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Phosphorus (P), an essential nutrient for crop and livestock production, can accelerate freshwater eutrophication, now one of the leading water quality impairments in the United States. In response, the U.S. Department of Agriculture and the Environmental Protection Agency proposed a new nutrient management policy, now addressing P as well as nitrogen (N), which each state must enact by 2008. There are three approaches that address P—agronomic soil test P recommendations, environmental soil test P thresholds, and a P index to rank fields according to their vulnerability to potential P loss. There are many versions of the P index now in use, demonstrating the robustness and flexibility of the indexing framework to better target remedial measures. Of the three P-based approaches, the P indexing approach has been most widely adopted with 47 states using this approach to target P management. This paper …