Seasonal changes of physical properties of an Oxic Paleustalf (Red Kandosol) after 16 years of direct drilling or conventional cultivation

Abstract

Oxic Paleustalf soils, as occurring in the Wagga Wagga region of New South Wales, are widely used for cereal and pulse cropping. However, the surface structure of these soils declines when frequently cultivated. In recognition of this problem, direct drilling of crops on these soils is advocated by agricultural advisory staff, and in 1977 a long-term trial was established to contrast differences that may develop over time in soil properties and in crop yield. The present study was conducted to evaluate the effect of 16 years of tillage practices, on these soils, on some physical and biological properties, such as macroporosity, water stability of aggregates from the 0–5 cm and 5–10 cm layers, and surface-layer saturated ( K sat) and unsaturated ( K-40) hydraulic conductivity. The tillage treatments at this site consisted of (i) conventional cultivation (CC), i.e. three cultivations before sowing; (ii) reduced cultivation (RC), i.e. one cultivation before sowing and a knockdown herbicide; and (iii) direct drilling (DD), i.e. no cultivation before sowing and a knockdown herbicide. Temporal changes in aggregate stability and hydraulic conductivity were observed during the growing season. Aggregates sampled at four time intervals from the surface (0–5 cm) layer were more stable under DD than under RC nor CC at each sampling ( P<0.05). Regardless of treatment, the stability of surface soil aggregates was lowest during winter (July) and increased in spring. Saturated hydraulic conductivity ( K sat) was significantly greater ( P<0.05) at sowing under CC than under RC or DD (276, 116 and 83 mm h −1, respectively), but decreased sharply from June to August relative to RC or DD treatments (89, 125 and 263 mm h −1, respectively). Unsaturated hydraulic conductivity ( K-40) was significantly greater ( P<0.05) in the DD than the RC or CC treatments at the beginning of the season (2.6, 1.6 and 1.3 mm h −1, respectively), but as the season progressed, this advantage diminished so that, in October, K-40 between the tree treatments were similar (0.8, 0.6 and 0.5 mm h −1, respectively). Earthworm populations (>1 mm) were significantly greater ( P<0.05) under the DD treatment than the RC or CC treatments (315, 220 and 140 earthworms m −2, respectively), but macroporosity (pores >2 mm and pores between 1 and 2 mm) did not differ significantly ( P<0.05) between treatments. These findings suggest that improvements in soil structural properties resulting from the adoption of direct drill-cropping practices may improve water conduction in the surface layer which may, in below average rainfall seasons, allow more water to be stored at depth for later use by crops.