Agricultural and forestry potentialities of the tropics

Only the very small areas (much less than 5%) of soils developed from certain volcanic products, being better drained and supplied with plant nutrients, measure up to the popular conceptions as to the potential fertility of the soils of the humid low latitudes. Consequently, generally held notions as to the unlimited fertility of soils of the tropics are quite wrong. Seldom can intensive agronomic and horticultural practices of Europe and the United States be used successfully on tropical soils. Where it is impossible to avoid exposing the soil to sun and rain, for the production of upland grains and fiber crops, a forest fallow of from 8 to 15 years is often effective in restoring soil productivity. Kaingining (shifting cultivation) is often the only way natives can produce food and fibers. Cogon grass and fire often prevent regeneration of the forest fallow. The Belgian Congo leads in rationalization of forest fallow methods. In humid equatorial forested regions tree crops are preferable, avoiding the clear felling and the destruction of the forest conditions during the transition, for once destroyed, forest conditions can seldom be restored. Partial thinning and under-story clearing of the native forest may preserve forest soil conditions and at the same time permit long time production of such forest crops as cacao, abaca, and coffee. Where coffee is grown without shade, serious chemical and physical deterioration of the soil results. Phosphorus, often deficient in humid tropical soils, is one of the limiting factors for both plant and animal growth. Pasture grasses on these poor soils are poor in nutrients. Production of livestock and dairy products is difficult: growth is apt to be both slow and unhealthy. Rapid increases in use of phosphorus as a fertilizer, exhausting the reserves, may be the factor limiting healthy population increase in the world. Where the soil can be kept at least slightly submerged under water during the growing season, and puddled to control weeds, a crop of padi (lowland rice) can be grown annually or oftener, even on soils very low in plant nutrients. In the long run, rice will feed increasing proportions of the world's population.