FACTORS OF SOIL FORMATION - Midlands State University

indicate that, besides the variables listed, additional soil formers may
have to be included in Eq. (4).
In selecting cl, o, r, p, and t as the independent variables of the
soil system, we do not assert that these factors never enter functional
relationships among themselves. We place emphasis on the fact that
the soil formers may vary independently and may be obtained in a
great variety of constellations, either in nature or under experimental
conditions. It is well known that various kinds of parent materials and
topographies do occur in various kinds of climates and that given
amounts of annual precipitations are found in association with either
low or high annual temperatures, and vice versa.
Soil Defined in Terms of the Fundamental Equation.—We are
now in a position to establish a differentiation—arbitrary, to be sure—
between soil and geological material. Soils are those portions of the
solid crust of the earth the properties of which vary with soil-forming
factors, as formulated by Eq. (4). As pedologists, we are interested
only in those strata on the solid surface of the earth the properties of
which are influenced by climate, organisms, etc. From this definition,
it follows at once that the depth of soils is a function of soil-forming
factors; in particular, it varies with humidity and temperature.
The Solution of the Fundamental Equation of Soil-forming
Factors.—Half a century ago, Hilgard had recognized the existence
of the soil-forming factors and discussed them at length in his classic
book on soils. Dokuchaiev (1) likewise realized the existence of soil
formers. He went a step further than Hilgard and formulated an
expression somewhat similar to Eq. (4). However, he did not solve it.
He wrote:
In the first place we have to deal here with a great complexity of
conditions affecting soil; secondly, these conditions have no absolute value,
and, therefore, it is very difficult to express them by means of figures; finally,
we possess very few data with regard to some factors, and none whatever
with regard to others. Nevertheless, we may hope that all these difficulties will
be overcome with time, and then soil science will truly become a pure science.
In these phrases Dokuchaiev expressed prophetic insight into one
of the most fundamental problems of theoretical soil science, namely,
the quantitative solution of the soil-forming-factor equation.
Curiously enough, Dokuchaiev's students have paid little attention to
the plea of their master for solving the function of soil-forming
factors. Russian pedology and international soil science have
developed in an entirely different direction and are stressing the
subject of classification of soils.
The fundamental equation of soil formation (4) is of little value
unless it is solved. The indeterminate function f must be replaced by
some specific quantitative relationship. It is the purpose of the present
book to assemble known correlations between soil properties and soilforming
factors and, as far as is possible, to express them as
quantitative relationships or functions.
Generally speaking, there are two principal methods by which a
solution of Eq. (4) may be accomplished: first, in a theoretical
manner, by logical deductions from certain premises, and, second,
empirically by either experimentation or field observation. At the
present youthful stage of soil science, only the observational
method—fortified by laboratory analyses of soil samples—can be
trusted, and it must be given preference over the theoretical
alternative.
The solutions given in this study are formalistic, as contrasted
with mechanistic. In simple words, we endeavor to determine how soil
properties vary with soil-forming factors. We shall exhibit but little
curiosity regarding the molecular mechanism of soil formation and