Soil-plant Relationships in the Tropics

As commonly defined, the tropics comprise that region of the earth lying between the Tropics of Cancer and Capricorn, constituting 38% of the larid surface (5 x 109 ha). The tropics may also be defined as those areas with mean temperatures greater than 18'C in all mor.ths of th. year. A third definition of the tropics specifies those areas with soils in which the tempera­ ture at a depth of 50 cm varies less than 5°C between summer and winter (39). Compared to the geogrepin-al definition, these latter definitions reduce the area considered t1 he tropical. In contrast to the temperate region where low temperature restricts plant growth at some period of the year, in the tropical regions the length of the growing season is determined by the amount of rainfall and its temporal distributioa. Dudal (13) has separated the tropics into five major ecological zor.es based on growing periods: humid, subhumid, =emiarid, arid, and wetland. The growing season varies from 12 months in the humid zone to nearly 0 months in the arid regions. In the tropics a wide range of climate and vegetation exists, owing mainly to variations in amount and duration of rainfali.