Phosphorus recycling - Characterisation of the effect of recycled phosphate fertilisers by field and pot experiments

The suitability of recycled P compounds as P fertilizer is important for their economic success because they are in direct competition to commercial fertilizers. The fertilizer effect depends on their interaction with the P dynamics in soil with regard to plant availability. This cannot be described satisfactorily by common chemical characterizations of P fertilizer solubility. Fertilizer effect has to be proofed in field experiments.

In three field experiments, the fertilizer effect of two magnesium-ammonium-phosphates of different sewage works, a heavy metal depleted sewage sludge ash, and an alkali sinter phosphate made from meat-and-bone meal are tested in comparison to two commercial fertilizers with different plant availability. All three experimental sites are poor in available P and differ with regard to their pH (acid through slightly alkaline) and texture. During a whole crop cycle of three years, yield, P uptake, CAL extractable P and P concentration in soil solution will be measured.

In addition to the field experiments, pot experiments are performed using the same soils with and without plants. The P dynamics in the pots will be determined in more detail by fractionated P extraction and the results will be related to plant P uptake. This allows an assessment of the fertilizer effect of the tested compounds. This can be important for the manufacturers to evaluate the market opportunities or to enhance the product quality with regard to agricultural use. The results are also important for farmers and agricultural extension services. Therefore, the experiments are performed in close contact to the manufacturers and the chamber of agriculture Lower Saxony.