A New and More Intuitive Structure for the New York Phosphorus Index.

Phosphorus indices (PI) are evaluated to ensure they rank fields based on their relative risk of loss of phosphorus (P), and encourage best management practices (BMPs) to reduce this risk. The current New York PI (NY-PI) includes a dissolved (DPI) and a particulate (PPI) index. Both indices have a common source score (consisting of soil test P [STP], manure, and fertilizer) multiplied by a transport score composed of flow distance, flooding frequency, and drainage class (for the DPI), or erosion and concentrated flow (for the PPI). If the current NY-PI score of a field exceeds 100, no P may be applied. Results from previous studies and feedback from nutrient management planners suggested opportunities for improvement. Here, we propose an alternative structure for the NY-PI: (1) the PI score reflects the inherent P transport risk of each field, (2) STP is used as a screening tool, and (3) the final PI score determines the P application rate (N-based, P-based, or no applications). Fields with very high PI scores (>100) may not receive P, regardless of the STP. Fields with high PIs may only receive P-based rates. Fields with medium PIs may receive N- or P-based rates (depending on the STP), while fields with low PIs may receive manure at N-based rates. After fields are ranked by their inherent P transport risk, implementation of BMPs reduce the field’s PI score. The PI scores can be reduced if (1) P is applied to ground-covered soils, (2) P is injected, incorporated, or surface-applied with setbacks, (3) concentrated flows are treated, and (4) buffers are established. The transport risk factors are the same as in the current NY-PI, but the coefficients for each factor were chosen to encourage that P could only be applied to fields with a high transport risk if BMPs are implemented.