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. A management tool is needed to identify those fields that have a high transport and source potential to deliver P to surface water. The Virginia P-Index 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 then given based on the level of risk. This is a new tool and additional research and testing are needed to determine the dependability and validity of the 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. Variability of the P-Index output was greater as the risk of P delivery increased and this could affect management recommendations.
A survey was completed to determine expert opinion as to the appropriateness of parameter estimation methods used in the Virginia P-Index. Thirty-eight surveys were returned, representing a diverse range of participants within and outside of Virginia. Comments from the respondents were used to evaluate the appropriateness of the parameter methods. All factors were determined to be 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. The Virginia P-Index was determined to be a well thought out management tool and implementation should identify fields with the greatest risk of P delivery to surface water. Recommendations for improvement were identified including a need for additional analysis and studies.
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