Geoscience Reference
In-Depth Information
wearing away of this parent rock into smaller and smaller particles by chemical or
mechanical processes. Weathering and soil formation are facilitated by climate
(temperature, rainfall, and humidity) and the colonization of microbes and simple
plants. These initial soil-forming processes are combined with the continuous addi-
tion and buildup of decomposing organic matter, mixing with small rocks and soil
fragments. Soil-forming processes are lengthy and complicated.
Tropical soils are complex and varied. They typically are highly acidic, older
soils with low fertility. Millions of years of constant heat and rainfall have shaped
these soils. Older, more acidic, and less fertile soils are dominant in South America
and Africa. The more fertile, younger soils share dominance in tropical Asia.
Soil Types
The most widespread tropical soils are ancient and deeply weathered oxisols and
ultisols. Most of the remaining soils are more recently derived from volcanic activ-
ity and the breakdown of volcanic rocks, where weathering has only begun.
Tropical soils are primarily oxisols, as identified in the U.S. Soil Taxonomy
Classification. The United Nations Food and Agriculture Organization (FAO) soil
classification uses the term ferralsols. Latosol or laterites are other names applied
to these tropical soils. Many of the tropical regions of the world have large areas of
oxisols, particularly South America and Africa. Millions of years of weathering,
facilitated by high rainfall and constant heat, created these ancient soils. Oxisols
have little to no organic or humus soil layer. The constant heat, abundant moisture,
and rapid bacterial decay prevent the accumulation of humus. Because of intense
chemical weathering and abundant rainfall, all soluble minerals, and even some
that are relatively insoluble, are leached. As a result, the A and B horizons may be
tens of feet thick and indistinguishable from one another. Iron and aluminum are
often high in oxisols. The excessive rainfall leaches out silica and important miner-
als and nutrients. In some regions, the high acidity of the soil combined with rain-
fall can lead to the release of soluble aluminum from the soil, making it toxic.
Oxisols are very porous and have very low fertility. Their potential for erosion is
low under natural conditions. Exposed to the tropical sun without the cover of
decaying organic matter or plants, oxisols can become dry, very hard, and brick
like. This process has been called laterization. This brick-like soil continues to be
used as a building material for houses in the tropics.
Oxisols in the tropics have a characteristic deep yellow or red color due to high
iron, or can be lighter due to higher aluminum content (see Plate I). They can also
contain quartz, kaolin, and small amounts of other clay minerals. Bauxite (aluminum
ore) is found within these soils in some areas and is mined, often with disregard to the
devastation left in the aftermath. Oxisols are typically found in low-lying or flat areas
within the tropics, but they occasionally can be seen on slopes. Oxisols are most
prominent in South America, where two-thirds of all tropical oxisols are found. They
appear mainly in the central and eastern Amazon Basin, derived from ancient Gond-
wanan basement rock. They also occur along the west coast of Colombia. In Africa,
