Geoscience Reference
In-Depth Information
African Climate, Vegetation, and Soils
Now that you have examined the three tropical soils, you
should review the relationship of climate, vegetation, and
soil orders by going to the
Geo Media Library
and selecting
African Climate, Vegetation, and Soils
. This brief anima-
tion illustrates how these variables are interrelated to form
a geographically distinct landscape. As you watch it, be
sure to look at the spatial patterns that exist on the African
continent. Once you complete the animation, be sure to an-
swer the questions at the end to test your understanding of
this concept.
Alfisols form by a specific soil-forming process called
podzolization
. Podzolization is similar to laterization because
both processes are associated with abundant leaching in their
respective humid environments. The primary difference
between the two processes is that organic matter decomposes
slowly where podzolization occurs because the environment
is cooler. Thus, a distinct O horizon forms in the regions
where podzolization is the dominant pedogenic process. This
O horizon consists mainly of deciduous leaves and conifer-
ous needles. This litter layer is slightly acidic, which causes
infiltrating soil water to become even more acidic. As a result,
organic acids are incorporated into the upper part of the soil
to form a distinct A horizon. From there, they are leached to
the B horizon.
The leaching of organic acids causes translocation of
the iron and aluminum sesquioxides, which are not moved
where laterization occurs. In this environment, podzoliza-
tion results in a very well-defined E horizon (Figure 11.26c),
which is ashy gray in color and is called an
albic horizon
because it is so light. Below this horizon, a reddish B hori-
zon forms where eluviated iron and aluminum collect. This
horizon is called a
spodic horizon
because of the high
sesquioxide content.
Alfisols are the soils that form in the warmer regions
where podzolization occurs, specifically in the areas where the
litter layer typically consists of deciduous leaves. Alfisols are
similar to Ultisols in that they contain an argillic Bt horizon
that is rich in clay. These soils are not as heavily weathered
as Ultisols, however, which results in Alfisols generally being
more fertile because they have higher concentrations of base
ions. Nevertheless, Alfisols contain a distinct E horizon that
is lighter, through extensive eluviation of minerals and clays,
than the overlying A and underlying B horizons. Because the
rate of decomposition is much less in this cooler environment,
the A horizon of Alfisols is generally better developed and
darker than in Ultisols.
Spodosols
Spodosols
are acidic soils that are also associ-
ated with the podzolization process. These soils occur in about
3% of the ice-free land surfaces on Earth (Figure 11.27a). In
contrast to Alfisols, Spodosols form in somewhat cooler, more
humid regions. As a result, the vegetation in these regions is
typically coniferous and hardwood forest (Figure 11.27b), and
the litter layer is thus a mix of needles and leaves. As with Alfi-
sols, this litter layer is acidic, which lowers the pH of percolat-
ing water sufficiently to mobilize organic acids and sesquiox-
ides. In places where the parent material is sandy, soil formation
occurs more rapidly because water percolates quickly through
the solum.
Spodosols have a very distinct sequence of soil horizons
that are readily visible (Figure 11.27c). The uppermost part
of the soil usually contains a thin O horizon (needle-leaf lit-
ter) over a thin A horizon, which is very dark due to the
slow decomposition of organic matter. In well-developed
Spodosols, the E horizon is prominent because iron, alumi-
num, and base ions have all been eluviated, resulting in a
distinct white or light-gray albic horizon. Directly beneath
the E horizon is a particular type of spodic horizon, which
is designated as a Bs horizon. This horizon is dark brown
or reddish in color because it contains high amounts of
Podzolization
A regional soil-forming process in cool, humid
environments that results in the eluviation of iron, aluminum,
and organic acids to form well-developed E and Bs horizons.
Spodic horizon
A mineral soil horizon characterized by the
illuvial accumulation of aluminum, iron, and organic carbon.
Spodosols
Soils in cool, humid regions that form through
podzolization and contain a spodic horizon enriched in eluvi-
ated iron, aluminum, and organic carbon.
Albic horizon
A diagnostic horizon of podzolization from
which clay and free iron oxides have been removed, resulting in
a light-colored E horizon.
