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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.
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.
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).
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.
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.
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.
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.
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.
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.
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