Phosphorus Sorption and Availability in Soils Amended with Animal Manures and Sewage Sludge

Soils that receive large applications of animal wastes and sewage sludge are vulnerable to releasing environmentally significant concentrations of dissolved P available to subsurface flow owing to the gradual saturation of the soilʹs P sorption capacity. This study evaluated P sorption (calculated from Langmuir isotherms) and availability of P (as CaCl2–P and resin P) in soils incubated for 20 d with poultry litter, poultry manure, cattle slurry, municipal sewage sludge, or KH2PO4, added on a P-equivalent basis (100 mg P kg-1). All the P sources had a marked negative effect on P sorption and a positive effect on P availability in all soils. In the cattle slurry– and KH2PO4–treated soils, the decreases in P sorption maximum (19–66%) and binding energy (25–89%) were consistently larger than the corresponding decreases (7–41% and 11–30%) in poultry litter–, poultry manure–, and sewage sludge–treated soils. The effects of cattle slurry and KH2PO4 on P availability were, in most cases, larger than those of the other P sources. In the poultry litter, poultry manure, and sewage sludge treatments, the increase in soil solution P was inversely related (R2 = 0.75) to the input of Ca from these relatively high Ca (13.5–42 g kg-1) sources. Correlation analyses implied that the magnitude of the changes in P sorption and availability was not related to the water-extractable P content of the P sources. Future research on the sustainable application of organic wastes to agricultural soils needs to consider the non-P- as well as P-containing components of the waste.