for example, in Cuba, where a high proportion of agriculture is devoted to sugar production.

The environmental characteristics of monocultural systems also raise important questions about their sustainability. The production and processing methods they employ are significant sources of pollution in many areas (Vincent and Hadi, Part Two, this volume). A high degree of biodiversity loss is incurred in establishing and maintaining monocultures. The fertile alluvial soils of the humid tropics are, in fact, so valuable for raising crops that the distinct and highly diverse lowland forests they once supported have virtually disappeared (Ewel, 1991). Because monocultures in the tropics concentrate species that under natural conditions were widely dispersed, they are more susceptible to pathogens and other pests than the same species in traditional mixed-crop systems or in natural forests. However, oil palm, rubber, sugarcane, and tea can be stable when grown in monocultures.

Despite these problems, monocultural systems are an important part of the mosaic of land uses in the humid tropics. With modifications, including reduced use of pesticides, enhanced recycling of nutrients, and more equitable distribution of productive land, these systems may continue to serve as important sources of food and agricultural production. Some monocultural crops, such as coffee, cacao, and rubber, have been produced in diversified small-landholder systems, making them more desirable both socially and environmentally. In the future, the challenge will be to better manage both the highly productive lands that are already in intensive use and the less productive lands that are used by many small-scale farmers. In advancing toward sustainability, a nation's agricultural system will need to be diverse to take advantage of available markets, to use more effectively its available natural and cultural resources, and to balance social, economic, and environmental needs.

The wide array of specific practices associated with sustainable agriculture includes the following:

• Low-impact land clearing techniques;

• Mulches, cover crops, and understory crops;

• Fertilizers and other soil amendments;

• No- and low-tillage planting techniques;

• Increased use of legumes as food crops, as cover crops, and in fallows;

• Improved fallow management techniques;

• Greater use of specially bred and alternative crops, grasses, shrubs, and trees (especially those tolerant of acidic, salinized, and high-aluminum soil conditions);