Effect of sheep stocking intensity on soil physical properties and dry matter production on a Pallic Soil in Southland

This 3-year study examined the extent of damage to soil physical properties of a Pukemutu silt loam (Pallic Soil) and the loss of ryegrass-white clover pasture production caused by intensive winter grazing at 1800 sheep ha(-1). Macroporosity, pore size distribution, bulk density, and hydraulic conductivity were measured at 5-cm incremental soil depths to 15 cm to assess changes in soil compaction. Soil smearing on intensively winter-grazed plots suggested that soil structural damage had occurred. Soil physical tests, three weeks after winter grazing, in August 1994 and 1995, however, showed only slight compaction at the surface. Macroporosity in the 0-5 cm soil depth was significantly reduced from 16.4% to 12.1% by the intensive winter grazing treatment. Soil pores were water-filled leading to plastic deformation rather than compaction. Spring pasture production was also significantly decreased (21%) following the 1994 winter grazing, but growth recovered the following summer. Macroporosity was generally greater than 10% so was unlikely to limit production for long at this site.