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PRINCIPAL USES OF COVERCROPS AND GREEN MANURES

Updated on November 12, 2010

PRINCIPAL USES OF COVER CROPS AND GREEN MANURES

Green manuring involves the soil incorporation of any field or forage crop while green or soon after flowering, for the purpose of soil improvement. A cover crop is any crop grown to provide soil cover, regardless of whether it is later incorporated. Cover crops are grown primarily to prevent soil erosion by wind and water. Cover crops and green manures can be annual, biennial, or perennial herbaceous plants grown in a pure or mixed stand during all or part of the year. In addition to providing ground cover and, in the case of a legume, fixing nitrogen, they also help suppress weeds and reduce insect pests and disease. When cover crops are planted to reduce nutrient leaching following a main crop, they are often termed: catch crop”.

Winter cover crop:A winter cover crop is planted in late summer or fall to provide soil cover during the winter. Often a legume is chosen for the added benefit of nitrogen fixation. In northern states, the plant selected needs to posses enough cold tolerance to survive hard winters. Hairy vetch and rye are among the few selections that meet clovers, vetches, medics, and field peas. They are sometimes planted in a mix winter cereal grains such as oats, rye, or wheat. Winter cover crops can be established by aerial seeding into maturing cash in the fall, as well as by drilling or broadcasting seed immediately following harvest.

Summer Green Manure Crop: A summer green manure occupies the land for a portion of the summer growing season. These warm-season. These warm-season cover crops can be used to fill a niche in crop rotations, to improve the conditions of poor soils, or to prepare land for a perennial crop. Legumes such as cowpeas, soybeans, annual sweetclover, sesbania, guar, crotalaria, or velvet beans may be grown as summer green manure crops to add nitrogen along with organic matter. Non-legumes such as sorghum-dunangrass, millet, forage sorghum, or buckwheat are grown to provide biomass, smother weeds , and improve soil tilth.

Living Mulch:A living mulch is a cover crop that is interplant with an annual or perennial cash crop. Living mulches suppress weeds, reduce soil erosion, enhance soil fertility, and improve water infiltration. Examples of living mulches in annual croppi9ng systems include over seeding hairy vetch into corn at the last cultivation, no-till planting of vegetables into sub cover, sweet clover drilled into small grains, and ryegrass broadcast into vegetables. Living mulches in perennial cropping systems are simply the grasses or legumes planted in the alleyways between rows in orchards, vineyards, Christmas trees, berriesm windbreaks, and field nursery trees to control erosion and provide traction.

Catch Crop: A catch crop is a cover crop established after harvesting the main crop and is used primarily to reduce nutrient leaching from the soil profile. For example, planting cereal rye following corn harvest helps to scavenge residual nitrogen, thus reducing the possibility of groundwater contamination. In this instance, the rye catch crop also functions as a winter cover crop. Short-term cover crops that fill a niche within a crop rotation are also commonly known as catch crops.

Forage Crop: short-rotation forage crops function both as cover crops when they occupy land for pastures pr haying and as green manures when they are eventually incorporated or killed for no-till mulch. Examples include legume sods of alfalfa, sweet clover, trefoil, red clover, and white clover, as well grass-legume sods like fescue clover pastures, for maximum soil-improving benefits, the forage should not be grazed or cut for hay during its last growth period, to allow time for biomass to accumulate prior to killing.

BENEFITS OF COVER CROPS AND GREEN MANURES

Organic matter and soil structure:A major benefit obtained from green manures is the addition of organic matter to the soil. During the breakdown of organic matter by microorganisms, compounds are formed that are resistant to decomposition such as gums, waxes, and resins. These compounds and the mycelia, mucus, and slime produced by the microorganisms help bind together soil particles as granule, or aggregates. A well-aggregated soil tills easily, is well aerated, and has a high water infiltration rate. Increased levels of organic matter also influence soil humus. Humus the substance that results as the end product of the decay of plant and animal materials in the soil provides a wide range of benefits to crop production. Sod-forming grass or grass-legume mixtures are important in crop rotations because they help replenish organic matter lost during annual cultivation. However, several years of sod production are sometimes required before measurable changes in humus levels occur. In comparison, annual green manures have a negligible effect on humus levels, because tillage and cultivation are conducted each year. They do replenish the supply of active, rapidly decomposing organic matter.

          The contribution of organic matter to the soil from a green manure crop is comparable to the addition of 9 to 13 per acre of farmyard manure or 1.8 to 2.2 tons dry matter per acre.

Nitrogen Production:Nitrogen production from legumes is a key benefit of growing cover crops and green manures. Nitrogen accumulations by leguminous cover crops range from 40 to 200 Ibs. of  nitrogen per acre. The amount of nitrogen available from legumes depends on the species of legume grown, the total biomass produces, and the percentage of nitrogen in the plant tissue. Cultural and environmental conditions that limit legume growth such as a delayed planting date, poor stand establishment, and drought will reduce the amount nitrogen produced. Conditions that encourage good nitrogen production include getting a good stand, optimum soil nutrient levels and soil pH, good nodulation, and adequate soil moisture. The portion of green-manure nitrogen available to a following crop is usually about 40% to 60% of the total amount contained in the legume. For example, a hairy vetch crop that accumulated 180 Ibs. N per acre prior to plowing down will contribute approximately 90 Ibs N per acre to the succeeding grain or vegetable crop. Dr. Greg Hoyt, an agronomist at North CarolinaStateUniversity, has estimated that 40% of plant tissue nitrogen becomes available the first year following a cover crop that is chemically killed and used as a no-till mulch. He estimates that 60% of the tissue N is released when the cover crop is incorporated as a green manure rather than left on the surface as a mulch. Lesser amounts are available for the second or third crop following a legume, but increased yields are apparent for two to three growing seasons.

Table 2. percent nitrogen in legume tops and roots

Crop                    Tops                     Roots

                             %N                       %N

Soybeans              93                         7

Vetch                    89                         11

Cowpeas               84                         16

Red Clover            68                         32

AlfaIfa                   58                         42

Soil microbial activity: A rapid increase in soil microorganisms occur after a young, relatively lush green manure crop is incorporated into the soil. The microbes multiply to attack the freshy nincorporated plant material.

          During microbial breakdown, nutrients held within the plant tissues are released and made available to the following crop. Factors that influence the ability of microorganisms to break down organic matter include soil temperature, soil moisture, and carbon to nitrogen (C:N) ratio of the plant material. The C:N ratio of plant tissue reflects the kind and age of the plants from which it was derived. (Table 3) as plants mature, fibrous (carbon) plant material increases and protein (nitrogen) content decreases. The optimum C:N ratio for rapid decomposition of organic matter is between 15:1 and 25:1. C:N ratios above 25:1 can result in nitrogen being “tied up” by soil microbes in the breakdown of carbon-rich crop residues, thus pulling nitrogen away from crop plants. Adding some nitrogen fertilizer to aid the decomposition process may be advisable with these high carbon residues. The lower C:N ratio, the more N will be released into the soil for immediate crop use. The C:N ratio is more a function of the plant’s N content than its carbon content. Most plant materials contain close to 40% carbon. To determine the C:N ratio of any plant material, divide 40% by its nitrogen content. For example let’s say hairy vetch contains 4.2% nitrogen: 40/42 = a C:N ratio of 9.5. a procedure for determining the nitrogen content of cover crop biomass was previously addressed in the section on nitrogen production. Estimating the nitrogen contribution of a cover crop is very helpful when adjusting N fertilizer rates to account for legume nitrogen.

Nutrient Enhancement:in addition to nitrogen from legumes, cover crops help recycle other nutrients on the farm Nitrogen (N), Phosphorus (KB), calcium (Ca), Magnesium (mg), Sulfur (S), and other nutrients are accumulated by cover crops during a growing season. When the green manure is incorporated, or laid down as no till mulch, these plant-essential nutrient become slowly available during decomposition. Dr. Greg Hoyt developed a method of estimating nutrient accruement by cover crops in order to reduce the soil test recommendation of fertilizer for the following crop. Table 4 shows the biomass and nutrients accumulated by several cover crops he worked with.

TABLE 4: BIOMASS YIELD AND NUTRIENT ACCRUEMENT BY SELECTED COVER CROPS.

           

Crop

Biomass

Nitrogen

Potassium

Phosphorus

Magnesium

Calcium

Ibs/ac

Ibs/ac

Ibs/ac

Ibs/ac

Ibs/ac

Ibs/ac

Hairy Vetch

3,260

141

133

18

18

52

Crimson Clover

4,243

115

143

16

11

62

Austrian Winter peas

4,114

144

159

19

13

45

Rye

5,608

89

108

17

8

22

          Dry weight of above ground material. Certain broad-leaved plants are noted for ability to accumulate minerals at high concentrations in their tissue. For example, buckwheat, lupine, and sweetclover are noted for their ability to extract P from soils. Likewise, alfalfa and other deep-rooting green manures scavenge nutrients from the subsoil and translocate them upwards to the surface rooting zone, where they become available to the following crop.

          The breakdown of green manures in soil influences mineral nutrient availability in another way. During decomposition of organic matter, carbonic and other organic acids are formed as a byproduct of microbial activity. These organic acids react with insoluble mineral rocks and phosphate precipitates, releasing phosphate and exchangeable nutrients.

          Rooting Action: the extensive roots systems of some cover crops are highly effective in loosening and aerating the soil. In Australian wheat experiments, the taproots of a blue lupine cover crop performed like a “biological plow” in penetrating compacted soils. When cover crops are planted after a sub-soiling treatment, they help extend the soil-loosening effects of the deep tillage treatment. The rooting depths of several green manures grown under average conditions are listed in table 5.

TABLE 5: TYPICAL ROOTING DEPTH OF SEVERAL GREEN MANURE CROPS.

Depth (feet)                   Green Manure Crop

5 to 7                    Red Clover, Lupine, radish, Turnips

3 to 5                    Common Vetch, Mustard, Black Medic, Rape

1 to 3                    White Clover, Hairy Vetch

Weed suppression:weeds flourish on bare soil. Cover crops take up space and light, thereby shading the soil and reducing the opportunity for weeds to establish themselves. The soil-loosening effect of deep-rooting green manures also reduces weed populations that thrive in compacted soils. The primary purpose of a non-legume green manure such as rye, millet, or sudangrass is to provide weed control. Add organic matter, and improve soil tilth. They do not produce nitrogen. Thus, whenever possible, annual grain or vegetable crops should follow a legume green manure to derive the benefit of farm-produced nitrogen. Providing weed suppression through the use of allelopathic cover crops and living mulches has become an important method of weed control in sustainable agriculture. Allelopathic plants are those that inhibit or slow the growth of other nearby plants by releasing natural toxins, or “allelochemicals”. Cover crop plants that exhibit allelopathy include the small grains like rye and summer annual forage related to sorghum and sudangrass. The mulch that results from mowing or chemically killing allelopathic cover crops can provide significant weed control in no-till cropping systems. Living mulches suppress weeds during the growing season by competing with them for light, moisture and nutrients.

Soil and water conservation:when cover crops are planted solely for soil conservation, they should provide a high percentage of ground coverages as quickly as possible. Most grassy and non-legume cover crops, like buckwheat and rye, fulfill this need well. Of the winter legumes, hairy vetch provides the least autumn ground cover because it puts on most of its above ground growth in the spring. Consequently, it offers little ground cover during the erosion prone fall and winter period. Sowing a mix of leguminous and grassy-type cover crops will increase the ground coverage, as well as provide some nitrogen to the following crop. The soil conservation benefits provide by a cover crop extend beyond protection of bare soil during non-crop periods. The mulch that results from a chemically or mechanically killed cover crop in no-till plantings increases water infiltration and reduces water evaporation from the soil surface. Soil cover reduces soil crusting and subsequent surface water runoff during rainy periods. Retention of soil moisture under cover crop mulches can be a significant advantage. Dr. Blevins and other researchers showed consistently higher soil-moisture levels for corn grown in a herbicide-killed, no-till bluegrass sod than for corn grown in conventionally plowed and disked plots. They concluded that the decreased evaporation and increased moisture storage under no-till mulch allowed plots to survive a short-term drought without severe moisture stress.

Vegetable Management to create Cover Crop Mulch: Herbicides are the most commonly used tools for cover crop suppression in conservation tillage systems. Non-chemical methods include propane flamers, mowing and mechanically tillage.

          Mowing a rye cover crop when it heads out in late spring sufficient kill. The rye must be in the pollination phase, or later, to be successfully killed. When the anthers are fully extended and you thump the stalk and pollen falls down, it is time to mow. If mowed earlier, it just grows back. Flail mowers generally produce more uniformly distributed mulch than do rotary cutters, which tend to windrow the mulch to one side of the mower. Sickle bar mowers create fairly uniform mulch, but the unchopped rye stalks can be more difficult to plant into. If late spring weather continues cool and wet, more rye re-growth is minimal and not a problem to crops grown in the moved mulch. In a Mississippi study, flail mowing, or rolling with rolling disk colters spaced at 4 inches, was usually as effective as herbicides in killing hairy vetch, crimson clover and subterranean clover. Timing is a key factor when using mowing or rolling to control cover crops. Mechanical control was most effective when the legumes were in the seed formation growth phase (mid to late April) or when stem lengths along the ground exceeded 10 inches. If mowing was followed with a pre-planting herbicide application of Atrazen, the legume kill was even more effective. Researcher at AhioStateUniversity developed a mechanical cover crop killing tool used to take out a cover crop without herbicides. They call it an undercutter because it uses wide V-blades which run just under the soil surface to cut off the cover crop from its roots. The bladed are pitched to 15 degrees, allowing the bladed to penetrate the soil and provide a slight lifting action. Mounted on the same toolbar behind the cutter bladed is a rolling basket to flatten and distribute the undercut cover crop. The undercutter was tried on several  cover crops and effectively killed crimson clover, hairy vetch, rye and barely. These undercutters could be made from locally available stock by innovative tinkers.

REFERENCE

Allison, F.E 1973. soil Organic Matter and its Role in Crop Production.

Elsevier Scientific Pub. Co. Amsterdam. 637 p.

Belvins, R.C., D. Cook, and S.H. Philips. 1971. influence of No-tillage on

oil moisture. Agronomy Journal. Volume 63. p. 593-596.

Dabney, S., N.W Bluehring, and D.B. Reginelli 1991. Mechanical control

of legume Cover Crops. P. 6. In: W.L. Hargrove (ed.). cover crops of Clean Water. Soil Conservation Society of America, Ankeny, 1A

Davidson, H., R. Meclenbrurg, and C. Petersdon. 1988. Nursery

Management: Asminiostration and Culture. 2nd ed. Prentice Hall, Englewood Cliffs, NJ. P 220.

May, J.T. 1981. Organic matter in nursery soils. P. 52059. In:

Proceedings of the 1981 Southern Nursery Conference.

Hoyt, G.D 1987. legumes as a green manure in conservation tillage. P.

96-98. In: J.F Powers (ed). The Role of Legumes in Conservation Tillage Systems. Soil Conservation Society of America, Ankeny, 1A.

Piper, C.V., ands A.J. Pieters. 1922. Green Manuring. USDA Farmer’s

Bulletin 1250. 45 p.


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