Fertilizer value of phosphorus in different residues

Abstract 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 5 weeks) 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 11 weeks) 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.