Cover Crop Mixes – A Tool to Build Soil Health
By Mark Henning, NRCS
Soil health is defined as the “capacity of the soil to function.” Producers expect their soil to grow a crop, whether that is wheat, grass, corn or alfalfa. Plants need water, nutrients, air and sunlight to be productive, but producers only have control over water and nutrients. For water that comes as rain or snow to get into the soil profile, that soil must be healthy and functioning, which means the life that lives in it—the soil microbes, must be healthy and functioning. As with any living creature, soil microbes need food, shelter and water to live. Soil microbes’ primary food comes from plants, whose roots exude sugars that microbes feed on. They will also feed on newly dead plants and older residues. Lastly, they will feed on organic matter, which is the worst possible scenario. Organic matter helps provide the home for soil microbes, and anything that disturbs this habitat means soil life will suffer and not function. Think of soil microbes as your workers in a factory that produces what your plants need to thrive—if you destroyed the factory the workers will leave or die, and your plants will suffer. Tillage is one of the primary ways this is done—soil structure is destroyed and organic matter is lost, which drastically reduces soil life and its’ diversity. We try to make up for this by supplying inputs—commercial fertilizer and pesticides, which end up being band aids and not solving the root problem—poor soil health.
We often hear about N, P or other nutrients being limiting to achieving a certain yield goal. In reality carbon (C) is the limiting nutrient in agricultural systems. This seems strange, but soil microbes need carbon based food sources to live—thus the sugar exudates (which are carbon based) from plant roots are tremendously important. In exchange for these sugars from roots, soil microbes supply plants with needed water and nutrients. Roots are very inefficient at scavenging for nutrients, but with a diverse soil life, or “soil foodweb,” plants get what they need when they need it. However, starve the soil life and your plants will starve also, thus leading to the need to apply commercial fertilizers. When commercial fertilizers are applied, plant roots will not put out those sugar based root exudates. But eventually the fertilizer runs out (and only 30-40% of what is applied actually gets to the plant), and the plants are left without soil microbes to help them as the root-microbe associations were never formed. Feeding the herd below ground (the “micro-herd”) is critical if the above ground life—whether it is plants or cows, are to thrive.
Cover crops, along with no-till and crop diversity, can be a useful tool to build soil health. A good carpenter or mechanic has many tools in his/her toolbox. Knowing how and when to use each tool separates the master from an apprentice. Because soil is a biological system, cover crops (plants) are a natural fit to help increase soil function. Cover crop mixes can serve multiple purposes:
- Use water that is in excess of the soil’s ability to store it (remember 2011?).
- Increase the soil’s aggregate stability to increase infiltration and reduce ponding of water on the soil surface.
- Increase the soil’s aggregate stability to increase infiltration to allow capture of incoming precipitation when it occurs.
- Increase the porosity of the soil to increase permeability so water can move through compacted soil and reduce surface soil saturation.
- Recover nitrate nitrogen that may have leached deep into the soil and retain it in cover crop residues until they decompose (turnips and radishes are especially good at this).
- Feed soil microorganisms to cycle nutrients and feed the next crop.
- Use excess water that might contribute to saline seeps.
- Keep the soil covered to reduce evaporation and salinization of the soil surface.
- Provide supplemental forage for livestock.
- Increase plant species diversity to improve soil health and increase crop nutrient cycling.
The first step in designing a cover crop mix is to ask “what is your purpose?” Other questions to ask are:
- When will it be planted?
- Will it be grazed?
- What cash crop was before the cover crop and what will come after it?
If you are interested in trying cover crops, a cover crop seed mix cost share program is available through the McCone Conservation District. As a part of the grant received, producers may be reimbursed up to $2,000.00 towards cover crop seed mixes. The seed mixes must include a minimum of 5 species covering 3 crop types. NRCS will provide technical assistance in designing cover crop mixes at your request. If you have any questions, please contact the Conservation District at 406-485-2744 x100.