EFFECT OF TILLAGE PRACTICES ON WHEAT PERFORMANCE IN A SEMIARID ENVIRONMENT

Sodosol soils are at risk of degradation under existing fallow managem ent practices involving tillage. Topsoil erosion exposes horizons with reduced infiltration and low concentrations of plant nutrients. Conse rvation management systems are needed on these soils to avoid a revers ion to low intensity grazing. This paper reports on a 4 year study (19 86-1989) of the effects of tillage practices on profile soil water and crop yield in a Sodosol (Typic Natrustalf ) in central Queensland, Au stralia. The tillage treatments were: zero till fallow (weed control b y herbicides), reduced till fallow (chisel plough/scarifier or herbici des) and conventional till fallow (chisel plough/scarifier) in two lin ked experiments. In the first experiment, wheat was grown in three con tour bays (approximately 1 ha), and in the second, wheat was grown in replicated plots (30 m x 6 m) to allow statistical comparisons. Zero t ill provided consistent advantages in grain yield in all 4 years compa red with conventional till. Zero till also outyielded reduced till as well as conventional till in the plot experiment. The average yield in crease of 0.5 t ha-1 in zero till compared with conventional till was associated with greater water use and increased water use efficiency. Tillage practice caused only marginal differences in the available wat er content in the root zone (0-100 cm) at sowing; zero and reduced til l contained, on average, an additional 4 and 8 mm, respectively, compa red with conventional till. The tillage treatments had no effect on pl ant available water capacity. Some of the soil water that accumulated during the fallow drained beyond the root zone in all treatments and w as not available to the following wheat crop. At the conclusion of the experiment, soil water accumulation in the 100-180 cm soil layer was 86 mm in zero till, 39 mm in reduced till and 40 mm in conventional ti ll. Results indicate that zero till can be a more productive wheat far ming practice than conventional mechanical tillage. The increase in wa ter storage below the root zone of the wheat crop shows that there may be benefit in using a deeper-rooting crop or pasture species in rotat ion with wheat, particularly after zero till fallows.