Geoscience Reference
In-Depth Information
atoms of oxygen for every two atoms of the metal), which results
in highly weathered A and B horizons that together are called an
oxic horizon
because the sesquioxides are oxidized (i.e., atoms
lose an electron). In addition, very little organic matter is added to
the soil because decomposition of surface litter is rapid.
Oxisols are the most weathered of the soils formed by lat-
erization. Given the oxidized character of these soils, they are
usually red, yellow, or yellowish brown in color (Figures 11.23b
and c). Well-developed Oxisols typically have a very thin A
horizon and a thick Bo horizon, with the “o” reflecting the pres-
ence of oxidized sesquioxides. Although Oxisols are associated
with the rainforest, they actually have very low fertility because
most nutrients have been weathered out of the system. In addi-
tion, Oxisols have low cation exchange capacity, which further
reduces fertility. Given this soil infertility, plants derive most
of their nutrients from standing and decomposing vegetation.
As noted in Chapter 10, farmers in Oxisol regions can tem-
porarily improve soil fertility by cutting and burning the rainfor-
est. Once the plant debris is burned, the resulting organic residue
collects in the upper part of the soil, providing nutrients for crops.
Unfortunately, this method of soil fertilization is effective for only
a few years in any given field, which requires that additional forest
stands be cut to maintain crop production. This cause and effect is
one of the principal driving forces leading to tropical deforestation.
Ultisols
Ultisols
form in warm, moist, subtropical environ-
ments like those on the southeastern coasts of the United States
and China. They also occur in the more humid parts within the
tropical wet/dry climate regions. Overall they occur on about
9% of the ice-free land surfaces on Earth (Figure 11.24a).
Recall from Chapter 10 that the vegetation in these tropical
and subtropical areas consists of forest. As a result of this
Global distribution of Ultisols
(a)
(c)
Figure 11.24 Ultisols.
(a) Generalized map of Ultisols on Earth. These
soils are found on 9% of Earth's ice-free land area. (
Source:
U.S. Depart-
ment of Agriculture.) (b) An Ultisol landscape in central Brazil. Again, note
the reddish color of this soil, which is due to extensive weathering in this
subtropical environment. This region was once covered by forest and is
now farmed. (c) A typical Ultisol. Note that most of the profile has an orange
hue, somewhat similar to an Oxisol, because it also forms by laterization.
Note the A horizon (dark zone) at the top of the soil and the increased
structure in the underlying Bt horizon (reddish zone) due to eluviated clays.
(b)
Oxic horizon
A diagnostic soil horizon in tropical and sub-
tropical environments rich in iron oxides.
Ultisols
Mineral soils in subtropical environments that formed
through laterization and thus are depleted of calcium and have
an argillic horizon.
