APPROPRIATE TECHNOLOGY TRANSFER FOR RURAL AREAS, P.O. Box 3657, Fayetteville, AR 72702; 1-800-346-9140 ATTRA Summary: Cover Crops and Green Manures The practices of cover cropping and green manuring can serve a number of important functions in cropping systems. Any field or forage crop grown to provide soil cover is a "cover crop." "Green manures" are crops tilled into the soil while green, or soon after flowering, to improve the soil. Since a crop grown as a cover crop may later be soil-incorporated as a green manure, the two practices are often referred to interchangeably. Cover crops are grown mainly to prevent erosion of soil by wind and water. But cover cropping and green manuring practices perform other important functions in cropping systems which: - improve soil quality and moisture-retention capacity - add fertility and cycle plant nutrients - aid in pest management The types of crops that can be managed as cover crops and green manures include annual, biennial, or perennial herbaceous plants. They can be legumes, such as clovers, vetches, and field peas and beans, or non-legumes like rye, buckwheat, and sorghum-sudangrass. Legumes that fix nitrogen are often selected as cover crops and especially as green manures to obtain the benefit of added nitrogen for subsequent crops. Cover crops and green manures may be included in cropping systems before or after a primary or "cash" crop. They may also be included during non-crop periods -- in some periods of traditional fallow, for instance, or included as a regular step in a cropping rotation. In some applications, cover crops are grown together with primary crops through forms of intercropping. Cover cropping and green manuring practices are used in a wide variety of farming systems. In many cases, they play a fundamental role in farming systems using or making a transition toward sustainable farming methods. Principal Applications The principal applications described below show the varied ways cover crops and green manures are used in cropping systems: 1) "Winter Cover Crops" are planted in late summer or in the fall to protect soils over winter. They must be cold tolerant. Legumes are often chosen in order to gain fixed nitrogen. Some applications combine a cool-season legume in a mixed stand with a winter cereal grain. 2) "Summer Green Manures" are warm-season crops which provide soil cover for part of the summer before being tilled in. They may be grown to fill a rotation interval, to improve poor soils, or to prepare land for a perennial crop. Summer green manures smother weeds, provide organic matter, improve soil tilth, and -- in the case of legumes -- add soil nitrogen. 3) "Living Mulches" are cover crops intercropped with annual or perennial cash crops to suppress weeds and to reduce erosion, enhance fertility, and improve water infiltration of soils. In annual cropping systems, a legume might be overseeded as a living mulch into a grain crop. Or, a legume may grown as a planting bed for vegetables that are transplanted into tilled or chemically killed strips in the legume stand. In perennial systems, sods of grasses, legumes, or grass-legume mixes are interplanted as living mulches between rows of berries or orchard trees. 4) "Catch Crops" are cover crops established to retain nutrients in cropping systems by reducing nutrient leaching from the soil following the harvest of primary crops. Catch crops may also double as winter cover crops. Short-term cover crops in a crop rotation are also commonly called "catch crops." 5) "Forage Crops" can function as cover crops while grown for grazing or haying, and then as green manures when later tilled in. A variety of forage-legume or legume-grass sods can fill this dual function. Forage crops used as green manures benefit soils most if allowed to accumulate topgrowth before being turned under. Benefits Cover crops and green manures perform important soil conservation functions in cropping systems. They also contribute much toward sustainable management of soil structure and moisture, crop pests, and fertility. Green manures in particular help fill crop fertility needs directly as well as improve soil structure. In large-scale farming systems, cover cropping and green manuring practices may be among the most effective ways to reduce soil erosion. Used to establish and cycle fertility on-farm, they may also be the most practical means available to large-scale operations for developing systems for sustainable fertility management. In small-scale, highly diversified, or intensively managed operations, cover crops and green manures may be one of several viable strategies for developing and cycling fertility on-farm. Reduced soil erosion: Soil erosion is a major environmental concern facing U.S. agriculture. Cover crops and green manures reduce the vulnerability of soils to erosion in many ways. The foliage canopies of cover crops directly block wind and rain erosion of soil. When used to maintain year-round soil cover, cover crops are effective in curtailing the erosion of soil in a wide range of farming systems. Cover crops that quickly produce canopies covering a high percentage of ground area yield the greatest soil protection benefit. Most grass and non-legume cover crops, like rye and buckwheat, fulfill this need well. The soil cover provided by living cover crops or by the mulch from cover crop residues can improve water infiltration of soil and limit surface soil erosion by rainwater. Cover crop root systems also limit soil erosion by actively holding soil in place. By adding organic matter, green manuring reduces soil vulnerability to surface erosion by improving soil structure and the capacity of soil to retain moisture. Soil improvement: Improving the physical structure of soil benefits field preparation and growing conditions by enhancing soil tilth, aeration, and water infiltration qualities. Cover cropping or green manuring practices for adding organic matter to soils can be highly effective in improving soil structure. By loosening and aerating soils, cover crops with extensive root systems also help remedy problems of soil compaction. Sod-forming grasses or grass-legume plant mixtures used as cover and forage crops are excellent suppliers of organic matter to soils in cropping systems. Frequent cultivation of soils accelerates organic matter decomposition. As part of a crop rotation, sods can help replenish organic matter lost from soils due to annual cultivation. To gain an increase in soil organic matter, however, several years of an established sod may be required. When established for the long term, sod systems such as native grass stands or mixed pastures can greatly improve soil structure. Consistent green manuring in more-frequently tilled soils is also an important way to replenish and maintain soil levels of organic matter. Whether from sods or green manures, organic matter supplies to soil are instrumental in building and renewing soil structure. As soil microorganisms break down organic matter from a freshly incorporated green manure, for example, compounds such as gums and lignins that resist decomposition are formed. These compounds, plus other substances the microorganisms produce, bind together soil particles as granules, or "aggregates," which provide the basis for good soil structure. Fertility: Cover crops and green manures help supply and cycle plant nutrients in cropping systems in a number of ways. Nitrogen accumulated and contributed by legumes plays a key role in sustainable farm fertility management. Amounts of nitrogen (N) fixed and accumulated by legumes can range from 40 to 175 pounds per acre. When legumes are used as green manures, about 40 to 60 percent of the nitrogen (N) they accumulate in plant tissue is usually available to a following crop. In rotations, forage legumes are valuable because they produce income from grazing or haying in addition to N from shoot regrowth and root residues. The nitrogen available from legumes depends on the total biomass produced and on the percentage of N in the plant tissue. Cultural and environmental conditions that limit legume growth -- such as delayed planting, poor establishment, or drought -- will reduce the amount of N produced. In addition to the nitrogen legumes fix, legume and non-legume cover crops accumulate and cycle phosphorus, potassium, calcium, magnesium, sulfur, and other important plant nutrients on the farm. When cover crops tilled in as green manures decompose, the nutrients accumulated are released and made available to other crops. Some cover crops, like buckwheat and sweetclover, accumulate certain minerals in their tissues even when soil availability of the minerals is low. Deep-rooting cover crops extract and translocate subsoil nutrients, making them accessible in surface rooting zones to subsequent crops. Green manuring stimulates greater plant nutrient availability indirectly, as well. Acids produced as green manures decompose react with insoluble minerals in surrounding soil material and release these in usable form as exchangeable plant nutrients. Another product which results from the partial decay of plant and animal materials in the soil is "humus." Humus also enhances the availability of plant nutrients. Water management: A key aspect of the soil conservation and improvement functions of cover crops and green manures in cropping systems is better management of water. Cover crop canopies reduce moisture evaporation from the soil surface. In low- and no-till systems, the mulch left from dead cover crop residues also limits evaporation. Soil moisture conserved through reduced evaporation helps keep surface soil from crusting. Less soil crusting in turn helps maintain the capacity for water to infiltrate the soil surface rather than be lost or cause erosion as run-off. By improving soil structure, organic matter supplied by green manures and sod-forming cover crops further expands the capacity of soil for water to infiltrate and be retained. Finally, by opening channels into subsoil or hardpans, the rooting action of deep-rooted cover crops encourages water penetration to deeper soil levels. Together, all of these effects improve the capacities of cropping systems to take advantage of moisture from rain as well as irrigation water. Pest management: The primary pest management benefit of cover crops and green manures is weed suppression. Cover crops take up space and light, thereby discouraging weeds that flourish on bare soil. By loosening soil, deep-rooting cover crops help reduce populations of weeds that thrive in compacted soils. In plantings of primary crops, cover crops interplanted as living mulches function as "smother crops" to suppress weeds by competing with them for light, moisture, and nutrients. Along with living mulches, cover crops such as rye and other small grains which exude natural toxins or "allelochemicals" are gaining importance as alternative forms of weed suppression for sustainable farming systems. In no-till systems, the mulch that results from mowing or chemically killing allelopathic cover crops can significantly suppress weeds. Rotating cover crops and green manures with primary crops creates additional opportunities for breaking pest life cycles. Cover crops can also aid insect pest management by serving as habitat for beneficial insects. Selecting and Managing Cover Crops and Green Manures Some important factors to consider when selecting crop types and varieties to use as cover crops and green manures are: 1) What cover cropping or green manuring use is planned? 2) What cover or green manure crops are best adapted to the soil and climatic conditions (including rainfall) of the farm site -- especially for the planned season of use? 3) Will irrigation be used? 4) How might the crops being considered interact with farm pest cycles? Climatic conditions that affect available moisture are important in determining when cover crops can be planted if no irrigation will be used. Soil and pest conditions, crop rotation stage, labor availability, and other factors will also determine the best time to plant. Correct seeding rate and, in cultivated systems, proper seedbed preparation are important to ensure good establishment. Because equipment and seeding methods used may be different (e.g., broadcasting vs. drilling), seeding rates may differ between large and small farm operations. When legumes are selected to supply nitrogen, the right rhizobial bacteria must be sufficiently available in the soil or used to inoculate seed at planting for nitrogen fixation to occur. Vegetation management is a basic requirement of using cover crops and green manures. In conservation tillage systems, herbicides are the most common means of suppressing a cover crop in preparation for the next stage in a crop cycle. In general, methods for managing cover crop vegetation range from conventional herbicide use to non-chemical or alternative practices like disking, mowing, rolling, and flaming. Methods of vegetation management may also vary between large- and small-scale farming operations. Limitations of Cover Crops and Green Manures Successfully including cover crops and green manures in a cropping system involves planning for establishment and other costs as well as for anticipated benefits. Whether benefits sought are immediate -- such as nitrogen from legumes, or long-term -- such as sustained soil health, farm plans to include cover crops and green manures should account for start-up and periodic costs of seed, establishment, and cultural management. They should also account for any short- or long-term crop income foregone by growing cover crops on acres that might otherwise be used for cash crops. Cover crops and green manures add both biological diversity and management complexity to a farm. In general, increased biological diversity can benefit farming systems in many ways. But some cover crops harbor pest communities that may pose problems for certain crops or geographical locations. Managing a greater number and variety of crops also entails increased farm cultural inputs, time, and expense. Water consumed by green manure crops can also be a concern. However, even in low-rainfall areas, native and adapted crop varieties may still be viable replacements for cultivation or herbicides during summer fallow. For More Information This publication summarizes material from the ATTRA Information Package: "Cover Crops and Green Manures," prepared by ATTRA Technical Specialists Steve Diver and Preston Sullivan. To request the full Package, or for help with a specific question, call ATTRA toll-free at: 1-800-346-9140. ATTRA Summary: Cover Crops and Green Manures Written by: Holly Winger Editorial and Project Director: Christine Rugen (PageMaker Version Design/Layout: Jim Tracy) March 1992 A word about ATTRA and NCAT ATTRA is a federally-funded service offering free technical information on sustainable agricultural production and marketing. ATTRA provides tailored responses to individual questions from farmers, agricultural researchers and extensionists, wildlife professionals, and others, and also distributes Information Packages, Summaries, and Resource Lists on a range of general topics in sustainable agriculture. ATTRA is funded by the U.S. Fish and Wildlife Service, Dept. of the Interior, and managed by the National Center for Appropriate Technology (NCAT) -- a non-profit organization incorporated in 1976 to help communities and individuals find sustainable ways to improve their quality of life using skills and resources at hand. The ATTRA staff includes technical specialists in horticulture, agronomy, livestock production, aquaculture, and wildlife management, and information specialists in agricultural information transfer. Their combined training and experience includes advanced degrees in agriculture and years in consulting, research, Cooperative Extension, large-scale farming, organic market gardening, agricultural writing, and orchard, nursery, and livestock management.