Green-Manure Cover Crops

Green manures, also known as cover crops, are plants grown primarily to benefit the soil rather than for consumption. Besides adding compost, cover cropping increases weed and erosion control, nutrient retention, water permeability, soil pore spaces, attracts beneficial insects, and reduces soil-borne diseases.

Benefits of Cover Crops

Green manures replace and hold nutrients in the soil with an increase of organic material that feeds microorganisms and improves soil structure.
Leguminous crops capture or fix atmospheric nitrogen in root nodules where specialized bacteria make it available in a form for uptake by spring and summer crops. Leguminous crops also attract pollinators.
- Clover develops shallow, fibrous root systems in summer that add biomass to soil.
- Alfalfa grows deep roots in summer to tap resources unavailable to some crops.
- Broad or fava beans have a tap root that breaks up heavy soil. They also offer a culinary advantage in the cool season when flowers are left to develop pods and beans. After harvest, foliage becomes green manure , but soil nitrogen moves to seeds when beans are allowed to develop.
The most commonly grown non-leguminous cover crop for cool-season planting is winter or grazing rye.
- Rye grows faster in cool weather than legumes and gives better weed suppression.
- Buckwheat is a non-leguminous, summer cover crop that attracts pollinators and moves phosphorus up from deep in the soil.
- Non-legumes tend to break down more slowly and add more organic matter to the soil but use some nitrogen in decomposition.
A combination of green manures balances the benefits of different types.

Planting and Using Green Manures

Broadcast seeds evenly across the planting area or sow larger seeds thinly in shallow drills and close rows.
Lightly rake in seeds and water thoroughly if it does not rain for the following 48 hours.
Once seeds germinate, allow crops to reach full size before turning them into the soil.
Never let green manure crops set seed or they will become a weed in later crops on the site.
Never let leguminous crops develop seed or nitrogen will be lost to the soil. When left to flower and set seed, nitrogen developed in root nodules is transferred from roots to developing seeds.
Just before they flower, cut small beds of cover crops with a string trimmer, hedge shears, or mower. Small pieces are easier to turn under and they break down fast in the soil.
When practicing no-till gardening, an option is to add the cover crop biomass to the compost pile. Once the green manure is well-composted, it can be layered on vegetable beds.

Additional Information

April 2022