E. de PauwFAO Consultant, Agricultural Zonation, Ethiopia
The agroclimatic potential of a soil can be expressed as the net total biomass production grown under a prevailing radiation and moisture regime. The period in which biomass production is possible can be estimated by growing period analysis, and the rate of biomass production by a summary radiation model. While the radiation regime can be assumed to be uniform, the moisture regime can, under the same climatic conditions, differ markedly according to soil type.
It is argued that Vertisols have a lower agroclimatic potential than other soil types because of their limited growing period. The single most important factor that reduces the available growing period below the estimates from climatic data is the waterlogging hazard in areas with concentrated surplus of rainfall over potential evapotranspiration. Improved surface drainage can prevent or alleviate waterlogging but cannot fully recover the growing period that would be available on other soils.
A second factor that reduces the available growing period is the need to sacrifice part of it to land preparation, which might not be possible at other times of the year because of inadequate power or inappropriate soil moisture.
Growing period losses are closely linked to the typical shrink-swell cycles of Vertisols. Deep and wide cracking in dry Vertisols leads to inverse moisture gradients at the beginning of the growing period, which bring topsoil moisture levels in the available range at a later time than in other soils. This necessitates delayed planting and is a limitation of particular relevance in areas with short growing periods.
A quantitative estimate of growing period losses by factors that delay plantings can be made by a comparing potential growing period from the climatic data with the growing periods available for different planting dates.
