Changes in soil test phosphorus and soil cations following application of sewage sludge ash and other recycled phosphorus fertilizers

Phosphorus (P) is essential to healthy plant development and is regularly applied in agronomic settings to ensure maximum crop yields. Sewage sludge incinerator ash (SSA) contains significant amounts of P and was investigated as a recycled P fertilizer in a 3-year field study. Untreated SSA was applied at a range of agronomically appropriate rates based on water- and citrate-soluble phosphate (WCSP) concentrations in a corn and soybean field study in Minnesota (Waukegan silt-loam, pH 6.8) to determine the soil chemical effects of SSA amendment individually and compared with triple superphosphate (TSP), biosolids, and struvite. Concentrations of Olsen-P and soil solution P ion fluxes from in-situ plant root simulator probes increased with increasing rates of SSA and were significantly greater than control plots. Compared to the other P sources, SSA amendment increased soil test P at 50%–70% the increase observed with TSP amendment, while total P concentrations increased at 1.7–2.5 times the rate of increase with other P sources. This was due to the lower WCSP concentrations in SSA and the experimental choice to apply based on WCSP. Soil-extractable concentrations and ion fluxes of copper (Cu) and zinc and total soil concentrations of Cu increased with SSA and biosolids application, indicating potential as a comprehensive nutrient source. Total concentrations of concerning elements including arsenic, cadmium, chromium, and lead did not increase above detection limits. Total mercury concentrations were statistically higher with SSA application compared with the control and other P sources, though the analytical sensitivity was significantly higher, and the increase was biologically marginal (an increase of 0.06 µg kg⁻¹ P applied as SSA). Overall, soil chemical responses indicated that SSA increased concentrations of soil test P and some plant nutrients with minimal increases in concentrations of elements of concern.