Influence of Water Treatment Residuals on Phosphorus Solubility and Leaching

Laboratory and greenhouse studies compared the ability of water treatment residuals (WTRs) to alter P solubility and leaching in Immokalee sandy soil (sandy, siliceous, hyperthermic Arenic Alaquod) amended with biosolids and triple superphosphate (TSP). Aluminum sulfate (Al‐WTR) and ferric sulfate (Fe‐WTR) coagulation residuals, a lime softening residual (Ca‐WTR) produced during hardness removal, and pure hematite were examined. In equilibration studies, the ability to reduce soluble P followed the order: Al‐WTR > Ca‐WTR ≈ Fe‐WTR >> hematite. Differences in the P‐fixing capacity of the sesquioxide‐dominated materials (Al‐WTR, Fe‐WTR, hematite) were attributed to their varying reactive Fe‐ and Al‐hydrous oxide contents as measured by oxalate extraction. Leachate P was monitored from greenhouse columns where bahiagrass ( Paspalum notatum Flugge) was grown on Immokalee soil amended with biosolids or TSP at an equivalent rate of 224 kg P ha −1 and WTRs at 2.5% (56 Mg ha −1 ). In the absence of WTRs, 21% of TSP and 11% of Largo cake biosolids total phosphorus (P T ) leached over 4 mo. With co‐applied WTRs, losses from TSP columns were reduced to 3.5% (Fe‐WTR), 2.5% (Ca‐WTR), and <1% (Al‐WTR) of applied P. For the Largo biosolids treatments all WTRs retarded downward P flux such that leachate P was not statistically different than for control (soil only) columns. The phosphorus saturation index (PSI = [P ox ]/[Al ox + Fe ox ], where P ox , Al ox , and Fe ox are oxalate‐extractable P, Al, and Fe, respectively) based on a simple oxalate extraction of the WTR and biosolids is potentially useful for determining WTR application rates for controlled reduction of P in drainage when biosolids are applied to low P‐sorbing soils.