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

Abstract

A field trial was carried out on a black earth (Waco series) at Cambooya on the Eastern Darling Downs to compare the effect of black gram, green gram, grain sorghum and a summer fallow on soil mineral nitrogen (NO3-N + NH4-N) and the yield of grain sorghum grown in the following summer. The initial sorghum treatment severely depleted soil mineral nitrogen to 120cm; even after a 173-d fallow, there was still 34kg/ha less nitrogen 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 29and 42kg N/ha, respectively, after a winter fallow. The fallow treatment accumulated 100kg N/ha of mineral nitrogen between January and October, but mineralization was markedly reduced from August to October. Sorghum grown on all plots in the second summer responded markedly to prior treatments, and grain yields and responses to nitrogen applied at 0, 34and 68kg N/ha reflected mineral nitrogen levels at planting. Yields of sorghum grain obtained without fertilizer after black gram, green gram and fallow were 8333, 7477and 9663kg/ha, respectively, compared with 4658kg/ha after sorghum. Prior crops of both grams increased sorghum yield as much as a fertilizer application of 68kg N/ha. © 1984 ASEG.