The appropriate use of wastes is a significant issue for the pig industry due to increasing pressure from regulatory authorities to protect the environment from pollution. Nitrogen contained in piggery pond sludge (PPS) is a potential source of supplementary nutrient for crop production. Nitrogen contribution following the application of PPS to soil was obtained from 2 field experiments on the Darling Downs in southern Queensland on contrasting soil types, a cracking clay (Vertosol) and a hardsetting sandy loam (Sodosol), and related to potentially mineralisable N from laboratory incubations conducted under controlled conditions and NO₃^– accumulation in the field. Piggery pond sludge was applied as-collected (wet PPS) and following stockpiling to dry (stockpiled PPS). Soil NO₃^– levels increased with increased application rates of wet and stockpiled PPS. Supplementary N supply from PPS estimated by fertiliser equivalence was generally unsatisfactory due to poor precision with this method, and also due to a high level of NO₃^– in the clay soil before the first assay crop. Also low recoveries of N by subsequent sorghum (Sorghum bicolor) and wheat (Triticum aestivum) assay crops at the 2 sites due to low in-crop rainfall in 1999 resulted in low apparent N availability. Over all, 29% (range 12–47%) of total N from the wet PPS and 19% (range 0–50%) from the stockpiled PPS were estimated to be plant-available N during the assay period. The high concentration of NO₃^- for the wet PPS application on sandy soil after the first assay crop (1998 barley, Hordeum vulgare) suggests that leaching of NO₃^– could be of concern when high rates of wet PPS are applied before infrequent periods of high precipitation, due primarily to the mineral N contained in wet PPS. Low yields, grain protein concentrations, and crop N uptake of the sorghum crop following the barley crop grown on the clay soil demonstrated a low residual value of N applied in PPS.

NO₃^– in the sandy soil before sowing accounted for 79% of the variation in plant N uptake and was a better index than anaerobically mineralisable N (19% of variation explained). In clay soil, better prediction of crop N uptake was obtained when both anaerobically mineralisable N (39% of variation explained) and soil profile NO₃^– were used in combination (R² = 0.49).