Soil water and crop production in Botswana

Abstract. This paper draws on the results of a five-year project on the agronomy, crop physiology and soil physics of the cropping of sorghum and subsidiary crops at several sites in Botswana. The low and erratic yields usually obtained stem largely from the low and erratic rainfall, the harsh physical properties of the soils and the unfavourable interactions between these factors. Deep ploughing is essential to allow root penetration and water retrieval from depth in soils of high bulk density; sorghum plant densities must be kept moderately low, at the price of increased surface evaporation, to ensure that plants are big enough to optimize harvest index; and intercropping with cowpeas should be avoided, since it decreases sorghum yield stability with little appreciable compensation. One essential for a more stable, higher-yielding cropping system is the year-round management of the soil as a water storage medium. This will require the alternation of short periods of bare fallow with a sequence of crops with short and long growth cycles.