FACTORS OF SOIL FORMATION - Midlands State University

system that are not universally recognized as soil properties. They are
the following: soil climate (soil moisture, soil temperature, etc.), kind
and number of soil organisms, and topography, or the shape of the
surface of the soil system. These properties will be denoted by special
symbols (cl' = climate, o' = organisms, r' = topography or relief),
which are included in Eq. (1)
F (cl', o', r', s l , s 2 , s 3 , • • • ) = 0 (2)
There is no essential difference between Eqs. (1) and (2) except that
some of the soil properties have been grouped together and given
special symbols. The reason for doing so will become obvious at a
later stage of our discussion. Emphasis should be placed on the fact
that in Eq. (2) the soil system is defined or described by its own
properties and nothing else. Moreover, soil is treated as a static
system. No reference is made that the properties may change with
time.
Soil Formation.—The transformation of rock into soil is
designated as soil formation. The rock may be gneiss, limestone,
shale, sand, loess, peat, etc. To avoid too liberal an interpretation of
the term "rock," soil scientists prefer to use the expression ''parent''
material or "soil'' material. The relationship between parent material,
soil formation, and soil may be conveniently expressed as follows:
Parent material———————> Soil
Soil formation
The foregoing formulation introduces a new factor or variable into our
discussion, namely, time. The states of the soil system vary with time,
i.e., they are not stable. Suppose we consider a piece of granite that is
brought to the surface of the earth. In the interior of the earth, the
granite may have been in equilibrium with its immediate
surroundings; but now, on the surface of the earth, it is in an entirely
new environment, and the rock system is highly unstable. It is
continuously changing its properties in a definite direction, namely,
toward a new equilibrium state. When the final equilibrium state has
been reached, the process of transformation, of soil formation, has
been completed, and the rock has become a mature soil. It is
customary to designate the intermediate, unstable states as immature
soils. We may define the phases of soil formation as follows:
Parent material————————> Soil (mature)
Initial state Intermediate states Final state
of system
of system
In this formulation, soil is treated as a dynamic system. Emphasis is
placed on the changes of the properties of the soil as a function of
time.
It may be well to point out that the foregoing concept of soil
formation is broader than that of a certain group of soil scientists who
sharply distinguish between weathering and soil formation. The
former process is said to be geologic and destructive, whereas the
latter is pedologic and creative. In the present treatise, we adopt a
more conservative viewpoint and consider weathering as one of the
many processes of soil formation.
Soil-forming Factors.—Agriculturists have long realized that
many important properties of soils are inherited from the underlying
rocks. Technical expressions like limestone soils or granitic soils are
encountered in the oldest textbooks on agricultural subjects. They
clearly convey the importance of parent material in soil formation.