The leaching of nitrogen and its uptake by wheat in a soil from southern New South Wales

Abstract

The changes in nitrogen concentration in the profile of a red brown earth soil, containing the equivalent of 160 lb mineral nitrogen per acre 18 inches at sowing, and to which ammonium sulphate at 0, 50, 100, and 150 lb nitrogen an acre had been added in early July, were measured for two seasons. A slow rate of nitrification resulted in the presence of high concentrations of residual ammonia-nitrogen 33 months later, equivalent to 55 and 30 per cent of the nitrogen added at the highest rate in fallowed and cropped treatments respectively. This residual ammonia-nitrogen persisted until the spring of the second season, a period of 13 months. Nitrogen was leached into the deeper subsoil, in the autumn and winter, when soil moisture was high and rain frequent. This leached nitrogen was retained in the surface 30 inches of the soil and subsequently taken up by the wheat crop. This uptake was responsible for differences in mineral nitrogen concentration between fallowed and cropped plots of approximately 82 and 96 per cent in the 0-18 inch and 0-30 inch depths respectively. It is suggested that leaching in low fertility soils, following heavy autumn and winter rains, could result in short term nitrogen deficiencies for crops in the Wagga Wagga region.