RESIDUAL EFFECTS OF IRRIGATED SOYBEAN ON THE GROWTH AND YIELD OF IRRIGATED GRAIN-SORGHUM

Irrigated grain sorghum was cropped immediately following irrigated soybean, to establish whether there were any residual effects on sorghum from the previous management of soybean. Results of experiment 1 showed that, where soybean residues were incorporated, sorghum DM accumulation and grain yields were greatest where soybean had been least frequently irrigated (after about 240 mm cumulative open pan evaporation), and least where the soybean had been grown with saturated soil culture (SSC). Extractable mineral nitrogen (N) at sowing of the sorghum was also least following the SSC soybean and greatest following the least-frequently irrigated soybean. The subsequent availability of N during sorghum growth appeared greater where the soybean was more frequently irrigated, although the release occurred too late to influence grain yield. In experiments 2 and 3 residual effects of conventional irrigation of soybean after 60 mm evaporation were compared with those of SSC. Where soybean stubble was uniformly removed (experiment 2), no significant effects of previous irrigation were observed. Where soybean stubble was either removed or incorporated (experiment 3), sorghum yields were lower following the SSC soybean. The incorporation of soybean stubble delayed the release of soil N, although this was not reflected in lower sorghum yields. Taken together, the studies suggested that, for a sorghum crop following soybean, more N fertiliser will be needed at sowing where the soybean stubble is incorporated; where the soybean had been well-irrigated; and the sooner after soybean harvest the sorghum is sown. Conversely, there were indications that more N may be available late in the growth of the sorghum crop where soybean had been well irrigated.