Rooting depth of wheat in the Victorian Mallee

In 1986 and 1987 wheat was sown in an experiment at the Mallee Research Station, Walpeup, at 2 times of sowing and with 3 rates of applied nitrogen. Soil cores were taken and trenches excavated to 1.5 m to measure wheat root growth and depth of rooting. Wheat roots penetrated to a maximum depth of 104 cm in crops sown in May, the optimum time of sowing for maximum yield, while delayed sowing reduced total root biomass and limited rooting depth to 73-83 cm. The application of nitrogen fertiliser did not affect either the rooting depth or growth and yield. Significant changes in total soil water content between sowing and harvest only occurred in 1987 with the early and late sown crops reducing the total soil water content by 47 and 99 mm respectively. In 1986, above average rainfall during the growing season caused the early sown crop to accumulate more water below 50 cm than the late sown crop. While total water use was increased only in 1986 with early sowing, crop water use efficiency and yield was greater in both years. The addition of nitrogen had no effect on crop water use or water use efficiency. A survey of wheat crops carried out in 1988 on 10 Mallee farms also found that shallow rooting is widespead. The field experiment and survey data show that, irrespective of sowing time, roots did not penetrate as far down the profile as might be expected, given reported rooting depths commonly in excess of 200 cm on similarly textured soils. This was shown to be associated with high soil pH and salt content. Poor rooting depth of wheat in this environment will restrict the use of stored water and accordingly, calls the practice of fallowing into question.