Influencing the Phosphorus Quality of Green Manure with Phosphate Rocks in Organic Farming Systems
Arcand, M. M. 2007. MSc. Thesis. University of Guelph
Abstract
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.
