Soil scientists employ a rather complex nomenclature for the great variety of soil types present at one place or another around the globe.

Fortunately, most of the names in these lengthy classifications can be avoided if one desires only a general understanding of a few major kinds of soils. For example, the soils that predominate in the eastern United States and southeastern Canada are called predalfers.

The name was fabricated to emphasize the fact that in such soils aluminium (al) and iron (fer) have been leached from the A horizon and deposited in the underlying B horizon. Pedaifers are clayey soils that develop in regions having an abundance of rainfall. As this water percolates through the humus, it becomes acid, and this accounts for its ability to leach aluminium and iron from the topsoil.

It is also effective in dissolving carbonates, which are then carried away by ground water. The acidity of some pedalfers may diminish their fertility, and so farmers often spread finely ground limestone (agricultural lime) on their fields to combat the actidity.

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As a consequence of the fact that the top soil or a horizon in pedalfers is leached, it is usually a lighter colour than the underlying subsoil where iron has accumulated. Pedalfers include several lesser categories of soils, which differ in their profiles. Most important among these are podzols, which typically have an ashy gray A horizon.

In the more arid regions of the world, less humus develops in soils, and there is less opportunity for solution and leaching. In the absence of water, chemical, weathering is slower, and less clay is produced. Usually, there is insufficient ground water to flush soluble materials such as calcite out of the B horizon, so it accumulates there as soil moisture is lost by evaporation. Because of the persistence of calcite in these soils, they are called pedocals (pedon, soil, and call for calcium carbonate).

Frequently in dry areas, there is not only insufficient water to cause downward leaching, but there is an upward movement of water because of the high rate of evaporation at the surface. Mineral matter dissolved in the diminishing soil water is precipitated at the surface as a hardpan or caliche layer.

In the hot and humid regions of the tropics, the characteristic soils are laterites. The term “laterite” is derived from the Latin word latere or brick, and originally referred to the use of this material to make bricks in India and Cambodia. Laterites have a telatively thin organic layer convering a reddish leached layer, which is often underlain by a still darker red layer. In laterites, oxidation and hydrolysis have been so intense that feldspars and ferromagnesian minerals are completely decayed. Not only is calcium carbonate removed, but also silica. Only the most insoluble compounds, mainly aluminium and iron oxides, accumulate in these soils.

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Although laterites may support a lush growth of natural tropical vegetation, they are not good agricultural soils. When forested areas with lateritic soils are cleared and plowed, the thin organic cover tends to be rapidly oxidized in the prevaling warm climates. A thick accumulation of organic matter such as that found in rich black soils of more temperate regions cannot develop. After a few years of tilling and planting, the organic component of the soil is so depleted that fields must be abandoned.

In some laterites, the concentration of either iron or aluminium may reach levels that permit the deposits to be profitably mined. The iron-rich laterites originate from the weathering of parent materials that also contained iron, although not concentrated into ore bodies. Extensive lateritic ores of iron occur in Cuba, Columbia, Venezuela, and the Philippines. If there is little iron in the parent material and an abundance of aluminum, then laterites rich in hydrated aluminium oxides map form.

Such materials are called bauxites. Ancient bauxite deposits are mined in Guyana, Ghana, northern Queensland and Arkanas. Concentration of bauxite takes place in tropical areas of low relief where temperatures exceed 25°C most of the time and where there is an abundance of water for leaching and chemical reactions. At the present time, bauxite is the only are from which it is economically practical to extract aluminium.