Box 22 Soils of the perhumid tropics

Kaolisols; this group includes most intensively weathered and leached zonal humid tropical soils and roughly corresponds to the 'tropical red earths' of earlier descriptions.

Modal kaolisols, clay kaolisols, basic oxidic clays, ultrabasic oxidic clays, limestone oxidic clays, alluvial kaolisols, ferricrete kaolisols. Non-kaolisol mature terra firma soils; all have low or very low nutrient contents.

Podzols develop from very sandy and quartzose raw materials including highly siliceous old volcanic deposits.

White sands have a thick mor layer of surface litter with deep bleached sandy subsoils, but lack an underlying humus pan.

Acid planosols are locally important in humid savanna; their subsoils are very compact and impermeable with little penetration by roots or water. Such soils have a sharp discontinuity between the pale, loose sandy upper horizons and the brightly mottled subsoils.

Immature terra firma soils; which have arisen on recent parent materials or on older deposits which have been recently truncated by erosion.

Andosols, recent colluvial soils, recent alluvial soils, skeletal soils, shallow calcareous clays.

Poorly drained soils; where water tables are high for at least part of the year and impede drainage. This results in restricted leaching, poor aeration, and the reduction of free iron from the trivalent to the divalent state. The reduced rate of decomposition of organic matter in these soils reaches its extreme with the formation of peat.

Freshwater gleys, ferruginous semi-gleys, saline gleys, acid sulphate soils, peats. Montane soils (not subdivided)

(This simplified listing is based on table 10.1, in Baillie (1996). This also gives the equivalents in international soil classifications of the terms used above.)

Betty J. Meggers, the Smithsonian archaeologist (Mann, 2002), implying that the lush forests grow on impoverished soils - but see the terra preta soils discussed in Section 2.5.1. Aluminium is often the dominant exchangeable cation in these acid soils. They have open porous structures; aeration is therefore generally good, and water infiltrates the surface rapidly and readily percolates downwards.

There are, however, many important tropical soils that are not acid, although highly weathered, deep, reddish, and well drained, as Box 2.2 indicates. Amongst these are the young soils which develop on alluvium (unconsolidated granular sediments deposited by rivers), and the colluvial soils that originate at the surface of recent hillwash deposits on lower slopes.

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