| Plant-available phosphorus (P) deficiency in organically managed soils is of increasing concern. This field study examines the combined use of green manure (GM) grown with phosphate rock (PR) as a source of P on organic farms. Buckwheat (Fagopyrum esculentum) GM was grown with five PRs on two organic farms and one conventional farm. The PRs of varying North American origin where characterized for their reactivity and heavy metal content. GM yield was not affected; however, GM P content was enhanced by applications of the more reactive PRs. GM mulch increased soil P flux by 56% and 0.5 M NaHCO3-extractable P by 24% in the following spring. Residual PR also increased cumulative soil P flux by 63% in the year following initial application. However, ryegrass (Lolium multiflorum) yield and tissue P content was not improved. Total initial GM P content explained 74% of the variation in P supply from the GM residues. Further research into soil P transformations following field application of organic materials is required to improve the P status in organically managed soils. |
