Tillage systems in the tropics [antikvár]

Foreword Soil, the most basic natural resource, is finite and fragile. It may take thousands of years to produce a few centimetres of fertile topsoil. Only 11 % of the earth's land area of 13.4 X 10' ha is currently cultivated. With rapidly increasing population, especially in tropical regions, the arable land per caput is rapidly decreasing. Densely populated Asia, with 75% of the earth's population, has little additional land to bring under cuhivation. Most of the land in Africa and Latin America that can be brought under cultivation is located within fragile and ecologically sensitive regions, for example tropical rainforest, acid savannah, drought-prone sahel, steepland of the Andes. Potentially productive agricultural land is either inaccessible, too steep, too shallow, too wet or too dry. Soil degradation is widespread, particularly in the tropics, where it is a major economic and ecological constraint. It leads to poor yields, poor standards of living, and contributes to hunger and malnutrition. The ecological consequences of soil degradation include pollution of wind and water by sediment and dust, and eutrophication and contamination of natural waters by sediment-borne pollutants. Processes of soil degradation most prevalent in the tropics include soil erosion and desertification, compaction and hard-setting, salt and water imbalance in the root zone, biological degradation including reduction in soil organic matter content, fertility depletion, and chemical degradation including leaching and acidification. Tillage, mechanical soil manipulation for seedbed preparation, affects the rate and trend of soil degradation. In addition to establishing the seed-soil contact, tillage is used to alleviate soil compaction and so improve infiltration capacity; to dispose of pathogen-infested crop residue; to incorporate fertilizer into the root zone, and to eradicate weeds. The kind of tillage and its frequency depend on the soil and its related constraints to crop production. A wide range of tillage implements has been developed to alleviate unfavourable soil conditions. Manual or anhnal driven tillage tools are widely used in small-scale agriculture, whereas motorized equipment is used in large-scale commercial or semi-commercial agriculture. The term "mode" refers to the type of tillage whether primary, secondary or tertiary based on tools for soil inversion, loosening, levelling, mixing or pulverizing. Similarly, the term "means" refers to the source of power and the type of machinery for tillage. Both mode and means of tillage have changed drastically since industrialization in the 19th century. Equipment has become larger and faster, so the risks of soil and environmental degradation have drastically increased. Recognition of the need to protect bare soil from soil erosion and erosion-induced degradation has led to the development of a wide range of practices involving the reduction of the intensity and frequency of tillage and the practice of leaving some crop residue or other biomass on the soil surface as a protective barrier against erosion. Reduced cultivation, an integral part of conservation tillage, has been widely adopted since the early 1950s. The widely recognized risks of soil erosion and environmental degradation in the tropics are only partly due to fragility of the soils. Their susceptibility to degradative processes can be