STATUS OF INORGANIC PHOSPHORUS IN SOILS IRRIGATED WITH MUNICIPAL WASTEWATER1

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Abstract
The effects of wastewater irrigation on the status of soil P were assessed. The influence of wastewater irrigation on changes in soil organic C, N and P and the sorption of Pi were evaluated. 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 were irrigated with secondary effluent for 11-12 yr. Soil analyses indicated that the majority of P added to soils during wastewater irrigation remained in the upper 30 cm of the Hublersburg clay loam soil. In contrast, appreciable amounts of Pi had leached to the 30-60 cm depth in the Morrison sandy loam. The equilibrium P concentration (EPC) increased markedly at the 0-15 cm depth as a result of wastewater irrigation, due to sorption of Pi added in the wastewater. As expected, the maximum Pi sorption capacity of soils was decreased by wastewater irrigation. A comparison of soluble Pi levels in leachate samples collected in the field at a 120 cm depth with soluble Pi predicted by EPC measurements indicated that laboratory measurements overestimate the ability of a soil to remove Pi from wastewater. Thus, wastewater may not interact with all potentially available Pi sorption sites in soils. The EPC and Pi 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.

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