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Introducing cover crops into the rotation between corn and soybeans can improve soil tilth, health, and productivity. Tilth is the physical condition of a soil and involves aggregation (the joining of individual soil particles into clusters), moisture and air content, and the rate of water infiltration and drainage. Soils with good tilth have increased aeration and water-holding capacity, drain well, resist crusting, and quickly take in water. Soils with stable aggregates can better resist water and wind erosion. Aggregates can be bound by clay particles, organic matter, fungal hyphae, and a protein called glomalin, a water-insoluble substance produced by mycorrhizal fungi.
Erosion and runoff prevention. Wind and runoff can lift and carry loose soil particles off site. The shear force of wind and water flow can be strong during periods between cash crops when the soil is fully exposed to these forces. The impact of raindrops can break apart weak soil aggregates resulting in smaller soil particles that are more easily carried away by wind and runoff. Runoff can also lead to off-site contamination to surface waters. Soil water recharge is also reduced when precipitation moves off site.
Soil that is moved off site is often topsoil that contains organic matter and nutrients. While the organic matter and nutrient levels of soils vary, each ton of lost topsoil could contain 20 pounds of nitrogen (N), 12 pounds of phosphate (P), and 9 pounds of potash.3 When soil loss occurs, nutrients and water often need to be replaced at an expense to the grower.
Cover crops and crop residue can slow or stop runoff and erosion. The canopy of cover crops shields the soil and reduces the impact of rainfall. Cover crop roots anchor the soil particles and cover crop residue increases the amount of organic matter near the soil surface, which helps to further stabilize soil aggregates, making them less likely to break apart upon impact. Cover crops also reduce the speed of water flow over the soil surface.
Nutrient retention. Nutrient loss from runoff, erosion, and leaching can be slowed or stopped with cover crop root and canopy growth. Nitrate-N is easily lost with water as runoff or leached through the soil profile. Cover crops take up or sequester excess N and reduce leaching into groundwater. Some deep-rooted cover crops can also pull nutrients from deeper in the soil profile and release them back into the soil nearer the soil surface as the cover crop residue degrades, making them more available to subsequent cash crops.
Nitrogen fixation. Legume cover crops can fix and release N for a cash crop to recapture in the spring. Cover crop seed cost and ability to supply N should be compared to fertilizer costs. The Midwest Cover Crop Council (MCCC) website can help select cover crops and estimate how much N is returned. The Cover Crop Decision Tool from MCCC can be found at http://mccc.msu.edu/covercroptool/covercroptool.php.
Impact on soil stability and structure. Along with decreased erosion and runoff, early spring growth of cover crops can help utilize excess moisture and stabilize the seed bed. This improved stability can allow machinery to pass over cover-cropped fields sooner compared to tilled fields. Termination of spring cover crop growth should be timed early enough to preserve adequate water for the cash crop. Herbicides, mowing, or another method besides tillage should be used to terminate cover crops.
Mitigation of compaction. Subsoil compaction can be broken up or ‘bio-drilled’ by deep-rooted cover crops such as oilseed radish or annual ryegrass. Soil structure is left intact when a cover crop, rather than tillage, is used to improve compaction issues. Tillage quickly negates the benefits of cover crops as the use of tillage machinery breaks down the soil structure. Additionally, soils may be re-compacted by machinery, sometimes worse than before deep tillage.
Restoration. Cover crops can be used to gradually restore lightly degraded soils. Salinization is the accumulation of water-soluble salts in the root zone of crops. This type of soil degradation is caused by poor quality irrigation water, or poor drainage in areas where there are naturally-occurring salts in the soil and water supply. As soil water evaporates, it pulls the salts from deeper in the soil profile up to the surface. The use of salt-tolerant cover crops, such as barley or sugarbeet, helps reduce soil water evaporation between the cash crops, thereby helping to minimize salinity problems.
Improved nutrient availability. When used long term, cover crops provide a food source for beneficial soil organisms, and in turn, soil organisms help with soil nutrient supply and plant uptake. Soils left fallow lack root growth, and mycorrhizal fungi that promote P uptake do not survive without host roots. Corn crops planted into fallow fields may have P deficiency symptoms, also known as ‘fallow syndrome’. Mycorrhizal fungi are important for supplying P for early corn growth and correction of ‘fallow syndrome’. Cover crops can favor the growth of mycorrhizal fungi and increase P levels within the soil comparable to if P was applied with corn seed at a rate of 6.3 lbs/ac.5 Legume cover crops, in particular, increase mycorrhizal fungi populations and can promote symbiotic relationships with subsequent crops.2
The cover crop survey report from the Sustainable Agriculture Research and Extension (SARE) program, Conservation Technology Information Center (CTIC), and American Seed Trade Association (ASTA) contains valuable statistics about the adoption and value of cover crops across the U.S. To access the latest version of this report, visit www.ctic.org/Cover%20Crops/