FACTORS OF SOIL FORMATION - Midlands State University

mantles, etc., are soils, unless they are being deposited under the very
eyes of the observer.
Soil Maturity and the Concept of Soil Equilibrium.—Marbut (12)
defines a mature soil as one "whose profile features are well
developed." This definition is strictly morphological and may be
applied directly in the field. Among students of soils, one frequently
encounters a second definition that is enjoying increasing popularity.
It rests on dynamic rather than morphological criteria and may be
expressed as follows: "mature soils are in equilibrium with the
environment." In this case, emphasis is not placed on profile
descriptions but on soil-forming processes. More specifically, the
concept has some bearing on the factor time in soil development. It
alludes to the familiar equilibrium idea employed by chemists and
physicists. It will prove profitable to analyze the equilibrium concept
of soil maturity in the light of time functions.
In the first place, it should be kept in mind that not all soil
components approach "maturity " at the same rate. In practice only a
few soil characteristics are taken into consideration when questions
pertaining to soil maturity are to be decided upon. They are, in the
main, soil reaction, organic matter, lime horizon, clay accumulations,
ortstein, or, more generally, the magnitudes of illuvial and eluvial
horizons. Obviously, we may be dealing with partial equilibria only;
the rest of the soil mass may still undergo further changes. In
Hissink's carbonate curve of Dutch polders, the value of calcium
carbonate becomes zero at t = 300 years; yet the remaining soil
properties continue to vary with time.
In the second place, we need reliable equilibrium criteria. How
are we going to decide whether or not equilibrium has been reached?
In a purely formalistic manner, we may postulate that a soil, or, more
specifically, a soil characteristic s, is in equilibrium with the
environment when its indicatrix no longer changes with time. The
component of the system is then at rest or stabilized. In
mathematical parlance, one would write
where ∆ s denotes a change in a soil property and ∆ t a time interval.
This definition is restricted to the soil as a macroscopic system. It
disregards the behavior of individual atoms and molecules such as
thermal agitation and Brownian movement, which never cease.
Applying the above criterion to soil-property-time functions,
equilibrium would be reached when the curves become and remain
flat, an indication that the rate of change is zero (Fig. 25a). It does not
necessarily follow that the soil characteristic is at an absolute
standstill, for, if we choose ∆ t sufficiently large, daily or seasonal
fluctuations would not affect the average slope of the curve (Fig.
256). Tamm's curves, and Hissink's calcium carbonate curve might be
cited as examples of functions that reveal equilibrium in regard to
certain soil characteristics.
A word might be said about the choice of time scales. For
absolute evaluation, the year, or any multiple of it, appears to be the
appropriate unit. Since certain soils reach maturity much more quickly
than others, it may become desirable, for purposes of comparison, to
use relative time units, e.g., fractions of maturity ages (Fig. 26).
Instead of speaking of young and old soils, one would use terms like
"immature," "mature," or "degrees of maturity" ("one-half mature,"
etc.).