Cover crops are typically planted in the fall and serve many purposes. They also accomplish one of the key soil health principles - always have a living plant growing in the soil.
They can be planted as a mixture of several plant species in late summer after wheat harvest or the cool-season garden vegetables have stopped producing. More often, they are planted in the fall after corn and soybeans are combined. Cereal rye can be planted into December if conditions are right.
First and foremost, cover crops protect the soil from eroding during rain events. The most precious resource on a farm or in a garden is the topsoil and we want to keep that soil in place. Did you know that a raindrop falls to the earth at 20 mph?
It has tremendous energy when it hits the ground, and when combined with thousands of other raindrops hitting a field, it has the power to move a lot of soil. In many places, this soil erosion has the potential to reach local streams, turning the water to muddy, chocolate brown - affecting the fish and insects that live in streams.
Cover crops feed the soil microbes. Most microbes cannot make their own food and they rely on plant roots to feed them. Certain microbes have a preference for certain plant species. So the more types of plants grown, either in a crop rotation or using cover crop mixes, the better the diversity of soil microbes and the healthier the soil.
This follows another key soil health principle - maximize diversity in the system.
A living plant root supports mycorrhizal fungi that help feed the crop and build soil structure. A healthy soil should be loose and crumbly with lots of pore space for air, water, and roots. The mycorrhizal fungi wraps its filaments around the soil particles and also makes biological glues, called glomalin, to create soil aggregates which give soils that crumbly texture.
More pore space allows water to soak into the ground instead of running off into local streams. This good soil structure also supports heavy farm equipment without compacting the soil.
Cover crops capture sunlight energy through photosynthesis. This process takes carbon dioxide out of the air and makes the precious oxygen that we need to breathe. The carbon dioxide is converted into simple sugars, then more complex carbon chains like amino acids and proteins. Eventually that carbon is incorporated into the leaves, stems, and roots of the plant. The roots and simple sugars that leak from roots to feed the microbes, are the foundation for building soil organic matter.
Safely storing the carbon as organic matter in the soil can help reduce atmospheric carbon dioxide and mitigate greenhouse gas emissions.
Cover crops also capture nutrients from leftover fertilizers or fall manure applications and prevent those nutrients from leaching into the groundwater and the streams. Most farms do their best to not over-apply fertilizers, but there are many variables during a growing season that affect how many nutrients are held in the soil or absorbed into the plant. If there is leftover nitrogen in the soil at harvest, a cover crop that survives the winter can capture it and keep it in the soil until the next spring when a new crop is planted.
There are very few downsides to planting a cover crop. Yes, it takes some effort and costs money for the seed, but the benefits gained and taking a key step for improving soil health is worth it.