Development and testing of a new phosphorus index for Kentucky

Abstract

The phosphorus index (PI) is a field-scale assessment tool developed to identify fields most vulnerable to phosphorus (P) loss. The USDA Natural Resource Conservation Service (NRCS) recently revised its 590 Nutrient Management Standard and Title 190 National Instruction requiring that all NRCS-approved PI tools meet certain criteria. A recent study evaluating the Kentucky PI showed that it did not meet several of the criteria established by NRCS. This paper describes the development and evaluation of a revised PI for Kentucky in response to the revised 590 Standard. Important revisions to the Kentucky PI include (1) use of a component formulation, (2) incorporation of erosion and P application rates, (3) use of continuous variables, and (4) use of empirically based weighting factors. The revised Kentucky PI was evaluated against measured P loss data reported in the literature. Output from the revised PI was well correlated (Spearman's ρ = 0.86; p < 0.001) with the measured P loss data. Results also indicated that the revised Kentucky PI correctly assigned the appropriate risk category to the majority of fields with P loss values below or above our predefined cutoff values for low and high risk fields. On the other hand, the revised PI only correctly categorized 43% of the fields deemed to be at moderate risk. To assess whether the revised PI provided improved estimates of P loss risk, output from both the original and revised Kentucky PIs was compared against a P loss data set collected in the southern United States. Both the original (after modifications to soil test P [STP] and runoff ratings) and revised Kentucky PIs were significantly correlated with the measured P loss data, though the revised PI (Spearman's ρ = 0.92; p < 0.001) was more strongly correlated than the original Kentucky PI (Spearman's ρ = 0.42; p = 0.03). The original and revised Kentucky PIs were also compared by calculating P loss risk for 46 fields in Kentucky using data recently collected for developing comprehensive nutrient management plans. The approach adopted in this study to revise the Kentucky PI should offer guidance to other states currently in the process of revising their PI.