Accelerated deployment of an agricultural nutrient management tool: the Maryland Phosphorus Site Index.

In 1998, the Maryland legislature mandated nitrogen (N) and phosphorus (P) nutrient management planning for nearly all of Maryland's commercial agricultural operations. State regulations required that a phosphorus indexing tool (P Index) be used for determining the potential for P losses from agricultural land, even though a reliable P Index did not exist. The development and assessment of the P Index as a dependable tool for the evaluation of the potential for P losses was constrained by a very aggressive implementation schedule imposed by state regulations. The Maryland Phosphorus Site Index (PSI) was evaluated on 646 state-representative field sites beginning in the spring of 1999 and continuing through the spring of 2000. Of the representative fields, 69% were determined to have a "low" P loss rating, 19% were in the "medium" P loss rating category, 8% were determined to be a "high" risk for P loss, and 4% rated as "very high" P loss potential. Fifty-five percent of the fields evaluated had soil test phosphorus (STP) levels less than the 75 mg kg-1 Mehlich-1 P environmental threshold established by state regulations. The frequency distribution of PSI performance was evaluated for several subcategories of the statewide data set. The Maryland PSI will be deployed for use in constructing farm nutrient management plans well before its predictive capabilities can be objectively and rigorously validated. Field validation is essential. In the meantime, the Maryland PSI should function adequately as a tool to assist in the prioritization of field P loss risk potential.