A cover crop is a crop specifically planted to:
protect the soil from erosion
maintain soil moisture
increase organic matter in the soil
recycle soil nutrients
Cover crops or “green manures” are not harvested, but rather contribute to soil improvement in the place where they are grown. Most gardens benefit from the use of cover crops when not planted, instead of leaving the garden fallow (unplanted). If the garden is in use for most of the year, arrange crops into warm- and cool-season groups. This makes it possible to rest a portion of the site for cover cropping. Cover cropping is a valuable component within a crop rotation plan for pest and disease management. Cover crops are divided into two primary groups: legumes and non-legumes.
Legumes have the ability to “fix” atmospheric nitrogen gas into “soil nitrogen”, which is available to plants. The fixed nitrogen will be available in the soil after the legume is terminated and starts to decompose. This fixed nitrogen allows the garden to rely less on added synthetic fertilizer. Cover crops will not supply all of the nitrogen needed for the following year, but over time will improve soil structure and nutrient levels for long-term sustainable growth.
Non-legumes are planted primarily to provide biomass, i.e. carbon-based plant material that includes stems, roots and leaves. This biomass improves the structure and water holding capacity of the soil while feeding beneficial soil microbes. Non-legumes consist mostly of grain crops such as oats, rye, and buckwheat, but several brassica crops like mustard, turnip, and daikon radish are also valued non-leguminous cover crops. Natural chemicals (glucosinolates) produced in the roots of particular brassica crops have shown promise for the management of some soil borne pathogens like nematodes, but results are inconsistent and research is ongoing.
Proper use of cover crops will improve the overall productivity of the soil. While the cover crop is growing, it will help prevent soil erosion and assist in weed control. The organic matter provided when a cover crop decomposes will improve soil structure and aeration, water and nutrient-holding capacity, and supply a portion of the nutrient requirements for subsequent crops. The type of cover crop and the length of time it is growing will determine how much organic matter and nutrients are returned to the soil. A legume may provide more nitrogen, but less total organic matter than a vigorously growing non-legume like sorghum-sudangrass.
Cover Cropping Basics
Before sowing the cover crop, turn over the garden with a rotary tiller. Clear the area of weeds and any remaining refuse from the vegetables or flowers that were not previously removed from the site. Level the soil with a garden rake. Sow seed by hand for small areas, broadcasting as evenly as possible. Broadcast back and forth over the area several times in an attempt to distribute the seed evenly. Lightly cover seeds by raking to ensure good seed to soil contact. Larger areas may require seeding equipment. In some years, irrigation may be necessary to ensure a good stand.
For maximum benefit, a cover crop should be terminated (killed) while in the flowering stage. At this point, the crop will return the greatest amount of biomass and nutrients to the soil. If cover crops are not terminated before seed formation, then their seeds may become weeds in a later crop. Once terminated, the cover crop can either be left on the soil surface to decompose as a mulch (known as “no-till”), or it can be tilled into the soil where it will decompose below the soil surface. If the no-till option is used, the cover crop will need to be mowed or crimped down prior to planting of the vegetable crop.
Cover crops that are tilled into the soil will usually need to be mowed or otherwise chopped prior to tilling. Time cover crop seeding so that the cover crop is terminated and tilled under about three weeks to a month prior to planting the vegetable crop. This will provide adequate time for the cover crop residue to breakdown before planting the vegetable crop.
Soil microbes convert the decomposing cover crop into organic matter for addition to the soil profile. Regular use of cover crops over a period of years will slowly raise the organic matter level in the soil and increase the activity of soil organisms such as earthworms and fungi in the soil. As these organisms decompose the organic materials, they help improve soil structure and tilth, making the soil a more favorable place for root development. It is important to understand that organic matter is continually decomposing and cannot be built up permanently in the soil. Soil building is a continual process in the garden.
Things to consider when choosing a cover crop:
Nitrogen fixation for the following crop
Reduction of synthetic fertilizer costs
Addition of organic matter
Improvement of soil health resulting in increased yield
Reduction of herbicide use
Prevention of soil erosion
Conservation of soil moisture
Water quality protection
Habitat for beneficial organisms
LayLa Burgess, Urban Horticulture Extension Agent, Clemson University Bob Polomski, Extension Consumer Horticulturist, Clemson University Debbie Shaughnessy, HGIC Information Specialist, Clemson University Cory Tanner, Horticulture Extension Agent, Greenville County, Clemson University