Canola is an effective phosphorus user – maybe too effective as far as crops that come after it in a rotation are concerned.
Recent studies have found a link between the interactions of soil microbes known as mycorrhizal fungi and the root systems of plants such as corn, flax and soybeans.
To tap the nutrients they need to grow, such crops are highly dependent on hyphae, the long needle-like appendages that the microscopic organisms use to access phosphorus bound in the soil.
Canola, on the other hand, does fine by itself. Its roots don’t seek out the interaction with mycorrhizal fungi and are able to access enough available phosphorus to sustain it throughout the growing season.
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Model-based estimates used by Statistics Canada showed the 2025/26 crop year has seen increases in canola, corn for grain, oats and lentils production while seeing dips in spring wheat, durum wheat, soybeans and barley in comparison to 2024/25.
“Canola has a root system that acidifies and solubalizes phosphorus tied up in the soil,” said Manitoba Agriculture soil fertility specialist John Heard.
“Other plants aren’t quite as efficient, so they have evolved this relationship with mycorrhizal hyphae. Canola never needed the relationship.”
He said canola doesn’t injure the mycorrhiza – it simply doesn’t invite them to the dance. Snubbed, the micro-organisms revert to a hardy spore state and wait for the next song – or crop season.
While some crops turn in unpredictable yields following canola, cereals appear to respond differently.
Studies of crop insurance data compiled over the years prove what farmers have long suspected: cereals tend to do better after canola in a rotation than other crops.
“It’s a logical rotation: cereals after canola, a grass crop after a broad leaf,” Heard said.
According to Marcia Monreal, a soil microbiologist at Agriculture Canada’s Brandon Research Centre, researchers are learning more about the link between flax, corn and possibly soybean performance and soil mycorrhizae.
In studies of soil micro-organisms, Monreal and colleague Cynthia Grant discovered clues as to why phosphorus supplementation for such crops often provides unpredictable results.
“That confused a lot of people, even experts on phosphorus application,” Monreal said.
“They didn’t know how to explain their completely different results.”
The complex interactions between soil organisms are difficult to understand even for scientists, she said, but all farmers need to know is that to get a good flax crop, one of the best ways to ensure success is to plant it after wheat in a rotation, and possibly barley.
Monreal and Grant found it didn’t matter how much phosphorus had been applied. Instead, the strongest predictor for flax performance was whether wheat was grown in the plot the year before.
“It came through in all the statistics: having the wheat before the flax was the main effect.”
