The P Index was proposed as a nutrient management tool in 1992 and has been implemented as such for the past decade. However, lack of water quality improvement in agricultural watersheds and discrepancies in P loss ratings between P indices have raised questions about continued use of the P Index. In response to these concerns, a symposium was held as part of the 2011 ASA, CSSA, SSSA annual meetings. This symposium produced a special collection of seven papers describing the role of P indices in P management, evaluation of P indices, new models for assessing P loss, methods to improve P indices, and changes in producer behavior resulting from P Index use. The objectives of this introductory paper are to provide background on the P Index concept, overviews of the special collection papers, and recommendations for future P Index evaluation and development research. The papers in this special collection conclude that P indices can provide accurate assessments of P loss but must be evaluated appropriately. Evaluation will require compiling large regional P loss datasets at field and small watershed scales. Simulation models may be used to generate P loss estimates; however, models must be calibrated and validated to ensure their accuracy. Further development of P indices will require coordinated regional efforts to identify common P Index frameworks and standardized interpretations. Stringent P Index evaluations will expand the utility of P indices for critical source area identification and strategic best management practice implementation by regulatory, education, and scientific communities alike.
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