Thermochemically Treated Sewage Sludge Ash From Pilot‐Scale Production as P‐Fertilizer for Spring Wheat and Maize in Different Soil Conditions

ABSTRACT Background P‐fertilizers made from sewage sludge ash (SSA) may be suitable substitutes for rock phosphate (RP)‐based fertilizers and contribute to sustainable use of waste. In this context, the thermochemical AshDec treatment of SSA (TC‐SSA) has been continuously improved and has emerged as a possible method to produce plant‐available and low‐pollutant P‐fertilizers. Aim Evaluation of the P‐fertilizer efficacy of TC‐SSA produced in pilot trials in vegetation experiments with different plants, soils, and fertilizer doses. Methods TC‐SSA was tested in pot experiments for spring wheat and maize on different soils and at two doses (0.2 and 0.4 g fertilizer P pot −1 ) and is compared to untreated SSA (USSA), RP, Struvite, triple superphosphate (TSP), and a control without P‐fertilization (Zero P). Pot experiments are complemented by an analysis of fertilizer P‐solubility. Results P‐solubility of SSA with different extraction methods increased due to the thermochemical treatment, whereas the water‐insoluble property remained. In contrast to RP or USSA, TC‐SSA enriched calcium–acetate–lactate extractable soil P to the same extent as TSP. Plant biomass and P‐uptake increased with TC‐SSA compared to Zero P, RP, and USSA and were in most cases equal to TSP fertilization under different soil conditions. In contrast, the effects of RP and USSA varied among soils but, mostly, did not exceed plant biomass and P‐uptake of the unfertilized control. Conclusion TC‐SSA has an increased fertilizer efficacy compared to USSA and is an effective P‐fertilizer for spring wheat and maize in different soil conditions.