Soils of rainforests Characterization and major constraints of dominant forest soils in the humid tropics

Rational use of forest resources should be based upon accurate knowledge of land and soil properties. Selected characteristics of 149 soil profiles of the humid tropics are analyzed. General environmental characteristics, physical, chemical and mineralogical soil data have been derived from ISIS, the ISRIC Soil Information System. The humid zone includes 6 major soil groupings: Acrisols, Arenosols, Cambisols, Ferralsols, Luvisols and Podzols. For these soils the arithmetic mean and standard deviation of selected key analytical characteristics, relevant for agronomic and ecological research, are presented for a standardized topsoil and subsoil. In addition, their major agronomic constraints were assessed. About 64 % of the humid tropics is covered by Acrisols, Arenosols, Ferralsols and Podzols of low fertility, all presenting various degrees of limitations when forests are cleared for (low input) arable farming. A low plant nutrient content, a low nutrient retention capacity and a high toxic exchangeable aluminium content are major constraints. The overall data presented in this paper show that Ferralsols and Acrisols, covering 57 % of the humid tropics, have rather similar key characteristics. The taxonomic separation is principally based on a relatively small increase in clay content, which will not determine the major vegetation type or crop productivity level. When considering conservation of the natural vegetative cover and its biodiversity, several special properties of the soils have to be taken into account, such as depth of the litter layer, rootable depth and the mineral composition of the deeper subsoil and the substratum, as well as the geomorphologic-pedologic history of the terrain units and the degree of short-distance variation in soil conditions. It is concluded that land assessment studies and soil-vegetation/bio diversity research should not rely on a soil taxonomic approach alone, but should also be based on measured key pedological characteristics. The study illustrates the usefulness of the ISIS database to correlate soil characteristics and to determine agronomic-ecological constraints of major soil groupings.