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Phosphorus indices: why we need to take stock of how we are doing
- Sharpley, Andrew H., Beegle, Doug, Bolster, Carl, Good, Laura, Joern, Brad, Ketterings, Quirine, Lory, John, Mikkelsen, Rob, Osmond, Deanna, Vadas, Peter
- Journal of Environmental Quality 2012 v.41 no.6 pp. 1711
- Natural Resources Conservation Service, animal manure management, guidelines, hydrologic models, nutrient management, phosphorus, risk, risk estimate, soil analysis, standards and grades, surface water, water quality, United States
- Many states have invested significant resources to identify components of their Phosphorus (P) Index that reliably estimate the relative risk of P loss and incentivize conservation management. However, differences in management recommendations and manure application guidelines for similar field conditions among state P Indices, coupled with minimal reductions in the extent of P-impaired surface waters and soil test P (STP) levels, led the U.S. Natural Resources Conservation Service (NRCS) to revise the 590 Nutrient Management Standard. In preparation for this revision, NRCS requested that a review of the scientific underpinnings and accuracy of current P Indices be undertaken. They also sought to standardize the interpretation and management implications of P Indices, including establishment of ratings above which P applications should be curtailed. Although some states have initiated STP thresholds above which no application of P is allowed, STP alone cannot define a site’s risk of P loss. Phosphorus Indices are intended to account for all of the major factors leading to P loss. A rigorous evaluation of P Indices is needed to determine if they are directionally and magnitudinally correct. Although use of observed P loss data under various management scenarios is ideal, such data are spatially and temporally limited. Alternatively, the use of a locally validated water quality model that has been shown to provide accurate estimates of P loss may be the most expedient option to conduct Index assessments in the short time required by the newly revised 590 Standard.