The desired outcome is an important consideration when selecting a cover crop. Keeping the end result in mind during the cover crop selection process goes a long way in making the best choice. Be sure the plants chosen are not a host for pests of the following cash crop. In addition, consider the following characteristics when selecting a plant or a mixture of plants for a cover crop [10]:
- ease of establishment
- early growth rate
- rooting depth
- biomass yield
- pest resistance
- ease of termination
- cost
- nitrogen fixation
Cover Crop Considerations to Enhance Benefits
Identify your primary objectives when choosing a cover crop(s). Examples of objectives include reducing soil erosion, improving soil moisture, service as a nitrogen source, providing beneficial insect habitat and/or wildlife habitat, etc., or a combination of objectives. Table 5.1 summarizes the relative benefits of common cereal and legume cover crops, and brassicas. Once you have identified your objectives, determine the species that best fit your particular soil type, climate and cropping window. Fortunately, many cover crops provide multiple benefits and are useful for a range of purposes.
Soil Erosion
Grass cover crops are a good choice for reducing runoff and consequently soil erosion because they are fast to establish and have deep fibrous roots. Their rapid establishment and early season growth can provide better than 50 percent ground cover in as little as 30 days. Good ground cover protects the soil from the impact of raindrops and provides protection against erosion due to wind. Mixtures of grasses and legumes may provide even better soil cover. As soil physical properties improve with long-term cover crop use, additional benefits are often observed due to increased water infiltration and further reductions in runoff.
The key to reducing soil erosion is making sure the soil surface is covered with a growing crop or crop residues all of the time. The long growing season and mild winters in the Southeast are well suited to year-round soil coverage by growing plants. Winter cereals and many brassicas often put on significant growth in the fall, even when temperatures drop into the 40s and 50s. Their rapid fall and early-winter growth make them good choices for reducing soil erosion. By slowing erosion and runoff, cover crops reduce nonpoint source pollution caused by sediments, nutrients and agricultural chemicals.
Nutrient Management
Nitrogen and phosphorus are the two nutrients most likely to be lost from cropping systems through runoff, leaching and, in the case of nitrogen, volatilization. Cover crops help reduce these losses in a number of ways:
- increasing infiltration—thus reducing surface runoff and erosion of soil particles containing adsorbed nutrients
- taking up nutrients—acting as a “catch crop”
- using water for growth—reducing the water available to leach nutrients
Cover crop roots can even help unlock some nutrients in the soil and convert them to more available forms. Fast-growing grasses and brassicas reduce nutrient losses because they are good at scavenging excess nutrients, especially nitrogen, left in the soil after cash crop harvest. Scavenging excess nitrogen can also improve water quality by preventing nitrogen leaching to groundwater. In the Southeast, cereal rye is effective at reducing nitrogen leaching because it is cold tolerant, has rapid growth and produces a large quantity of biomass. Legume cover crops are not as effective at scavenging nitrogen before the winter leaching season, but they do take up some soil nitrogen and fix large amounts of atmospheric nitrogen. As a result, legume cover crops can provide 30–60 percent, and sometimes more, of the nitrogen needed by the following crop [10].
Winter annual weeds vary in their responsiveness to nitrogen: some species accumulate as much nitrogen as small grains while others take up relatively little residual soil nitrogen. However, winter weeds are not a good substitute for either a monoculture or mixed species cover crop because of the potential for increased weed seed density and management complexity. The amount and availability of nutrients from cover crops will vary widely depending on such factors as species, planting date, plant biomass and maturity at termination date, residual soil fertility, and temperature and rainfall conditions.
Soil Moisture
In addition to increased water infiltration from terminated cover crop residue, evaporation is also reduced, resulting in less moisture stress during short-term droughts. Winter cereal cover crops such as rye, oats and wheat, and late-summer/early-fall grasses like a sorghum-sudangrass hybrid are especially effective at covering the soil surface.
Pest Management
Cover crops that produce high levels of biomass help manage weeds by competing with the weeds for water, light and nutrients. Cover crop residues or a growing plant canopy block light, alter the frequency of light waves and influence surface soil temperatures. All of these negatively impact weed seed germination. Many cover crops produce root exudates and organic compounds that provide natural herbicidal (allelopathic) effects against weeds.
In addition to suppressing weeds, some cover crops help reduce damage from diseases, insects and nematodes. A growing cover crop adds root exudates and organic compounds that encourage diverse populations of soil microorganisms. The increased diversity creates an inhospitable soil environment for many soilborne diseases and helps suppress certain disease organisms. Root compounds may also reduce harmful nematode populations and encourage beneficial nematode species. Brassicas release bio-toxic chemicals and metabolic byproducts that have shown some activity against bacteria, fungi, insects, nematodes and weeds. However, the level of activity is low compared to traditional soil fumigants.
Cover crops provide habitat for beneficial insects and wildlife. Beneficial insects and parasitoids that prey on pests can reduce insect damage below economic thresholds. Tillman et al. [11] showed that mixed cover crops increased the prevalence of insect predators, especially big-eyed bugs (Say, Geocoris punctipes) and red imported fire ants. This led to a reduction in the level of budworms and bollworms (Heliothine moths, Lepidoptera: Heliothinae) in conservation-tilled cotton compared to conventional-tilled cotton without cover crops. Cover crops provide both food and habitat that help reduce large fluctuations in insect populations and imbalances between pests and beneficial insects.
Cover crops also serve as sources of food and habitat for wildlife, and hunting leases can provide an additional source of income for growers. Cover crop benefits can be enhanced by increasing the diversity of cover crops grown through mixtures, the frequency of use between cash crops and the length of time that cover crops are growing in the field.
Improve Soil
Cover crops contribute indirectly to overall soil fertility and health by catching nutrients before they can leach out of the soil profile or, in the case of legumes, by serving as a nitrogen source. Soil organic carbon (SOC), a key indicator of soil quality, can be increased by using high-residue cover crops. Soil chemical and physical improvements associated with increased SOC contents are well documented. Recent interest in climate change highlights the potential that high-residue cover crops possess for carbon sequestration and potential government payments. The Soil Conditioning Index (SCI) is a tool used by USDA Natural Resources Conservation Service (NRCS) to predict how SOC levels are affected by cropping and tillage systems [7]. Positive SCI values predict SOC levels will increase, while negative SCI values predict SOC levels will decrease [6]. There is more information about the SCI in Chapter 3.
Table 5.2 summarizes SCI values for various scenarios and highlights the importance of crop rotations and maintaining residues. Government programs, such as the Environmental Quality Incentives Program and the Conservation Securities Program, do not currently use SCI values, but future payments related to carbon sequestration could potentially be based on SCI levels. Balkcom [1] examined the relationship between measured SOC values for various tillage and cover crop combinations after six years and predicted SCI values for one location in the Southeast. Although a reasonable relationship between the SCI and measured SOC values was observed, there were discrepancies that indicated opportunities to improve the SCI for the Southeast. In lieu of this information, growers should be aware of how their cropping and tillage practices may be evaluated in the future with regard to potential carbon sequestration.
Choosing a Cover Crop
As with any good crop rotation, it is more desirable for grass cover crops to precede legume cash crops and for legume or broadleaf cover crops to precede grass cash crops. This practice helps reduce insect and disease problems attributed to monoculture systems and helps ensure good nitrogen management. A legume cover crop following a legume cash crop has the potential for excess nitrogen accumulation, and a grass cover crop following a grass cash crop has the potential for significant nitrogen immobilization. In the Southeast, choose cover crops to maximize biomass because the warm, humid climate promotes crop residue decomposition, resulting in loss of organic matter needed to maintain soil productivity.
Cover crop mixtures enhance benefits associated with each plant type. For example, a legume/grass mixture provides the benefits of nitrogen fixation from the legume and greater biomass production associated with the grass. Combined residues may result in nitrogen release that more closely matches the nitrogen needs of the following crop. Another example is combining two legume species with different times to maturity to extend the flowering period. This provides an extended period of enhanced beneficial insect habitat. Cover crop mixtures can increase seed costs and do require greater management.