Physical impact of grazing by sheep on soil parameters in the Nama Karoo subshrub/grass rangeland of South Africa

The direct short-term impact of three rates of stocking (4, 8 and 16 small-stock units [SSU] ha−1) was quantified in terms of soil characteristics of arid Nama Karoo vegetation (subshrub/grass). Mature Merino wethers grazed in the experimental plots throughout May in 1995 and 1996 (the plots were not subjected to grazing at any other time). Stocking rate proved inversely related to initial infiltration rate. Light trampling (4 SSU ha−1) loosened the topsoil sufficiently to increase the initial infiltration rate: infiltration capacity of soil in fields stocked at 4 SSU ha−1 and 16 SSU ha−1 was 17% higher and 14% lower respectively than that of soil of ungrazed rangeland over the two grazing periods. Increased soil compaction and greater bulk density due to higher stocking rates significant decreased the infiltration rate. Compared to ungrazed rangeland stocking rates of 4, 8 and 16 SSU ha−1 over the two grazing periods increased bulk density respectively by 2.73%, 6.67% and 8.945% and compaction by 10.90%, 16.78% and 20.90%. No grazing also increased bulk density and soil compaction and decreased infiltration rate. Light stocking (4 SSU ha−1) influenced all soil parameters most favourably. From a hydrologic point of view, grazing levels and rotation schemes need to be tailored for sustainable utilization of arid subshrub/grass vegetation by livestock.