Soil Phosphorus and Nitrogen Fixation on Organic Dairy Farms

by Michael Main, M.Sc.

Excessive soil phosphorus (P) on livestock farms has become a major environmental concern. However, on some long-term organic dairy farms in Ontario, low soil P is more likely to be the concern. A recent study conducted by Dr. Derek Lynch, Canada Research Chair for Organic Agriculture, and Cory Roberts and Paul Voroney at the University of Guelph showed that soil test P ranged from moderate to very low on surveyed Ontario organic dairy farms. While organic dairy farming in Ontario has been proceeding successfully, low soil P could limit productivity if it is not addressed.

It is known that lack of available P can reduce yields and nitrogen (N) fixation in common forage legume crops such as alfalfa and clovers. Since N fixation is the cornerstone of N fertility on organic farms, any factor that reduces N fixation can hurt farm productivity. It is not yet clear whether low soil P is affecting N fixation and crop productivity on organic dairy farms in Ontario. A recently launched study will address this question.

Dr. Lynch in collaboration with Drs. Paul Voroney and Ivan O’Halloran (University Guelph and Ridgetown), among others, are undertaking a study to examine the affects of soil P levels on yield and N fixation in forage crops, and also to evaluate organic methods of improving crop P uptake. The study is jointly funded by the Natural Sciences and Engineering Research Council, Organic Meadow Cooperative and OMAFRA’s New Directions program. The project commenced in the autumn of 2007 and will continue until the year 2011.

One objective is to draw a relationship between soil test P and productivity of legume forage stands across a range of soils on organic dairy farms. A second objective is to evaluate the effectiveness of organic amendments and mycorrhizae fungi in increasing the supply of P to organically grown crops. As an outcome, farm advisors and farmers will have new information to decide when and how to act effectively to improve P uptake, using organic approaches.

This study will serve to evaluate if current recommendations on P fertility are a good guide for forages grown in organic systems. Healthy biological activity in soils could mean that forage crops require less ‘available’ P than previously assumed. For instance, growth of mycorrhizae fungi on crop roots is known to enhance P uptake, but studies suggest that mycorrhizae become less abundant when P fertilizers are applied.

The first phase – surveying soil fertility levels and forage growth – is well underway. For 2008, soil and forage samples have been taken from across 28 fields on organic dairy farms – 18 fields in Ontario, and 10 in Nova Scotia. Within each field, samples are taken in a straight line across the field at 40 meter intervals, generating up to 840 samples per year in total. This repeated sampling approach attempts to capture the real variability across farm fields, and provides more realistic results. Each sample is sorted into alfalfa, clovers, grass and weeds to determine the yield contribution of each type of plant. The research team is employing a new method that uses analysis of natural nitrogen isotopes to determine the rate of N fixation by the legume plants. This novel approach is likely to lead to the broadest scientific evaluation of N fixation in forage legumes ever conducted in Canada. Relating forage N fixation to soil P fertility under farm field conditions is also unique.

The very rainy summer of 2008 has resulted in an abnormal harvest in Ontario, with two to five weeks delay in the first harvest, and in most cases, two rather than the usual three cuts of forage. Hopefully, the growing season in 2009 and 2010 will be closer to normal.

Other aspects of the project are getting underway as well. Master of Science (M.Sc.) candidate, Amanda Ward of the Nova Scotia Agricultural College has commenced a greenhouse study of the role of organic phosphorus amendments on growth of legumes in P deficient soils. Kim Schneider, a Ph.D. candidate at the University of Guelph, is currently in the planning phase for a field study on the role of mycorrhizae fungi and its enhancement in crop phosphorus uptake.

Updates on the project will be available on this website as results become available.

 

Michael Main, M.Sc., is a Research Associate at the Organic Agriculture Centre of Canada (OACC). Please send comments or questions by phone to 902-893-7256 or by email to mmain@nsac.ca or dlynch@nsac.ca

 

Posted January 2009