FACTORS OF SOIL FORMATION - Midlands State University

identical properties, one placed in the arctic region, the other near the
equator. In those localities, the two rocks are, strictly speaking, not
identical, because they differ, among other things, in temperature,
which affects some of their properties, such as the refractive indexes
of the mineral components. To overcome this shortcoming, we shall
introduce an extension of Eq. (8) by stating that parent material is the
initial state of the soil system referred to some arbitrary standard state
(e.g., normal temperature and pressure). Thus, the two granites
mentioned above become identical if referred to the arbitrarily chosen
temperature of 18°C. Parent material so defined will be designated as
p. In practice, the distinction between π and p is rather subtle and, in
most cases, may therefore be neglected.
Determination of Parent Material.—It is only under special
circumstances that the lower strata of a soil profile permit an exact
quantitative evaluation of parent material as defined in the previous
section. The exact composition of the initial state of a soil may be
determined if the history of the soil, or, more precisely, the time
functions of its properties are known. Such studies are restricted to
soil families of known age. Another avenue of approach is given by
soil-climate relationships, particularly soil-moisture functions. An
illustration will be presented on page 121 in which the lime-rainfall
curves of Alway are extrapolated to zero rainfall. The resulting value
of 17.2 per cent CaO represents the calcium content of the parent
material. Naturally, the reliability of such a determination depends
entirely on the degree of accuracy of specific soil-property-climate
functions. Generally speaking, the exact evaluation of the composition
of the parent material involves considerable speculation and is the
source of much uncertainty in the elucidation of soil-forming
processes.
Weathering and Soil Formation.—Some of the leading
pedologists of today lay great stress on the distinction between
weathering processes and soil-forming processes. The former are said
to be geologic; the latter are pedologic. Weathering includes solution,
hydrolysis, carbonization, oxidation, reduction, and clay formation.
Among the soil-forming processes, the following are listed:
calcification, podsolization, laterization, salinization, desalinization,
alkalization and dealkalization, formation of peat and poorly drained
soils, including gleization (33). Since these are merely special types of
chemical processes, their separation into geologic and pedologic
groups appears neither convincing nor fruitful. The entire issue might
be dismissed as being of purely academic interest, were it not for the
fact that a practical consequence is involved, namely, the
determination of parent material.
Pedologists who distinguish between geologic and pedologic
processes do not regard granite, basalt, limestone, and consolidated
rocks in general as parent materials. They maintain that only the
weathered portions furnish material for soil-building purposes, and
only these deserve the name "parent material." To mention a case in
point, it has been contended, that the relationship between clay
content of soil and annual temperature, to be discussed on page 150,
refers to a geologic and not to a pedologic problem, because it deals
with the formation of parent material rather than of soil. Whatever the
merits of this new approach may be, it frustrates the climatic theory of
soil formation. Since weathering is controlled by moisture and
temperature, it follows that the formation of parent material also
becomes a function of climate. Parent material could no longer be
treated as an independent variable and therefore would cease to be a
soil-forming factor. All soil property-climate functions to be
discussed in the following chapters become meaningless, because