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The humid tropics ecosystem covers about 1.5 billion ha of land area, has a total human population of about 2 billion distributed over some 60 countries, and contains about I billion ha of the remaining tropical rainforest of the world. Relative distribution of the humid tropics comprises 45% in Latin America, 30% in Africa, and 25% in Asia. Rainfall of the humid tropics ranges from 1500 to 6000 mm/yr, with constantly high temperatures exceeding 18°C throughout the year. Predominant soils of the humid tropics are Oxisols (525 million ha) and Ultisols (413 million ha), comprising 63% of the total land area. These soils are old, highly weathered, and low in inherent soil fertility. Soils of intermediate fertility include Entisols (212 million ha) and Alfisols (53 million ha), comprising about 18% of the land area. Soils with good fertility and high productivity include Inceptisols (226 million ha), Histosols (27 million ha), and Mollisols (7 million ha), and comprise only 17.5% of the total land area. Other soils also are found in the humid tropics, but in very small proportions. The rate of tropical deforestation for the decade ending in 1990 was estimated at 0.9% for Latin America, 0.8% for Africa, and 1.2% for Asia. Deforestation is estimated to contribute between 1.1 and 3.6 pa-C as CO2 per year to the atmosphere, a major factor in the so-called "greenhouse effect".
Predominant farming systems of the humid tropics include shifting cultivation and bush fallow systems in Africa and Latin America, rice based cropping systems in southeast Asia, tree crops and plantations in lowland wet forest and pre-montage moist and wet forest ecoregions, and livestock-based systems in Central and South America. The resource-based agricultural systems are subsistence and sustainable only at low levels of productivity and low demographic pressure. Introduction of intensive agricultural systems can lead to problems of soil and environmental degradation. Principal soil degradative processes include soil erosion, leaching, fertility depletion and nutrient imbalance, decline in soil organic matter content and soil biodiversity, deterioration of soil structure, and disruption in mineral recycling mechanisms.
Consequently, sustainable use of soil and water resources is a major concern. Because sustainable development implies meeting the needs of the present without compromising future needs, it is important to manage soil and water resources successfully to satisfy changing human needs. In this context, there are several improved and innovative technological options for sustainable management of soil resources in the humid tropics. These include ecologically compatible methods of deforestation and land development, and science-based techniques of soil and crop management of existing lands. If deforestation is inevitable, it must be done either manually or by shear-blade methods followed by in situ burning. It is desirable that cleared land is sown with an appropriate cover crop for providing an immediate ground cover. Installing erosion control measures and adopting erosion-preventive systems is a high priority. Mulch farming, conservation tillage, and vegetative hedges and strips established on the contour are conservation-effective measures.
Nutrient management is crucial for sustainable use of those soils with low inherent fertility. Nutrient recycling is important so that losses can be reduced, and requirements for chemical fertilizers can be minimized through returning crop residues and other organic wastes. Crop residues contain large quantities of nutrients, including 5 to 35 kg of N, 1 to 4 kg of P, 5 to 30 kg of K, 4 to 25 kg of Ca, 1 to 8 kg of Mg, and 30 to 100 kg of total nutrients per Mg of crop residue. In addition to crop residue, appropriate cover crops can be grown to produce in situ mulch. The biomass from leguminous crop cover is rich in N. P. and other plant nutrients, and biological nitrogen fixation is important to enhancing the nitrogen supply of these soils.
Agroforestry systems are also important in erosion control and nutrient recycling. There are several tree species with high net biomass production. These trees are also efficient in nutrient recycling from the sub-soil horizons. Pruning and foliage from these trees in one year may contain 100 to 800 kg/ha of nutrients. With proper management, 20% to 30% of these nutrients can be available for crop production.
Research conducted in different agro-ecoregions has shown that substantial increases in production of crops, livestock, and trees can be achieved by adopting these improved and science-based technologies. Synergistic effects are achieved if supplemental doses of chemical fertilizers are used in conjunction with organic residues and pruning from woody perennials and leguminous trees. For each hectare of already cleared land on which these improved techniques of soil and crop management can be adopted, several hectares of tropical rainforest can be saved from the urgent need for deforestation and conversion for food crop production.
Sustainable systems of soil and water management have distinct attributes and characteristics. Principal attributes include erosion control, maintenance of soil organic matter content, enhancement of soil structure, replenishment of nutrients harvested through a judicious combination of organic residues and chemical fertilizers, and improvement of soil resilience and quality. Major characteristics of sustainable systems are highenergy flull that leads to an increasing trend in per capita productivity, and soil restorative ability to sequester carbon in soil and biomass, thus Improving environmental quality.
Considerable progress has been made in developing improved and creative technologies for increasing and sustaining production in the humid tropics. However, a lot remains to be done in understanding the basic principles and processes that govern soil degradation and soil quality; developing and adapting soil restorative technologies; developing local adaptive research networks in fine-tuning problem-solving technologies; disseminating existing knowledge through training; and identifying policy considerations that promote the use of improved technologies.
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