tropics and are closer to a neutral pH (though still
acidic), with less overall leaching than typical oxisols.
It is estimated that ultisols, oxisols, and alfisols, taken
together, make up about 71% of the land surface in the
humid tropics worldwide.
Semiarid and arid regions in the tropics, because of
climatic differences, have different soil types from those
of humid and semi- humid regions. Some of these soils
are dark, heavily textured, and calcareous, and some
are subject to salt accumulation. Because of frequent
occurrences of burning, and sometimes animal grazing,
litter is thin and poorly developed on savanna soils, and
the decomposer ecosystem is more limited. Termites,
however, can be particularly abundant in arid, grassy
areas (termites are discussed below).
To summarize, the general pattern throughout much
of the humid tropics is that heat and heavy moisture
input result in formation of oxides of iron and
aluminum (which are not taken up by plants), giving
the soil its characteristic reddish color. Clay content
is normally high, evident as you slip and slide your
way over a wet trail. Mountain roads become more
dangerous and often impassable during rainy season,
because wet clay makes the roads slippery. Clay also
has reduced porosity, impeding penetration by water.
Thus clay soil enhances flooding potential.
In the Amazon Basin, sediments eroded from
highland areas during the Late Tertiary period were
deposited in the western end of the basin, forming a
flat surface about 250 m (820 ft) above sea level. Much
of this surface, called the Amazon Planalto, is made up
of kaolinitic clay, a substance devoid of most essential
minerals but rich in silicon, aluminum, hydrogen,
and oxygen. In the eastern part of Amazonia, soils are
sandy, though remaining acidic and nutrient poor.
Laterization
A process called laterization, which results from the
combined effects of intensive erosion and heat acting
on soil, is associated with some tropical soils. If
vegetation cover is removed and bare soil is exposed to
heavy downpours and heat, soil may be converted into
a brick- like substance, preventing future plant growth.
Laterization has long been utilized by tropical peoples
around the world for making bricks used in buildings
as impressive and venerable as some of the ancient
temples in Cambodia. Though laterization has been
widely reported as demonstrating the extreme delicacy
of tropical soils and thus the futility of farming such
soils, such a generalization is unfounded. Laterization
occurs only with repeated wetting and drying of the soil
in the absence of any vegetative cover. The loss of roots
(which utilize and indeed produce aeration channels
in the soil) and the repeated wetting and drying act to
break up soil aggregates of bound clay particles. Only
when these aggregates are broken up and eliminated,
and the soil is thus subject to extreme compaction,
does laterization ensue. In Amazonia, only about 4%
of the soils are at actual risk of laterization.Plate 6- 9. Soils known as red oxisols are common throughout
much of the tropics. Photo by John Kricher.Plate 6- 10. This small subsistence farm along the Amazon
River is sustained by the rich várzea soil (ultimately from the
Andes Mountains) that is annually renewed in the flooding
cycle. Note the grove of bananas. Photo by John Kricher.86 chapter 6 essential dirt: soils and cycling