Proposed Revisions to the Ontario P Index.

The P Index has been part of the NMAN nutrient management software program in Ontario since the late 1990s.  It was modified from the Maryland P Index, following an additive model, and the output was restrictions on P application rates and setbacks from surface water.  Since that time, the understanding of mechanisms for P loss have advanced significantly, which led to a review of the current P Index to create a more robust and accurate tool to identify areas of greatest risk for P loss to surface water.  The proposed revisions include accounting for dissolved as well as particulate P losses, the impact of application method and timing on losses from applied P, and the net impact of tile drains on P movement from agricultural land.  A component model is used to facilitate prediction of losses from various sources and through various pathways with the correct relative weighting.