Effectiveness of recycled P products as P fertilizers, as evaluated in pot experiments

World phosphorus (P) resources are limited and may be exhausted within 70-175 years. Therefore recycling of P from waste materials by chemical or thermal processes is important. This study evaluated the effectiveness of recycled P products from sewage sludge and animal wastes as P fertilizer. Four products were obtained from chemical processes, three magnesium-ammonium-phosphates (MAP) of different sewage treatment plants and a Ca phosphate precipitated from waste-water (Ca-P) and four from thermal processes, an alkali sinter phosphate (Sinter-P), a heavy metal depleted sewage sludge ash (Sl-ash), a cupola furnace slag made from sewage sludge (Cupola slag) and a meat-and-bone meal ash (MB meal ash). The effectiveness of these products as P fertilizers compared with triple superphosphate (TSP) and phosphate rock (PR) was determined in a 2-year pot experiment with maize (Zea mays L., cv. Atletico) in two soils with contrasting pH (pH(CaCl2) 4.7 and 6.6). The parameters used to evaluate the effectiveness were P uptake, P concentration in soil solution (C-Li) and isotopically exchangeable P (IEP). MAP products were as effective as TSP in both soils, while Ca-P was only effective in the acid soil. Sinter-P was as effective as TSP in the acid soil, while Cupola slag was in the neutral soil. The products Sl-ash and MB meal ash were of low effectiveness and were comparable to PR. The effect of the fertilizers on IEP, but not on C-Li, described their effectiveness. Recycled P products obtained by chemical processes, especially MAP, could be directly applied as P fertilizers, while products such as Sl-ash and MB meal ash are potential raw materials for P fertilizer production.