When cover cropping first started getting a hold in Western Canada it was triggered by tillage radish. In the United States it was being used with great success between corn and soybean, or after a winter wheat crop harvest to help loosen up the soil, activate soil calcium for the next crops, tie up nutrients to reduce the amount lost from the field to be released to the next year’s crop, and help suppress weeds. Similar results were seen in Western Canada, but there were some failures.
In the United States most of the rotations are dominated by corn and soybean, namely a grass and legume. In Western Canada, canola contributes a significant amount of some producers’ rotation. Radish is part of the Brassica family, the same as canola. This means they will share many of the same pests, including insects and diseases. There are numerous times radish seedlings did not survive. Or, we ran out of time for fall growth and they were killed before they could improve the soil. So where do Brassicas fit in cover cropping in Western Canada?
One of the keys for improving soil health is to increase plant diversity that has been grown in rotation or in the cover crop blend. The main five functional plant group families are grasses, legumes, Brassicas, non-Brassica broadleaf and forbs. Brassicas are one part of the functional plant families. Other species in the Brassica group are turnip, radish, kale, collards, forage rape, mustard, as well as weeds like wild mustard.
If canola or mustard are part of the rotation, especially if in a tight rotation with canola it will be difficult to find a spot to use other Brassicas in a cover crop. Even if there is a lot of Brassicas in the area, there may be challenges establishing seedlings due to high flea beetle pressure.
Adding another Brassica into a rotation already with a Brassica does not add any diversity and will create new issues. The idea is to get a plant with similar function of increasing plant-available calcium in the soil from a different functional plant group, like beets. Beets will produce a large tuber that when it rots will increase calcium levels in our soils.
Increasing our calcium is important even though a soil test will show we have a lot in most soils. We need it in a more plant and microbial friendly form, that is, calcium with carbon attached to it.
The best place for a Brassica in a cover crop is at least one year away from a Brassica cash crop, such as mustard and canola. In a rotation including a cereal, pulse and canola, look at using a turnip or radish in between the cereal and pulse. Ideally, it would be seeded after the pulse before the cereal. This way the pulses that are normally harvested earlier in the fall will have some residue rot, releasing nutrients. The Brassica can then absorb the nutrients, hold them until the next spring, then release the nutrients as the Brassica cover crop rots.
Like the rest of cover cropping, do not just use a Brassica as a cover crop alone. Include a cereal, broadleaf, and forb plants in the mix. This will further reduce potential problems, increases plant diversity, and allows the soil aggregation to continue if the plants are alive.
Overusing Brassicas in cover crops can cause other issues. Without balancing the calcium with some magnesium, the soils can become very loose and can be prone to erosion. We need to have magnesium on our base saturation around 15 to 20%. Another issue of Brassicas is the high nitrogen content of the residues. Like the rumen, there needs to be balance of carbon to nitrogen for it to work properly.
Brassica residue when breaking down will release a lot of nitrogen into the soil. For the microbes to utilize this burst of nitrogen, they need a good carbon source, like root exudates. If there is not enough liquid carbon being injected into the soil, the microbes will then use the next source of carbon — our soil organic matter — and this will start the decline of soil organic matter.
—Elmy is founder of Cover Crops Canada. His books Cover Cropping in Western Canada and Not Just Dirt are available through Friesen Press, Amazon, and digitally through Apple Books, Kindle, and Google Play. For more info on Cover Crops Canada visit covercrops.ca
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