Residues from a buckwheat (Fagopyrum esculentum) green manure crop grown with phosphate rock influence bioavailability of soil phosphorus

Low soil test phosphorus (P) concentrations are common in organically managed soils in Canada. This field study examined the effect of residues from a buckwheat (Fagopyrum esculentum) green manure (GM) crop grown with an igneous and a sedimentary source of phosphate rock (PR) on in situ soil P supply, Olsen P, and soil microbial biomass P on an organic farm in Ontario, Canada. Phosphate rock application did not increase GM dry matter production, but did increase above-ground tissue P concentration with applications of the sedimentary PR (Calphos). In the following spring, in situ soil P supply and Olsen P were increased in GM residue-applied soils and in soils containing the Calphos PR, while microbial biomass P was largely unaffected. Release of P was detected when GM P concentration was greater than 2.9 g P kg-1. The results suggest the quality of the GM residues had more influence on P availability than the quantity applied to the soil; however, the low changes in available P (P supply and Olsen P) were not agronomically significant.