Tillage effects on soils - Physical and hydraulic responses to direct drilling at Lockhart, NSW

Abstract

The effects of cultivated fallowing (maximum disturbance), direct drilling (intermediate disturbance) and direct drilling using narrow sowing points (minimum disturbance) on soil physical and hydraulic properties were assessed after 3 years of cropping treatments at Lockhart in the southern wheat-belt of New South Wales. Infiltration was measured in the field using simulated rain, before and after the site had been uniformly grazed by sheep. Minimum disturbance produced some of the highest soil strengths, aggregate stabilities and bulk densities in the top 50 or 100 mm of soil, whereas intermediate disturbance gave the lowest bulk densities. Field infiltration of simulated rain was enhanced by reducing the level of tillage disturbance. Grazing the site with sheep generally reduced infiltration, although the ranking of treatments remained similar. Measurements of hydraulic conductivity, porosity and pore continuity on soil cores recovered from the field site failed to reflect treatment differences in field infiltration, suggesting that the properties of the bulk soil were not responsible for these differences. Laboratory assessment of surface crusting using repacked top soil (0-50 mm) showed that slaking on wetting and raindrop impact produced highly impermeable crusts or surface seals that control infiltration with simulated rain. The formation of crusts or seals after 30 min of simulated rain at 42 mm/h caused a 100-fold reduction in the effective hydraulic conductivity of the surface soil. Although the tillage treatments had no significant effect on crust hydraulic conductivities, we suggest that crusts or seals formed on cultivated soils were responsible for differences observed in field infiltration. In particular, the reductions in infiltration produced by grazing the field site with sheep suggest that the surface soil may have become crusted and compacted.