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ARS Home » Midwest Area » Bowling Green, Kentucky » Food Animal Environmental Systems Research » Research » Publications at this Location » Publication #313419
Title: Phosphorus Indices: What is the water quality goal?

Author
item OSMOND, DEANNA - North Carolina State University
item Bolster, Carl
item CABRERA, MIGUEL - University Of Georgia
item FEAGLEY, SAM - Texas A&M University
item HAGGARD, BRIAN - University Of Arkansas
item MITCHELL, CHARLES - Auburn University
item MYLAVARAPU, RAO - University Of Florida
item OLDHAM, LARRY - Mississippi State University
item SHARPLEY, ANDREW - University Of Arkansas
item WALKER, FORBES - University Of Tennessee
item ZHANG, HAILIN - Oklahoma State University

Submitted to: Waste to Worth Conference
Publication Type: Proceedings
Publication Acceptance Date: 2/10/2014
Publication Date: 4/2/2015
Citation: Osmond, D., Bolster, C.H., Cabrera, M., Feagley, S., Haggard, B., Mitchell, C., Mylavarapu, R., Oldham, L., Sharpley, A., Walker, F., Zhang, H. 2015. Phosphorus Indices: What is the water quality goal?. Waste to Worth Conference. http://articles.extension.org/pages/72818/phosphorus-indices:-what-is-the-water-quality-goal.

Interpretive Summary: In this study we used water quality data and land use information from field-scale experiments to evaluate the accuracy of phosphorus risk assessment tools used throughout the South. Measured P loads and land use/management information such as the amount and timing of P applied as fertilizer and/or manure and tillage, as well as site characteristics such as rainfall, soil series, and crop or forage management were collected from the following states: AR, GA, MS, NC, OK, and TX. This information was used to run each southern P Index. We then compared the ratings from each state’s phosphorus index and compared them against the total and soluble P loads that were measured from each study site. In order to compare load losses with qualitative P indices, measured total P loads were transformed based on USDA-NRCS tentative guidelines of Low (0-2 lb P/ac), Medium (2-5 lb P/ac), and High (>5 lb P/ac) P loss. Not surprisingly, there were some notable differences between state-P Indices for the same set of data, but there was often considerable uniformity. However, what was less clear is what the P-Index ratings mean for water quality protection. The analysis left us with many difficult questions on how to relate edge-of-field P loss to more complex water quality criteria and thresholds.

Technical Abstract: We have collected water quality and land use data from plot- and field-scale studies throughout the South (AR, GA, MS, NC, OK, and TX). The water quality data provide information on runoff and P concentrations and loads. Land use data provide information on management practices, including the amount and timing of P applied as fertilizer and/or manure and tillage, as well as site characteristics such as rainfall, soil series, and crop or forage management. This information was used to run each southern P Index. Four of the indices are considered component, in that the rating is in lbs P/ac/year. The remaining eight P Indices are either additive or multiplicative and final ratings are qualitative. We then compared the state ratings against each other and against the total and soluble P loads that were measured from each study site. In order to compare load losses with qualitative P indices, measured total P loads were transformed based on USDA-NRCS tentative guidelines of Low (0-2 lb P/ac), Medium (2-5 lb P/ac), and High (>5 lb P/ac) P loss. When we compared the data, there were expected differences between state-P Indices for the same set of data, but there was often considerable uniformity. However, what was less clear is what the P-Index ratings mean for water quality protection. The analysis left us with many difficult questions on how to relate edge-of-field P loss to more complex in-stream or lake P criteria and thresholds.