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Influencing the Phosphorus Quality of Green Manure with Phosphate Rocks in Organic Farming Systems

Arcand, M. M. 2007. MSc. Thesis. University of Guelph


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. Green manure yield was not affected; however, GM P content was enhanced by applications of the more reactive PRs. Green manure mulch increased soil P flux by 56% and 0.5 M NaHCO3-extractable P by 24% in the following spring. Residual PR also increased pre-tillage soil P flux by 146% in the year following initial application. However, ryegrass (Lolium multiflorum) yield and tissue P content was not improved. Total initial GM P content explained 70% of the variation in P supply from the GM residues in early spring in the season following application. Further research into soil P transformations following field application of organic materials is required to improve the P status in organically managed soils.