Phosphorus Regulation of Legume Biological Nitrogen Fixation on Ontario Organic Dairy
Farms
Amanda Ward1,2, Shelly Juurlink2, Derek Lynch1,2, Ralph Martin1,2 and Andy Hammermeister2
1
2
Nova Scotia Agriculture College, PO Box 550, Truro, Nova Scotia, B2N 5E3
Organic Agriculture Center of Canada, PO Box 550, Truro, Nova Scotia, B2N 5E3
Biological nitrogen fixation (BNF) is a key process providing nitrogen in organic production
systems as a result of the legume-rhizobia symbiosis. Phosphorus (P) is one of a number of
factors limiting symbiosis establishment and function but the mechanism of P limitation of BNF
is not well characterized. From preliminary soil samples taken in the fall of 2007, it has been
shown long-term organic dairy farms in Ontario have low soil available P. The available P in the
samples was between 1.2 and 18.7 ppm having an average of 3.4 ppm with soil test P below 10
ppm considered low. A better understanding of phosphorus dynamics in these soils and its
regulation of legume BNF is necessary to ensure the long term sustainability of organic dairy
production systems. To better understand this relationship, soil and forage samples will be
collected on ten long-term organic farms in Ontario and transitional/organic farms in Nova
Scotia. Soil samples will be analyzed for available N, P and S content while forage samples will
be analyzed for P uptake and %N obtained from BNF (using 15N techniques) between 2008 and
2010. From this data we hope to relate low soil available P to the amount of N obtained from
BNF. For a more in depth study, the effects of low soil test P on legume BNF greenhouse trials
will be used. Preliminary results from spring 2008 for forage and soil samples will be presented.