CHANGES IN SOIL STRUCTURE INDUCED BY WHEEL TRAFFIC AND GROWTH OF PERENNIAL GRASS

Soil structure was characterised in a grassland soil that had been compacted in the first, second and fourth years after establishment by a range of tyre-soil contact stresses. The largest of the stresses consistently reduced the annual yields of herbage during the 4 year period. Topsoil properties were assessed by bulk density (from the layer 0-120 mm deep), vane shear strength measurements and quantitative image analysis of macroporosity in soil blocks (from the layer 0-100 mm deep). Each method showed generally better soil structure in the relatively uncompacted treatments; dry bulk density at the 30 mm depth had decreased in each treatment between the first and fourth years after sward establishment. In the fourth year, core sample density and pore structure attributes from the image analysis procedure showed that the soil close to the soil surface was similar in all treatments. This similarity could be attributed to an increase in pore space in initially heavily compacted soil and could be attributed, at least in part, to the presence of pore space associated with the crownal area of the living sward.