CULTIVATION TECHNIQUES AND GRAZING AFFECT SURFACE-STRUCTURE OF AN AUSTRALIAN HARDSETTING SOIL

The tillage practices appropriate for the transition from a grass-dominant pasture phase to a cropping phase with oats (Avena sativa) were evaluated on a hardsetting sandy loam soil at Cowra, N.S.W., Australia. Soil physical conditions, including bulk density, mechanical impedance, organic matter distribution, aggrepte stability, water infiltration, and air-filled porosity, were measured. Comparisons were made between plots that were either grazed by sheep during winter when soil moisture content was high, or left ungrazed. The effects of different tillage practices were compared with an undisturbed seedbed (direct drilled) and demonstrated a greater reduction in bulk density and mechanical impedance by the disc and scarifier than the chisel (or ripping) implement. The mechanical impedance 35 mm below the soil surface was 0.3 and 3.7 MPa in the scarify and undisturbed seedbeds, respectively, just prior to sowing. Organic matter contents of the disc, scarify and chisel seedbeds in the 25 mm surface layer were 60%, 45% and 28%, respectively below that of the undisturbed seedbed. Surface aggrepte stability at the end of the season was least on the disc seedbed, which had the greatest soil mixing in the cultivated layer. Some recompaction of this soil occurred without grazing, but grazing of all seedbeds with sheep during winter compacted the soil and offset any soil benefits achieved with cultivation. The greatest compaction occurred on the disc seedbed to a depth of 140 mm following grazing for 22 days. The least compaction occurred on the chisel seedbed.