Development of phosphorus indices for nutrient management planning strategies in the United States

ABSTRACT: 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 charts the development of the indexing approach, which ranks site vulnerability to P loss by accounting for source (soil test P, fertilizer, and manure management) and transport factors (erosion, runoff, leaching, and connectivity to a stream channel) and outlines modifications made among states that reflect local conditions and policy. Additional factors include flooding frequency, STP modifiers (texture, pH, P sorption, reactive aluminum [Al]), conservation practices, and priority of receiving waters. While computation of the final index value is additive in 20 states, 17 multiply source and transport factors to define critical source areas. Most states (47) have maintained the original indexing approach of assessing site vulnerability to P loss, with indices in three states quantifying P loss. We demonstrate using three management scenarios (changing the time of applied manure, riparian buffer establishment, and reduced feed P ration) that overall P index ratings can be decreased, giving farmers more options for manure management than by simply reducing application rates.