A cycle of land use between crop and fallow years is needed to work around the infertility of tropical soils. After clearing tropical forests, a farmer may get only a few years of crops before there is no fertility in the soil and must move on to clear more land.
In the meantime, the forest reclaims the previously farmed plots. Where new lands are not available, fallow periods on old fields are reduced or eliminated, resulting in soil deterioration. Also known as swidden or slash-and-burn cultivation.