Availability of Compost P to Fescue Under Nonlimiting N Conditions.

Nutrient management plans require an accurate assessment of P fertilizer additions to soil. The fertilizer equivalents of manures and composts must be estimated in order to comply. A previous study comparing triple superphosphate(TSP) to poultry litter compost (PLC) as a source of P for rescue indicated that overall they were equal, but when N became limiting during the final harvest, PLC supplied more P to rescue than TSP at comparable rates. A subsequent study was initiated to determine if P from PLC was more available to rescue than TSP when N was not limiting. Sassafras soil was amended with PLC and TSP at rates of 0, 50, 100 and 150 kg ha^-1. Nitrogen was supplied to be uniform across all treatments taking into account the N mineralization rate of PLC. Two harvests of rescue were taken and analyzed for yields and P uptake. Nitrogen was added to all treatments at the original application rate of 120 kg ha^-1 and two more harvests plus roots were collected. Yields were affected by harvest date but not by P rate or source of P. P uptake was affected by date and rate but not source of P. The P uptake response to PLC and TSP was curvilinear and linear respectively, but within the range of P application rates used, the source of P was not a significant factor in P uptake by fescue. Based on the current study testing the effect of unlimited N, addition of supplemental N did not affect P supply from either source. Poultry litter compost is considered equal to TSP in supplying P to rescue and N availability did not affect these conclusions. [ABSTRACT FROM AUTHOR]