Learn about cover crops that are grown to benefit the following crop, benefit the soil, and how to choose the right ones.
Benefits of Cover Crops
Cover crops are those grown to benefit the soil and the crop to be planted in the field after the cover crop. They are normally not intended for harvest and are sometimes called green manure since their purpose is to improve the soil and the following crop. Cover crops protect the soil, feed the soil ecosystem, increase organic matter, supply nutrients, and help retain water for the next crops planted. Properly managed cover crops also protect the soil surface from water and wind erosion and remediate soil compaction. Selecting the right cover crops can improve the yield and profitability of the next crop sown.
When selecting a cover crop, keep in mind that growing plants feed the soil ecosystem by releasing compounds such as sugars, organic acids, amino acids, and more from their roots. These compounds bind soil particles into aggregates, which improve the soil structure. When the plants decompose, their residue becomes organic matter that provides food for organisms including bacteria, fungi, and arthropods and increases the water storage capacity of the soil.
Cover crops improve soil fertility in several ways. Legumes and their associated bacteria fix nitrogen that becomes available to the next crop grown as the cover crop decomposes. The cover crop takes up unused nutrients from the previous crop, reducing leaching losses. Deep-rooted cover crops scavenge nutrients from deep in the soil. These nutrients become available to crops the next growing season as the residue decomposes.
Cover crops also often affect pest pressures in the cropping system. Thick plant residue helps suppress weed growth by blocking sunlight and physically slowing weed seedling growth. Some cover crops produce allelopathic chemicals that inhibit the growth of weed seedlings. Others suppress parasitic nematodes by repelling, confusing, or starving them. However, some cover crops can support insect pests, such as yellow- margined leaf beetles and armyworms on brassicas, nematodes (such as hairy vetch), and diseases. For example, field peas are susceptible to sclerotinia and should not be used in succession with potatoes, sunflowers, mustard, or beans as it is harmful to following crops. This is one reason it is so important to select the right cover crop for the cropping system.
Cover crops can make an economic difference to your operation, mainly by reducing production costs and risks. Improved water infiltration and storage can reduce irrigation costs and the effects of droughts. Nitrogen production and scavenging of nutrients can reduce fertilizer needs. The suppression of weeds and pests, costs of pesticides and their application, and the reduction of erosion and soil compaction lessen land preparation and tillage expenses.
Choosing the Right Cover Crops
Several cover crops are suited for Alabama cropping systems. Choose ones based on your unique growing system. Asking yourself what you need a cover crop to accomplish and what your time frame is between cash crops and when you can grow a cover crop is a good place to start. In general, choose a cover crop of an opposite type to the one you will plant next (broadleaf versus grass, for example).
If planting peanuts or soybeans, use any winter small grain, although cereal rye is usually best. If planting corn, use any small winter grain, a winter legume, or both. For cotton, use cereals or legumes. For vegetables, most cover crops are good choices, including winter and summer cereals, legumes, brassicas, and others. Do not use a legume cover before legume vegetables, however.
Crimson clover can be an effective cover crop, and brassicas can be added as winter covers to any of the crops above. Do not use a brassica cover before a brassica cash crop, however.
You can grow one cover crop species or a mix of two or more. Farmers across the country are experimenting with a variety of mixtures, called cover crop cocktails, to serve a variety of needs; however, research into the use of mixtures in Alabama is limited. Adding additional covers may dilute the benefits of each component. If a mixture is used, covers with similar maturities will be easier to terminate at the same time.
It is important that you know how and when you are going to terminate the covers before you plant. A cover that does not die when it is supposed to can be a problem. Green or half-dead covers can pull moisture from deep in the soil, interfere with planting, use up nutrients, and provide a bridge for pests to move directly to following crops. Generally, allow at least 2 to 3 weeks after termination before planting another crop. Cover crops that have entered bloom or seed development are generally much easier to kill.
What Cover Crops Provide
There are numerous reasons a cover crop might be needed, including the need for nitrogen or weed control, compacted soil, nematodes, attracting beneficials, or as forage.
Legume cover crops, such as sunn hemp, clovers, and hairy vetch, can capture nitrogen and add it to the soil. Some cereals, especially cereal rye, are good at scavenging unused nitrogen from a previous crop. Many cover crops, including buckwheat, mustards, radish, and rye, can scavenge unused nitrogen, phosphorus, and potassium from deep in the soil and move it closer to the surface. When the cover crop is terminated and decomposes, these nutrients are returned to the soil and are available to the cash crop.
Cover crops that produce a lot of biomass can help control weeds. Cereal rye, wheat, black oat, sorghum- sudangrass, sunn hemp, iron clay cowpea, radish, and buckwheat can smother small weeds. Cereal rye and sorghum-sudangrass also produce allelopathic chemicals that inhibit weed growth. These same chemicals can also affect crop germination and growth, however, so a waiting period may be needed before planting the next crop. Residual herbicides from the main crop can also affect cover crops, so check pesticide labels carefully for planting intervals.
Crops that have deep taproots, such as tillage radish and canola, can break through a compacted soil layer. Other crops that have dense root growth (cereal rye, sunn hemp, sorghum-sudangrass) add organic matter to the soil and improve the soil structure, reducing compaction and slowing recompaction.
Some cover crops, such as lupin, sunn hemp, velvet bean, black oat, sorghum-sudangrass, and some brassicas, repel, confuse, or starve nematodes. In a healthy soil ecosystem fueled by cover crops, parasitic and predatory organisms can help suppress nematodes. Although some cover crops help suppress pests, others can also harbor pests, so selecting the right ones for a cropping system is important.
Attracting Beneficial Insects
Beneficial insects pollinate crops and eat pests. Some covers that attract them are hairy vetch, lupin, canola, sunflowers, perennial peanut, and buckwheat. Flowers that have open petals attract a more diverse group of beneficials, while flowers with closed petals target very small bees and large-bodied insects that can push through the petals. Plant the cover crop that will deliver the desired outcome.
Some cover crops provide good-quality forage for grazing early on and still recover to benefit the soil system. Clovers, sorghum-sudangrass, and wheat are especially good for this. However, removal of biomass through grazing can weaken root systems and organic matter, and during wet periods, grazing animals can compact the soil. If too much cover is removed, many benefits, such as prevention of soil erosion, infiltration of rain, prevention of runoff, and shading of the soil, will be lost.
Managing a Cover Crop
Carefully managing your cover crop will ensure dividends. Plant quality seed at the recommended date, rate, and depth. Plant winter covers at least 2 to 4 weeks before the average first killing frost and summer covers when favorable soil temperatures and moisture are expected. Some cover crops may need supplemental irrigation after planting until the crop is established. If in an area prone to drought, a drought-tolerant cover crop such as sorghum-sudangrass or sweetclovers may be recommended.
Sources of Cover Crop Seed
Local seed suppliers and cooperatives often have commonly used seeds for your area.
Below are sources of some crops that are harder to find. Check their websites.
- Alabama Crop Improvement Association, Headland, Alabama – (334) 693-3988
- Petcher Seeds, Fruitdale, Alabama
- Adams-Briscoe, Jackson, Georgia
- Seven Springs Farm, Check, Virginia
- Johnny’s Seeds, Winslow, Maine
Another cover crop source is Arnold Caylor, who has been farming in Cullman, Alabama, and has been using cover crops for many years. As the past director of the North Alabama Horticulture Research Center, Caylor used cover crops for vegetable cropping systems, which call for covers in the summer as well as in the winter. He used a variety of mixtures, growing cereals for biomass and weed control, legumes for nitrogen, and brassicas for beneficials and pest control.
For mixes, Caylor recommends about 50 pounds per acre of cereals and 20 to 25 pounds per acre of legumes. One of his winter cover mixtures is rye, hairy vetch, and crimson clover. Another winter mix is crimson clover and 5 to 10 pounds per acre of canola/rapeseed or oilseed radish. These are planted from September through November and terminated March through April. When mixing a cereal with a legume, cut the cereal seeding rate 25 to 50 percent so it does not smother the legume, which is slow growing. Plant mix by the cereal’s optimum planting date.
Summer mixes can include tall, strong plants such as sunflower and sorghum sudangrass paired with viny legumes such as cowpeas, soybeans, and velvet bean. These are usually planted from late April through June and terminated from July through August.
- Sustainable Agriculture Research and Education website
- Sustainable Agriculture Research and Extension Program, University of California – Davis Resource Page
- Midwest Cover Crops Council
- USDA-ARS Northern Great Plains Research Laboratory Cover Crop Chart
- Cornell University Cover Crops for Vegetable Growers
- Alabama Cooperative Extension System
- USDA National Soil Dynamics Laboratory
Cover Crop Seeding Rate Table
E = Excellent; VG = Very Good; G = Good; F = Fair; P = Poor/None (W) = Winter annual; (S) = Summer annual; (P) = PerennialSource : aces.edu