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Sustainable Agriculture and the Environment in the HUMID TROPICS
through protective plant cover. Physical deterioration, compaction, and loss of structure through rainfall can be equally damaging, reducing productive potential. Continuous crop or crop residue cover from appropriately managed systems is crucial to maintenance of productive potential.
The efficiency and degree of utilization of sunlight and soil andwater resources. With increasing limitations on the extent of natural resources in many populous countries, the selected agricultural systems must be managed for optimal use, including continuous crop cover, good crop and animal genetic potential, minimal pest damage, and optimal nutrient supply.
A small offtake (harvested removal) of nutrients in relation to totalbiomass. This factor is especially important on the more fragile soils. Where soils are erosive, have poor nutrient status, or are otherwise chemically or physically fragile, the maintenance of high biomass systems is critical.
Maintenance of a high residual biomass in the form of wood, herbaceousmaterial, or soil organic material. A carbon source for both energy and nutrient retention is critical to the support of biomass in the soil and to crop and animal productivity.
The structure and preservation of biodiversity. The efficiency of nutrient cycling and the stability of pests and diseases in the system depend on the amount and type of biodiversity as well as its temporal and spatial arrangement (structural diversity). Traditional systems, particularly those in marginal production environments, often have significant stability and resiliency as a result of structural diversity. Research is only now beginning to quantify these effects.
TECHNICAL NEEDS COMMON TO ALL LAND USE OPTIONS
Three scientific areas, interwoven throughout the report, are an essential part of every land use option and its application to any given environment. The degree to which a land use is sustainable often depends on the success in dealing with pest management, nutrient cycling, and water management.
Pest Management
Plant and animal protection is crucial to the productivity of any land use system. Although many land uses have an inherent stability or resiliency with regard to pests and diseases, additional steps may be needed to protect plants and animals from damage due to insects, weeds, pathogens, or nematodes. Pest-induced losses to crops before
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