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 und isturbed bare soil surface and soil cultivated twice per annuum, only for t he last four years. Evapotranspiration was determined by quantifying the so il-water balance equation with the aid of runoff plots and soil- water cont ent measurements. Crude protein content calculated front N-content (Kjeldah l-method) of the leaves, stems and seed was determined. Though the percenta ge crude protein content of grassland in good condition was generally lower (P<0.01) than that of grassland in poor condition, crude protein productio n was still significantly (P<0.01) higher when expressed as total quantity of above-ground phytomass produced. Water-use efficiency declined significa ntly (P<0.01) with grassland degradation. Grassland in good condition avera ged a WUE of 0.29 kg crude protein.ha(-1).mm(-1) during four growing season s. Higher surface runoff occurring in grassland in poor condition due to le ss vegetation cover, caused soil-water content to be much lower than that o f grassland in good condition. Soil-water storage increased by 31% due to c ultivation. 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 s oil-water management, which is only possible when the veld is in good condi tion.