EXTRACTABILITY AND SOLUBILITY OF PHOSPHATE IN SOILS AMENDED WITH CHEMICALLY TREATED SEWAGE SLUDGES

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Abstract
Soils were amended with anaerobically digested sewage sludges that resulted from treatment of sewage with Ca(OH)2, Al2(SO4)3, or FeCl3. The sludges were applied annually to field plots situated on a loamy sand (pH 7.3) and a loam (pH 7.4) and cropped to corn (Zea mays L.) or bromegrass (Bromus inermis Leyess) for 7 yr. Extractability of soil P by NaHCO3 was in the order Ca-sludge .mchgt. Fe-sludge .gtoreq. Al-sludge. Equilibrium P concentration was highest in the loam treated with Ca-sludge and in the loamy sand treated with Fe-sludge. The solubility of P in Ca-sludge-amended soil apparently is limited by the precipitation of octacalcium phosphate. The Fe-sludge decreased soil pH, and this increased P solubility. Soil Fe and Al oxide content was increased by sludge application; this increased P retention in soil through sorption and occlusion by the oxides. Calcite in the Ca-sludge-amended soil provided additional sorption sites. Fractionation of soil P showed that just over 50% of inorganic P in the Ca-sluge treatment and about 70% in the Al- and Fe-sludge treatments were Al- plus Fe-P. The relatively high Al- plus Fe-P in the Ca-sludge treatment resulted from the use in 1977 of a sludge treated with Ca and Fe. More P was present as loosely bound P (NH4Cl-extractable) and as a Ca-P in the Ca-sludge treatment than in Al- and Fe-sludge treatments.