Evaluation of the New York Phosphorus Index.

The New York Phosphorus Index (NY-PI), developed in 2001, includes a dissolved (DPI) and a particulate (PPI) index. Both indices have a common source score that is multiplied by a transport score that ranges from 0 to 1. If the NY-PI score of a field exceeds 100, no manure or fertilizer P may be applied. In 2014, nutrient management planners were asked for feedback on the NY-PI. The planners suggested to not drastically change the format of the NY-PI but to update the weight of some input factors and add a credit system for setbacks, buffers, and cover crop use on corn fields. The present study evaluated the main drivers of the current NY-PI and explored how updating the weight of factors (as suggested by planners) would impact the NY-PI. A database of 33,327 individual farmer’s fields across 40 NY counties was collected. Of all fields, 87, 8, 3, and 2% were classified as low, medium, high, or very high risk for runoff, respectively. The DPI tended to exceed the PPI, consistent with saturation excess as the dominant mechanism for runoff generation in NY. The source score (particularly soil test P) was the main driver of the NY-PI, while the transport score was mainly driven by the flow distance to the stream. Increasing the weights of manure application rate, timing, and method of application greatly impacted the distribution of fields across the NY-PI categories. In contrast, updating various transport factors only marginally impacted the final NY-PI score if the transport score was not allowed to exceed 1.0. Future work includes alternative structures of the current NY-PI, taking into account planners’ feedback and results of water quality monitoring.