Applicability of phosphorus site indices to wastewater effluent-irrigated cropland

Phosphorus (P) indices, originally developed to assess the risk of P export to aquatic systems from agricultural fields receiving inorganic fertilizers and livestock manures, are being used for other P sources with different characteristics and methods of application. This study determined Pennsylvania P index scores for eleven crop fields of Penn State University's Living Filter system receiving 130 kg P₂O₅ ha^-1 (115 lb P₂O₅ ac^-1) in 2008 via sprinkler irrigation of municipal wastewater effluent. A baseline assessment of P index treated effluent irrigation as surface application of completely soluble P source material with maximum potential of runoff P loss. Three fields located ≤22 m (200 ft) from streams ranked high and very high in P loss rating under baseline assessment. Modified assessments involved changing the application method multiplier and P source coefficient to more accurately reflect the year-round intermittent application of P via sprinkler irrigation. All fields were ranked as having low risk of P export using the modified parameters. Site monitoring data from nearly 50 years of effluent irrigation show no evidence of excess P transport to groundwater or off-site surface water, suggesting that baseline assessment might be overestimating P loss rating of some fields. Results underscore the need to tailor P indexing tools to address the differential risk of P loss for specific P sources and application methods.