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Multiple Impacts Cover Crops

Contents

 Cover Cropping Practices
 Enhancement of Soil Structure
 Contributions to Soil Fertility
 Environmental Impacts of Cover Crops
 Pest Management
 Summary
 References cited

Multiple Impacts of Cover Crops
in Farming Systems

John Luna
Department of Horticulture
Oregon State University, Corvallis, OR

Cover crops are crops which are not usually grownfor harvest, but which serve multiple functions in crop rotation systems. Although cover crops are commonly usually grown to prevent soil erosion and for improvement of soil tilth, other important roles include the enhancement of soil structure, improvement of soil fertility, enhancement or preservation of environmental quality, and contributions to the management of weeds, insect pests, and plant pathogens (Fig. 1). Not all of these benefits are associated with a specific cover crop use, however many of these benefits can occur simultaneously.

Although many advantages are associated with growing cover crops, there are also potential problems, including increased tillage operations and production costs, possible delays in planting, nutrient immobilization, soil moisture depletion and increased pest incidence. Clearly, a decision to utilize cover crops in a farming system must weigh potential advantages and disadvantages. But because of the multiple impacts of cover crops, particularly those affecting long-term soil and water quality, calculation of total benefits becomes extremely difficult, if not impossible. Long-term changes in soil tilth can be observed, but are not easily quantified. Use of fertilizers for crop production can also mask the contribution of cover crops to nutrient cycling and soil fertility. Except in severe soil erosion situations, effects of soil erosion on decline in soil productivity can be gradual, making the economic evaluation of soil conservation practices such as cover crops quite difficult.

Cover Cropping Practices

Winter annual cover crops such as rye, barley, oats, ryegrass, vetch, Austrian field peas, and crimson clover are commonly grown during the cold season in northern latitudes. These crops are typically planted in the fall and killed in the spring prior to planting a summer cash crop. They can be incorporated into the soil as a green manure crop, or killed and left on the soil surface as no-till mulch.

Warm-season annual cover crops such as buckwheat, foxtail millet and sudangrass can also be used in the summer to fill openings in crop rotation sequences. Perennial grass and legume cover crops are commonly used in orchards and vineyards as living mulches. Living mulch systems involving chemically or mechanically suppressed cover crops have also been used successfully in vegetable and field crop systems through temporary suppression of the cover crop with herbicide or mowing.

Enhancement of Soil Structure

Cover crop impacts on soil structure relate both to their role as a protective cover and to their contributions to soil organic matter and to the biological processes occurring within the soil. The physical covering of the soil surface by a cover crop provides protection from raindrop impact and the shearing force of overland water flow. Cover crops can reduce the soil compaction impact of rainfall as well as preventing the crusting and sealing of the soil surface. Water infiltration is increased, reducing the potential for erosion.

The incorporation of cover crops into the soil as green manures can accelerate the beneficial formation of soil aggregates. During the microbial degradation of organic material, polysaccharide gums are released which serve to glue soil particles together into stable aggregates (Burns and Davies 1986). This aggregate stability helps reduce soil erosion and improves soil structural properties related to soil aeration, water infiltration and water holding capacity. Channels formed by decayed roots and enhanced earthworm activity are principal factors responsible for the increase in water infiltration capacity (Lal et al. 1991).

Contributions to Soil Fertility

Legume cover crops such as clovers, vetch, and Austrian field pea have long been grown because of their ability to biologically "fix" nitrogen through a symbiotic relationship with Rhizobia bacteria living in nodules on the roots. These cover crops can biologically fix or accumulate (as in the case of non- legumes) from 35 to 200 pounds of nitrogen per acre, and can replace or greatly reduce the need for manufactured nitrogen fertilizer (Table 1).

The amount of nitrogen actually contributed to following crops varies considerably, and is dependent on environmental conditions, carbon to nitrogen (C:N) ratios of the cover crop, available nitrogen in the soil and soil microbial activity. The "N-fertilizer equivalency" of winter cover crops has been used as an estimate of the N-supplying capacity for summer crops following the cover crops. These N fertilizer equivalency values of legume cover crops commonly range from 50 to 120 lbs N/acre (Doran and Smith 1991).
In addition to the quantity of nitrogen available in a legume cover crop, the rate of decomposition, or mineralization, must be matched with crop N- uptake requirements for optimum yield. For example, in a Virginia study (Sullivan et al. 1991) which evaluated the contribution of hairy vetch and a mixture of hairy vetch and bigflower vetch to corn production, N-uptake rates in corn following the vetch cover crops closely paralleled N-uptake rates of 70 kg N/ha (62 lbs/acre) (Fig. 2). But N-uptake in the corn was considerably higher for 210 kg/ha, which also produced higher corn yields. In this situation, the addition of some supplemental nitrogen fertilizer to the vetch crop treatments would have been required to achieve maximum yield.

In a Kentucky study (Frye and Blevins, 1989), corn grown following a hairy vetch crop produced higher yields than any rate of manufactured nitrogen fertilizer; however the addition of 100 kg N/ha to the corn following a vetch crop produced the highest corn yields in the experiment, as well as generated the highest net return to the producer.

The N-fertilizer equivalency of non legume cereals, such as oats, rye, wheat, and barley, is low or negligible (sometimes negative) due to a lower N content and higher C:N ratio, which often results in the tie up, or immobilization of N during the cropping season. These non-legume species may not contribute nitrogen to the crop immediately following the cover crop, however they play an important role in scavenging nitrate nitrogen from the soil that may be lost to leaching (see later section on environmental impacts) and cycling of nutrients.

Although most research on cover crop contributions to soil fertility have concentrated on nitrogen cycling, cover crops also store other major plant nutrients in their tissue. Microbial decomposition of this plant tissue following incorporation as a green manure crop helps make these nutrients available for subsequent crop uptake. Organic acids released by the decomposition of organic matter in the soil can also accelerate the transformation of mineral phosphorus into more plant available forms (Stevenson, 1986).
Cover crops comprise a major energy source for the soil biota. Just as forage crops such as alfalfa capture solar energy through photosynthesis and transform this energy into carbohydrates for animal consumption, cover crops transform solar energy into food for a diverse community of detritivorus arthropods, earthworms, and microorganisms. Additions of organic matter to soil, particularly fresh green plant material, stimulate microbial growth and development. This microbial activity typically causes a relatively short (1-3 week) lag period in nutrient availability (immobilization), however there is a net gain in nutrient accumulation in the soil, as well a "banking" of nutrients within the living and dead bodies of the microbial complex. The diversity and abundance of the living biota of the soil is a critical component in the assessment of soil quality.

Environmental Impacts of Cover Crops

Although most research on cover crop contributions to soil fertility have concentrated on nitrogen cycling, cover crops also store other major plant nutrients in their tissue. Microbial decomposition of this plant tissue following incorporation as a green manure crop helps make these nutrients available for subsequent crop uptake. Organic acids released by the decomposition of organic matter in the soil can also accelerate the transformation of mineral phosphorus into more plant available forms (Stevenson, 1986).
Cover crops comprise a major energy source for the soil biota. Just as forage crops such as alfalfa capture solar energy through photosynthesis and transform this energy into carbohydrates for animal consumption, cover crops transform solar energy into food for a diverse community of detritivorus arthropods, earthworms, and microorganisms. Additions of organic matter to soil, particularly fresh green plant material, stimulate microbial growth and development. This microbial activity typically causes a relatively short (1-3 week) lag period in nutrient availability (immobilization), however there is a net gain in nutrient accumulation in the soil, as well a "banking" of nutrients within the living and dead bodies of the microbial complex. The diversity and abundance of the living biota of the soil is a critical component in the assessment of soil quality.

Controlling Soil Erosion.

Soil erosion continues to be a threat to agricultural productivity worldwide, with soil losses in the United States alone exceeding 3 billions tons annually. Wind and water erosion of soil cause dramatic declines in soil productivity. Cover crops play a vital role in filling open gaps in crop rotation sequences where the soil is left bare, and in providing protective mulches in no-till and conservation tillage systems. Even after cover crops are incorporated as green manures, the increased aggregate stability (Table 2) helps reduce soil erosion.

Water Quality. Grasses and grains, because of their ability to become quickly established in the fall and establish an extensive root system, have been shown to be more efficient than legumes at capturing soil nitrate and preventing late fall and winter leaching to the ground water (Meisinger et al. 1990; Muller et al. 1987).
In a western Oregon study of nitrate movement in the soil profile following harvest of a broccoli crop fertilized with 250 lbs N/acre, Hemphill and Hart (1991) demonstrated the effectiveness of a fall-planted rye cover crop in reducing the movement of soil nitrate into the soil profile (Fig. 3). Areas with cover crop contained 0.18 ppm nitrate at 30-40 inches of soil depth compared to 5.38 ppm nitrate at the same depth where no cover crop was grown. Although no ground water measurements of nitrate contamination were made in this study, the implications of nitrate capture by cover crops is clear.

Pest Management

Cover crops have been shown to have both beneficial and detrimental effects on crop pests. Incorporating green manures as cover crops has been shown to reduce incidence of a wide array of plant pathogens. Rothrock and Kendig (1991) reported that hairy vetch cover crops reduced the incidence of black root rot on cotton (Thielaviopsis basicola) (2% disease incidence compared to 29% incidence on fallow treatments).
Ingham (1993) has demonstrated the value of specific cultivars of rapeseed and sudangrass in suppression of the Columbia rootknot nematode (Meloidogyne chitwoodi ) in potatoes. Although the use of Trudan 8 sudangrass and Humus rapeseed cover crops significantly suppressed nematode levels in potato crops following the cover crops, the integrated use of cover crops and a non-fumigant nematicide, Mocap, was required to reduce tuber damage below economic threshold levels (Table 3).



Surface residues of rye and other cover crops have been shown to provide substantial weed suppression through the release of natural herbicides, or "allelochemicals" (Barnes and Putnam 1983; Putnam 1986 and 1990). Mangan et al. (1991) reported substantial weed control provided by rye and vetch cover crop mulches, and several cereal cultivars have been shown in Oregon to suppress weeds (Ray William, OSU Dept. of Horticulture, personal communication).

Cover crops can influence arthropod pest abundance in several ways: (1) serving as a host for alternative prey for beneficial arthropods, (2) providing favorable habitat for generalist predators such a carabid beetles, (3) interfering with the host-finding abilities of the pest species, (4) serve as a trap crop to attract pests away from the primary crop. Bugg (1991) has provided an excellent review of the role of cover crops in arthropod pest management.

Cover crops can also exacerbate pest problems by serve as hosts for insects which are disease vectors, provide a host source for pathogenic nematodes, and also attracting insect pests into a cropping system. Rodent and slug population increases are frequently reported as problems in reduced tillage plantings into cover crops.

Summary

Using cover crops in horticultural production systems offer the possibility for multiple benefits of enhancing soil quality, nutrient cycling, and pest management, as well as improving soil conservation and water quality. Cover crops can also produce deleterious effects, including delaying of planting, nutrient immobilization, increased labor requirements and other costs, and increased risk of certain pests. Clearly the selection and management of specific cover crop cultivars must involve a broad knowledge of both beneficial as well as detrimental effects, and must be tailored to specific cropping systems. Current research and demonstration projects are currently underway by Oregon State University research and Extension personnel, in collaboration with innovative farmers, Oregon Department of Agriculture, and other groups to gain more knowledge necessary for improved cover crop management.

References cited

Barnes, J. P. and A. R. Putnam. 1983.

Rye residues contribute weed suppression in no-tillage cropping systems. J. Chem. Ecol. 9:1045-1057.

Blevins, R. L., J. H. Herbeck, and W. W. Frye. 1990.

Legume cover crops as a nitrogen source for no-till corn and grain sorghum. Agron. J. 82: 769-772.

Burns, R. G. and J. A. Davies.

The microbiology of soil structure. Biol. Agric. and Hort. 3: 95-113.

Chapman, H. D., G.F. Leibig, and D. S. Rayner. 1959.

A lysimeter investigation of nitrogen gains and losses under various systems of cover cropping and fertilization, and a discussion of error sources. Hilgardia 19:57-102.

Doran, J. W. and M. S. Smith. 1991.

Role of cover crops in nitrogen cycling. Pages 85-90 inW. L. Hargrove (ed.) Cover crops for clean water. Soil and Water Conservation Society, Ankeny, Iowa.

Ebelhar, S.A., W. W. Frye, and R.L. Blevins. 1984.

Nitrogen from legume cover crops for no-tillage corn. Agron. J. 76:51-55.

Frye, W. W. and R. L. Blevins. 1989.

Economically sustainable crop production awith legume cover crops and conservation tillage. J. Soil and Water Cons. Jan-Feb. pp. 57-60.

Hargrove, W. L. 1986.

Winter legumes as a nitrogen source for no-till grain sorghum. Agron. J. 78:70-74.

Hemphill, D. D. and J. Hart. 1991.

Effect of nitrogen rate and placement on broccoli yield and nitrogen uptake. Report to the Oregon Processed Vegetable Commission. 5 pp.

Ingham, R. 1993.

Report to the Oregon Potato Commission, 1993. Dept. of Botany and Plant Pathology, Oregon State Univ., Corvallis, OR.

Lal, R., E. Regnier, D. J. Eckert, W. M. Edwards, and R. Hammond. 1991.

Expectations of cover crops for sustainable agriculture. Pages 1-11 inW.L. Hargrove (ed.) Cover crops for clean water. Soil and Water Conservation Society, Ankeny, Iowa.

Mangan, F. X., S. J. Herbert, and G. L. Litchfield. 1991.

Cover crop management systems for broccoli. Pages 178-179 inW. L. Hargrove, (ed). Cover Crops for Clean Water, Soil and Water Conservation Society, Ankeny, IA.

Meisinger, J. J., W. , W. L. Hargrove, R. L. Mikkelsen, J. R. Williams, and V. W. Benson. 1991.

Effect of cover crops on groundwater quality. Pages 57-68 In Hargrove, W. L. (ed). Cover crops for clean water. Soil and Water Conservation Society. Ankeny, IA.

Meisinger, J. J., P. R. Shipley, and A. M. Decker. 1990.

Using winter cover crops to recycle nitrogen and reducing leaching. Pages 3-6 in J. P. Mueller and M. G. Wagger (eds) Conservation Tillage for Agriculture in the 1990's. Spec. Bull. 90-1. N. Carolina State Univ., Raleigh.

McVay, K. A., D.E. Radcliffe, and W. L. Hargrove. 1989.

Winter legume effects on soil properties and nitrogen fertilizer requirements. Soil Sci. Aoc. Am. J. 53: 1,856-1,862.

Mitchell, W. H. and M. R. Teel. 1977.

Winter annual cover crops for no-tillage corn production. Agron. J. 69:569-573.

Muller, J. C., D. Denys, G. Morlet, and A. Mariotti. 1987.

Influence of catch crops on mineral nitrogen leaching and its subsequent plant use. In . D. S. Jenkinson and K. A. Smith. (eds). Nitrogen efficiency in agricultural soils, Vol. 2. Elsevier, New York.

Murray, H. and D. McGrath. 1992.

OSU cover crop trial update. Pacific Northwest Sust. Agric. 4: 4-5.

Putnam, A. R. 1986.

Allelopathy:can it be managed to benefit Horticulture? HortScience 21(3):411-413.

Putnam, A. R. 1990.

Vegetable weed control with minimal herbicide inputs. HortScience 25(2):155-159.

Rothrock, C. S. and S. R. Kendig. 1991.

Suppression of black root rot on cotton by winter legume cover crops. Pages 155-156 inW. L. Hargrove (ed.) Cover crops for clean water. Soil and Water Conservation Society, Ankeny, Iowa.

Shennan, C. 1992.

Cover crops, nitrogen cycling, and soil properties in semi-irrigated vegetable production systems. HortScience 27:749-754.

Stevenson, F. J. 1986.

Cycles of soil. John Wiley & Sons. New York. 380 pp.

Sullivan, P.G., D. J. Parrish, and J. M. Luna. 1991.

Cover crop contributions to N supply and water conservation in corn production. Amer. J. Alternative Agriculture. 6: 106-115.