Plant Availability of Phosphorus in Sewage Sludge Compost

Field and greenhouse studies were conducted to compare the effectiveness of Blue Plains sewage sludge compost (BLU), Parkway sewage sludge compost (PAR), and triple superphosphate (TSP) as sources of P for corn. These amendments were applied to a Sassafras sandy loam (Typic Hapludults) in a field study and to the Sassafras soil and a Christiana silty clay loam (Typic Paleudults) in a greenhouse study. First-year application rates in the field study ranged from 0 to 1500 kg total P/ha. Plots were split the second year with one-half receiving no additional P and one-half receiving P equal to 1st-yr rates. Nitrogen as NH₄NO₃ was applied at variable rates to plots so that the total assumed available N equaled 180 and 219 kg N/ha for the 1st and 2nd yr, respectively. Dry matter yield and P uptake were measured at the eight-leaf stage (V8) and at maturity. Below the 100 kg P/ha rate, there were no significant differences in P uptake or yield among the different sources and rates of P. Above the 100 kg P/ha rate, TSP but not the composts, significantly increased growth and P uptake by corn in both field and greenhouse studies. The TSP was approximately four to seven times more effective than either compost in raising tissue P concentration in corn (Zea mays L) ear leaves. A maximum of 2.05 and 11.2% of the P applied as compost and TSP, respectively, was taken up by the corn. Soil P extracted by the dilute double acid (0.5 M HCl and 0.0125 M H₂SO₄) and 0.5 M NaHCO₃ methods increased with increasing P application from all sources. Extractable soil P by both methods correlated well with total P uptake from TSP but did not correlate well with P uptake from the composts. A P fractionation indicated that most of the compost P was associated with Fe and Al and only 2 to 5% was in organic forms. The Fe-bound P fraction of the amended soils increased with increasing P application from all sources. Two factors appeared to control the P uptake response of corn: the treatment of the wastewater to remove P at the sewage treatment plant, and the P fixing capcity of the soil.