Environmental Engineering Reference
In-Depth Information
The Seventh Approximation, or New Soil Taxonomy, divides the soils of the world into
ten main categories, or orders, based on distinguishing characteristics.
The Great Soil Groups of the World
General
Before the development of the New Soil Taxonomy, soil scientists considered soils to be of
three main types, as follows:
•
Zonal soils
constitute the Great Soil Groups of the world in which climate is the
major factor in development. Described below, they are subdivided into two
groups by climate. In humid regions, soils are acidic, termed as pedalfers to
emphasize the removal of aluminum (al) and iron (fer) from the leached A hori-
zon. In arid regions there is little leaching and all dissolved matter is precipitated
as the water evaporates and layers of carbonates are formed. Soils containing
carbonate layers are termed pedocals.
•
Interzonal soils
reflect some local conditions that cause a variation in the zonal
soils, such as muck.
•
Azonal soils
are without profile development.
Tundra soils
develop under Arctic type of vegetation at high altitudes and lati-
tudes. Drainage conditions are usually poor and boggy. Underlain by a permanently
frozen substratum, the profile is shallow and much decomposed matter is found at the
surface.
Podzol soils
possess well-developed A, B, and C horizons. The surface material consists
of organic matter under which a whitish or grayish layer develops. The name derives from
two Russian words meaning under and ash. Below the gray layer is a zone in which iron
and aluminum minerals accumulate. The A and B horizons are strongly acid. They
develop under coniferous and mixed hardwood forests.
Laterites
are soils formed under hot, humid conditions, and under forest vegetation.
They have a thin organic cover over a reddish leached layer, which in turn is underlain by
a still deeper red layer. Hydrolysis and oxidation are intense. They are rather granular
soils and are confined mainly to tropical and subtropical regions, although some soils in
the middle latitudes have been described as alteration (see also
Section 7.7.2.)
.
Chernozems
originate under tall-grass prairie vegetation. The name is the Russian word
for black earth and suggests the color and high organic content of the A horizon. The B
layer exhibits an accumulation of calcium carbonate rather than leaching. Columnar struc-
ture is common, and they are the most fertile soils, typically developing from loess
deposits.
Chestnut soils
are brown or grayish brown soils that develop under short-grass vegeta-
tion in areas slightly drier than those that produce chernozems. Secondary lime is found
near the surface and the profile is weakly developed.
Brown aridic soils
are found around the margins of deserts and semiarid regions. They
have a low organic content and are highly calcareous.
Gray desert soils
(sicrozems) and
reddish desert soils
develop under desert or short-grass
vegetation. Calcium carbonate accumulates near the surface. The sicrozems are found in
continental deserts and the reddish soils are found in what are termed as subtropical
deserts.
Noncalcic brown soils
form in areas which originally had forest or brush vegetation. Weak
podzolification makes the surface layer slightly acidic.
