Fertilizer value of phosphorus in different residues

The phosphorus (P) fertilizer effect of a range of commonly available manure, waste treatment and by-product residues was tested in pot, field and incubation experiments. The effect of the residues on P offtake was compared with that of commercial mineral P (super phosphate) to calculate the mineral fertilizer equivalent (MFE). Possible relationships between MFE and P extractable from residues using different agents (ammonium lactate, citrate, water) were examined. Dry matter yield and P concentration were measured in ryegrass grown in pots amended with 14 different residues. The effect on the first cut (after 5weeks) was significantly higher for residues with a low organic matter content, for example ash and biogas residues (MFE=74-85%), than for many other products with higher organic matter content, for example meat meal (MFE=44%), cattle slurry (MFE=57%) and sewage sludge (MFE=0-37%). However, the effect on two combined cuts (after 11weeks) was more similar between residues (MFE=40-60% for most residues). Ammonium lactate-extractable P (P-AL) in residues correlated better with MFE (r(2)=0.48) than water-extractable or citrate-extractable P. Grain yield and P concentration were measured in a field experiment with spring wheat fertilized with four different residues. Pelleted meat meal had a similar effect on yield and P offtake as mineral fertilizer P, whereas two different sewage sludge and chicken manure had approximately 50% of the mineral fertilizer effect. The effect of residues on soil P-AL (the Swedish measure of easily available soil P) in the incubation experiment showed no correlation with MFE from the pot experiments.