FACTORS OF SOIL FORMATION - Midlands State University

concentration, etc., are also altered. The simplest method of
expressing the foregoing assumptions of interrelationships is in form
of an equation, written as follows:
F (s 1 , s 2 , s 3 , s 4 , • • • ) = 0 (1)
It means that the soil is a system the properties of which are
functionally related to each other.
Equation (1) is far too general to be of practical value to soil
science. In fact, it is merely the definition of any natural body written
in symbolic form. In order to distinguish soil from other natural
systems, certain limits must be given to the properties s 1 , s 2 , etc. The
magnitudes of the properties must not exceed or fall below certain
characteristic values. To quote an example, the water content of a soil
must be below, say, 95 per cent on a moist basis, or else the system
would not be called "soil" but "swamp," or "lake," or "river."
No general attempt as yet has been made to assign quantitative
limits to soil properties, and, therefore, it is not possible at the present
time to contrast soils sharply with other natural bodies on the basis of
Eq. (1). However, if we specify for the moment that soils are portions
of the upper weathering layer of the solid crust of the earth, the limits
of some of the properties of Eq. (1) are roughly set.
Soil and Environment.—The latter contention requires a closer
examination. Just where on the surface of the earth are the soils
proper, and what constitutes the boundaries between soils and other
natural bodies that also are part of the upper portion of the earth?
It is generally realized that the soil system is only a part of a
much larger system that is composed of the upper part of the
lithosohere, the lower part of the atmosphere, and a considerable part
of the biosphere. This larger system is illustrated in Fig. 8.
A number of Russian soil scientists designate this larger system
as soil type (climatic soil type). Glinka (3) writes:
Contrary to the majority of foreign soil scientists, Russian pedologists
choose the soil type as a unit of classification instead of soil masses,
regarding the soil type as a summary of the external and internal properties of
a soil. Thus, speaking of chernozem, for instance, the Russian pedologist saw
not only a natural body with definite properties, but also its geographical
position and surroundings, i.e., climate, vegetation, and animal life.
This Russian idea of soil type is nearly identical with the soil
geographer's concept of natural landscape.
Most soil scientists deal only with part of this wider system,
namely, the soil per se. The remaining part is called "environment."
Often it is not sufficiently realized that the boundary between soil and
environment is artificial and that no two soil scientists have exactly
the same enclosure of the soil system in mind. A glance at Fig. 8
provides a felicitous illustration. Suppose we approach the surface
from the atmosphere, just where does the soil begin? Is the forest litter
a part of the soil or of the surroundings? If one is inclined to discard
the freshly fallen leaves or needles and include only the decomposed
organic material in the soil system, what degree of decomposition is
necessary? Similar difficulties are encountered in the sampling of
virgin prairie soils. Living grasses are not soil, but dead parts
gradually become incorporated into the soil matrix. In practice, a
rather arbitrary separation between vegetative cover and soil becomes
necessary.
If we approach soils from below, similar problems are
encountered. There is no sharp boundary between undecomposed
rock, weathered rock, soil material, and soil. Although criteria for
distinguishing soils from unconsolidated geological deposits may be