Phosphorus Indices and the 590 Revision: Why We Need to Take Stock of How We Are Doing.

Since its inception nearly 20 years ago, the phosphorus (P) Index has morphed from an educational tool to a Best Management Practice targeting and implementation tool, a manure-scheduling tool, and in many cases, a regulatory tool.  A great deal of research has been conducted across the U.S. to derive, validate, and support components of the P Indexing concept, particularly those related to source factors.  As different versions of the P Index have emerged, ostensibly to account for local topography, hydrology, soils, land use, and individual state policies and agendas, so too have differences in the P management recommendations that are made using the P Index.  As a result, there are many variations in P Indices now in use as part of the NRCS 590 Nutrient Management Conservation Standard.  This variation is both a strength and weakness of the P Indexing concept.  However, the inconsistency among P Indices in terms of level of detail and scientific underpinnings among states, as well as in recommendations and interpretations based on site risk, prompted a review and possible revision of the 590 Standard and P-Indexing approach.  The need for revision has been heightened by a slower than expected decrease in P-related water quality impairment and, in some cases, an increase in soil P to levels several fold greater than agronomic optimum due to the inability of the P Index to prevent the continued over-application of P to soils.  While the basic scientific foundations of the P-Indexing approach are sound, these concerns are real.  In this presentation, we propose the use of lower and upper boundaries of P Index use and describe an approach to evaluate individual State P Indices.