Applicability and Evaluation of Pennsylvania Phosphorus Index for Effluent Irrigated Cropfields.

Phosphorus (P) indices are field evaluation tools for assessing P loss from agricultural landscapes amended with manures and fertilizers. Because off-site P migration is differentially impacted by the P source, this study was undertaken to evaluate the suitability of the Pennsylvania P index for fields irrigated with P-containing municipal wastewater effluent for 26 yr at Penn State's Living Filter (LF) system.  Annual loadings of 217 kg P₂O₅ ha^-1 from year-round application of effluent containing 3.5 mg P L^-1 at 2 in wk^-1 (maximum regulatory rate) were used to evaluate index rating for cropped fields as a “worst case” condition. Based on the index application factor (AF), two different ratings were determined: (1) maximum AF (1.0) throughout the year, (2) maximum AF for frozen months (Dec, Jan, and Feb) and AF = 0.4 (corresponding to incorporation within 1 wk) for the remaining months. For the first situation, the P loss ratings of all the cropfields were “high” or “very high” which would, respectively, restrict effluent application to P-based rates or prohibit it altogether. Using different AF values for frozen and unfrozen conditions, only fields located within 61 m (200 ft) of the receiving tributaries had “high” P loss ratings (P-based application). Index evaluation for actual conditions (irrigation restricted to non-frozen months with an annual application of 130 kg P₂O₅ ha^-1) results in “low” and “medium” P loss ratings for most fields, suggesting that current effluent practices will be sustainable under P-based nutrient management. We conclude that typical P-indexing algorithms need to be modified to fairly evaluate wastewater irrigation sites where effluent runoff is prohibited by law and applied soluble P is effectively incorporated below the surface with the percolating effluent.