Supplying the nitrogen needs of cereal crops with swine anaerobic lagoon sludge

Swine anaerobic lagoon sludge is a potential source of nutrients for crop production. In order to best utilise nitrogen (N) contained in the sludge, prediction of plant available N is required. This was estimated from two field experiments conducted on contrasting soil types. The sludge was applied as dry stockpiled sludge at rates of 6 and Is t/ha, and wet sludge at rates of 20 and 65 t/ha on a sandy loam soil, or 40 and 120 t/ha on a clay soil. In addition to field measurements anaerobic and aerobic laboratory incubations were conducted to estimate available N. Sludge source and soil type influenced net release of available N. Preplant soil nitrate plus anaerobically mineralizable N were significantly correlated with plant N uptake plus residual soil nitrate, with R-2 values of 0.49 and 0.80 for the clay and sandy soils, respectively. In spite of the level of variation explained by the two variables the inclusion of anaerobically mineralizable N did not greatly improve the relationship on the sandy soil. Measurement of preplant nitrate to 1.5m depth did not explain enough of the variation in field measured plant available N in order to use this single measurement to predict plant available N. Combining anaerobically mineralizable N, as an indicator of potentially mineralizable N for the growing season, with preplant nitrate concentrations was not st useful index to predict the N supply on soils receiving applications of swine anaerobic lagoon sludge.