THE MORE I FARM, the more I appreciate cover crops and green manures. Though they don’t go to market with us and are therefore never seen by our customers, these crops play a vital role in the overall health and productivity of our farm. Cover crops and green manure are seldom grown for human consumption. Nor, on a vegetable farm, are they likely to be used to feed animals, unless the farmer employs horses or oxen in place of tractors, in which case their above ground parts may come in very handy as fodder. The most obvious purpose of a cover crop, as the name implies, is to keep the soil covered while it’s not being used for other agricultural purposes. But a green manure will often do the same thing.
So, what exactly is the difference between a cover crop and a green manure? Often, not much. Both help replenish and maintain healthy soil and both perform many overlapping, often identical functions. It comes down to a matter of emphasis. With cover crops, the emphasis is on protecting the soil from erosion. With green manures, the emphasis is on adding organic matter and sometimes nitrogen to the soil, but these things might also be asked of a cover crop.
Here’s a brief rundown of the some of good things that cover crops and green manures can do for a farmer, or a gardener, for that matter. Don’t worry too much about which is a cover crop or which is green manure—depending on the goal, one might use either name for the same crop.
Cover crops and green manures, like all forms of vegetation, hold the soil together; they protect it from erosion by water and wind. This is especially important in the winter months and during spring thaw when the land might otherwise be bare. Erosion rates in the U.S. have declined over the past 25 years but they are still alarmingly high. According to the USDA, we are losing about 1.7 billion tons of topsoil to erosion every year on farmland alone. More widespread use of cover crops and green manures would greatly reduce the loss of this invaluable and finite resource.
Add organic matter and humus
When they are returned to the soil from which they came, cover and green manure crops add significant amounts of organic matter that were not present before. The decomposing plant material provides food for the trillions of organisms that live in a healthy soil. Once they have fully digested this windfall of organic matter, worms and other soil critters leave behind a substance called humus. Humus is light and fluffy, and has a pleasant, earthy smell. It’s mildly gummy quality helps bind soil particles together into aggregates, making them less susceptible to erosion. A soil rich in humus is said to have good structure, or tilth.
Humus also absorbs and retains water, a big plus for a farmer during droughty periods. And it acts like a magnet, holding on to certain nutrients for plants to use. Without a good supply of humus, these nutrients can leach out of the soil and be lost. Most organic farmers would agree: The more humus, the better. But remember, the road to humus often begins with a cover crop or green manure. (Decomposing weeds or applications of compost will do the same.)
Capture and recycle nutrients
While they are alive and growing, cover crops and green manures capture and hold on to nutrients before they leach down below the reach of most plant roots—something that can easily happen during periods of prolonged rain, especially if a soil is left bare. When the cover crop or green manure dies and decomposes, the nutrients it has captured become available to subsequent crop plants that reside in the upper reaches of the soil.
Deep-rooted cover and green manure crops that are permitted to grow for several months, or longer, also can acquire minerals and other nutrients at levels that most vegetable roots would never reach. These ingested nutrients rise up through the crop’s roots and vascular systems. When the crop dies and is left to decompose, the upper levels of soil are enriched.
Many cover crops and green manures excel at suppressing weeds. They grow fast and commandeer available nutrients, moisture and sunlight. The weeds are essentially starved of what they need in order to survive. This is especially helpful in the summer months, when weeds can easily get the upper hand and start going to seed, creating even more weed problems down the road.
Add nitrogen to the Soil
Nitrogen is an essential plant nutrient. It is also a nutrient that is often in short supply, which is why leguminous cover and green manure crops (clover, vetch and alfalfa) can be a farmer’s best friend. These crops enter into a symbiotic relationship with soil bacteria called rhizobia. The rhizobia, which attach themselves to the roots of the legume, are able to capture the atmospheric nitrogen that resides in pore spaces in the soil and convert it into a form that both the host legume and other plants can use. In return, the rhizobia receive carbohydrate from the legume crop. The process is known as nitrogen fixation. It is a good way to increase a soil’s supply of nitrogen without adding animal manures or resorting to the use of synthetic fertilizer. If your primary intention is to add nitrogen to the soil, then the term green manure is more appropriate than cover crop.
Attract beneficial insects
Many cover and green manure crops provide habitat for beneficial insects like ladybugs, lacewings and parasitic wasps. They may also harbor insect pests—not necessarily a bad thing, since the pests are likely to be prey species for the beneficials. After all, for the beneficials to stay around, there has to be something for them to eat. The important points are balance and diversity. When you have a healthy population of predatory beneficials, you are unlikely to suffer a serious outbreak of pests.
Not all cover and green manure crops produce flowers, but those that do provide a valuable source of pollen and nectar for bees. And bees, aside from producing honey, are important plant pollinators, which makes them a good insect to have around.
Each year, we seed at least four or five different cover and green manure crops in our fields. It’s a pleasure to watch them grow and count the many good things they do for our land and us. There are numerous cover crops and green manures to choose from, each offering its own set of benefits, many of which are shared in common. Deciding which one to use in a specific situation is not always easy. It will depend on what your primary goal is, along with variables like the time of year and the length of time the crop will be in the ground. The cover crops and green manures listed here are the ones we grew on our farm in 2010; next year the lineup may be different.
This fast-growing, broad-leaved annual thrives in the summer heat and does a great job of blocking out weeds. It can blossom in five or six weeks and set viable seed in a few more. Because buckwheat plants are tender, green and succulent, they can usually be turned into the soil without the need for mowing. Another of buckwheat’s attributes is that it breaks down quickly once incorporated. This means a farmer can transplant vegetables into ground that had buckwheat living on it only a week or two before.
An acre of buckwheat in flower is a captivating sight. It will also attract thousands of bees that come to dine on the nectar. It’s a delight to see them hovering over and gently probing the white blossoms. But a wise farmer will not allow the bees to partake for very long: Once flowers appear, buckwheat seeds are not far behind. To let the seeds develop to maturity would mean a second round of volunteer buckwheat not long after tilling in the first—a case of unintended and unwanted consequences. So a farmer has to adopt a tough love attitude and turn in the little white flowers, even though it means disturbing the bees and shortening their stay at the dinner table.
If you’re looking for a cover crop that will block weeds and give you major biomass production in the hot, dry days of summer, sorghum—sudangrass would be a good choice. This cross between sorghum, a forage or grain crop that looks a lot like corn, and sudangrass, can grow over ten feet tall in a couple of months, depending on the variety. If mowed a few times and allowed to re-grow, an acre of sorghum sudangrass can produce more than 15,000 pounds of dry material. That’s a serious contribution of organic matter. With this crop, mowing prior to incorporation is essential; but even then, sorghum sudangrass can take a couple of months to break down. We often use it as both a cover and green manure on land that will be planted to garlic in late October.
Winter, or cereal rye, is an annual, unlike the perennial rye grass you might have in your lawn. It is widely used as a cover crop in the northeast. Winter rye is usually seeded in the fall, after summer crops are harvested. It grows rapidly in the cool, late-season weather and provides excellent erosion control over the winter. When spring arrives, winter rye resumes growing and can reach a height of five feet by late May, at which point it is prudent to mow the crop and turn it in. If you give the plant enough time to develop mature seeds, you can have rye coming back at you as an unwanted and difficult-to-eradicate weed for years to come.
Winter rye has a fibrous and far-reaching root system, which enables it to scavenge nutrients at deeper levels and recycle them into the growing zone that most vegetables occupy. This is a major plus. Though it cannot capture atmospheric nitrogen, as some cover crops and green manures can, rye is adept at finding and storing nitrogen that is already in the soil before it can leach down below the root zone. It also contains allelopathic compounds, both while it is growing and while it is decomposing. This means it will inhibit the germination and growth of many other plants, including weeds and vegetables. We always wait three or four weeks after incorporating rye before seeding any cash crop.
Oats prefer cool weather. In the northeast they can be seeded in April or May for early season cover, or in late August or September for winter cover. Oats planted in the spring can produce a not-too-shabby 6,000 to 8,000 pounds of dry weight per acre.
Seeding oats in late summer or fall is a different story. Because they are less hardy than rye, oats will die in a northeastern winter when temperatures get too cold for them. This winter kill is not necessarily a bad thing. We often seed oats in the fall in fields that are scheduled to be planted to vegetables in April or May of the following season. Because the oats are already dead, they are easy to incorporate into the soil and will break down rapidly. Sometimes, we skip the incorporation phase altogether and let the dead oats act as a mulch that we transplant right into. With winter rye, our other major winter cover, this would not be possible.
Crimson clover is a fast-growing, cool-season annual with a striking crimson flower. Aside from facilitating the capture of atmospheric nitrogen (100 to 200 pounds per acre in a season is possible), crimson clover can provide a few thousand pounds of organic matter, per acre, as well.
In general, clovers are used as green manures when the intention is to add nitrogen to the soil, rather than simply the largest amount of organic matter possible. Because they take longer to establish than buckwheat, oats, rye or soreghum sudangrass, they are seldom used as a short-term cover and weed-suppressing crop. There are several differnet clovers available to northeastern growers, some perennial and some annual. Depending on the goal we have in mind, we commonly use the red, white and crimson species. To gain maximum benefit, the perennial clovers are best left in place for at least a year.
Cover crops and green manures—let’s just call them green crops this one time—can do much good for farmers and gardeners who are motivated to use them. They will also bring beauty and diversity to the land.