Processed agrifood residues for phosphorus recycling in the field

Processing of organic residues may affect plant-availability of phosphorus (P) and thus the potential to recycle the nutrient, i.e., recyclability, but empirical evidence in the field is lacking. In field experiments in clay and silt loam soils with low available P, impact on P recyclability by cattle manure and sewage sludge processing methods (composting, anaerobic digestion, lime-stabilization, acid-oxidizer) and three application rates were assessed. Synthetic nitrogen (N) and potassium (K) fertilizers were supplied in surplus and NPK served as a reference. The differences in plant response were small at relevant application rates and not consistently explained by solubility of fertilizer P. Least P was required in composted manure for the same P uptake in silt loam, and composting was beneficial to plant response in clay as well. Lime-stabilization of sewage sludge had an adverse effect on P uptake in silt loam. Increasing application rates of sewage sludge hardly enhanced but did not lower P uptake or yield even at an excessive rate. Soil water-extractable P in the autumn liable to leaching was increased by NPK only. In clay soil, sewage sludges performed better than manures obviously due to anaerobic conditions caused by high precipitation, but in silt loam the contrary was the case. In conclusion, the availability of P in processed residues is more susceptible to weather and soil variables than in synthetic fertilizer. P fertilization benefits in cereal cropping in current north European conditions appear to be generally small.