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 (T> pic Hapludults) in a field study and to the Sassafras soil and a Christiana silty clay loam (Typic Paleudults) in a greenhouse study. Firstyear 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 Ist-yr rates. Nitrogen as NH<NO3 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 eightleaf 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 HC1 and 0.0125 A/H.SO,) and 0.5 A/NaHCO, 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 1 Contribution from the Agronomy Dep. and Maryland Agric. Exp. Stn., Univ. of Maryland, College Park, MD 20742 and the SoilMicrobial Systems Lab., AEQI, USDA-ARS, Bettsville, MD 20705. Scientific Article no. A-3906, and Contribution no. 6887 of the Maryland Agric. Exp. Stn. Received 12 Aug. 1985. 2 Graduate Research Assistant, Univ. of Maryland; Microbiologist, USDA-ARS-Beltsville Agricultural Research Center; and Associate Professor, Univ. of Maryland; respectively. of the compost P was associated with Fe and A1 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. Additional Index Words: f fractionation, fertilizer efficiency, Febound P, triple superphosphate, corn, Zea mays., yield, P-uptake. McCoy, J.L., L.J. Sikora, and R.R. Weil. 1986. Plant availability of phosphorus in sewage sludge compost. J. Environ. Qual. 15:404-409. In a survey of sewage sludge from eight states, Sommers et al. (1976) reported that the mean total N and P contents were 3.3 and 2.3%, respectively. Because the crop requirement for P is usually l/10th to one-fifth that for N, excessive quantities of P are applied when sewage sludge products are used to supply the N requirement of the crop. For example, assuming that 10% of the N in sewage sludge compost is mineralized during the 1st yr after application (Tester et al., 1982), then approximately 45 Mg/ha would need to be applied to supply a crop with 150 kg N/ha. This amount of compost would contain approximately 1500 kg P/ha, or 10 to 100 times normal agricultural P application rates. Even if only 25 to 40% of the total P in sewage sludge compost is plant available (as reported by Sikora et al., 1982), the amount of P applied would still be substantially in excess of crop needs. Little additional information is available on the plant availability of P in sewage sludge composts. The plant J. Environ. Qual., Vol. 15, no. 4, 1986 403 Table 1. Chemical analysis of sewage sludge compost. Compost P N K C Fe A1 pH Blue Plains 12.6 14.5 Parkway 13.3 16.1 Blue Plains 14.1 12.2 Parkway 12.5 11.4 "g]kg (dry weight basis~