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Functions of green manuring

Green manuring is the tilling of green, growing plant residues into the ground to improve the condition of the soil. Green manures improve soils in several ways:

1. Addition of organic matter

2. Addition of nitrogen

3. Conservation of nutrients

4. Protection of soil against erosion

Turning a green manure into the soil adds one or two tons of dry matter per acre and is the surest method of increasing soil organic matter to give improvements in soil structure. Although some effects on soil conditioning, such as aggregation, may last for several years, most of the benefits occur in the season following incorporation of the green manures. Green manures decompose rapidly due to their succulence, and about 75% of the organic matter will be rotted away in the first season.

Nitrogen is added to the soil if the green manure crop is a legume. Legumes are the major agronomic crops that fix nitrogen. Nitrogen fixation is a symbiotic process by which legumes incorporate nitrogen gas (N2) from the air into ammonia. The symbiosis occurs in root nodules formed by infection of roots with bacteria (Figure 8). The amount of nitrogen added varies with the kind of legume, depending largely on the amount of dry matter that is produced (Table 7). The benefits from the added nitrogen parallel those derived from the addition of organic matter, that is, most of the benefits are in the first season of cropping after the green manure is turned under.

Figure 8. Illustration of legume with root nodules in which nitrogen fixation occurs.  

 Table 7. Nitrogen fixation by some leguminous crops.


Nitrogen fixed, lb/acre

Sweet clover








Red clover


Garden beans and peas




Nonlegumes used as green manures do not add nitrogen to the soil. Nonlegumes, however, will help to conserve nutrients. The roots of these crops will intercept nutrients that might otherwise be lost by leaching (Figure 9). Nitrates are likely lost by leaching in all soils, and leaching of potassium may occur in sandy soils. Also, green manures help to stabilize soil against erosion. Nonlegumes grown to lessen leaching or to stabilize soil are called catch crops or cover crops. Grasses because of their shallow, fibrous root systems are excellent for this function. Frequently, cover crops are grown after the main crops have been harvested and are turned under at the beginning of the following season. To obtain maximum yields, some crops such as vegetables often are fertilized in excess of the amounts that they remove in the growing season. A catch crop will utilize these nutrients and protect against their loss and potential entry into groundwater or surface water. Most vegetable crops leave little residues on the ground after harvest. A cover crop following vegetable crops will protect the soil from erosion by fall, winter, and spring rains.

Figure 9. Illustration of fibrous root system of a catch crop or cover crop, which absorbs nutrients and restricts their leaching.

Selection of a crop for green manuring

An ideal green manure has several desirable characteristics (Table 8). A green manure crop should be easy to establish and to grow in poor soil. Infertile soils are the ones that will benefit most the green manure crop. Green manures are needed to enhance the water-holding and nutrient-retention capacities of sandy soils. Clay soils will benefit from the aggregation that will be promoted by the additions of organic matter. In addition, the crop should require a low level of fertility with respect to plant nutrients. To keep the cost of production low, one does not want to fertilize the green manure crop to get it to grow. However, if the soil is so infertile that the green manure crop will not grow, fertilizers should be applied. Otherwise, growth of the green manure will be so poor that no benefits occur.

The crop should be rapid growing so that it will fit conveniently into a crop rotation. No more than one year should be required to produce the green manure crop. Any longer period of time will take the land from production of the main crops for too long. An ideal situation occurs when the supplemental crop can be grown between the time that one main crop is harvested in the fall and the next one is planted in the spring. This situation occurs most often in warm climates, such as those in the southeastern United States. The green manure crop should cover the ground quickly to protect the soil from erosion. The total growth should be at least 18 to 24 inches to produce enough organic matter that will improve the conditions of the soil. Almost any plant that meets these criteria is suitable as a green manure crop. Some plants that are used commonly as green manures are listed (Table 9). Most of these crops are adaptable as green manures in a wide range of climatic conditions. Crimson clover, some vetches, and lespedeza may be suitable only in warm climates and should be treated as annuals. Hairy or smooth vetch are adapted to most regions.


Table 8. Characteristics of an ideal green manure crop.


 Is cheap to seed

 Is established easily

 Grows well in poor soil

 Covers the ground quickly

 Grows rapidly

 Produces abundant and succulent topgrowth

 Adds nitrogen to the soil



Table 9. Some plants that are suitable as green manure crops.










Sweet clover




Ladino clover




Crimson clover




If all of these characteristics are equal between a legume and a nonlegume, the legume is the better choice. A legume will add nitrogen to the soil by fixation. Nitrogen fixation is accomplished by legumes and specialized bacteria living in symbiosis. The bacteria infect the roots of legumes and form structures called nodules. The bacteria are specific for a particular legume or group of legumes. If the legumes have not been grown in a soil for several years, the bacteria may not be present. Commonly, inoculum consisting of spores of the bacteria mixed with peat is purchased for treatment of the seeds. The inoculum and seeds are mixed to coat the seeds with inoculum before planting. After the seeds germinate, the bacteria infect the root, and the symbiotic relationship is formed. The plant benefits from the nitrogen that is fixed, and the bacteria benefit from the carbohydrates that are provided by the legume. Legumes will not fix nitrogen if they are fertilized abundantly with ammonium or nitrate fertilizers. The legume will use the inorganic nitrogen, and the nodules will not be active.

Generally legumes are more expensive to seed, harder to establish, grow more slowly, and require better soils than nonlegumes. Legumes might require more than one year in a crop rotation. Also, a good crop of a legume may be more valuable for hay on large acreages, and it would be poor management to turn it into the ground. Hay may be harvested for a year or two, and then the green manure can be plowed under to give the full benefits of a green manure crop.

A leguminous crop obtains about one-third of its nitrogen from the soil and two-thirds from nitrogen fixation. The crop has about two-thirds of its nitrogen in its shoots, so if the shoots are harvested, the amount of nitrogen remaining in the roots equals that removed from the soil.

A mixture of legumes and grasses makes a good green manure crop. The mixture ensures some success in establishing a stand. The grass will help to support the legume and may provide some winter protection. The resulting green manure will reflect the high organic matter production by shoots and fibrous roots of the grass and the addition of nitrogen from the legume. Neither organic matter addition nor nitrogen addition may be as great as that which may occur in pure stands, but the compromise may be more benefical in total addition than that which would occur from the individual crops.

Incorporating the green manure into the soil

The green manure should be turned into the soil when the combination of growth and succulence of the crop are at a maximum. This condition generally occurs when the crop is at its half- height. Knowledge of the growth habit of the crop is needed to recognize this stage of growth. Another and probably more useful guideline is to turn the crop under at the initiation of bloom. After blooming, the crop begins to transport materials to the fruits and seeds at the expense of the vegetation of the plants. The C:N ratio of the vegetation will widen, and the organic matter will not be as suitable to incorporate into the soil as it was before the crop bloomed. Also, after blooming, the flowers begin to form seeds. If the seeds mature sufficiently before the crop is turned under, seeds of the green manure may germinate and become weeds in the succeeding main crop.

The amount of seed needed, the time of seeding, and the time of incorporation of green manures into the soil varies with the kind of crop and the climate (Table 10).

Table 10. Guidelines for seeding and incorporation of green manures.


Green manure (kinds & examples)

Amount of Seed

lb/1000 sq ft lb/acre


For seedling For plowing

Small-seeded legumes, perennial (alfalfa, clovers)




Fall or spring

Small-seeded legumes, annual (crimson clover, lespedeza)





Medium-seeded legumes (hairy or smooth vetch)




Fall or spring

Large-seeded legumes (soybean)




Late summer

Nonlegumes (rye, oats, buckwheat)





Mustards (rape)







Problems with green manures

A major problem with green manures is that their production takes the land away from the production of a main crop. The green manure should occupy the land for no more than one year in three. Unless the green manure is a valuable hay crop, any more time devoted to its production will cause the costs from loss of productivity to outweigh the benefits of improved soil condition. One should use a rapid-growing crop to avoid this problem or alternatively consider using manures and composts to condition the soil. Manures and composts should be used to maintain soil organic matter on small plots of land, because area of land is at a premium and should be used for production of a main crop.

Crops which have large amounts of after-harvest residues may increase the organic matter as effectively as a green manure crop. Fertilization with some additional nitrogen to avoid immobilization is sometimes a good practice when residues of these crops are turned in. Sometimes, leaving these residues on top of the ground after harvest is more effective than trying to establish a cover crop. More soil erosion may occur between the time that the residue is turned under and the cover crop is established than would occur if the residue were left on the soil surface until the soil is tilled for the next main crop.

Green manures should be used with caution or avoided in dry climates or on dry soils. Precipitation should exceed 18 inches per year before one considers using a green manure crop. Transpiration by the green manure will deplete the soil of water. Deep-rooted crops, such as some of the perennial legumes, will desiccate the soil to an extent that several years may be needed to recharge the soil with water.

The green manure should be tilled into the ground at the proper stage of maturity. Turning under a mature crop may fill the soil with seeds that will produce weeds in the following season. As the green manure crop matures, its C:N ratio widens. Turning under of a mature crop may cause immobilization of nitrogen and result in nitrogen deficiency in the succeeding main crop. Many growers fertilize the soil with additional nitrogen after a nonleguminous green manure crop is turned under. This added nitrogen ensures that nitrogen deficiency will not occur and accelerates the rate of decay of the organic matter so that the benefits of improved soil structure are achieved rapidly. About 50 lb N/acre (1.25 lb/100 sq ft) are recommended to accelerate the decomposition of nonleguminous, green manures.

Maroon Divider
Description | Syllabus | Notes |Guide | Internet | Lab Manual|Exams and Quizzes|Results|More|
Maroon Divider

Produced and maintained by Your Name Allen V. Barker
University of Massachusetts, Amherst.
last updated - April 6, 1999