Against the background of resource conservation, many recycled products have been developed in context of phosphorus recovery. Due to their properties some of them can be used directly as fertilizers. Phosphorus is subject to complex sorption and precipitation processes in the soil, which have a significant influence on solubility and plant availability. Therefore, in this work comparative studies on the fixation of phosphorus of recycled and conventional fertilizers in soil and experiments on solution equilibrium and solution kinetics were carried out. For the investigation of the solution equilibrium and the solution kinetics, batch and pH-stat experiments were conducted. The determined concentrations and pH values were compared with model simulations in Visual MINTEQ. To investigate the fixation of phosphorus in the soil, 8-week incubation experiments were carried out, in which soils of different properties were mixed with a respective fertilizer per batch. For the experiments two struvite products (Crystal Green and Berliner Pflanze), a thermally processed ash (Ash Dec), a rock phosphate (Dolophos 26) and the conventional fertilizers TSP and DAP were selected. Solution equilibrium of all fertilizers in water was achieved after seven days of shaking. Conventional fertilizers showed significantly higher P solubilities in water than recycled fertilizers. Because there was a very high level of agreement with the Visual MINTEQ model simulations, the software is a good tool for first orientation of the dissolution behavior of different P fertilizer products. In terms of solution kinetics, the lowest half-life was observed for struvite, with an average of 9.5 minutes. While the half-life of Dolophos increased from pH 5 to 7, no pH dependence was observed in the dissolution kinetics of the recycled fertilizers. In the incubation experiment, a higher decrease in plant-available phosphorus with conventional fertilizers tended to be observed toward the end of the experiment. Although this could be an indication of increased fixation in soil, this cannot be attributed only to chemical sorption and precipitation processes due to the increased microbial activity. Therefore, under these experimental conditions no clear statements can be made on the fixation of phosphorus of conventional and recycled P-fertilizers in the soil. Further investigations are required here.

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