An analysis of the physical condition of two intensively grazed Southland soils

Abstract

The treading effects of high-density winter grazing of sheep associated with all-grass wintering systems in Southland were studied for evidence of resultant soil physical degradation. Typical examples of Pukemutu (YGE) and Waikiwi (YBE) soils, extensive areas of which have been intensively managed in Southland for many years, were selected for assessments of mechanical impedance, bulk density, porosity, air permeability, and hydraulic conductivity. The results showed that intensive winter grazing in this environment resulted in significant losses of large soil macropores. As a result, the transmission of water through the root-zone was significantly restricted. After rain, this can lead to waterlogging and soil oxygen deficiencies. Probably as a result of cumulative soil damage over several winters, soil had become more compacted to nearly the full depth of the A horizon. Natural ameliorative processes may therefore be too slow to overcome fully the effects of treading. Measurements of mechanical impedance and bulk density, while widely used, proved inappropriate for examination of the effects of treading. Both are insensitive to small, but important, changes in soil porosity. Air permeability and hydraulic conductivity were good indicators of relative degrees of soil compactness of the sites examined. Both are sensitive to small changes in effective macroporosity, and are useful measurements to diagnose soil structural damage. Keywords all-grass wintering, Southland, sheep, treading damage, soil physical degradation