Fallow period requirements
In their natural state, many soils, in particular in the tropics, cannot be continuously cultivated without undergoing degradation. Such degradation is marked by a decrease in crop yields and a deterioration of soil structure, nutrient status and other physical, chemical and biological attributes. Under traditional low input farming systems, this deterioration is kept in check by alternating some years of cultivation with periods of fallow. The length of the necessary rest period is dependent on inputs applied, soil and climate conditions, and crops. Hence, the main reason for incorporating fallow into crop rotations is to enhance sustainability of production through maintenance of soil fertility.
Regeneration of nutrients and maintenance of soil fertility under low input cultivation is achieved through natural bush or grass fallow. At somewhat higher inputs to soils, soil fertility is maintained through fallow, which may include for a portion of time a grass, grass-legume ley or a green-manure crop. Factors affecting changes in soil organic matter are reviewed in Nye and Greenland (1960) and Kowal and Kassam (1978). They include temperature, rainfall, soil moisture and drainage, soil parent material, and cultivation practices. The fallow factors used in the present Global AEZ land potential are based on earlier work done in the context of FAO’s regional assessments (Young and Wright, 1980) and the Kenya AEZ study (FAO/IIASA, 1991).
The fallow factors have been established by main crop groups and environmental conditions. The crop groups include cereals, legumes, roots and tubers, and a miscellaneous group consisting of long term annuals/perennials. Fallow requirements have been assumed to be negligible for olive and oil palm. The environmental frame consists of individual soil units, thermal regimes and moisture regimes. The thermal regimes are expressed in terms of annual mean temperatures of > 25°C, 20-25°C, 15-20°C and <15°C. The moisture regimes are made up of five broad LGP ranges:<90 days, 90-120 days, 120-180 days, 180-270 days, and > 270 days.
presents fallow-land requirements by thermal and moisture regimes imposed to
maintain soil fertility. This factor is expressed as percentage of time during
the fallow-cropping cycle the land must be under fallow. For Fluvisols and Gleysols,
fallow factors are lower because of their special moisture and fertility conditions.
At high levels of inputs and management fallow requirements are uniformly
set at 10%. At intermediate level of inputs the fallow requirements are set
at one third of the levels required under low level of inputs. In the present
study the fallow requirement factors have been applied for the estimations of
annually available arable land.
