A Review of the Pennsylvania Phosphorus Index ... - PennFuture

estimating available manure P using a WEP test is strong but there is evidence that the currentanalytical procedure could be improved.The real issue with agricultural P management based upon a P-Index system is at what level theallowable P losses from farm fields are set within each P-Index ranking. The model upon whichthe P-Index is based seems scientifically sound. It estimates the quantity of available P andmoves that P across the landscape. However, the model end point setting of allowable P loss isindependent of the model structure. The determination of allowable P loss from farm fields is apolicy decision. However, this critical decision has instead been made by the P-Index modeldesigners. A policy discussion about the appropriate level of allowable farm field P loss shouldbe held by policy makers with stakeholder input. However, that discussion needs to be informedby the knowledge of how the aggregate loading of P from farm fields will be affected by theallowable farm field P loss. Currently, it is not known how much P would be loaded into awatershed from agricultural fields by the current P-Index ranking structure.Furthermore, the scientific basis of the current P-Index ranking index is based upon sourceinput variables for manure P availability using Book value PSC’s or only adjusting manure Pavailability by loss factors for application and timing. The relationship between P-Indexrankings and field edge P losses needs to be updated with current manure PSC values. It may beargued that the Book value PSC’s overestimate manure P availability but the scientific basis forthe current P-Index ranking incorporates those overestimates in the formulation of itsrelationship between PI ranking and field edge P loss. If the availability of manure P is to becorrected with WEP analyses, the relationship of the P-Index ranking to field edge P loss alsoneeds to be corrected.Recommendations1. Model the P load from farmlands in aggregate across watersheds based upon current andalternate P-Index ranking thresholds.o The allowable P loss inherent in the P-Index rankings is a policy decision that isburied within the P-Index model structure. Information is needed on the impactof setting the allowable P loss at various levels before an informed policy decisioncan be made.o The research supporting the current P-Index ranking thresholds seems to bebased upon estimates of manure P availability that are higher than likely beingused today. Therefore the relationship between manure as a source of availableP, and the P runoff from farm field edge needs to be reexamined.2. Pennsylvania should collect nutrient management information in a centralized location(with protections in place for confidential business information, if needed) so acres offarmland with P restrictions can be determined. This would also provide informationabout how much P was being applied by these farms, the values of the WEPmeasurementbased PSC’s, and what activities are being employed to manage manuredifferently to avoid P loss.3. All farms applying manure should calculate a P-Index value. All P-Index informationshould be gathered at a centralized location to allow analysis.Page 13