Cover crops can provide many benefits for the soil in crop productions systems. Benefits can include conserving soil and water as well as improving the soil.
An obvious effect of growing cover crops is covering the soil surface, which can significantly reduce the potential for wind and water erosion. This is particularly true when precipitation and/or winds are intense and traditional cash crops aren’t actively growing, such as in early spring and late fall.
Plant residue, both living and dead, is critical to minimize the impact of rainfall and wind on soil erosion. The estimated amount of residue remaining after corn harvest is often in the 75 to 90 percent range. However, depending on the region and conditions, fields can lose up to 40 percent or more of their residue cover during the winter. This loss leaves the field more susceptible to erosion during heavy and intense spring rains.
Depending on the species grown the planting method, and days of growth, cover crops have the potential to increase soil cover to almost 90 to 100 percent during times of the year when fields are most susceptible to erosion. Minimizing soil erosion to around 100 pounds will lead to an increase in soil depth because the rate of soil formation is greater than the rate of soil loss. Greater soil depth, in turn, results in greater soil and crop production resiliency, sustainability, and productivity.
In addition to adding aboveground plant material, the root systems of cover crops help hold the soil in place and reduce erosion. Many soil health advocates recommend planting several species of crops in a single planting to capitalize on species’ differences in the root architecture (think taproot vs. fibrous root systems, shallow vs. deep-rooted plants). They argue that the diversity in root system growth will help hold soil in place, create pore space, scavenge and recycle nutrients and water, and move carbon deep within the soil profile.
Increased Soil Organic Matter
Soil organic matter is the soil fraction consisting of plant, animal, and/or microbial tissues in some stage of decomposition. The amount of soil organic matter depends primarily on natural factors such as temperature (cool locations often accumulate more organic matter due to less decomposition), soil texture (clayey and silty soils tend to have more organic matter than sandy soils), and drainage (poor drainage slows decomposition).
Native levels of organic matter in the soil (pre-European settlement) were likely much higher, possibly more than twice as high, than they are today. Over time, as we have opened and managed these soils for agriculture, organic matter has been lost via erosion or decomposition.
Benefits of Increased Soil Organic Matter
Soil organic matter plays several beneficial roles, including enhanced aggregation and aggregate stability, increased soil fertility, and greater biological activity.
Increased soil organic matter typically results in an increase in biological activity and the production of organic glues that hold soil aggregates together. Glomalin, a glycoprotein secreted by mycorrhizal fungi, is believed to be one of the primary organic glues for bonding and stabilizing aggregates.
The benefits of increased soil aggregation with increased soil organic matter trickle down, leading to increased pore space, improved water infiltration, and reduced runoff, as well as increased water-holding capacity, gaseous exchange, root growth, and microbial activity.
Although the concept of soil aggregation seems quite straightforward, farmers’ perspectives on the impact of cover crops on aggregation and soil water dynamics may appear contradictory. Farmers report that increased soil organic matter leads to greater water-holding capacity, yet their fields are drier and can be worked/planted earlier in the spring. Large pores move water and air through the soil, while small pores hold water. The use of cover crops increases both sizes of pores. Root channels, worm channels, and an increased number of larger aggregates increase the number of large pores in a soil, draining away excess, saturating water. The increase in soil organic matter associated with cover crops also increases the number of smaller aggregates within the soil matrix. These smaller aggregates store up to approximately 25,000 gallons of plant-available water per acre per 1 percent organic matter. The combination of large and small pores allows soil to drain excess water properly and introduces oxygen back into the root zone, while simultaneously holding additional plant-available water. Increasing soil organic matter by only a percentage point or two can have a huge impact on the ability of that soil to support crop growth in times of water stress—both too much and too little.
Soil organic matter also increases cation-exchange capacity (CEC), which is the soil’s ability to hold and supply nutrients over time. With an increase in CEC, more nutrients are stored in the soil profile, leading to a decrease in nutrient loss. Increased CEC also improves the soil’s buffering capacity (ability to resist change), providing a more chemically stable environment for plants and microbes.
Additionally, soil organic matter is itself a source of nutrients for plants, particularly nitrogen. As organic matter is decomposed, nutrients become dissolved and available for plant uptake. Predictions of the amount of nutrients released from soil organic matter are complicated, but research is underway to quantify the amount of nitrogen we can expect to come from our soils.
Improved Nitrogen Cycling
Cover crops can be used to capture available soil nitrogen, which is stored in previous crop plant tissue. This helps decrease nitrogen leaching. Once the cover crop is terminated and starts to decompose, this nitrogen is released back into the soil system where it can be used by the subsequent crop.
A study in Oregon’s Willamette Valley found that a cereal rye cover crop reduced nitrate leaching by 32 to 42 percent over a three-year period, as compared to fallow. Such reductions in nutrient leaching not only reduce the fertilizer requirements in the year following the cover crop, but protect ground and surface water quality as well.
Legume cover crops such as peas, vetches, and clovers can “fix” nitrogen from the atmosphere. Legumes are estimated to contribute anywhere from 40 to 200 pounds of nitrogen per acre. Current research into nitrogen fixation may help farmers utilize legumes more effectively in their rotations.
The rate and amount of nitrogen released from a cover crop is a direct result of the cover crop’s carbon to nitrogen (C: N) ratio. Microbes decomposing the cover crop like to maintain a 8:1 C: N ratio. When plant residue is added to the soil, the population of soil organisms increases to take advantage of the added food source. However, if the residue is relatively low in nitrogen, the microbes consume more nitrogen from the soil system to maintain their 8:1 C: N ratio. As a result, this nitrogen is temporarily unavailable for plant use until the microbes die. As the microbes decompose, nitrogen is released back into the soil system, making it plant-available again.
Residues with low C:N ratios, such as legumes and young plant tissues, typically does not result in immobilization but often release nitrogen back into the plant-available pool quickly. In fact, legume nitrogen can be quickly mineralized, sometimes even before the subsequent crop has a high demand for it.
Achieving synchrony of nitrogen release from decomposing residues and crop nitrogen demand is difficult, but can be achieved through effective timing of cover crop termination and an ideal mixture of cover crop species to spread nitrogen mineralization over the growing season. Growing mixtures of quickly mineralized cover crops (legumes and some brassicas) with slowly mineralized cover crops (the grasses) can often achieve this goal.
Enhanced Soil Biology
We’re just starting to realize how important large and small soil organisms are to soil health and crop productivity. A healthy soil has a large and diverse population of microorganisms.
Soil organisms play an important role in decomposition. By growing cover crops and improving soil physical and chemical properties, farmers can enhance the microbial populations found in their soils and reap the benefits of a functioning, diverse soil ecosystem. These benefits include nutrient cycling, residue degradation, and pore and aggregate creation, among others.
Cover crops can be used to suppress weed growth through:
- Promotion of a weed’s natural enemies, such as seed predators and pathogens
- Physical suppression (i.e., mulch) to reduce weed seed germination and growth
- Allelopathy or chemical inhibition to reduce weed seed germination and growth
- Competition for space, nutrients, and light to reduce weed growth and seed production
Cover crops planted for weed suppression should be aggressive growers that cover the ground quickly, shading the ground to prevent or suppress weed seedling growth. Cereal rye, hairy vetch and red clover are well-known for their quick growth and ability to suppress weeds. The impact of cover crops on weed suppression can be enhanced through proper species selection; timely establishment appropriate seeding rates; and proper row spacing, fertilization, and use of no-till techniques.
Manipulation of cover crops for the control of insect pests is not as simple as it sounds. The underlying principle is to attract beneficial insects and repel pests. However, cover crops may provide both beneficial and pest insects with a supplemental food source and/or shelter. In fact, some insect pests, such as armyworms, wireworms, seed corn maggots, slugs, and white grubs are attracted to the high residue cover of cover-cropped fields in early spring and can become a crop production issue. Insecticides, both surface applied and seed treatments, are often effective in controlling these pests.
It is important to note that soil arthropods, springtails, and other soil insects also benefit from cover crops. These small soil dwellers help in organic matter decomposition and serve as predators, keeping microscopic organisms in check while aiding in nutrient cycling.
The role of cover crops in insect support and suppression has not been fully researched, but anecdotal accounts support the role they can play in enhancing pest control.