Revising the Maryland P Index: Direction and Magnitude of Changes.

The Maryland Phosphorus Site Index (PSI) has been used as a tool to evaluate potential for phosphorus (P) loss from agricultural fields in Maryland since the adoption of the Water Quality Improvement Act of 1998.  This act required agricultural operations to institute nitrogen and P nutrient management planning. The final PSI scores categorize fields into four loss categories (low, medium, high, very high) with different P management practices associated with each category.  The PSI has been used in Maryland for the past twelve years with only one previous validation study attempt, based on a dataset of PSI final scores collected from across Maryland. From that study, and the trend of scores throughout the state, revisions to PSI were proposed. Proposed changes include a calculation of the potential for P loss through possible loss pathways then summing the potential loss from these pathways. The current study assessed 391 fields state-wide, where soils samples were collected, slope steepness and length, and distance to surface water were measured, and nutrient management data were collected.  Both the current and the proposed revised PSI were calculated for all fields sampled. More fields had higher numerical PSI scores with the new index, however 287 fields remained in the same loss category. Eighty fields had scores which increased categorically while 24 fields had scores that decreased from a higher to lower loss category. Most fields had scores that changed marginally (difference of approximately 25 points),  however there were some fields with a score difference of over 100 points between the two indices. The new PSI may be better at evaluating the movement of P, as seen by the increase in scores, by identifying the specific pathways by which P moves to water. Further research will investigate why some fields had a large score difference between the indices.