Kansas Rural Center COVER CROPS
AGRICULTURE AND LEGUMES
AUGUST 1998 MG1A.1
Nature continually works to provide a natural cover of these particles by organic or inorganic agents.
over the soil. Farmers can enhance the soil fertility of Improving soil aggregation is important to improve
their fields by using as many legumes and cover crops water infiltration and to create more favorable soil con-
as they can within their rotations. Legume and sod ditions for growing crops. It is also thought by some
grasses in rotations will increase soil organic matter, or agronomists that the use of cover crops which stimulate
at least maintain it at relatively higher levels, over row soil microbial growth is a larger key to better soil
crop production (Smith and Varco). Further advantages aggregation than just additional organic matter alone
of cover crops are less exposure to soil erosion and (Smith and Varco, 1987).
reduced surface runoff through greater water infiltra- An ideal plowdown time for legumes is between
tion in the soil. According to research at Purdue early bud and early bloom. A legume cover crop that
University, fields with a winter cover crop that were begins flowering will increase the amount of carbon
used as a green manure crop in the spring had 55% less which will slow down the availability of nitrogen for
water runoff and 50% less soil loss than fields without the next crop. The nitrogen produced by the legume
a winter cover. When corn or soybeans are no-tilled will remain in the plant tissues whether the plant is liv-
into a vigorous cover crop of rye or wheat, soil losses ing or dead. As long as the legume sod attained its
are 90-95% less than conventionally tilled soils maximum growth prior to fall dormancy, it should not
(Brunoehler, 1989). matter whether the plant regenerates in the spring or if
Soil productivity can be increased with long term use it has winter killed (“Green Manures,” 1983).
of cover crops. Organic matter content increases with It is best after plowdown of a mature, carbon-rich
cover crops and conservation tillage. Legume cover green manure to wait until the breakdown is well
crops enhance the sustainability of crop production by underway before planting the following crop. Soil
increasing soil organic matter, improving the long-term microbes multiply rapidly to break down the carbona-
nitrogen reservoir of the soil, improving soil structure, ceous plant matter, and they consume a lot of nitrogen
conserving soil water and reducing runoff and soil ero- in the process. This process leaves little nitrogen avail-
sion. Disadvantages of cover crops are the cost of seed- able for plant growth until breakdown has been com-
ing; loss of economic production while the cover crop pleted to the extent that the microbes begin to die and
is growing; lowered soil temperature in spring, and release their nitrogen reserve back to the soil. A gen-
depletion of soil water at planting time (Frye and eral rule of thumb applicable to most crops and cli-
Blevins, 1989). mates is to allow breakdown of the green manure crop
Approximately 2.2 tons per acre of crop residue per for at least two weeks. When the tilled-in materials
year is considered adequate to maintain soil organic break apart or crumble easily, the breakdown may be
matter at constant levels in continuously cropped soils. considered far enough along for the next crop to be
Annual green manure crops have a negligible effect on planted (“Green Manures,” 1983).
total soil organic matter levels if cultivation is continu- Deep-rooted legumes, such as alfalfa, used in crop
ous, although they do replenish the supply of active, rotations, are believed to cycle nutrients upwards from
rapidly-decomposing organic matter (Frye and Blevins, subsoils. Winter cover crops trap nutrients that other-
1989). wise might have been lost from the root zone and recy-
The main benefit of legumes in soil organic matter cle them for the next crop. Legumes have an advantage
management is the periodic supply of residue which over other soil-conserving crops because of their abili-
soil microorganisms decompose and in the process ty to decompose more rapidly due to their lower car-
release binding agents that are important in improving bon-nitrogen ratio.
soil aggregation - the physical process of putting soil Cover crops can reduce nitrate leaching through the
particles together followed by the chemical cementing soil profile. Winter cover crops prevent this leaching
Page 2 Cover Crops and Legumes
by depleting both soil nitrates and water. This occurs Brunoehler, Ron. September, 1989. “Cover Crops:
most actively in late winter and spring when the poten- Conservation vs. Cost.” Farm Industry News.
tial for leaching is often greatest. Some research indi-
cates that nitrogen accumulated in legumes is less sub- Frye and Blevins. January, 1989. “Economically
ject to leaching than commercial fertilizer during the Sustainable Crop Production with Legume Cover
summer growing season (Smith and Varco, 1987). Crops and Conservation Tillage.” Journal of Soil and
One-third of agriculture’s 3% share of national ener- Water Conservation.
gy consumption is used in the manufacturing of nitro-
gen fertilizer. According to Dr. Larry King, agronomist “Cover Crops and Green Manures.” Appropriate
at North Carolina State University, the substitution of Technology Transfer for Rural Areas.
biologically fixed nitrogen from a potential hairy vetch
green manure crop that fixed 100 pounds of nitrogen Hofstetter, Robert. February, 1985. Rodale Research
credits uses only 7.2% of the energy required to pro- Center.
duce a similar amount of commercial fertilizer.
The portion of green-manure nitrogen provided to a “Green Manures - A Mini-Manual.” 1983. Johnny's
following crop is usually about 50-60% of the total Selected Seeds.
amount contained in the legume. Approximately 40%
of the plant tissue nitrogen becomes available the first Goldstein, Walter. 1989. “Thoughts on Drought
year following a chemically burned, no-till legume Proofing Your Farm; A Biodynamic Approach.”
mulch. Approximately 60% of the tissue nitrogen is Michael Fields Agriculture Institute.
released when the cover crop is incorporated as a green
manure rather than left on the surface as a mulch. CREDITS
Lesser amounts are available the next two growing sea-
sons, but increased yields are apparent. Nutrients from The author of this publication is Jerry Jost with the
decaying plant material are more readily available for Kansas Rural Center.
use by succeeding crop plants than those nutrients Funding for this management guide came from
derived from soil minerals or particles. During decom- USDA’s Environmental Quality Incentives Program.
position of organic matter, carbonic and other organic Additional funding is from the Clean Water Farms
acids are formed. These organic acids react with insol- Project, a project of the Kansas Rural Center in coop-
uble mineral rocks and phosphates precipitates, releas- eration with the Kansas Department of Health and
ing phosphates and exchangeable nutrients (“Cover Environment (KDHE), and funded by the U.S.
Crops and Green Manures”). Environmental Protection Agency (EPA) Section 319
If a cover crop is put up as hay, the nitrogen in hay is Non-Point Source Funds.
removed with the crop. If it is grazed, the animals The U.S. Department of Agriculture (USDA) pro-
return about 80% of the nitrogen in their excrement hibits discrimination in all its programs and activities
(Goldstein, 1989). on the basis of race, color, national origin, gender, reli-
gion, age, disability, political beliefs, sexual orienta-
Publications tion, and marital or family status. (Not all prohibited
bases apply to all programs.) Persons with disabilities
Kansas State University has published a useful bul- who require alternative means for communication of
letin called “Using Legumes in Crop Rotations,” 1988. program information (Braille, large print, audiotape,
For more information about interseeding legumes into etc.) should contact USDA’s TARGET Center at (202)
cool-season grasses, Kansas State University has pub- 720-2600 (voice and TDD).
lished “Interseeding Legumes into Cool-season To file a complaint of discrimination, write USDA,
Grasses,” Publication No. 40, 1978. Another publica- Director, Office of Civil Rights, Room 326W, Whitten
tion is “Performance of Grass-Legume Mixtures in Building, 14th and Independence Ave, SW,
Eastern Kansas,” Bulletin 649, 1986. Washington, DC 20250-9410, or call (202) 720-5964
(voice or TDD). USDA is an equal opportunity
REFERENCES provider and employer.
Smith, Scott and Varco. 1987. “Legume Winter Cover
Crops.” Advances in Soil Sciences.
Cover Crops and Legumes Page 3
Percentage of Nitrogen in Tops Legume Forage Nitrogen Production
and Roots of Legumes Under Optimum Growing Conditions at
Mound Valley, Kansas in 1990-1991
Legume Tops Roots
Soybeans 93 7 Legume #N/acre
Vetch 89 11 Winter peas 139
Cowpeas 84 16 Hairy vetch 127
Red clover 68 32 Sweetclover 82
Alfalfa 58 52 Black medic 73
Crimson clover 120
Source: McCleod, Edwin. 1982. “Feed the Soil.”
Arrowleaf clover 62
Carbon: Nitrogen Ratios for Plants L.S.D. (0.05) 31
Young rye 20:1 Source: Kansas State University Research.
Flowering rye 38:1
Mature rye 350:1 Winter Survival of Eight Cover
Legumes 25:1 Crops Grown at Two Locations
Corn stalks 40-60:1 in Eastern Kansas
Powhattan Mound Valley
Source: May, Jack. 1981. “Organic Matter in Legume 1990 1991 1990 1991
Nursery Soils.” Survival Percentage %
Winter peas 69 0 92 67
Hairy vetch 75 94 100 73
Sweetclover 100 73 71 59
Black medic 44 0 39 88
Crimson clover 0 2 50 80
Arrowleaf clover 98 0 77 97
Winter wheat 88 90 54 80
Rye 68 94 77 97
L.S.D. (0.05) 23 32 27 20
Source: Long, J.H., J.L. Moyer, and B.H. Marsh.
“Winter Annual Legumes and Grass Use as Cover
and Green Manure Crops in Eastern Kansas.”
Kansas State University.
The Kansas Rural Center is a private, non-profit organization that promotes the long
term health of the land and its people through education, research and advocacy.
The Rural Center cultivates grassroots support for public policies that encourage
family farming and stewardship of soil and water. The Rural Center is committed
to economically viable, environmentally sound, and socially sustainable rural
culture. For more information, contact the Kansas Rural Center at PO Box 133,
Whiting, Kansas 66552 or (785) 873-3431.