FACTORS OF SOIL FORMATION - Midlands State University

compare pieces with equal surface areas. Since we know of no
reliable method to determine surface area of alumino-silicates, the
weathering series mentioned above can have no claim to finality.
In nature, the extent of chemical rock decay may be very
deceiving. Niggli's (20) decomposed gneiss of San Vittore-Lumino,
Misox (T = 10.9°C, N S quotient = 624) has all the appearances of a
completely weathered rock; yet chemical analysis can detect little
change other than hydration and oxidation.
Significance and Behavior of Cations.—The nature of the
elements released during rock decay has a specific bearing on soil
formation. Silicon and aluminum furnish the skeleton for the
production of clay colloids; iron and manganese are important for
oxidation-reduction processes, and they strongly influence soil color;
potassium and sodium are dispersing agents for clay and humus
colloids, whereas calcium and magnesium have high flocculating
powers and assure soil stability.
Acid igneous rocks contain considerable amounts of quartz and
are rich in monovalent cations, whereas basic igneous rocks are high
in calcium and magnesium contents. One would expect that these
chemical and mineralogical differences would be reflected in the
trend of soil formation. Indeed, Hart, Hendrick, and Newlands (9) in
their studies on the soils of Scotland found that, under conditions of
identical climate and topography, the basic igneous rocks produce
brown earth and the acid igneous rocks produce podsolized soils.
It is a common saying that acid igneous rocks give rise to soils of
good physical condition, whereas soils from basic rocks possess
favorable chemical characteristics that ensure abundant plant growth.
Aside from the fact that well-known exceptions exist (e.g., serpentine
soils), it is probable that such belief originated in regions where
chemical weathering is not pronounced. In warmer climates with
extensive leaching and removal of bases, the chemical influence of
the parent rock is likely to be less marked. Cobb's work seems to
support this conclusion.
Cobb (4) has published chemical analyses of igneous rocks and
soils derived therefrom that were collected in the North Carolina
section of the Piedmont Plateau. He arranged the data in the form of
two development series as follows:
Cobb's analyses, recalculated to molecular values, are
plotted in Figs. 31 and 32.
The ba values, or the base-alumina ratio that reflects the
leaching of potassium, sodium, and calcium, and the relative