Abstract

An experiment to examine the effect of tillage practice on yield, nodulation and nitrogen fixation of soybean in a podsolic soil on the far north coast of New South Wales is reported. Soybeans were sown into either a cultivated or no-tilled seedbed following pasture in 1983 and 1984, and following soybeans in 1984 and 1985. Results over the 3 seasons indicated substantially improved nodulation under no-tillage. The mean nodulation index (nodule mass as a percentage of shoot mass) was 4.5 for no-tillage soybean and 2.3 for the cultivated crops. Nitrogen fixation, assessed using the ureide technique, was higher in the no-tillage plots, especially during early plant growth, Plant growth and seed yield were increased by no-tillage in the wetter 1984 season; the reverse occurred in the other 2 less favourable years. Data on crop N, seed N, and fixed N (estimated by partitioning N accumulated by the crops during successive periods of growth, according to the relative ureide values) were combined to calculate N balances. Potential gains of soil N were greatest under the no-tilled soybeans (as much as 110 kg Nha; mean over all seasons was 80 kg Nha). The cultivated crops showed a maximum gain of 86 kg N/ha and an average gain of 30 kg N/ha. The data indicate that soybean has the potential to improve the N fertility of podsolic coastal soils, particularly when grown using no-tillage practices.