Effect of nitrification of a black earth soil on legume nodulation

Abstract

Black soil from the Darling Downs, Queensland, nitrified to 130-159 p.p.m. nitrate nitrogen in the glasshouse when subjected to 8 cycles of wetting and air-drying. The highest level in the field was 43 p.p.m. nitrate nitrogen. Nodulation inhibition in soybeans by nitrate and ammonium ions was compared. The nitrate ion was more active and nitrate levels close to 168 p.p.m. N were necessary to prevent nodulation, while ammonium ions as high as 224 p.p.m. N were only partly inhibitory. Interplanting soybean with sorghum in the field reduced the nitrate levels in the vicinity of the legume but did not overcome a nodulation failure from inoculated seed. Nitrate inhibition is an unlikely explanation for soybean nodulation failures on black soils.