COMPARATIVE EFFECTS OF BLACK AND GREEN GRAM (MUNG BEANS) AND GRAIN-SORGHUM ON SOIL MINERAL NITROGEN AND SUBSEQUENT GRAIN-SORGHUM YIELDS ON THE EASTERN DARLING DOWNS

A field trial was carried out on a black earth (Waco series) at Cambooya on the Eastern Darling Downs [Queensland, Australia] to compare the effect of black gram, green gram, grain sorghum and a summer fallow on soil mineral N (NO3-N + NH4-N) and the yield of grain sorghum grown in the following summer. The initial sorghum treatment severely depleted soil mineral N to 120 cm; even after a 173-d fallow, there was still 34 kg/ha less N present than initially in this treatment. Black and green gram also reduced levels of soil mineral nitrogen during crop growth, but these recovered to exceed pre-trial levels by 29 and 42 kg N/ha, respectively, after winter fallow. The fallow treatment accumulated 100 kg N/ha of mineral N between Jan. and Oct., but mineralization was markedly reduced from Aug.-Oct. Sorghum grown on all plots in the 2nd summer responded markedly to prior treatments, and grain yields and responses to N applied at 0, 34 and 68 kg N/ha reflected mineral N levels at planting. Yields of sorghum grain obtained without fertilizer after black gram, green gram and fallow were 8333, 7477 and 9663 kg/ha, respectively, compared with 4658 kg/ha after sorghum. Prior crops of both grams increased sorghum yield as much as fertilizer application of 68 kg N/ha.