Development and testing of a new phosphorus index for Kentucky

Authors: Bolster, Carl; HORVATH, T - Natural Resources Conservation Service (NRCS, USDA); LEE, B - University Of Kentucky; MEHLHOPE, S - University Of Kentucky; HIGGINS, S - University Of Kentucky; Delgado, Jorge

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/22/2013
Publication Date: 5/7/2014
Citation: Bolster, C.H., Horvath, T., Lee, B., Mehlhope, S., Higgins, S.F., Delgado, J.A. 2014. Development and testing of a new phosphorus index for Kentucky. Journal of Soil and Water Conservation. 69(3):183-196.

Interpretive Summary: The phosphorus index (PI) is a tool developed by USDA-NRCS to evaluate a field’s risk of P loss and has been adopted by most states, including KY, in their 590 Nutrient Management Standard. USDA-NRCS recently revised their 590 Standard, now requiring that states test the accuracy of their PI. A recent study evaluating output from the Kentucky PI highlighted several important deficiencies. To address these limitations, a committee consisting of scientists from federal, state, and local government agencies was formed to formulate a new PI for KY. Significant modifications were made to the KY PI which resulted in a much better correlation between PI risk assessment and measured P loss data compared with the original PI. 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 still be allowed at N-based rates. To make it easy to use, the new PI was integrated into an existing graphical user interface (GUI) and combined with a new nitrogen index so that risk assessments can be conducted for both of these nutrients simultaneously.

Technical Abstract: 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 across 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. This paper describes the development and evaluation of a revised PI for Kentucky. The revised Kentucky 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. The approach adopted in this study to revise the Kentucky PI should offer guidance to other states that are currently in the process of revising their PI.