Cover crops are becoming more popular. But how do you plan your first cover crop?
The first part of the planning process is setting goals, says Kevin Elmy, owner of Friendly Acres Seed Farm at Saltcoats, Saskatchewan. Elmy has been growing cover crops for seven years and now grows and sells cover crop mixes.
“With any cover crop, the first goal is to have plants growing in the soil all the time throughout the whole growing season,” says Elmy. “The next goal is to increase plant diversity by having a wider array of plants in the soil, and the third goal is to make the soil better than it is right now.”
Cover crop mixes can also meet other needs. They could increase production by keeping soil cool, retaining moisture, and feeding the microbes and earthworms that make healthy soil.
Including deep-rooted plants in the cover crop mix can help address physical issues such as compaction or erosion, as their roots can break through hardpan layers and improve water infiltration to prevent runoff. Soil chemical issues such as salinity or low nutrient levels can be addressed with plants such as legumes, buckwheat and tuber crops which release potassium and phosphorus.
Cover crop plant groups
There are three main plant groups: grasses, broadleafs and legumes. Cover crop mixes ideally contain all three groups.
Grasses build organic matter and produce lots of biomass, so for green feed or a grazing crop, or to keep soil covered, grasses are ideal. They have fibrous root systems and are a nitrogen (N) sink. Grasses also support mycorrhizae in the soil (the relationship between fungus and plant roots).
Broadleafs are great nutrient scavengers that will help improve soil quality and build soil bacteria.
Legumes are nitrogen fixers. They add protein to the soil to feed soil biology and more legumes in a rotation will build up mycorrhizae populations quickly.
A good cover crop not only includes a grass, a legume and a broadleaf, but also warm and cool season plants of each of these types. The more diversity in the mix, the better.
“In a monoculture the roots are all growing at the same time, at the same depth and are after the same moisture and nutrients so there is lots of competition,” says Elmy. “When you have a cover crop, plants grow at different times and roots are at different depths, so they are not competing in the same soil, at the same time, for the same nutrients. It spreads risk and takes the stress out of the soil.”
Living material in the soil, mainly bacteria, fungi and nematodes, breaks plant organic material into plant-usable nutrients.
Bacteria is the party animal of the group, says Elmy. “When you plough down green material the bacteria population goes crazy because that’s providing carbon and protein for them and they will recycle nutrients fairly fast.”
Fungi grow slowly and like to eat more straw and humus in the soil. Fungi are the preferred food source for every other microbe that lives in the soil. “Their fine filaments — hyphae — can get into places the plant roots can’t get into, and by doing that they can reach more micronutrients, such as stable carbohydrates, which is what other soil critters want to take in,” says Elmy.
Both legumes and grasses are dependent on mycorrhizae, their relationship with fungi, to absorb nitrogen from the soil. In exchange, the fungi get sugars from both plants.
Earthworms are indicators of healthy soil. They prefer to eat fungi, and they burrow and eat soil, producing castings that are rich in carbohydrates and protein, which attracts bacteria and fungi to keep the nutrient cycle going.
Earthworm populations will be reduced through tillage and growing too many brassicas, such as canola, which reduces mycorrhizae. Repeat applications of fungicides deplete earthworms’ food source by killing the fungi they feed on.
The first question Elmy asks when he helps a farmer develop a cover crop blend is, “What’s your goal?”
“If you tell me you are going to grow a cover crop and you don’t have goals I can’t give you the proper information for the best species,” says Elmy. “If the goal is to deal with high salinity, sugar beets, safflower, some sunflower, and some fall rye, seeded early in the season will help. If a producer wants high biomass and to build organic matter, lots of grasses and some legumes are best. To scavenge nutrients, grasses, legumes and broadleafs will absorb nitrogen. For weed suppression, use broadleafs and grasses.”
As a base for the blend, know what grows in the area and start with those plants. Then add diversity based on what the goals are, but be sure to think about plant growth above and below ground.