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Phosphorus Management: An Analysis of the Virginia Phosphorus Index

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  032156,  2003 ASAE Annual Meeting . (doi: 10.13031/2013.14021) @2003
Authors:   Julie Jesiek, Mary Leigh Wolfe
Keywords:   phosphorus management, sensitivity analysis, expert opinion survey

Excess phosphorus (P) that is transported into water bodies can cause water quality problems. A high potential for P delivery occurs when there is a high transport potential from erosion, runoff, and/or leaching coupled with high soil test P and/or high rate of fertilizer P application. Identifying those fields that have a high transport and source potential to deliver P to surface water is the first step in reducing water quality problems due to excess P. The Virginia PIndex is a mass-based tool that estimates the annual risk of delivery of P from a given field to surface water. Guidelines on P application rates are based on the level of risk. Research and testing are needed to determine the dependability and validity of this recently developed index. The overall goal of the research was to contribute to the continued development of the Virginia P-Index as an effective P management tool.

A sensitivity analysis was completed to identify the parameters to which the P-Index was most sensitive under a range of conditions. In low erosion and runoff conditions, the P-Index was most sensitive to P management factors including application rate. As erosion and runoff potential increased, the P-Index was most sensitive to the erosion risk factors including soil loss. Under conditions with subsurface leaching, the P-Index was most sensitive to the subsurface leaching factors and Mehlich I soil test P. A stochastic analysis was also conducted to determine the effects of parameter variability as a function of input parameter variability on P-Index values and subsequent application recommendations. Variability of the P-Index output was greater as the risk of P delivery increased.

A survey was completed to determine expert opinion as to the appropriateness of parameter estimation methods used in the Virginia P-Index. Thirty-eight survey participants from within and outside of Virginia Indicated that all factor estimation methods were appropriate, given the state of the science. Estimation methods for the following factors were determined to be less appropriate than the other sub-factors by the survey respondents: soil texture/drainage class, subsurface dissolved reactive orthophosphate (DRP), runoff delivery, and sediment delivery. Recommendations for improvement were identified including a need for additional analysis and studies.

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