Many farmers know seeding flax into canola stubble leads to lower yields. Some try to offset ill effects by pouring on the phosphorus. But research out of Manitoba shows applying phosphorus yields few benefits for flax.
“For the most part, flax doesn’t respond really, really well to the fertilizer phosphorus anyway. Unless the soil is extremely deficient in phosphorus, you often will not see a response in flax to the phosphorus fertilizer,” says Cindy Grant, soil fertility expert with Agriculture and Agri-Food Canada (AAFC). Grant and her colleagues conducted field research examining the effects of crop rotation, phosphorus application, and tillage on flax.
Flax roots don’t access fertilizer granules the same way as crops such as canola and wheat. When canola plants find an area in the soil with a high concentration of fertilizer, they send more roots there to extract nutrients.
But flax “hits it, shrugs, and keeps on going through. It doesn’t really proliferate its roots very well,” says Grant.
Flax leans heavily on micorrhizae to access nutrients and water. Micorrhizal fungus inhabit a plant’s roots, sending out filaments that pull in vital nutrients such as phosphorus, nitrogen, potassium, copper, zinc, and calcium.
Many crops, including legumes, wheat and barley, host the fungus, but crops such as canola and mustard do not host micorrhiza. Seeding flax after growing a non-host such as canola decreases the flax crop’s ability to take in nutrients.
Marcia Monreal is a soil microbiology scientist with AAFC and lead on the micorrhiza portion of the flax research. Monreal used an image analysis program to precisely measure the micorrhizae in the flax roots. When flax followed wheat, Monreal found more micorrhizae in the flax roots. The flax roots were larger as well.
“Flax after canola has micorrhiza at six weeks, but a very different percentage,” says Monreal.
Weather also played a role in micorrhiza establishment. During one cool, dry spring there were even more micorrhizae in the wheat-flax rotation. Root biomass and lateral growth increased.
“One year that it rained a lot, the effect of micorrhiza was less,” says Monreal. Monreal says previous research shows rain creates favourable conditions for mites and other soil organisms which eat micorrhiza.
Wheat and barley aren’t the only crops that benefit the following year’s flax. Monreal says legumes and micorrhizae share common pathways, making legumes ideal micorrhiza hosts.
“They are very friendly to each other. So a legume before flax would be excellent,” says Monreal.
Grant says researchers looking at corn have seen similar results. Corn grown after summerfallow doesn’t perform as well as corn grown after a micorrhizal host.
Monreal says it isn’t known if canola forms relationships with any soil microorganisms. Her study didn’t look at canola grown after flax, but she doubts canola would benefit from being planted into flax stubble.
“It wouldn’t have any benefit because canola is not micorrhiza. So from a microbiology point of view, flax before canola, it wouldn’t change anything for canola. Canola would still need fertilizers,” says Monreal.
Premier Tech produces a micorrhizal inoculant for field and horticultural crops. The commercial inoculant uses a micorrhizal specie that has proven effective throughout Canada. Monreal wanted to see if other micorrhizal species could measure up to the commercial inoculant, so she obtained 10 species from a colleague in Ottawa. Last winter she inoculated flax roots, one species at a time, in a growth chamber.
The inoculated plants produced fewer capsules. But capsules from the inoculated plants weighed 40 per cent more than those in the control group, more than making up for the lower capsule numbers.
Monreal then picked the highest performing micorrhizae for field trials this summer. Flax plants were inoculated with the commercial inoculant and other micorrhiza species one at a time. Other plants were inoculated with combinations of different species.
Monreal is still analyzing the results, but she says the research should provide a glimpse into how micorrhizae interact. Some studies suggest different micorrhizae don’t compete, while others suggest some plants favour one species over others.
Rotation affects quality
Some food markets are sensitive to cadmium levels in flax seed. Previous research has shown micorrhizae produce chemicals that interact with cadmium, so AAFC researchers measured zinc and cadmium levels in flax tissue and seed.
Rotations consistently affected cadmium and zinc concentrations in flax. Flax following wheat had lower cadmium levels and higher zinc levels than flax following canola. Applying phosphorus also increased cadmium and cut zinc concentration in flax, which was consistent with previous research.
Phosphorus fertilizer does contain cadmium, but even if the fertilizer contains very low cadmium levels, Grant says there is still an increase in cadmium concentration. Researchers think there may be reactions in the soil, or something going on inside the plant, causing cadmium levels to rise and zinc levels to drop.
“Because cadmium and zinc are very closely related chemically, cadmium seems to hitch-hike a ride on the zinc transporters that the plants have at the root surface and also probably within the plant for translocation. And so it may be that part of the effect for phosphorus on cadmium is because the phosphorus reduces zinc, and by reducing the zinc it allows more cadmium to sneak in.”
Don’t skimp on phosphorus
Monreal and Grant also co-operated with Mohammad Khakbazan, who studied the economics of flax rotations over four years. Khakbazan found higher flax yields and net revenues in the wheat-flax rotation. The most economical practices were to grow flax after wheat, apply less phosphorus, and use reduced tillage.
Though applying phosphorus to the flax didn’t cure the lack of micorrhizae, Grant doesn’t advise skimping on phosphorus year after year. Alternately, building excessive phosphorus could cause environmental problems.
“This whole idea of managing phosphorus through a rotation is becoming more and more of an issue… do we need to boost up the amount of phosphorus in the crops that can accept large amounts of phosphorus so that we balance through for the crops that don’t do well with phosphorus?” †