Revising the Phosphorus Index for Kentucky.

The phosphorus index (PI) is a field-scale assessment tool developed by the USDA Natural Resources Conservation Service (NRCS) to identify fields most vulnerable to P loss that has been adopted in some form in 48 states.  Due to concerns that the use of the PI has not resulted in intended reductions in P loss from agricultural fields, the USDA NRCS recently revised their 590 Nutrient Management Standard requiring that states demonstrate their PI ratings are correlated with actual risk of P loss from agricultural fields using either measured or modeled P loss data. Earlier research evaluating the Kentucky PI against modeled P loss data indicated several limitations with the index highlighting the need to update the PI for Kentucky. A revised PI was developed by incorporating a component formulation with each component based on published relationships between observed P loss and P source and transport factors. Output from the revised PI was well correlated (r² = 0.77) with a measured P loss data set (n = 260) published in the literature.  To assess whether the revised PI provided improved estimates of P loss risk compared with the original PI, output from both PIs was compared with a subset of the P loss dataset used to evaluate other PIs in the South. Linear regressions on the predicted and measured P loss values yielded r² values of 0.04 and 0.06 for the original PI for the untransformed and log-transformed data whereas r² values for the revised PI were 0.76 and 0.84 for the untransformed and log-transformed data, respectively, indicating an improvement with the revised PI in assessing P loss risk. The original and revised PIs were also compared by calculating P loss risk for 46 fields using data recently collected for developing comprehensive nutrient management plans in Kentucky. These results suggest that while the new PI will likely result in somewhat more restrictive P applications compared with the original PI, for the majority of fields in Kentucky, manure applications will be allowed at N-based rates.