The U.S. Department of Agriculture’s Natural Resource Conservation Service (USDA-NRCS) has recently revised its 590 Nutrient Management Conservation Standard. As part of this revision, USDA-NRCS is requiring states to test the accuracy of their phosphorus (P) index using either measured P loss data or simulated P loss data generated from process-based models. With this in mind, the objective of this study was to critically evaluate the KY P index by comparing index output with simulated P loss data generated using the Annual P Loss Estimator (APLE) model. Furthermore, the general formulation of the index was evaluated against current research on the processes controlling P transport in the environment. Results suggest 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.