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COG Organic Field Crop Handbook
1.5 Green Manures
A green manure cover crop, or plowdown crop, is any crop that is turned into the soil to add organic matter, nitrogen or other nutrients.
1. Advantages of green manure crops
Green manures, in the case of legumes, fix nitrogen and contribute to farm nitrogen needs.
Green manure crops help discourage the growth of weeds. The slow release of mineralized nitrogen from green manures favors the crop rather than the weeds.
Green manures tend to be vigorous plants that can out-compete weeds and effectively displace them from their ecological niche.
When green manures are turned into the soil and decompose, they provide nutrition for soil organisms, thus protecting and enhancing the soils biological activity.
The root mass of a green manure crop loosens and aerates the soil, consequently improving the soil structure.
The roots and top growth maintain or increase the organic matter content of the soil, which improves soil tilth.
Green manure crops cover the soil in between successive grain crops, thus protecting the soil against wind and water erosion. As little as 30 per cent ground cover will significantly reduce soil losses in winter.
The rooting activity of green manures facilitates drainage in wet areas, and the shading and mulching restricts soil moisture evaporation which is particularly useful in dry conditions. However, this advantage needs to be balanced with the loss of moisture that can result from transpiration in drier climates.
Green manure crops reduce soil compaction. They lessen the impact of rainfall and vehicle traffic.
Green manures soak up potentially-leachable nutrients thus tightening up the on-farm nutrient cycle. The nutrients are then held by the green manure until it is turned under. During decomposition, the nutrients will be released to the following crop.
Green manure crops provide a habitat for pollinators and other beneficial insects.
The choice of green manure and whether to seed a green manure must also take into account the following factors:
Seed cost and availability vary considerably. For example, red clover is inexpensive, imported hairy vetch seed is currently (1991) more costly, requiring an investment of $20-30/acre because of increasing demand and lack of availability. Oilradish seed is less expensive and with a seeding rate of 17 kg/ha, cost is not prohibitive. Many farmers are producing their own seed to reduce costs.
Seeding, maintenance and incorporation requires extra time, labor and fuel.
Green manures may harbor pests and plant disease so a good rotation should be followed. For example, do not grow a cruciferous green manure if growing main crops of the same family. Also note that some diseases such as white mould (Schlerotinia) can have several host families such as crucifers, beans and sunflowers.
Some cover crops are difficult to turn under and may require repeated tillage which will accelerate organic decomposition and soil erosion. An example is rye which has a heavy residue and should be incorporated fairly early in its growth to avoid this problem.
While the green manure residue decomposes, there may be a short period when nitrogen will be unavailable to the following crop. This may temporarily interfere with seeding. Waiting 2-3 weeks before planting the main crop usually minimizes this problem.
Residue of any sort can become allelopathic (exude toxic chemicals) to the following crop and may interfere with seed germination. Again, delaying planting will minimize this problem.
Living or winter-killed green manure can retard spring soil-warming by acting as a mulch. This, in turn, can delay or retard growth of temperature-sensitive crops such as corn.
Some green manure crops, such as oilradish and buckwheat will become a weed in the succeeding crop if they are allowed to set seed.
Questions are often raised about the feasibility of taking land out of production for a crop that will not bring direct revenue. Despite the lack of immediate financial gain from growing green manures, organic farmers consider this material an essential part of their farm ecosystem. Crop rotations are designed so that the green manure crops are grown in between cash crops, thus still gaining soil-conserving benefits without detracting from the cash crops.
Examples of practices that balance the needs of cash crops with green manures include:
1. Undersowing a green manure such as red clover with small grains. On the Prairies, yellow sweet clover is underseeded into cereals, flax and safflower.
2. Overseeding a green manure such as red clover or rye grass into corn at last cultivation.
3. Interseeding a crop such as hairy vetch into a maturing winter cereal stand.
Using these methods, the green manure does not compete with the crop but can get established and will continue to grow after the grain crop is harvested, at a time when the soil would otherwise be vulnerable to erosion and weed encroachment.
[photo 5.1: Underseeding of red clover]
3. Green manure crop categories
Green manures are referred to by a number of names, according to their primary function at any one time. But, green manures always perform a multitude of functions on the farm and when choosing which to grow, the more functions the green manure can perform, the better.
Cover crops protect the soil from wind and water erosion by covering it and, because they form a mulch, they greatly reduce annual weeds in the next growing season. They are frequently used to cover the soil over winter either alive or as a dead, dense mat. They can also be used in summer, especially when a crop fails because of adverse weather. Examples include red and sweet clover, hay or pasture seedings, hairy vetch, winter cereals and buckwheat. A volunteer crop seeded from harvest losses, can also be a cover crop. On the Prairies, organic farmers use annual legumes as cover crops for at least part of the season rather than leaving summer fallow bare. Indian Head lentils and Sirius peas have been developed for this purpose.
Catch crop / nutrient conserving crops
A catch crop has a brief period of growth and is either worked in after the main crop has been taken off or planted between two main crops. The crop protects the soil from erosion and minimizes nutrient loss from the soil through leaching. Also, because it is not harvested, it can enrich the soil by adding organic matter, nitrogen or other nutrients. Examples of catch crops are oilradish, red clover and buckwheat.
Applications of compost or liquid manure are made to catch crops which soak up a lot of nutrients and immobilize them in their tissues. These nutrients become available to the next crop in the rotation as the plant residues decompose. Farmers have found that oilradish is particularly effective in storing nutrients. Within the forages, reed canary grass is good but it can be difficult to control, so rye or rye grass may be a better choice.
Some rapidly growing catch crops are useful because they accumulate large amounts of organic matter in a short time. Again, oilradish is an good example; under ideal conditions, it is possible to yield 10 tons of green matter per acre, 45 days after seeding.
[photo 5.2: Oilradish and rye grass in October]
A green manure that breaks the life cycle of pests, weeds or diseases is know as a break crop. Effectiveness increases with the length or frequency of the breaks. The term is not commonly used but is a useful concept to remember when planning rotations.
N-fixing green manures
Legumes are the most important of the green manure crops. They fix nitrogen from the air, add organic matter to the soil and also enhance the cycling of phosphorus. Examples are red clover, alfalfa, sweet clover, vetch and most hay and pasture legume species. An additional benefit of the deep-rooted alfalfa, vetch or sweet clovers is their ability to loosen compacted subsoils.
When a green manure is grown primarily to control weeds, it is called a smother crop. It is characterized by its extremely dense, vigorous and rapid growth and is usually chosen with specific weeds in mind. For example, fall rye is used against quack grass because its vigorous growth in spring coincides with the growth cycle of quack grass. Other examples of smother crops are oilradish, reportedly good against yellow nutsedge, and buckwheat.
Some crops produce natural chemical toxins which retard germination and inhibit the early growth of weed species. Examples are rye and yellow sweet clover.
Examples of green manures that provide good bee habitats are sweet clover and buckwheat.
4. Nutrient benefits of green manures
Nutrients absorbed by green manure crops or those retained within crop residues after harvest, are gradually released or "mineralized" when the crop is incorporated into the soil and subsequently decomposes. The time when the nutrients will be released can be influenced by the carbon:nitrogen ratio, moisture content, particle size of the soil, method of incorporation, soil nitrogen levels and temperature.
The ability of legumes to fix nitrogen from the air is invaluable. Alfalfa and sweet clover will probably fix more nitrogen than hairy vetch or red clover but the amount is variable dependent on several factors. About 30-50 per cent of this nitrogen is available to the next crop while much of the remainder is incorporated into the soil organic matter and is released gradually in succeeding years.
Timing of plowdown will influence the total amount of nitrogen that is fixed. For example, if you plan to plow red clover in the fall, delay as late as possible as it continues to fix nitrogen until late October.
A rule of thumb is that incorporating a vigorous stand of alfalfa will supply most of the nitrogen for a following crop of corn, however, the amount of nitrogen fixed will vary depending in the residual soil nitrogen. Legumes are "lazy" crops and prefer to take up their nitrogen from the soil rather than going through the energy-expensive process of nitrogen fixation. As available soil nitrogen levels increase with soil biological activity, the rate of nitrogen fixation is reduced.
Some examples of nitrogen contributions reported by researchers include:
3-year-old stand of alfalfa -- 110 kg/ha
red clover plowdown -- 45 kg/ha, 75 kg/ha
hairy vetch -- 65 kg/ha
yellow sweet clover -- 56 kg/ha, 140 kg/ha
Rye, crucifer and grass crops can also absorb excess soluble nitrogen and other nutrients from the soil that would otherwise be lost to leaching. This not only improves soil nutrient efficiency, but also reduces nitrate contamination of groundwater.
It should be noted that the N from the green crop can be leached out of a warm sandy soil if there is a long, wet period between incorporation of the green manure crop and the establishment of the the following crop.
[Illustration - legume line drawing, no caption[
Nitrogen-fixing legumes, buckwheat and oilradish are very efficient at utilizing insoluble soil phosphorus, which is accumulated in their tissues and made more accessible to the following crop. Green manures also accumulate potassium, calcium and micronutrients.
If a green manure includes a high percentage of legumes, or if it is an immature green plant, the carbon:nitrogen ratio is narrow. Decomposition will be rapid, causing a swift release of carbon dioxide. This benefits crop growth and by producing carbonic acid, helps lower the pH of alkaline soils, increasing the availability of phosphates and micronutrients.
Enough seed should be used to insure a complete cover. Generally a greater quantity should be used than would be needed under theoretically-ideal conditions. When broadcasting rather than drilling, higher seeding rates are used. Higher rates should also be used when planting later than recommended, in a rough seed bed or in weedy or sloping erosion-prone land. Rates for individual crops are given in Table 3. For mixtures of legumes and non-legumes, reduce the non-legume rate by 50 per cent and the legume by 20-35 per cent.
6. Methods of incorporation
The beneficial effect of the green manure crop on the following crop will depend upon the microbial activity in the soil when the green crop is incorporated, and whether there is sufficient air present in the soil for aerobic decomposition. Some farmers pre-cut green manure while others simply incorporate it directly into the soil.
If the green manure is not pre-cut, partially incorporate it into the top few inches of the soil with a disc or chisel plow.
When there is lush growth, clip the green manure crop down to the root area.
If the green manure has tough stems or stands more than 15 cm (6 inches) tall, it may be necessary to chop it before working it in. A rotary mower is well suited for this job. The chopped material is left lying 2-8 days on the surface before incorporation into the soil.
Once the crop is dry, use a disc or chisel plow to work it into the top 8-10 cm of the soil.
In lighter soil, the decomposition proceeds more rapidly. To avoid too rapid decomposition the green manure is worked in at a deeper level (10-20 cm).
If a moldboard plow is used in heavy soils decomposition can slow down. Do not plow more than 15 cm (6 inches) deep. Leaving the furrow on edge will improve the air supply.
Avoid seeding the next crop into the plowdown for 2-3 weeks to allow the incorporated material to decompose and for the nutrients (especially nitrogen which is tied up during the decomposition) to become available.
[photo 5.3: Incorporating a green manure crop]
Altieri, Miguel, Agroecology: The Scientific Basis of Alternative Agriculture, 2nd ed. Boulder, CO, Westview Press, 1987
Pieters, A. J., Green Manuring, Principles and Practice, John WIley & Sons Inc., New York, NY, 1927
Reid, K., Cover Crops in Conservation Farming, OMAF Factsheet, August 1990
The New Farm's Cover Crop Guide, The New Farm Magazine, Emmaus, PA, April 1988
Warman, P., The Basics of Green Manuring, MacDonald J. 40(1): 3-6, 1981
Table 3 Common green manure/cover crops
(information for Ontario unless otherwise noted)
Crop Seeding rate* Seeding and plowdown dates
Alfalfa 11-17 kg/ha From time soil is dry to Aug 1, if moisture adequate. With early seeding in fine seedbed, reduce rate by 25 per cent.
Buckwheat 55-110 kg/ha From June 10 to Aug. 15. Plowdown at 10 per cent bloom. Late seeding will winterkill.
Hairy vetch 16-28 kg/ha From time soil is dry to Aug. Will overwinter and /or reseed. Must be drilled or harrowed in.
Oats 60-110 kg/ha From time soil is dry to Aug. Late planting will winterkill.
Oilradish 10-17 kg/ha From time soil is dry to 1st week Sept. After Aug. 15, frost should kill plant before seed set. Cut or plowdown before seed set.
Red clover DC 11-17 kg/ha From time soil is dry to Aug. 1, if adequate moisture.
Rye 70-110 kg/ha From time soil is dry to 2nd week Sept. Seed Aug. 15 onwards for spring plowdown. Latest plowdown at heading.
Indian Head 34 kg/ha Early spring or late June (not recommended if dry lentils year); incorporate approx. 60 days later.
Sirius peas 112 kg/ha Early May to 2nd week June.
Yellow sweet 6-11 kg/ha Underseed with any crop; plowdown at 10-20 per clover cent bloom following year.
* To convert kg/ha to lb/ac multiply by 0.89
Copyright © 1992 Canadian Organic Growers. Inc
Reprinted with permission. All rights reserved.
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