Phosphorus Indices: Why We Need to Take Stock of How We Are Doing

• Published in: Journal of environmental quality 2012-11, Vol.41 (6), p.1711-1719
• Main Authors: Sharpley, Andrew, Beegle, Doug, Bolster, Carl, Good, Laura, Joern, Brad, Ketterings, Quirine, Lory, John, Mikkelsen, Rob, Osmond, Deanna, Vadas, Peter
• Format: Article
• Language: English
• Subjects: animal manure management; chemistry; Conservation of Natural Resources; Ecosystem; Environmental Monitoring; Environmental Monitoring - methods; Environmental Pollutants; Environmental Pollutants - chemistry; Environmental protection; Geological Phenomena; guidelines; hydrologic models; Manures; methods; Natural resources; Natural Resources Conservation Service; nutrient management; Phosphorus; Phosphorus - chemistry; Resource conservation; risk; risk estimate; Risk Factors; Soil; Soil - chemistry; soil analysis; Soil testing; standards and grades; Surface water; United States; Water; Water - chemistry; Water Movements; Water quality
• ISSN: 0047-2425; 1537-2537
• DOI: 10.2134/jeq2012.0040

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.