Decomposition of Sewage Sludge Compost in Soil: II. Phosphorus and Sulfur Transformations

Laboratory incubation studies were conducted in which sewage sludge compost was mixed with three soils (Evesboro loamy sand, Christiana silty clay loam, and Fauquier silt loam) and a sand at rates of 0, 2, 4, and 6% (0, 44.8, 89.6, and 134.4 metric tons/ha, respectively) of the dry weight. The mixtures were incubated at 22°C under a flow of CO2‐ and NH3‐ free air. Extractable P in the compost‐sand mixtures ranged from 48 to 81% of the total P. The Evesboro‐compost‐sand mixtures showed an initial net P mineralization trend followed by a slight immobilization trend at 54 days. The Christiana‐compost mixtures had less extractable P than the Evesboro‐compost mixtures, but did show initial increase in extractable P followed by a decrease during incubation. The iron and aluminum components in the Christiana soil were probably responsible for net immobilization of extractable P. The Fauquier‐compost mixtures had the least extractable P and the highest free iron oxide content. Contrary to the other soils, the Fauquier‐compost mixture showed initial net immobilization of extractable P followed by mineralization at 54 days. Ammonium acetate extractable S in the sand‐compost mixtures was 8 to 11% of the total S. In the Evesboro‐compost mixtures extractable S increased after 54 days incubation. The Christiana‐compost mixtures immobilized extractable S early in the incubation but net mineralization of S was subsequently evident where the soil was amended with 4 and 6% compost. The Fauquier‐compost mixture had the least extractable S and had immobilization‐mineralization trends similar to the extractable P data. Both extractable P and S are in sufficient quantities at the 44.8 metric tons/ha application rate to sustain plant growth and, therefore, sewage sludge compost could be used to correct P or S deficiencies in most soils.