that are washed by heavy rains. Calcium and potassium are leached from the soil by rain. Iron and aluminum form insoluble compounds with phosphorus and, if present in high concentrations, will decrease the availability of phosphorus to plants. When forests are removed, rapid degradation in soil fertility can occur because of the dependence of these soils on nutrient cycling by deep-rooted plants (Buol et al., 1980).

Inceptisols, young soils of sufficient age to have developed distinct horizons, comprise the third most widespread soil type in the humid tropics. Three major kinds occur: Aquepts (poorly drained), Andepts (well drained, of volcanic origin), and Tropepts (well drained, of nonvolcanic origin). Among the Inceptisols, Aquepts are dominant in humid tropic America and Africa, and Tropepts are dominant in humid tropic Asia. Most of the Aquepts, or wet Inceptisols, are of high to moderate fertility and support dense human populations. In tropical America, they occur in the older alluvial plains along the major rivers and inland swamps of the Amazon Basin. About half have high potential for intensive agriculture. In Africa, large areas of wet Inceptisols (known locally as hydromorphic soils) long remained undeveloped because of human health hazards, although many of these hazards have been overcome and settlement has advanced. In Asia, many of the Tropept soils are used for lowland rice production. More than 90 percent of the world's rice is grown and consumed in Asia (where about 55 percent of the earth's people live). Inceptisols of volcanic origin (Andepts) are important in the volcanic regions of Asia, in parts of Central and South America, and in parts of Africa. They are generally fertile and have excellent physical properties.

Entisols are soils of recent development that do not show significant horizon layers. Within this soil type, well-drained, young alluvial soils (Fluvents) not subject to periodic flooding are considered among the best soils for agriculture in the world. Fluvents account for only 2.7 percent of the soils of the humid tropics and most are already cultivated; about two-thirds (25 million ha) are found in Asia where they are under intensive lowland rice production. Where forests remain on these soils, their preservation will be difficult due to their high agricultural potential.

Biological constraints on agriculture in the humid tropics include insect and other pests, pathogens, and weeds; a lack of improved germplasm for the common crops of the region; and the loss of domestic and wild biodiversity. The hot and humid climate provides