Geoscience Reference
In-Depth Information
Regional Pedogenic Processes
At this point we have completed our discussion of soils that gen-
erally form in association with regional environmental variables
and pedogenic processes. These processes are particularly well
suited for animation to more completely illustrate how they work.
Go to the
Geo Media Library
and select
Regional Pedogenic
Processes
. This animation illustrates how the soil-forming pro-
cesses laterization, podzolization, calcification, and salinization
influence the development of soils at a regional level. When you
complete the animation, be sure to answer the questions at the
end to test your understanding of this concept.
sodium is easily leached from soils because it is one of the
more soluble minerals, and water is sufficiently abundant to
move it out of the solum. In semi-arid to arid environments,
however, sodium is not completely leached from soils and
instead is moved only part way down the soil profile when
infrequent rains do fall. When the soil dries out during the
extended dry periods that occur in this environment, the par-
tially leached minerals rise back up through the profile in a
process called
wicking
.
This wicking process is most active in enclosed basins
where pools of water collect when it does rain. Sodium in these
low spots can be leached many centimeters when the water
percolates through the profile following a hard rain. When the
site dries out, however, the salt rises back up and forms a salty
crust at the surface called an Az horizon. If this crust reaches
a thickness of 15 cm (6 in.) and contains at least 20 g/kg of
salt, it is called a
salic horizon
. These salts are highly toxic
to most plants. In regions where calcification is the dominant
process associated with Aridisols, the soils have thick Bk, or
calcic, horizons that can become completely cemented with
time. If they do, the horizon is designated as a Bkm horizon
(Figure 11.31d).
At this time, each of the three major soil orders that
occurs within the midcontinental United States—Alfisols,
Mollisols, and Aridisols—have now been discussed. This
juncture thus marks a good point to examine holistically
how soils vary along a study transect that extends from Ohio
to central Colorado (Figure 11.32). Which side of this tran-
sect has the most moisture? Which side is drier? Do you
know why? You should know that the eastern side of this
transect is more humid due to the influx of mT air from
the Gulf of Mexico. As you proceed westward, however, the
climate generally becomes drier due to the influence of the
Rocky Mountain rain shadow. The response of vegetation is
that trees populate the eastern side of the transect. As you
move westward, the vegetation shifts to tall grass and then
to short grass.
How do soils differ along this transect? The primary agent
of change is that more leaching takes place on the eastern
side of this transect than on the west. This geographic pat-
tern should make sense because the climate in Ohio is gen-
erally more humid, with more precipitation, than in eastern
Colorado. As you can see in Figure 11.32, this distribution of
soil water causes a distinct E horizon in Ohio, which fades
away as you move farther west. The thickest A horizons are in
the tall-grass region of the Mollisol belt because the biomass
is so high, and thus decomposition and leaching are less than
to the east. In this zone, the depth of illuviated carbonate (lime,
CaCO
3
) is the greatest and the B horizon is the thickest. As you
move deeper into the rain shadow, in the western part of the
Mollisol belt, the A horizon thins significantly and the depth
of the B horizon becomes shallower. This pattern culminates
within the zone of Aridisols with a very thin or nonexistent
A horizon and a very shallow, carbonate-rich B horizon. This
discussion is an excellent example of how many geographic
variables come together to produce a distinctive pattern—in
this case, regarding soils.
Entisols
Finally, it is time to examine the soils that are
not related to any specific environmental variable or pedo-
genic process. In other words, these soils can be found in
many different places on Earth and are a product of specific
soil-forming factors. The most widespread of these more
randomly distributed soils are
Entisols
, which means
young
soils
. These soils occur on about 16% of Earth's ice-free land
area (Figure 11.33a).
Although Entisols are soils in the sense that they can
support plants, they are the least developed soils because
they have no diagnostic horizons such as an E or a B horizon
(Figure 11.33b). They may contain a thin A horizon. This
overall lack of development can occur for several reasons:
(1) the climate is extremely dry, making the processes of soil
additions and translocations very weak; (2) the parent mate-
rial is quartz sand, which is a resistant deposit that does not
Salic horizon
The diagnostic horizon of salinization that
forms due to the recrystallization of secondary salts.
Entisols
Soils that are very weakly developed and thus have
no distinct horizonation.
