Areas used for intensive (high-productivity) agriculture in the humid tropics generally are resource-rich lands that have adequate water supplies, naturally fertile soils, very low to modest slope, or other favorable environmental characteristics. These areas range from the flat lowland delta or river valley areas to gently rolling uplands, and include the broad continental, high rainfall plains of the Amazon and of Central Africa. They can support input-intensive management systems and yield multiple harvests of crops at high levels of productivity. Crops are usually planted in rapid sequence, using improved varieties. With adequate water and good growing conditions the crops are responsive to fertilizer inputs. However, crop yields are constrained during periods of high rainfall and by seasonal flooding in some river and delta areas. Pest management usually prevents economic loss but often entails heavy pesticide use that can have adverse environmental and health impacts.

Intensive agriculture is agronomically feasible for most Oxisols and Ultisols of the humid tropics. This alternative may interest farmers near urban areas where favorable marketing infrastructure ensures that fertilizer-based continuous food crop production is viable. Large Amazonian cities import most of their food from other areas. Farmers would have a potential comparative advantage in growing food crops near these cities. In Peru and Brazil, respectively, sustained yields have been obtained with continuous cropping trials for 41 crops (17 years) in Yurimaguas Ultisols and 17 crops (8 years) in Manaus Oxisols (Alegre and Sanchez, 1991; Sanchez et al., 1983; Smyth and Cravo, 1991). The key to continuous production is effective crop rotations and the judicious application of lime and fertilizers.

Intensive agricultural production in the humid tropics has historically concentrated on the highly fertile lowlands. These lowlands constitute only a small portion of land. For example, lowland areas comprise only 20 percent of the estimated 510 million ha of the Amazon located within the national territory of Brazil (Serrão and Homma, Part Two, this volume). They account for between 10 and 40 percent of the total land areas of Southeast Asian countries (Garrity, 1991). In some river bottom and delta areas, annual flooding and receding water cycles deposit enriching organic and inorganic sediments. However, these flooded areas represent an even smaller portion of the total land base.

Soil characteristics coupled with water availability make these areas especially suitable for the intensive production of high-value food crops. Paddy rice production in Southeast Asia is one well-