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

overestimation is incorporated into the establishment of the P-Index ranking thresholds atcertain quantities of allowable P loss. When WEP-based PSCs are used, the P-Indices that resultare lower than those calculated using Book value P-Indices, all else being equal, however, theactual P loss would not have changed. Hence, the amount of P loss across the P-Index rankingswould be higher than that modeled by Sharpley et al (2001).While it seems appropriate that construction of the P-Index model be undertaken byagricultural technical experts, the setting of the allowable P loss from the farm field edge is apolicy decision, beyond the technical modeling of P delivery. Beegle (pers. comm.) suggestedthat the P-Index model thresholds were set with reference to the amount of P allowed to bedischarged by waste water treatment plants with strict effluent limits. Further, Beegle offeredthat the P-Index was designed to identify risk levels of P runoff not specific loss rates of P.However, it is appropriate to ask whether setting farm field runoff at a P concentrationequivalent to waste water treatment plant effluent is appropriate? (Given the use of WEPmeasurementbased PSC’s, the P losses could be even higher.) The answer to this question maybe best addressed by trying to better understand the impact of setting the P-Index rankingthresholds at certain levels of P runoff. What would be the loading impact if all the farms thathave P-Indices which require consideration of phosphorus loss, have P runoff at 200 or 350 gP/ha? The Sharpley et al (2001) model also assumes some specific volume of runoff from farms;what is that volume and how does that value influence the estimate of loss of P from farms? Toallow a better assessment of the impacts of P loading from manure application, the loss of Pfrom farm fields using WEP-measurement based PSC’s in aggregate across a watershed shouldbe modeled.Pennsylvania changed the variables by which it allows nutrient managers to determineappropriate P management but there has been no coordinated effort to monitor how this changehas affected actual P management. The Commonwealth needs to collect data on P management,P-Indices and PSC’s so it can evaluate how the current P-Index management approach isworking. Models relating P-Index rankings to P runoff need to be recalculated using the newparameters. Furthermore, targeted watershed monitoring of actual farm field P loss andtransport in rivers based upon the current management strategies should be undertaken.Over the long term, the current approach to agricultural P management is unsustainable. MoreP is being imported into counties with high livestock and poultry densities within feed grainsthan is being exported as agricultural products (Gburek et al 2000.) This imbalance means Pwill continue to accumulate in soils and the problem of loss from farm fields will continue to getworse. Attention needs to be given to a long term solution that restores the import/export Pbalance within a watershed.ConclusionsChanges in the P-Index between Version 1 and Version 2 are likely to result in: a) more farmfields assessed for risk of P loss given the addition of 2 conditions to the Part A Screening test,but; b) fewer fields having P application restrictions or having lesser P restrictions as the WEPmeasurementbased PSC’s are likely to lower P-Index rankings. The scientific support forPage 12