Site-specific considerations for managing phosphorus

Nutrient management can be difficult for land receiving nonuniform and sometimes very large applications of animal manure. The objective of this research was to document spatial variability in soil P status and develop a management strategy to help producers avoid problems related to past manure management practices. A 60-ha center-pivot sprinkler irrigated cornfield was intensively 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 application would be sufficient; however, 74% of the individual data points indicated a positive response to P fertilizer could be expected, A map revealed unusually high soil P concentrations around a previous farmstead, Other spatial patterns were related to depositional areas with relatively high organic matter content. Consultants are beginning to collect soil samples on a 100 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 single point in the center of each hypothetical cell and compared to the map using data from the cell corners, These maps were similar in some respects but would result in considerably different variable-rate P application maps, Results document the limitations and potential fallacies associated with grid sampling. A more reasonable approach might be to use a bare-soil photograph, crop canopy image, or yield map to identify sampling areas with similar characteristics to define sampling zones. Every effort should be made to identify past manure history prior to soil sampling.