or biotic factors. As an example, one might point to the multitude of agricultural crops that are frequently planted side by side under identical conditions of climate, topography, parent material, and time. Each crop or sequence of crops (rotation) acts as a biotic factor.
b. The problem of treating the biotic factor as a "constant" occurs whenever the general equation
s = f (cl, o, r, p, t, . . . . ) (4) is to be solved for either cl, r, p, or t. Expressing s as a function of the climatic variable, we have, in the preceding chapter, written Eq. (4) as follows:
s = f (climate) o, r, p, t
Here we are confronted with the task of evaluating o as a constant while climate varies. The general solution of this problem has been suggested by Overstreet.*
* Personal communication.
FIG. 100.—Illustration of a simple biotic factor. Distribution of Atriplex in a desert landscape (playa).
The reader's attention is directed to Fig. 100, which shows a section of a desert landscape. The white barren area represents a playa, i.e., an old lake bed incrusted with alkali salts. The borders and sand elevations are covered with saltbush (Atriplex). Although the playa lacks vegetation, it cannot be said that it lacks a biotic factor, for the seeds of the saltbush certainly are scattered over the salt beds.
Plant growth on the playa is nil because of unfavorable physiologic conditions and not because of absence of potential vegetation. Both the playa and the elevations possess the same biotic factor, namely, saltbush. It is conceivable that additional plant species reach this particular landscape but fail to survive because of lack of sufficient water. These species also should be included in the description of the biotic factor.
Figure 101 portrays a segment of the semiarid portion of the Coast Range in California. Oaks grow in the canyons and depressions, and grasses cover the slopes and ridges. Assuming constancy of climate and parent material, the distribution of grasses and trees is clearly a function of topography and therefore constitutes a dependent variable. In this specific landscape the biotic factor consists of oaks and grasses.
FIG. 101.—Vegetational factor consisting of oaks and grasses (Coast Range, California).
In certain parts of the limestone regions of the Alps, slightly weathered rock has a cover of basophilous Dryas octopetala, whereas adjacent acid soil derived from the same material as a result of
intensive leaching supports acidophilous Carex curvula. Since the seeds of both species have access to both kinds of soil, the biotic factor is the same for both soils, namely, Dryas plus Carex. In this instance, the differentiation in the actual vegetational cover is brought about by the factor time or the degree of maturity. Ultimately, the slightly weathered material will be converted into a strongly acid soil with a simultaneous displacement of Dryas octopetala by Carex curvula (compare page 216).
Proceeding to more general cases, we may enunciate the following ecologic principle: If in any region the plant species or plant communities are dependent variables, their distribution being a function of one or of several soil-forming factors, the biotic factor may be obtained, as a first approximation, by enumerating all plant species growing within the area.
A more detailed discussion of the dependent and independent nature of vegetation will be presented in succeeding sections.
Animals.—A similar line of thought may be applied to the influence of animals in soil formation. Because of lack of sufficient observational data covering wide areas, the discussion of animal life is omitted in this treatise.
Man.—Like vegetation, man may appear in the role of a dependent as well as an independent variable. With respect to man's dependency on soil-forming factors, some scientists (43) go so far as to attribute the origin of different human races to the influence of soil and climate. Hilgard and especially Ramann (47) have emphasized the relationships between social structures and soils, as indicated by despotic governments found in ancient irrigation areas and the more democratic institutions developed on soils of humid regions.
Huntington stresses the importance of weather on health and efficiency of human beings and traces the rise and fall of certain civilizations to climatic changes. Abbott (1) calls attention to widespread nutritional anemia among children who live on home- grown food from poor soil that is deficient in iron. Medical science in general knows countless examples of correlations between pathology of man and environment.
A considerable number of human influences on soil appear to stand in no direct relationship to soil-forming factors. Lands in all parts of the world are plowed and are subjected to numerous cultural treatments. Stable manure is added to the soil wherever cattle are raised. Crops are harvested universally. Deforestation occurs on all continents, and burning is practiced whenever needs arise. In all these enterprises, man acts, as far as the soil is concerned, as an
independent variable or soil-forming factor. Some of the consequences will be discussed in subsequent sections.