Geoscience Reference
In-Depth Information
One of the distinctive characteristics of Vertisols is that
prominent cracks can form in the ground (Figure 11.25c, d)
during long dry periods because the clays within the soil shrink
so much. Because of the constant shrinking and swelling that
occurs in these soils, they are slowly but constantly mixed, with
much of this
pedoturbation
occurring when fragments of soil
fall into the cracks. The results of this shrinking and swelling
can be seen in a profile through the presence of shiny clay skins
on peds. These glossy surfaces, or
slickensides
, are created
when the long-term expansion and contraction of the soil cause
clays to be polished as they rub against one another. As a result,
a diagnostic horizon of a Vertisol is a Bss horizon, with the “ss”
reflecting the presence of slickensides.
If you have ever noticed that cracks form in the ground
during long droughts around your home, then the soils in your
neighborhood are likely Vertisols, or closely related to them.
In most places, Vertisols occur in isolated pockets that are too
small to show up on the global soils map. Significant regions
of Vertisols form in the tropical and subtropical regions of the
world, however, including the Deccan Plateau of western India,
along the Nile River in eastern Africa, and much of eastern
Australia. Although these soils are often high in base content,
they are difficult to farm because they are hard to cultivate.
Alfisols.
Alfisols.
form in cooler parts of the moist continen-
tal climate zones that lie poleward of the subtropical regions.
These soils occur on about 10% of Earth's ice-free land area
over an extremely wide range of latitude, extending from as
high as 60° N in North America to the equatorial zone in South
America and Africa (Figure 11.26a). Large areas of these soils
occur in places like the American Midwest (Figure 11.26b),
northern Canada, Europe, Asia, and the east coast of China.
Smaller pockets are found in Central America, Australia, and
Africa. Given the relatively high moisture of these regions, the
natural landscape is largely covered with deciduous forest, but
extensive areas of boreal forest occur as well.
Alfisols.
Soils generally found in seasonal midlatitude regions that
formed through podzolization and have an alkaline argillic horizon.
Global distribution of Alfisols
(c)
(a)
Figure 11.26 Alfisols.
(a) Generalized map of Alfisols on Earth.
These soils occur on about 10% of Earth's ice-free land area.
(
Source:
U.S. Department of Agriculture.) (b) A typical Alfisol land-
scape in central Missouri. Prior to cultivation, the vegetation of
this region was a mix of forest and tall-grass prairie. (c) An Alfisol
from the northern Midwest. Note the light E horizon (white arrow),
which is present in well-developed Alfisols. The reddish zone
below the E is a Bt horizon rich in eluviated clays.
(b)
