Soil solution chemistry of sewage-sludge incinerator ash and phosphate fertilizer amended soil

The chemical composition of the soil solution provides useful information on the feasibility of amending agricultural land with municipal and industrial waste, because the soil solution is the medium for most soil chemical reactions, the mobile phase in soils, and the medium for mineral absorption by plant roots. The soil solutions studied in this research were from plots in a 4-yr field experiment conducted to evaluate the effects of the trace metals and P in sewage-sludge incinerator ash. Treatments compared ash with equivalent P rates from triple-superphosphate fertilizer and a control receiving no P application. Ash and phosphate fertilizer were applied annually at rates of 35, 70, and 140 kg citrate-soluble P ha^-1. Cumulative ash applications during 4 yr amounted to 3.6, 7.2, and 14.4 Mg ash ha^-1. Soil solutions were obtained by centrifugation-immiscible liquid displacement using a fluorocarbon displacing agent. Following chemical analysis, a chemical speciation model was used to determine possible solubility- controlling minerals for trace metals and P, and correlations between solution composition and plant uptake were analyzed. Ash increased soil solution pH, Cd, and Zn, but had no significant effect on solution concentrations of other trace metals. Ash increased soil solution P and S, but P increases were less than those from equivalent citrate-soluble P rates of phosphate fertilizer. Soil solution Ba appeared to he in equilibrium with barite (BaSO₄). Solubility data did not indicate that any discrete mineral phases controlled Cd, Zn, Cu, Ni, Pb, or P solubility. Soil solution P concentration was strongly correlated (r = 0.92) with P accumulation in sweet corn (Zea mays L.) plants, but solution trace metal concentrations were either weakly correlated (r = 0.49 for Zn and 0.36 for Cd) or not significantly correlated (r = 0.09 for Ni and -0.25 for Cu) with plant accumulation.