Phosphorus Indices: What is the water quality goal?

Phosphorus indices provide relative loss ratings that then have a corresponding management response.  Because most state Phosphorus Indices are qualitative it is not clear how the relative loss rating corresponds to actual phosphorus inputs into the receiving water and how the receiving water would react to these additions.  Even with qualitative Phosphorus Indices, unless the water resource has a specific Total Maximum Daily Load, it is not clear how losses correspond to water quality outcomes.  These issues will be discussed in the context of the 590 Natural Resources Conservation Standard for nutrient management.

Why Examine the Phosphorus Index?

The purpose of our work was to determine, within the southern region (AL, AR, FL, GA, KY, LA, MS, NC, OK, SC, TN, and TX), the relationship between state P-Index ratings to measured water quality P losses, and each other.

What did we do? 

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.

What have we learned?            

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.

Future Plans 

To answer these questions, we are going to run state P Indices in different modes: against annual water quality and land treatment data; against averaged water quality and land treatment data; using erosion rates from sediment generated from the experiment, and; using erosion rates using RUSLE2. We will compare these P Index ratings against each other, the water quality data, USDA-NRCS ratings, and EPA ecosystem nutrient criteria, to help us better understand the relative value of P Indices in protecting water resources.


Deanna Osmond, Department Extension Leader, NC State University Soil Science Department

C. Bolster, M. Cabrera, S. Feagley, B. Haggard, C. Mitchell, R. Mylavarapu, L. Oldham, A. Sharpley, F. Walker, and H. Zhang


Thanks to our sponsor, USDA-NRCS grant 69-3A75-12-182.

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