Cover Crops for Organic Farms
Benefits and Drawbacks of Cover Crops
The use of cover crops can result in substantial benefits as well as be a source of unforeseen problems in organic crop production. The choice of which species, as well as the decision whether or not to plant a cover crop, is based in large part on the benefits desired. Problems associated with cover crops are often a result of unsuitable species or inappropriate management practices keeping in mind the best choice of species and farming practices can be highly site-dependent and often evolves through trial and error. Because species or organic crop production practices may have benefits and limitations, the best choice is often a compromise.
Benefits of Cover Crops
Cover crops can provide protection during those periods when a primary crop is not present. Grasses (e.g., rye, wheat, and sorghum) are easy to establish and form extensive root systems, thus reducing soil erosion and improving soil structure.
Organic Matter and Soil Structure
A major benefit obtained from cover crops is the addition of organic matter to the soil. During the breakdown of organic matter by microorganisms, compounds are formed that are resistant to decomposition—such as gums, waxes, and resins. These compounds—and the mycelia, mucus, and slime produced by the microorganisms—help bind together soil particles as granules, or aggregates. A well-aggregated soil tills easily, is well aerated, and has a high water infiltration rate. Increased levels of organic matter also influence soil humus. Humus—the substance that results as the end product of the decay of plant and animal materials in the soil—provides a wide range of benefits to crop production.
One of the most significant contributions of legume cover crops is their ability to fix atmospheric nitrogen in their plant tissues in relationship with rhizobium bacteria. In association with legume roots, the bacteria convert atmospheric nitrogen into a form that plants can use. The amount of nitrogen fixed is directly related to the amount of biomass produced by the cover crop.
Recycling or Scavenging Unused Nutrients
Cover crops can capture leachable nitrogen and making it available to the following crop after manure or compost applications. In general, legume cover crops do not scavenge nitrogen as well as grasses since they acquire nitrogen from the atmosphere, making them less aggressive at scavenging soil nitrate.
The use of cover crops in well-planned crop rotations can help to interrupt the life cycle of many fungal, bacterial, insect, or nematode pests. To interrupt pest life cycles, it is important to select cover crops of a different family than that of the future cash crop so that they do not harbor pests that can negatively impact the cash crop that will follow.
Cover crops partially control some weeds by competing with them for light, moisture, nutrients, and space, which can be particularly helpful for suppressing winter annual weed growth or certain cool-season perennials. In addition, cover crops and their residues can act as mulches or physical barriers by smothering weeds, suppressing weed seed germination and growth, and lowering soil temperatures.
Soil Microbial Activity
Organic matter is a food source for microbiological life, which thrives in healthy, well-aerated soil. When a green manure crop is incorporated into the soil, microbes multiply to break down the fresh plant material, resulting in a rapid increase of microorganisms.
Soil Water Management
Cover crops enhance water infiltration by improving soil structure and reduce sealing and crusting of an otherwise bare soil surface. If the cover crop is killed without tillage by using a roller/crimper, the surface mulch will also reduce the evaporation rate of water from the soil during summer growth of the next crop.
Cover crops can significantly alter soil temperatures. Cover crops decrease the amplitude of day and night temperatures more than average temperatures resulting in less variability.
Drawbacks of Cover Crops
Deplete Available Water Supply
One important consideration when using overwintering cover crops is their potential to deplete soil water.
There are additional costs above and beyond normal cropping practices that must be considered in systems that include cover crops.
Rates of Mineralization of Nutrients for the Subsequent Crop
Winter cover crops are used in part for their ability to scavenge nutrients, particularly nitrogen, which are then maintained in their biomass during the rainy season. Whether the cover crop is grown in winter or summer, it takes up and thus immobilizes nutrients, which then need to be mineralized for the subsequent crop.
Cover crops may attract and provide shelter for pests, potentially increasing the pest and/or disease populations in the field. Consider specific pest/crop interactions that may become a problem. For example, cereal rye or orchardgrass can attract armyworms. Clover root curculio, a pest common to red clover, also can attack alfalfa.
Interference with Primary Crop
Unmanaged cover crops can act as weeds by competing with the primary crop for light, moisture, nutrients, and space. In a dry year, cover crops can rob primary crops of valuable soil moisture. In other years, they may also compete for other resources such as nitrogen if not managed properly. For most cropping systems with cover crops, the use of starter fertilizer during planting of the primary crop should compensate for nutrients used by the cover crop. Immobilization of nitrogen by the cover crop generally is negligible, especially if manure has been applied.
Risk of Frost Damage
A bare, firm, moist orchard or vineyard floor absorbs heat during the day and releases it at night, increasing the air temperature by as much as 3 to 4 degrees F (1.6 to 2.2ºC).
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Topics Within This Chapter:
- Introduction to Cover Crops for Organic Farms
- National Organic Program Standards for Cover Crops
- Benefits and Drawbacks of Cover Crops
- Life Cycle of Cover Crops
- Types of Cover Crops
- Cover Cropping Systems
- Nitrogen Fixation by Legumes
- Green Manures
- Catch Crops
- Managing Pests with Cover Crops
- Cover Crop Strategies with Crop Rotations
- Selecting Cover Crop Species
- Buliding Complimentary Cover Crop Mixtures
- Cover Crops in Perennial Systems
- Establishment of Cover Cropss
- Termination of Cover Crops