Fertility Status of Organically Managed Fields
Organic Agriculture Centre of Canada
For modern organic farmers, particularly those involved in large scale operations, balancing the nutrients that leave the farm with those that are returned to the soil is an ongoing challenge. Recognizing this, Dr. Diane Knight and a team of researchers from the University of Saskatchewan, in partnership with Agriculture and Agri-Food Canada, worked with 39 of Saskatchewan’s organic producers on 60 individual fields to determine which of four pre-determined organic management styles (Perennial, Summerfallow, Diverse and Cereal based) had the least impact on soil nitrogen, potassium, phosphorous and sulphur levels. Their results, published in the paper “” in the Canadian Journal of Soil Science, were, Diane acknowledged during a recent phone interview, “very, very surprising”.
Diane noted that, in terms of overall nutrient loss, the perennial system, in which the inclusion of perennial forages is one of several criteria, is “the most depleting of them all.” She went on to explain that part of the reason for this depletion is that alfalfa (the forage species observed in the experiment) lowers soil pH and allows plants to access nutrients, like phosphorous, much more easily than would otherwise occur.
On the other hand, Diane acknowledged that conventional measurements of plant available nutrients don’t fully take into account all of the soil microbial inhabitants and biological processes.  She pointed out that the perennial system may remove the most nutrients but it is “leaving more roots and organic matter”, adding that, over the long term, the system may play an important role in the “soil building aspects” of agricultural lands.
She also explains in the paper that soil tests for available nutrients provide an idea of the fertility status of the soil at the time of sampling but may not adequately account for potential nutrients mineralized over the season, which may be of greater importance in an organic system than a conventional system.
When asked which organic management system had the least impact on soil fertility she answered that “the diverse system tended to be a bit better than the others”. The diverse system is classified as one that includes broad leaved plants, non-legumes, annual legumes and summerfallow with less cereal. The system also uses more post-seeding and pre-emergent tillage. Diane was quick to add, however, that the diverse system “was by no means head and shoulders above the rest”.
The “rest” in this study included the Cereal Based rotation that “included occasional summerfallow, biennial and green manure crops” and the Summerfallow system which exhibited the least diversity of all. This is reflected in the system’s near zero incidences of broad leaved plants, non-legume crops, biennial crops and green manures.
Diane was also extremely surprised to learn that over half of the organic Summerfallow fields were located on Saskatchewan’s most fertile, Black soil zones. She warned that Summerfallow can result in excessive erosion, loss of topsoil and has been associated, in part, with the region’s dustbowls of the dirty 30s.
So what’s a Saskatchewan organic farmer to do in order to prevent the loss of soil nutrients that, according to Diane’s study, are “threatening the long-term sustainability of organic farms in this region”?
Diane stressed that organic farmers need to incorporate more animals on the farm. For example, alfalfa fed to livestock on the farm will eventually return to soil as nutrients and organic matter.
Diane was surprised to find out during the study, however, how little animal manure or amendments of any kind are used on Saskatchewan’s organically managed agricultural soils. She noted that many organic farmers feel that that by utilizing little to no extraneous amendments they are farming much as their grandparents and great grandparents would have done.
“Unfortunately, farmers can’t compare farming now to farming 100 years ago”, says Diane. One hundred years ago the majority of what was produced on the farm was used on the farm, creating a nearly closed and, therefore, sustainable system. Today, nearly the entire crop is exported off the farm and the nutrients that accompanied it need to be replaced.
Diane concludes that farmers need to remember that “the soil is alive and needs to be fostered no matter if the system is organic or conventional.” Fortunately, she and fellow researchers will continue to work with Saskatchewan’s organic farmers to ensure that their soils remain productive in the long term.
This article was written by Tanya Brouwers on behalf of the OACC with funding provided by Canada’s Organic Science Cluster (a part of the Canadian Agri-Science Clusters Initiative of Agriculture and Agri-Food Canada's Growing Forward Policy Framework). The Organic Science Cluster is a collaborative effort led jointly by the OACC, the and industry partners. For more information: oacc@dal.ca or 902-893-7256.
Posted March 2011