COVER CROPPING BENEFITS
A Brief Essay
March 02, 2001
Robert M. Dixon, PhD
Cover cropping is a common process that falls within the realm of the agronomic sciences. This process is employed in sustainable agriculture, ecological restoration, mined-land reclamation, revegetation, weed suppression, and soil conservation. Cover crops are often used on barren land that is highly susceptible to wind and water erosion. They often are considered to be a temporary means for establishing more permanent vegetation.
Cover cropping has many soil and societal benefits including:
- Soil erosion control
- Water infiltration control
- Soil structure enhancement
- Topsoil formation acceleration
- Wildlife habitat improvement
- Soil invertebrate food
- Greater soil macroporosity
- Greater soil surface roughness
- Greater soil water holding capacity
- Increased soil organic matter
- Soil fertility modification
- Weed control or suppression
- Secondary succession acceleration
Each of these thirteen benefits and their interactions will now be discussed on this and following pages.
1. Soil Erosion Control
In a narrow sense, a cover crop is literally a plant shield for a barren soil to protect it from the hazards of wind and water erosion. Quick growing annual grasses provide excellent control of wind and water erosion of barren, structureless soils by reducing the velocity of these two fluids as they flow over the soil surface. Topgrowth absorbs the energy of raindrop impact, thereby preventing splash erosion of the soil surface. The fibrous root system of grass binds the soil together to resist rill and gulley erosion. Soil protection from erosion is perhaps the most common benefit of cover cropping although there are close rivals.
2. Water Infiltration Control
Infiltration is greatly accelerated by a grass cover crop. The topgrowth increases hydraulic roughness of the soil surface, thereby ponding the surface water required to drive the infiltration process, especially preferential water flow into the open soil macropores. Topgrowth also increases soil macroporosity by surface feeding the soil invertebrate animals that burrow deeply into the soil. A grass cover crop not only enhances both surface roughness and surface macroprosity but it stabilizes these two soil properties as well, ensuring high sustained infiltration rates. High infiltration means low water runoff, erosion, flash flooding and sedimentation-not to mention a flourishing cover crop.
3. Soil Structure Enhancement
The fibrous root system of grass starts building structure in a degradated soil. Leaf litter adds organic matter to the soil which in turn builds stable soil structure. Decomposition of organic matter releases complex sugars and gums that provide the glue for stable aggregates. Soil aggregation increases water infiltration, soil water holding capacity, and soil aeration-conditions contributing to biomass productivity. Soil water movement and soil fertility also depend on soil aggregation.
4. Topsoil Formation Acceleration
A cover crop of either grasses or legumes adds organic matter and structure to the surface soil to build a fertile productive topsoil. The infiltration and aeration benefits of cover crops also serve to accelerate topsoil formation. Some of the best topsoils in the world were originally formed by prairie grasses such as in the U.S. Cornbelt region. Again the unique topgrowth and root system of grasses is instrumental in topsoil formation.
5. Wildlife Habitat Improvement
A cover crop, by providing food, greatly enhances the habitat for wildlife ranging from large mammals to soil microbes. As previously noted, cover crops increase soil invertebrate burrowing, soil macroporosity, and water infiltration. More soil moisture means greater forage production for wildlife.
6. Soil Invertebrate Food
An important function of cover crops is to provide food (plant litter) for surface feeding invertebrates such as earthworms, ants and termites. Burrows of these invertebrates contribute greatly to both infiltration and aeration-processes essential for rapid growth of the cover crop and any other species seeded with the cover crop.
7. Greater Soil Macroporosity
The way in which cover crops stabilize macroporosity has been discussed previously, however the prevention of surface clogging of the macropores by splash-eroded soil should be stressed as a cover crop benefit. Likewise the role of surface-connected macropores in enhancing both infiltration and soil aeration has already been described. Enhancement of these two soil properties accelerates the growth rate of the cover crop and any other seeded species.
8. Greater Soil Surface Roughness
Besides soil surface macroporosity, the hydraulic roughness of the soil surface is the other principal property controlling water infiltration through the air-earth interface or soil surface. Grass cover crops increase hydraulic roughness in two main ways. The closely spaced grass stems and leaves at the soil surface impede surface water flow and the existing microroughness of the soil surface is maintained by preventing the smoothing action of splash erosion. Microroughness comes from burrowing activities of animals, foot/hoof prints, wheel tracks, and tillage implements such as land imprinters.
9. Greater Soil Water Holding Capacity
The ability of soils to hold more water against the pull of gravity because of cover cropping can be deduced directly from the comments made under the foregoing benefits. Improvements in topsoil, soil structure and organic status all add to soil water holding capacity and water availability to plants. Accelerated infiltration also increases subsequent water availability and the growth rate of plants especially in arid regions and during short-term droughts in more humid areas. Not only is water infiltration accelerated but water penetration is deeper where it is less susceptible to loss by surface evaporation.
10. Increased Soil Organic Matter
As discussed previously cover cropping builds soil organic matter. The fibrous root system of grasses adds organic matter directly to the topsoil whereas the topgrowth (leaves and stems) if not eaten by herbivores eventually accumulates on the soil surface as a plant litter mulch to increase infiltration, suppress surface evaporation and feed the soil burrowing invertebrate animals. The mulch and soil organic matter have soil fertility ramifications that will be discussed in the next cover cropping benefit.
11. Soil Fertility Modifications
Cover cropping can profoundly affect soil fertility because of changes in plant-soil-water relations resulting from the growing plants, surface mulch and soil organic matter. Fertility can either increased or decreased depending on the stage of decomposition of the mulch and soil organic matter. This fact can be useful in manipulating the natural plant succession (pushing it forward or backward for weed control), sustainable agriculture and ecological restoration. More about this under the 12 th and 13 th benefit.
12. Weed Control or Suppression
Weeds are not eradicated by using cover crops but they may be managed, suppressed or controlled to the point that they all but vanish except for the seeds in the soil bank which may persist for many years, depending on the weed species. Fast growing cover crops such as annual ryegrass can out compete the weeds, keeping them in the dark and out of water even though the weed seeds may germinate and produce small severely stressed seedlings. The cover crop can then serve in the roles of nurse, mulch and green manure crops to establish perennial plants that will keep the weeds in the dark and out of moisture indefinitely into the future unless a land disturbance gives the weeds a chance to recolonize and even thrive.
Weeds can be displaced by pushing the natural succession ahead of the weed stage or pushing it backward in case the weed is a late successional type. But usually common weeds are early or mid-successional and can be displaced by accelerating the succession rather than regressing it. Several cultural practices that have proven to be effective in advancing the secondary succession include land imprinting, cover cropping and the application of dolomitic limestone or the sulfates of calcium and magnesium. These practices work in concert to improve the soil moisture regime enough for the establishment of late seral species that replace the weeds. Keep in mind that the weed seeds are still waiting patiently in the soil for a land disturbance to come along to allow them to again flourish. A common disturbance is the use of chemical herbicides which may be effective in killing the current crop of weeds but do little to control weeds arising from the soil seed bank in future years. Chemical herbicides are not to be recommended because of the often unforeseen collateral damage they do to the environment. Remember Agent Orange and Vietnam not to mention the super weeds that herbicide use often produces.
13. Secondary Succession Acceleration
As noted previously cover crops can accelerate the secondary succession of plant types mainly by improving the deep soil moisture regime which allows the later seral species to displace the cover crop and other early seral plants such as weeds. As noted previously, succession can be promoted by land imprinting seeders and the application of soil amendments. The mulch produced by the cover crop aids succession by nitrogen stressing any weeds that may be present. Fungal decomposition of the mulch immobilizes the soil nitrogen that would otherwise be available for weed growth.
My first experience with cover/nurse crops occurred when I was a Kansas farm boy during the dirty thirties. A common farm practice to exclude weeds from a new stand of alfalfa was to use oats (an annual grass) as a cover crop for the under-seeded alfalfa. The alfalfa and oats were seeded together in early February. The oats a cool season grass, germinated and grew rapidly, however it was seeded at about one-half the normal rate to allow some light to reach the alfalfa which started to grow before the normal spring weed season. By the time the soil became warm enough for weed germination the soil was completely shaded and dried out by the two-story canopy of oats and alfalfa. When the oats completed its life cycle in June it was harvested by cutting off the seed heads just above the top of the alfalfa seedlings. The result of this cover crop strategy was a good stand of alfalfa free of the weeds that normally plague new stands. I have since learned that this is a common practice for establishing weed free alfalfa in United States and that it was brought to this country by the European pioneers. This practice now fits the category of agroecology or sustainable agriculture.
This concludes my brief essay on some of the many benefits of cover cropping. For the reader who may want to do some cover cropping a good source of seed is: Peaceful Valley Farm Supply, P.O. Box 2209 , Grass Valley , CA 95945 , (888) 784-1722, www.groworganic.com
Finally, did you know that our farmer founding fathers, Washington and Jefferson, were especially fond of cover cropping and the cover crop, hairy vetch? More information on cover crops and cropping can be obtained from the seed source given previously.