STATUS OF INORGANIC PHOSPHORUS IN SOILS IRRIGATED WITH MUNICIPAL WASTEWATER1

Abstract

A cooperative study was conducted with Pennsylvania State University to assess the effects of wastewater irrigation on the status of soil phosphorus. The major objectives of the research were to evaluate the influence of wastewater irrigation on changes in soil organic carbon, nitrogen, and phosphorus and the sorption of orthophosphate (P). Soil samples were obtained in 1974 and 1975 from Hublersburg silty clay loam cropped to corn (Zea mays L.), Reed canarygrass (Phalaris arundinaceae L.), or a mixed forest and from a Morrison sandy loam with a mixed forest stand. The sites had been irrigated with secondary effluent for 11 to 12 years. Soil analyses indicated that the majority of phosphorus added to soils during wastewater irrigation remained in the upper 30 cm of the Hublersburg clay loam soil. In contrast, appreciable amounts of P had leached to the 30 to 60-cm depth in the Morrison sandy loam. The equilibrium phosphate concentration (EPC) increased markedly at the 0 to 15-cm depth as a result of wastewater irrigation, due to sorption of P added in the wastewater. As expected, the maximum P sorption capacity of soils was decreased by wastewater irrigation. A comparison of soluble P levels in leachate samples collected in the field at a 120-cm depth with soluble P predicted by EPC measurements indicated that laboratory measurements overestimate the ability of a soil to remove P from wastewater. Thus, wastewater may not interact with all potentially available P sorption sites in soils. The EPC and P sorption capacity of soils irrigated with wastewater was reduced by extracting soils with oxalate and citrate-dithionite-bicarbonate. These results indicated the significance of hydrous iron oxides in P retention by soils irrigated with wastewater.