DEVELOPMENT OF PHOSPHORUS INDICES FOR NUTRIENT MANAGEMENT PLANNING STRATEGIES IN THE U.S.

Authors: Sharpley, Andrew; Weld, Jennifer; BEEGLE, DOUG - PENN. STATE UNIV.; Kleinman, Peter; Gburek, William; Moore, Philip; MULLINS, GREG - VA. POLYTECH. UNIV.

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2003
Publication Date: 7/20/2003
Citation: Sharpley, A.N., Weld, J.L., Beegle, D., Kleinman, P.J., Gburek, W.J., Moore, Jr., P.A., Mullins, G. 2003. Development of phosphorus indices for nutrient management planning strategies in the U.S. Journal of Soil and Water Conservation. 58(3):137-152.

Interpretive Summary: Phosphorus, an essential nutrient for crop and livestock production, can accelerate freshwater eutrophication, one of the leading contributions to water quality impairment in the U.S. In response to these impairments, the U.S. Department of Agriculture and the U.S. Environmental Protection Agency jointly issued a new nutrient management policy, addressing nitrogen and phosphorus, which each state must enact by 2008. The strategy outlined three management approaches managing phosphorus based upon: (1) agronomic soil phosphorus thresholds, (2) environmental soil phosphorus thresholds, (3) a Phosphorus Index that ranks fields according to their vulnerability to potential phosphorus loss. Forty-seven states have chosen to adopt the Index to address phosphorus management. The widespread adoption of the Phosphorus Index reflects its dynamic and flexible structure, which can be modified to regional conditions and state requirements. A summary of the national development of the Phosphorus index demonstrates that modifications extend beyond the inclusion of unique source or transport factors to the incorporation of different mathematical approaches to calculate the final Index value. Despite the numerous modifications that have been made to the Phosphorus Index, the original intent to educate farmers and advisors that careful manure application timing, riparian buffer establishment, and animal ration development can not only reduce the potential for phosphorus loss, but provide flexible options to meet new guidelines to protect water resources.

Technical Abstract: Phosphorus, an essential nutrient for crop and livestock production, can accelerate freshwater eutrophication, now one of the leading water quality impairments in the U.S. In response, the U.S. Department of Agriculture and Environmental Protection Agency proposed a new nutrient management policy, now addressing P as well as N, which each state must enact by 2008. Of the three P-based approaches, agronomic soil test P (STP) recommendations, environmental STP thresholds, and a P Index to rank fields according to their vulnerability to potential P loss, the P Indexing approach has been most widely adopted with 47 states using this approach to target nutrient management. This paper charts the development of the indexing approach, which ranks site vulnerability to P loss by accounting for source (STP, 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 Al), conservation practices, and priority of receiving waters. Computation of the final Index value is additive in 20 states, while 17 multiply source and transport factors to define critical source areas. Most states (44) 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. Finally, the fact that there are many versions of the P Index now in use demonstrates the robustness and flexibility of the Indexing framework to better target remedial measures.