Soils of the Humid Tropics and Subtropics

The term "tropics" generally refers to areas of the world with high rainfall, dense forests, and many infertile soils. The tropics occur at low elevations within the equatorial zone, while the subtropics extend to the latitudes of 30°N and 30°S. Fifty-one percent of the world's soils occur in the tropics and subtropics. Lowlands in the tropics have a mean annual temperature of greater than 75°F (24°C), and the mean monthly temperature of the coldest month is more than 65°F (18°C). In the low-lying subtropics, the mean annual temperature is between 62 and 75°F (17 and 24°c), and the coldest month averages between 50 and 65°F (10 and 18°C).

Freshly deposited alluvium and soils that are stony, shallow, eroded, poorly drained, or deep and sandy can be found in all humid regions throughout the world. Soils of minimal soil development can be found in temperate, subtropical and tropical regions. The primary differences between the characteristics of soil in the tropics and subtropics and their characteristics in temperate regions result from differences in temperature and major geological events such as glaciations. In the tropics and subtropics, soil temperatures are high every day of the year, whereas in most temperature regions, freezing of the soil interrupts the chemical weathering of minerals and soil profile development, even though there is some physical weathering by freeze-thaw effects. In general, for every 18°F (-10°C) rise in temperature above freezing, the chemical reaction rate is doubled, which means that tropical soils weather much faster than temperate or arctic ones. Weathering in the tropics can be at least eight times faster than in the Arctic and Antarctic regions, and four times faster than in temperate regions. Weathering rates in subtropical regions average about half those of the tropics. Products that remain from the decomposition - iron, aluminum, and some silica - recrystallize to form hydrous oxides of iron, aluminum, and sometimes titanium, plus some kaolinite clay. In soils of the tropics, many composite particles the size of sand granules consist of altered minerals cemented together by iron, in contrast with the composition and structure of sand particles in temperate and arctic regions, which are mostly primary minerals such as quartz and feldspars. Predominant soil orders that develop only in the tropics and subtropics are Oxisols, Ultisols and Vertisols.

Organic matter is rapidly lost from tropical soils following the clearing and cultivation of land because mixing and aeration increase the rate of decomposition; organic matter loss is a primary cause of decreasing crop yields in the tropics. A decrease in soil organic matter results in soil structure deterioration, lower plant nutrient reserves from organic matter, and a reduced cation exchange capacity. Ninety percent of all soils in Western Africa, Latin America and India are deficient in available phosphorus. Particularly large amounts of phosphorus fertilizers are needed on Oxisols, Ultisols, and Vertisols, as well as on tropical soils that have developed from volcanic and parent materials. Rates of phosphorus as high as 143-1,069 pounds per acre (160-1,197 kg ha-1) are needed to increase food crop yields.

Oxisols have a total cation exchange capacity of less than 10 meq per 100 grams of soil; when compared with other soils containing the same levels of clay and organic matter, this is lower than soils from any of the other nine soil orders. Since plant-available potassium is stored as part of the overall soil exchange system, levels of potassium are often deficient for the satisfactory growth of many tropical crops. Oxisols and Ultisols may require treatment with lime to correct the soil pH or to reduce the toxic effects of aluminum. Vertisols do not need lime because they usually develop from calcareous materials in a wet-dry climate, and the high clay content and the subhumid climate retard the loss of lime by leaching. Vertisols are not acid enough to develop toxic levels of aluminum. Oxisols and Ultisols have more kaolinite, more gibbsite and more goethite than soils from the other soil orders. These clay minerals do not absorb lime cations, calcium or magnesium as strongly as montmorillonite, which predominates in soils from the drier temperate regions. Tropical soils with pH values of less than 5 are not productive because of deficient levels of nitrogen, phosphorus and frequently potassium, as well as some micronutrients; high soil-solution aluminum and high exchangeable aluminum, which are toxic to the roots of many plants, such as cotton, tomato, alfalfa, celery, barley, corn, grain sorghum, and sugar beets; high exchangeable manganese, which is toxic to many crops; as well as serious calcium, magnesium and molybdenum deficiencies. Shifting cultivation is the pragmatic solution to the problems of cropping tropical soils used in primitive conditions: land is cleared and burned, planted with crops until the soil fertility is exhausted, then abandoned to return to native woody vegetation and rejuvenation, while a new forested site for planting is sought.

In arid and semiarid areas, crop production depends on the conservation of soil moisture. Data on soil water available for plant growth during the growing season form the scientific basis for deciding upon improved cropping systems.

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