Sewage Sludge — Soil Conditioner and Nutrient Source: I. Phosphorus availability and its uptake by ryegrass (Lolium perenne L.) grown in a pot experiment

The plant availability of municipal sewage sludge-borne phosphorus (MSS–P) and its effects on various soil P forms were investigated in a pot experiment using single superphosphate P (SSP–P) as the standard P source. Unlimed (control) and limed (pH 6.64, 7.19) samples of a silt loam soil [pH(H₂O) = 5.72] were mixed with 134 mg P kg–1 dry soil in the form of SSP or MSS, labelled with 32P tracer and gown with ryegrass (Lolium perenne L.). After harvesting the rye-grass plants at the earing stage, the soils were sampled and extracted sequentially with resin in the Cl– form followed by 0.5 M NaHCO3 (pH 8.5), 0.1 M NaOH and 1.0 M HCl solutions for P analysis. The sum of resin- and NaHC03–P levels as well as NaOH– and HCl–P levels were designated as the labile and nonlabile P pools, respectively. Relative MSS–P uptake established by 32P tracer averaged 8% (range 7.4–8.9), whereas the corresponding average MSS–P uptake established by the difference method was 7.5% (range 3.8–14.5), with lower values recorded in the limed soils. Liming and applications of SSP and MSS significantly (P = 0.05) increased soil levels of resin- and inorganic NaHC03‱P. Liming decreased organic NaOH‱P levels, whereas organic NaHC03–P and HCl–P levels did not significantly (P = 0.05) differ from that in the control treatment. MSS–treatments were, on average, less effective than the corresponding SSP–treatments at elevating the labile P pool sizes, whereas they were more effective than the latter at increasing nonlabile P pool sizes.