Click on the following links to be taken to the section of this page describing each benefit.
The role of cover crops in reducing soil erosion is probably the simplest benefit to explain. In a cropping system, even no-till, a crop is planted, grows, and is harvested. In the case of corn and soybeans this harvest is in the fall. The land then lies fallow until it is planted next spring. If anything does grow, it is weeds.
A field without cover crops is less protected from all sorts of erosion. Take wind erosion. As wind blows across bare soil, it raises dust. If cover crops are planted, it blows across vegetation.
However the real soil savings comes from water erosion. Erosion occurs in several ways. One of the most difficult to recognize is known as sheet and rill erosion. Here true gullies do not form. Very small gullies known as rills do but much of the erosion is caused by the relatively uniform passage of water down a slope. Despite this not being very visible, it can result in significant soil loss.
Larger gullies are more visible and caused by concentrated surface flows of water. Some of these are ephemeral, meaning they do not form in the same place in a field year-after year. Classical gullies; the very large, often deep damage caused by water moving in a channel, are another cause of significant soil loss.
One of the most devastating periods of erosion in Indiana occurs during thaws following extended cold periods. Say there is a foot or more of frost in the ground. Several warm days follow. Now the top few inches of soil have thawed out. If a significant rain event occurs, that water can do nothing but flow across the surface. It can’t percolate into the soil as there’s an (nearly) impermeable frost layer. Instead it runs across the surface, carrying soil, often in significant quantities. With cover crops, the root system will hold soil in place, reducing soil loss.
Another source of erosion comes from rainfall itself, even when relatively gentle. When a raindrop strikes bare soil, the impact splashes small particles of soil out of place. These small particles are much more easily carried away by water.
So how do cover crops help? By having vegetation in place outside of the normal growing season, the root systems hold soil in place, drastically reducing the amount lost. In the case of raindrops, the rain falls on vegetation rather than bare soil.
The root system of cover crops penetrate into the soil and leave small channels water can pass through. Instead of running across the surface, water can percolate through these passages. In addition, a field with cover crops will have significantly higher levels of earthworms and other insects and grubs living beneath the surface. These organisms also create subsurface channels which can hold water. Over time, the use of cover crops often increases soil organic matter. Higher organic matter levels are associated with increased permeability and soil water-holding capacity. If heavy rains occur, fields which have grown cover crops typically see less surface runoff and therefore less erosion than their “naked” neighbors. These benefits continue throughout the growing season after the cover crops have been terminated.
Weeds are very opportunistic. In a typical farming system, once the crop is harvested in the fall and the soil surface is left bare, various weed seeds begin to germinate. Some of these may be part of a “seed bank” already present in the soil, germinating when growing conditions such as soil moisture and temperature are right. Other weed seeds may enter the field through wind, farm equipment or be carried in by wildlife such as birds. Whether these weeds are perennials, winter annuals, or annuals that germinate in the spring, they will need to be controlled the following season either before or shortly after planting in order to have a successful crop.
Cover crops act to suppress weeds in several ways. Once cover crops are established and grow, they will shade out sunlight from reaching weed seeds or seedlings, preventing them from germinating and growing. The cover crop will be using available soil fertility, meaning the weeds will not find the nutrients they need in order to thrive.
Some cover crops are allelopathic. This means their root systems give off chemicals which discourage other plants, including weeds, from germinating and/or developing roots, thus retarding weed growth. Caution must be used in identifying cover crop allelopathy to ensure that it does not negatively impact the subsequent, desirable crop as allelopathic effects may persist for several weeks after the cover crop has been terminated.
Cover crops and their root systems can alter soil microbial communities or microflora. In many cases this causes various soil factors to become less favorable for weed growth and development. Some cover crops host microbes that are actively harmful to weed species. Once again, caution must be used so the microbial community is not harmful to the subsequent, desirable crop.
Once a cover crop is terminated, the resulting plant material remains in the field. For some species, such as annual ryegrass, perennial rye and cover wheat, the resulting biomass layer is sufficient to act as a mulch, preventing weed seed germination by not allowing sunlight to penetrate to the soil surface.
Improved Soil Fertility
The use of cover crops over a period of time can have such an impact on soil fertility levels that it is impossible to cover this topic in any depth here. Cover crops are often considered to be a “soil builder.” This term encompasses a wide variety of soil characteristics including some, such as soil health and soil water-holding capacity which will be discussed below. For this section I will focus on soil fertility as a product of nutrients available to plants for uptake during the growing season although soil fertility actually covers much more ground than this.
One of the functions of cover crops is to serve as a scavenger of nutrients available in the soil, particularly nitrogen. As nitrogen is relatively ephemeral, any which is not used by the cash crop during the growing season is generally lost over the winter. Farmers do not apply fertilizer so the crop runs out during the growing season as this would reduce yields so there is generally some remaining in the soil after harvest. A cover crop will take up and hold this nitrogen. When it is terminated, either during the winter or in the spring, nitrogen is released back into the soil and is available for the subsequent growing season. This reduces the amount of nitrogen being lost into the atmosphere or into surface water. The same principle holds true for other nutrients, though these are not as susceptible to loss as nitrogen.
Soil organic matter levels are an important component of determining overall soil fertility. Higher organic matter content is associated with improved soil tilth and water-holding capacity. In addition, organic matter breaks down in the soil and can be a source of various nutrients. Through a process known as mineralization, nitrogen from organic matter can become available for crops to use during the growing season, often reducing the amount of fertilizer which needs to be applied to produce a high-yielding crop. In the top six inches of a soil profile, a difference of one percentage point in soil organic matter will increase the amount of organic matter present by about 10 tons per acre. Organic matter contains about 5% nitrogen so a 3% organic matter field contains approximately 1,000 more pounds of nitrogen per acre than a field with 2% organic matter. Not all of this will be available to the crop and how quickly this mineralizes varies significantly depending on a number of factors, including soil type. However many farmers report that they are able to reduce their nitrogen fertilizer applications without negatively impacting yields after growing cover crops for several years.
Cover crops add organic matter in several ways. The most obvious is that they grow, produce biomass, are terminated, and are incorporated into the soil as they decompose. Most cover crops also produce prodigious root systems. These root systems also decompose into organic matter.
However the most significant addition to soil organic matter may come from the microbial communities mentioned in a previous section, along with other organisms living in the soil. With an active, growing root system for a greater portion of the year, cover crops provide a host site for a substantially higher level of these microbes. As they move through their life cycle, microbes are continuously dying and adding to organic matter levels. Some of these, known as mycorrhizal fungi, enable soil phosphorous to become more readily available for plant uptake often reducing the need for fertilizer applications. There are more microbes contained in a teaspoon of undisturbed soil than there are people on Earth. Each acre of soil will contain between 8 and 15 tons of bacteria, fungi, protozoa, nematodes, earthworms, and arthropods.
Cover crops are influential in many other ways related to soil fertility. By increasing water-holding capacity, less water runs off the surface of a field or into field tile, reducing nutrient loss from leaching. By reducing soil erosion, nutrients applied to a field stay in the field rather than blowing or washing away. The very structure of soil changes over time, which will be discussed in the section on soil health, increasing the ability of the soil to retain nutrients. While erosion reduction is the most visible benefit of cover crops and most easily explained, many farmers consider improved soil fertility to be the most important benefit.
The impact of cover crops on reducing plant disease has been well-established in the vegetable world. Recently some efforts have been made to determine if this same impact can be seen in corn and soybeans. While more studies need to be done, based on an University of Illinois study led by Dr. Darin Eastburn, it appears very possible that this does take place, at least for soybeans.
Plant disease pathogens can be introduced into a crop in several ways. One of those is the presence of soil-borne pathogens. As discussed previously, the use of cover crops increases the presence and activity of microorganisms in the soil. These beneficial microorganisms compete directly with soil-borne pathogens for all of the resources they need to survive. In addition, some microbes produce what amount to antibiotics which reduces the ability of pathogens to survive.
For additional information on this study see this article in AgriNews or you may take a look at a more detailed description of the study on the Sustainable Agriculture Research and Education (SARE) website.
Crop rotation has been used for a long time as a tool in managing insect pests. The basic premise is simple; by alternating crops, pests that feed on or otherwise damage primarily one crop will not be able to maintain their numbers with another crop. For example, soybean cyst nematode is primarily a pest of soybeans. When corn is used in a crop rotation, these nematodes lose their food source and their numbers drop dramatically so they are much less of a problem the next time soybeans are grown.
Cover crops add another plant to interrupt the life cycle of a pest. In may cases, cover crops may be used to help control pests even when crop rotation is not used. For example, if a producer wanted to plant continuous soybeans, a cereal rye cover crop has been shown to reduce nematode populations. Trials have also shown that certain cover crops can reduce western corn rootworm levels in continuous corn. More research in this area needs to be done however it appears that in some cases cover crops may be able to be used in place of pesticide applications in certain situations.
Soil health is such a complex issue that for the purposes of this page, I’m going to include a couple of definitions and just a very brief description of what these mean. The Cornell Soil Health Page uses the following to begin its description:
Soil Health is the capacity of the soil to function to sustain life. A healthy soil can be used productively without adversely affecting its future productivity, the ecosystem or the environment. Soil health emphasizes the integration of biological with chemical and physical measures of soil quality (used synonymously with “soil health”) that affect farmers’ profits, risks, and the environment.
The Natural Resource Conservation Service (NRCS) uses the following description on its Soil Health Page:
Soil health, also referred to as soil quality, is defined as the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans. This definition speaks to the importance of managing soils so they are sustainable for future generations. To do this, we need to remember that soil contains living organisms that when provided the basic necessities of life – food, shelter, and water – perform functions required to produce food and fiber.
Only “living” things can have health, so viewing soil as a living ecosystem reflects a fundamental shift in the way we care for our nation’s soils. Soil isn’t an inert growing medium, but rather is teaming with billions of bacteria, fungi, and other microbes that are the foundation of an elegant symbiotic ecosystem. Soil is an ecosystem that can be managed to provide nutrients for plant growth, absorb and hold rainwater for use during dryer periods, filter and buffer potential pollutants from leaving our fields, serve as a firm foundation for agricultural activities, and provide habitat for soil microbes to flourish and diversify to keep the ecosystem running smoothly.
Cover crops contribute significantly to Soil Health. As discussed previously, cover crop root systems substantially increase the number of microbes such as bacteria and fungi and invertebrates such as earthworms and grubs in the soil. As the root systems decompose, organic matter is added to the soil which helps these same organisms. Root systems and organisms create channels in the soil improving permeability and water penetration. And reducing erosion always improves the health of soils.
Healthy soils promote plant growth and agricultural productivity, improve the buffering and filtration capacity of the soil, increase nutrient availability, improve soil physical structure and stability, and improve water-holding capacity, as well as providing other benefits.
The role cover crops play in creating channels in the soil, and promoting insects to create channels as well, has been discussed previously. A term often used to describe this is macroporosity. This, along with increased soil organic matter levels, allows soil to absorb and hold much greater amounts of water than bare soil. The USDA-NRCS Engineering handbook states that bare soil contains 1.7 inches of water while soil with a cover crop contains 4.2 inches.
There are many benefits of increasing soil water levels. For one thing, as discussed in the section on soil erosion, any water which penetrates the soil surface is that much less which travels across the surface causing erosion, carrying away nutrients, and adding sediment to streams. This soil water is available to be accessed by crops. No-till farming practice increases the ability of soil to hold water and cover crops improves this even more. Crop rooting depth is increased and plants find more available water as they grow. There are some possible negatives as in a very dry spring cover crops may reduce soil moisture levels when new seedlings need it, however wet springs are far more common and cover crops can help farmers get into a field more quickly in the spring, particularly in wet years.
There are many ways cover crops improve water quality. A few of these are:
Reduced levels of sediment reaching rivers and streams. Sediment is agriculture’s top pollutant. By helping to reduce soil erosion, cover crops reduce the amount of soil which ends up in water. Sedimentation is one of the most commonly listed impairments for Indiana water bodies. Cover crops can reduce the amount of sediment deposited in Indiana water by as much as 75%.
Reduce the amount of nutrients entering water bodies. Nutrient loading of rivers, streams, ponds and lakes occurs many ways. One of these is that when soil particles are deposited into water, nutrients they may carry, such as phosphorous, is also deposited. Reducing soil erosion is an obvious way to lower the levels of these nutrients in water.
Nitrogen is an interesting product as it is not very stable in the soil. When farmers apply nitrogen to a crop, they do not plan to have the plant use it up completely but try to leave a little margin for error in case conditions occur which may cause plants to use more than intended. This means that in many years, some nitrogen remains in the soil after the crop, particularly corn, is harvested. Cover crops reduce nitrogen loss in several ways. By allowing greater amounts of water to be absorbed by the soil, the nitrogen is also carried into the soil rather than across the surface as runoff which may make its way into surface water. Many cover crops are known as nitrogen scavengers. By planting these after harvest, these plants use the available nitrogen for their own growth. This prevents the nitrogen from leaving the field and allows the cover crop to be used as a green manure or fertilizer the following spring as, once it’s terminated, much of that nitrogen will be available for next year’s crop.
It is estimated that cover crops can reduce the amount of nutrients reaching water by as much as 50%.
Reduce the amount of pesticides entering water. These days most pesticides are not very persistent, meaning they break down fairly quickly in the environment. Still, some chemicals often remain in the soil following the growing season. Once again, by reducing the amount of water flowing across the soil surface, fewer pesticides will be carried with it. Many of the soil microbes which cover crops encourage can play a role in breaking down pesticides. It is estimated that cover crops can reduce the amount of pesticides reaching water by as much as 50%.