Soil-water utilisation and sustainability in a semi-arid grassland

The impact of different botanical composition classes;, viz. poor, moderate and good. on soil-water balance, water-use efficiency (WUE: crude protein produced per unit of evapotranspiration), productivity and soil temperature were determined in a semi-arid grassland during four growing seasons(1995/96 to 1998/99), In addition, the same measurements were also made on an undisturbed bare soil surface and soil cultivated twice per annuum, only for the last four years. Evapotranspiration was determined by quantifying the soil-water balance equation with the aid of runoff plots and soil- water content measurements. Crude protein content calculated front N-content (Kjeldahl-method) of the leaves, stems and seed was determined. Though the percentage crude protein content of grassland in good condition was generally lower (P<0.01) than that of grassland in poor condition, crude protein production was still significantly (P<0.01) higher when expressed as total quantity of above-ground phytomass produced. Water-use efficiency declined significantly (P<0.01) with grassland degradation. Grassland in good condition averaged a WUE of 0.29 kg crude protein.ha(-1).mm(-1) during four growing seasons. Higher surface runoff occurring in grassland in poor condition due to less vegetation cover, caused soil-water content to be much lower than that of grassland in good condition. Soil-water storage increased by 31% due to cultivation. Veld degradation resulted in soil temperature increasing up To 8.5 degrees C at a depth of 50 mm during December. An important requirement for sustainable grassland production in semi-arid climates, is effective soil-water management, which is only possible when the veld is in good condition.