Cover crops and green manures are basically the same term. When these plants are alive they are cover crops. When the plants are decaying they are green manures. Green manuring has all the soil benefits of classic composting, plus other benefits:
1. Green manures can fertilize large acres of land cheaper and easier than hauling in tons of finished compost.
2. You can eat the produce of certain cover crops like beans, peas, or radishes. It is harmful to humans to eat compost!
3. The roots of certain legume green manures can supply tons of free atmospheric nitrogen per acre to the topsoil after the cover crop plants are tilled, mowed down, or smothered. Rhizobacteria live inside the legume roots creating a unique relationship that actually converts atmospheric nitrogen into organic nitrogen for the legume to use. This extra nitrogen fixation built up by the rhizobacteria can not be beneficial to other plants near by, or future crops in the soil next season, until that legume is dead and recycled into the soil by the green manuring process. Compost can't fix nitrogen in the soil.
4. All green manures supply extra organic matter to feed and breed beneficial soil organisms for soil fertility and soil health.
5. The roots of certain cover crops can go down several feet below the topsoil and into the subsoil to break up hardpan and pull essential nutrients up to the topsoil level at green manuring time. After 24 months of continuous growth, alfalfa roots can extent over 20 feet down, that can turn it extra OM down into the subsoil! No regular tractor or mechanical tiller can plow that deep! Without tilling it in, compost can't do that.
6. Some cover crops can weed out other plants. Buckwheat, oats, and sunflowers are good types of these allelopathic plants.
7. Some cover crops can attract beneficial insects and repel bad insects like marigolds and crimson clover. Compost can't do that either.
8. Some special cover crops can help control diseases in the soil or on the foliage of nearby plants. Examples are garlic, onions, hot peppers, basil, marigolds, thyme, and other herbs. These plants can protect plants like tomatos from diseases. They can also control bad nematodes or other soil problems. Some of these cover crops can do these functions as living plants or as decaying green manures.
9. Some legume cover crops, like white clover, can be planted next to your crops during the warm season to be used as a living mulch.
10. Green manures work best when mixed with legumes and non-legumes. That way you get the nitrogen fixing benefit from the legumes, but also you maximize the fast growth of the expansive root development and tall foliage height that is characteristic of grasses and grains.
11. Did you know that about 95% of all the bulk and biomass of all non-legume cover crops is a direct production from only water and photosynthesis! The other 5% is straight from the soil. Legume cover crops go farther and pull free nitrogen from the air also. That means that all cover crops will add more humus to the soil than what was there before just from energy from the sun and the atmosphere. Since humus is mostly carbon, hydrogen, and oxygen from the air, that means that cover crops greatly benefit the biosphere and the soil microherd, just from above! Plain compost can't do that!
******************************************************************************** Green Manures have other nicknames also beside Cover Crops. Sometimes they are referred to as Smother Crops, Catch Crops, Nurse Crops, Living Mulches, or Compost Crops. Green manures also help conserve nutrients from the topsoil and even the subsoil, in bad weather, varying temperatures or seasons or climates. Green manures help solve crop rotation issues for small or large garden spaces. Green manures can even help heal soil diseases. Green manures can help buffer soil pH with its organic matter in the soil just like compost for the next planting season. Green manures are also great at controlling erosion and soil water moisture content. A nurse crop is a fast growing cover crop like rye that is grown close to a slower legume cover crop like hairy vetch in order to help support the plant's foliage as it grows. You can grow a cover crop in order to supply green nitrogen rich organic matter for your compost bins. Next to classic composting, green manuring is the greatest single thing you can ever do improve your soil and your gardening environment. ********************************************************************************
Mostly all cover crops can be grown easier by sprinking or lightly covering seeds on top of 1-2" finished compost or rich garden soil.
This a partial list of some of our favorite cool season and warm season cover crops and their basic purposes:
A. Crimson clover - cool season annual legume. More cold hardy than most other clovers. Grows great on all soil types in 1-2 months. Supplies lots of foliage and roots OM. Crimson clover and winter peas can be found in deer plot mixes. Attracts lots of beneficial insects. Easy to green manure. The favorite legume green manure of the south.
B. Hairy vetch - cool season annual legume. It is the most cold hardy of the popular legumes. Can grow in temps below -5 degrees F. Can be found in some deer plot mixes. Great legume weed suppressor. Grows great with a non-legume like rye as a nurse crop. The favorite legume green manure of the north.
C. Rye, winter wheat, and oats - cool season fast growing grass/grain non-legumes. Rye is the most cold hardy of all green manures, and it can grow almost anywhere. Rye can grow in temps below -20 degrees F. Rye and oats are a great non-legume weed suppressors. These crops can grow fast in a few weeks. Supplies lots of OM. Rye must be killed at least 4-6 weeks before planting spring crops to not hinder seed germination. You can find these cover crops in deer plot mix or horse or cattle feeds.
D. All beans and peas (legumes) - All beans are warm season except fava beans. Most peas are cool season. The most cold hardy peas are winter peas. Most dry beans and peas sold in bags in grocery stores will sprout and grow fine.
E. Radishes, mustard greens, rape, kale - Great cheap fast growing cool season annual non-legumes for lots of foliage OM. A thick crop of mustard, rape, or kale is a great non-legume weed suppressor. a thick crop of radishes are great for repelling many bug pests all year round.
F. Buckwheat and sunflowers - Great fast growing warm season annual non-legumes that can break up hardpan, kill weeds, and pull up insoluble subsoil phosphorus from minerals up to the topsoil to make available for next crops. Buckwheat can grow fast on any soil type (4-6 weeks to maturity).
G. Marigolds and other herbs - warm season annual non-legumes. Marigolds when planted thick, it's great for nematode control and other pest controls. Garlic, onions, and hot peppers control many soil pests.
H. All weeds - This unknown class of green manures recycle much needed foliage and root OM and nutrients back to the topsoil.
All cover crops should be either tilled in, mowed down almost to the ground, or smothered by organic mulches before they go to seed, to prevent sprouting later in the year and becoming a weed themselves. The no-tilling option is the best way to get optimal soil health, texture, and soil microbial activity. Recent research has proven that excessive tilling, or tilling too deep, can kill off beneficial fungi in the soil and create soil texture problems and some soil fertility issues. If you decide to go with a no-till garden, you can poke holes in the soil around crop roots with your spade fork, to get more oxygen in the soil to further increase organic matter decomposition and increase microbial activity in the soil. Year round green manuring practices can not only improve your soil health and texture, but also decrease extra periodic plant fertilizers and soil amendments.