Site-specific considerations for managing phosphorus

Nutrient management can be difficult for land receiving nonuniform and some times very large applications of animal manure. The objective of this resea rch was to document spatial variability in soil P status and develop a mana gement strategy to help producers avoid problems related to past manure man agement practices. A 60-ha center-pivot sprinkler irrigated cornfield was i ntensively sampled (12.2 by 24.4 m alternate grid) and analyzed for Bray-l P, The average Bray-l P concentration showed that only a maintenance applic ation would be sufficient; however, 74% of the individual data points indic ated a positive response to P fertilizer could be expected, A map revealed unusually high soil P concentrations around a previous farmstead, Other spa tial patterns were related to depositional areas with relatively high organ ic matter content. Consultants are beginning to collect soil samples on a 1 00 by 100 m grid (1.0-ha cells), The study field was separated into 1.0-ha cells from which P availability maps were generated using data from a singl e point in the center of each hypothetical cell and compared to the map usi ng data from the cell corners, These maps were similar in some respects but would result in considerably different variable-rate P application maps, R esults document the limitations and potential fallacies associated with gri d sampling. A more reasonable approach might be to use a bare-soil photogra ph, crop canopy image, or yield map to identify sampling areas with similar characteristics to define sampling zones. Every effort should be made to i dentify past manure history prior to soil sampling.