Phosphorus Indices.

A Phosphorus (P) Index is an applied assessment tool used to identify agricultural fields most vulnerable to P loss by accounting for the major source and transport factors controlling P movement.  The Index was proposed as a voluntary educational tool to help farmers identify fields with a high probability of P loss in runoff.  In the two decades since its introduction, the P Indexing concept has evolved and expanded and there are now P Indices that serve as Best Management Practice (BMP) selection and targeting tools, manure application scheduling tools, manure application rate calculators, and the P Index has been adopted as a regulatory tool in many states.  In addition, the P Index has been integrated into the Natural Resources Conservation Service (NRCS) 590 Nutrient Management Standard since 1999 and currently, 48 U.S. states have adopted a P Index as a site assessment tool to identify critical source areas and to target practices to reduce P loss, and in most of these states the P Index is required by the NRCS 590 Nutrient Management Standard and other state and federal programs.  The development, evolution, and current status of the P-Indexing framework will be discussed.