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EAP Publication - 51
by Professor P. Warman, Soil and Land Resources, Department of Renewable Resources
Green manuring is the practice of turning into the soil undecomposed green plant tissue. The function of a green manure crop is to add organic matter to the soil. As a result of the addition, the nitrogen supply of the soil may be increased and certain nutrients made more readily available, thereby increasing the productivity of the soil.
The practice of green manuring is very ancient. The Greeks turned under broad beans around 300 B.C., and the planting of beans and lupines for soil improvement was a common practice in the early years of the Roman Empire. The Chinese wrote about the fertilizing value of grass and weeds hundreds of years ago and the early colonists in North America commonly used buckwheat, oats, and rye to add organic matter to the soil. Farmers in the southeastern United States recognized early in the 18th century the value of green manure crops, especially the legumes, but the usage of green manure in North America peaked in the 1940s, with a notable decline in planted acreage since that time. With the current trend toward the use of "organic' fertilizers, many people are again looking at green manuring as an economical, practical, and even aesthetically pleasing method of restoring productivity to idle or overworked land.
Before you plant your field into sweet clover, however, I'd like to present some of the positive and negative aspects of green manuring as well as some soil science basics which I feel are necessary to the use of this method of fertilization.
The major benefits to the use of green manures in a crop rotation system include organic matter and nitrogen addition, nutrient conservation, and protection of the soil surface du ring erosion-prone periods of the year.
Organic Matter and Nitrogen Addition -- The amount of organic matter that may accumulate through the decay of plant material by the addition of green manures may be considerable. One to two tons of dry matter per acre (20-40 kg/100m2) is not an unusual amount to be added (wet weight of green manure crops are 4-10 times the dry weight).
The organic matter added when young succulent plant tissue is incorporated into the soil encourages the microbial action of not only the heterotrophic microorganisms responsible for general organic matter decomposition but also the "free-living" organisms (e.g. Azotobacter. Clostridium) which are able to fix atmospheric nitrogen. The organic residues from green manures also help to stabilize the soil structure, increase the water holding capacity of the soil, and increase the infiltration of moisture into the soil and percolation through the soil.
Agronomists have argued that green manuring will increase either the humus content or the supply of available nitrogen in the soil, but rarely both at the same time. The humus content is only increased appreciably if material fairly resistant to decomposition is added to the soil (high Carbon:Nitrogen ratio), and this type of plant material is typically low in nitrogen (less than 1.5 per cent on a dry-weight basis). The available nitrogen supply is only increased if readily decomposable material high in nitrogen, such as immature green plants, is incorporated into the soil. The amount of organic matter that may accumulate will vary with the soil, climatic conditions, and the age and type of crop.
The accompanying diagram shows that mature residues, whether legume or non-legume, have a much higher C:N ratio than do the younger succulent materials. Also, the legume crops, which have two to three times the nitrogen content of the non-legumes, have lower C:N ratios throughout their life cycles, and even if turned under when mature (e.g. 3rd year alfalfa, 2nd year clover) will yield their nitrogen more quickly than if oats or buckwheat straw were ploughed under.
The non-legume supplies a large amount of energy (carbohydrate) material readily, but the bacteria responsible for the decay of this material must have access to N. If the N carried in the green manure crop isn't sufficient (that is, a C:N greater than 30:1, or less than 1.5 per cent), the bacteria will draw on the available soil N and may deplete it to such an extent as to ruin the following crop; for example, turning under a nearly ripe rye crop will act as a detriment to the following crop, but a sweet clover crop carries with it sufficient N for its decay, and this excess becomes available to the following crop. If there is a long wet period between the incorporation of the green manure crop and the establishment of the following crop, especially on warm, sandy soils, much of the available N from the green crop will be leached out of the soil, with little benefit to the following crop. On the other hand, the main crop should not be sown too soon after the green manure has been turned under, since the initial stage of decomposition is very unfavourabe for germination and the growth of young plants. Therefore, the incorporation and subsequent decay of the green manure or cover crop should occur at the time when it will best serve as fertilizer for the crop it is to benefit.
Not all of the plant material turned under becomes part of the soil humus; a large part is lost as carbon dioxide during decay. Under some conditions, as on sandy soils in very warm weather, the loss may be excessive, leaving no contribution at all to the soil humus. Generally, green manure crops are not as effective as farmyard or animal manure, per unit of carbon, in increasing the organic master content of the soil, because of their greater decomposability. On a Woburn sandy loam in England, researchers found 25 per cent of the carbon remained after 19 annual dressings of 25 t/ha farmyard manure, but only 14 per cent of the carbon remained when green manure was used, although it was estimated that 20 per cent less Org.-C/ha was incorporated as green manure. In Tennessee, Mooers found that 2.5 t/ha dry master of cowpeas grown as an autumn catch crop in continuous wheat over a 20-year period decreased the soil organic master content 0.11 per cent; whereas, a farmyard manure, also containing 2.5 t/ha dry master increased soil organic master 0.11 per cent. On the other hand, researchers in Ohio found that sweet clover in a two-year rotation of corn followed by oats seeded with sweet clover maintained the productivity of the soil and produced as high yields of corn as were obtained by ploughing under 6 t/a (12.4 tonnes/ha) of manure or applying commercial N up to, at times, as much as 100 Ibs/a(110kg/ha).
Additional Nutrients Added-- Some green manure crops may utilize less available forms of phosphate and micronutrients than the main crop in a rotation: hence, when they are ploughed under, they may increase the availability of some nutrients for the subsequent crop (a concept which is being evaluated at Macdonald College for the 1978 buckwheat crop). In addition, by decomposing rapidly, the plant residues liberate large quantities of carbon dioxide and weak acids, which act on insoluble soil minerals to release nutrients for plant growth. The increase in the availability of phosphates in soils high in calcium is a good example of this effect. Since green manure crops are rarely fertilized, their value is strongly dependent on their ability to use available nutrients in the soil. Alfalfa and sweet clover, for example, have high calcium, potassium, and phosphorus requirements, and most legumes have higher than average requirements for the micronutrients boron, molybdenum, and cobalt. Most green manure crops respond well to some fertilization.
Nutrient and Soil Conservation--These two other benefits of green manuring are closely related. Since green manure crops are often sown in the fate summer or fall after a primary crop, the green manure cover or catch crop utilizes excess fertilizer not taken up by the main crop. When these catch crops are incorporated into the soil the following spring, rather than harvested, they not only protect the soil from erosion attributed to fall and early spring rains and winter runoff, they also conserve nutrients which might have leached away or contributed to the contamination of streams and lakes.
Generally, ideal green manure crops should: --be inexpensive to plant; --be easily established; --produce succulent tops and roots rapidly; --generate good ground cover quickly; --be capable of growing on poor soils (sands and clays benefit most).
It is more advantageous to seed a legume than a non-legume because of the nitrogen gained by the soil; however, generally the legume seed is more expansive, it is more difficult to establish, and the crop may be so valuable as animal feed that one would not want to turn the stand under.
The most suitable crops for use as green manures in Canada are listed in the accompanying table titled Typical Green Manure Crops. I have modified the table originally presented in the Garden Way Publication on "Green Manures" to be more suitable for our Canadian climate. Even so, some of the crops are unsuited to certain soils or climatic regions; whereas, other crops are too valuable for haylage to be ploughed under in an immature stage.
There are certain advantages to growing two crops together for green manuring. If both crops are adapted to soil and climatic requirements, and to each other, both crops may gain by the association. One crop may offer physical support to the other. If one is a legume, the fixed nitrogen might benefit the other. Good examples of green manure combinations are oats and peas, and rye and vetch. This concept of companion planting is being more widely investigated, although it is not a new practice.
The inoculation of leguminous plants is the practice of introducing bacteria into the soil which are capable of fixing free nitrogen from the soil air and eventually transferring this N to the host plant, thereby increasing crop production and improving the soil organic matter content.
Legume bacteria are single-celled microorganisms which are highly selective. Legume seeds may be innoculated with N-fixing bacteria by the use of commercially available pure cultures or by use of soil from a field that has recently grown the crop it is desired to inoculate. As the roots of the plant develop, the bacteria enter the root hairs and cause nodules to be formed. The legume supplies food in the form of a carbohydrate energy to the bacteria and in return the bacteria furnish nitrogen to the plant when they die.
Leguminous plants are arranged in groups of cross-inoculation bacteria. One species of nodule bacteria will successfully nodulate each member of a given group. The nodule bacteria that work on alfalfa and sweet clover, for example, will not successfully nodulate any other legume. Each cross~inoculation group requires its own special inoculant.
Groups of cross-inoculation bacteria which are recognized are:
Alfalfa group--alfalfa, yellow, and white sweet clover
Clover group--common clovers such as alsike, red, and white Pea group--field, garden, and sweet pea, hairy vetch, broad bean, lentil Bean group--garden and field beans Lupine group--blue and yellow lupine, serradella
Soybean group--soybeans Cowpea group--cowpea, lima bean, peanut Lotus group--birdsfoot trefoil and other lotus species
When a mixture of seed is to be sown, each type of seed must receive its own inoculant. Several manufacturers produce a mixed inoculant containing bacteria suitable for the inoculation of garden legumes (peas, beans) or for plants in both the alfalfa and clover cross-inoculation groups.
For example, Legume-Aid@ or other seed inoculants are available from Stokes Seeds Ltd., William Dam Seeds, Bishop Farm Seeds, J. Labonte & Fils, and others.
NOTE: Seeds treated with dusts containing copper or mercury compounds are not suitable for inoculation because of toxicity towards the bacteria. Also, the fertilizers which cannot safely be mixed with inoculated seed include superphosphate, ammonium sulfate, sodium nitrate, muriate of potash, and potassium sulfate. Lime, superphosphate mixed with equal quantities of lime, and rock phosphates are considered safe. For more information on legume seed inoculation refer to Ontario Ministry of Agriculture and Food Publication No. 212.
Anyone who plans to grow a legume crop without making use of the specialized inoculum is sorely misquided. Not only is the inoculum easy to apply, it is considerably cheaper than nitrogen fertilizer. The only difficulty I have found in using inoculum is finding it! There are many seed companies which self garden and field legumes, but don't carry the inoculum. Some seed dealers have the appropriate inoculum for the species, but they "forget" to mention it when you purchase. Make sure you ask for it when you buy, and save yourself the money which you would have to spend on N fertilizer.
The amount of nitrogen fixed by the legume bacteria depends on the physical and chemical conditions of the soil. The prime conditions are the drainage, aeration, moisture, pH, active calcium, and the amount of readily available N in the soil.
Not all legumes fix the same amount of N. An average crop of alfalfa might fix 200-250 kg/ha/yr.; sweet clover, 150-175 kg/ha/yr.; red clover, 100-150 kg/ha/yr.; soybeans, 80-100 kg/ha/yr.; beans, 50-60 kg/ha/yr., and peas 40-50 kg/ha/yr.
Bishop Farm Seeds (P.O. Box 338, Belleville, Ontario) has introduced for 1979 a green manure mix called Bearss Plow-Down Mixture. The preinoculated mixture consists of 60 per cent Red Clover, 20 per cent Yellow Sweet Clover, and 20 per cent Ryegrass. l am sure other seed companies have mixtures designed for green manure plough-down. It is possible for you to buy your own components and mix them to suit yourself. J. Labonte & Fils Inc. in Longueuil is an example of one source that has a wide variety of green manure type seeds in stock.
Problems with Green Manures
There are potentially some problems with the use of green manures. These are:
-- Tilling in a heavy nonleguminous crop with a high C:N ratio can result in a depressed nitrogen uptake by the following crop.
- In areas of low rainfall, green manure crops may deplete soil moisture to the point that the succeeding main crop will suffer from drought.
- Depending on the soil, the green manure crop, and the rotation, succeeding crops may not benefit from the expense, time, and energy devoted to green manuring. -- On land of high market value, giving over the entire cropping season to green manure crops is seldom profitable. However, it is justified when the following crop is fairly permanent and the establishment and good growth of the seedlings or young plants are of prime importance, e.g. for orchards or lawns. In addition, with the cost of N fertilizer increasing at 20 per cent plus per year, and the quality of our soils deteriorating under monoculture cropping, green manuring might just be the best method of saving our good farmland.
If the reader is unfamiliar with some of the terms or concepts which I have presented in this article, please refer to any standard Soil Science or Field Crop Production text book. In addition, the best and most complete book on green manures was published in 1927 by A. J. Pieters. Your local library may still have a copy.
1. BRADY, N. C. 1974. The Nature and Property of Soils. MacMillan Publ. Co. Inc., N.Y. 8th Ed.
2. CHATER, M. and J.K.R. Gasser, 1970. J. Soil Sci. 21: 1 27-1 37.
3. JORDAN, D. C. 1975. Legume Seed Inoculation. Ont. Min. of Agr. & Food. Publ. No. 212.
4. MOOERS, C. A. 1927. Tenn. Agric. Expt. Sta. Bull. No. 137.
5. PIETERS, A. J. 1927. Green Manuring. John Wiley & Sons, Inc. N.Y.
6. RUSSELL, E. W. 1973. Soil Conditions and Plant Growth. Longman Gp. Ltd. N.Y. 10th Ed.
7. WILLIARD, C. J. and E. E. Barnes, 1959. Ohio Agric. Expt. Sta. Bull. No. 839.
8. WOLFE, T. K. and M. S. Kipps, 1959. Production of Field Crops. McGraw-Hill Book Co. Inc. N.Y. 5th Ed.
Copyright © 1980