Shifting cultivation is a traditional method of cultivating tropical upland soils, used mainly for subsistence purposes. During the fallow periods of rest intervening between crops, the natural fertility of the soil is restored for renewed utilization in a subsequent period of crop growth. This traditional system of cultivation is in ecological balance with the environment and does not irreversibly degrade the soil resource, provided a sufficient length of fallow is allowed for soil restoration. However, increasing population pressures have necessitated more intensive use of land, particularly in the humid tropics of Asia and in the savanna and forest zones in Africa. The consequence is extended cropping periods and shortened fallows, which are inadequate to restore the soil's productive capacity. The subsistence farmers in the tropics are thus caught in a cycle of increasingly falling yields, more poverty and even less opportunity to subsist, let alone to improve their standard of living. In addition to this perpetuation of human misery, shifting cultivation, as currently practised in many areas, is wasteful of scarce land resources and frequently leads to intolerable erosion, particularly of hillsides and sloping lands.