The U.S. Department of Agriculture's Natural Resource Conservation Service (USDA-NRCS) is currently revising its 590 Nutrient Management Conservation Standard. As part of this revision, USDA-NRCS is considering requiring states to test the accuracy of their phosphorus (P) indices using either measured P loss data or simulated P loss data generated from process-based models. The objective of this study was to critically evaluate the KY P index by comparing index output with simulated P loss data obtained from a validated P loss model. Furthermore, the general formulation of the index was evaluated against current research on the processes controlling P transport in the environment. Results suggested that in some areas the index does a good job in assigning P loss risk; however, this analysis also showed some important deficiencies in the index, primarily the neglect of important factors known to affect P loss (e.g., soil erosion and P application rates) and how the different factors in the index are weighted. To reduce the amount of P that is exported from agricultural fields to waterways within Kentucky, resources should be devoted to revising the KY P index to address these limitations as well as developing long-term monitoring sites where the P index and more process-based models can be evaluated against measured P loss data.