In this article we will discuss about:- 1. Benefits of Using Green Manures 2. Place in Farming System 3. Green Manures in Agroforestry 4. Management of Green Manuring 5. The Choice of Green Manure.
Contents:
- Benefits of Using Green Manures
- Place in Farming System
- Green Manures in Agroforestry
- Management of Green Manuring
- The Choice of Green Manure
1. Benefits of Using Green Manures
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Green manuring offers an inexpensive way of improving crop yields and it takes little extra effort. Green manures are especially important on farms where there is not enough animal manure available, and when it is not possible to bring in natural fertilizers from elsewhere.
Although the use of green manures may seem to create extra work, they provide following benefits:
1. Enhance Soil Fertility:
Green manures recycle nutrients and add organic matter to the soil. They help prevent nutrients being washed out of the soil. The nutrients are taken up by the green manure and held inside the plant. When the nutrients are needed for the next crop, the plants are dug into the soil or used as a mulch on top of the soil. This helps to increase crop yields. Legumes and other nitrogen fixing plants, which take nitrogen from the air to the soil are particularly beneficial.
2. Supplement for Nutrients:
Different green manures and grain- legumes are used to increase the nitrogen content and texture of the soil. Among the green manures Sesbania aculeata accumulated the largest amount of bio-mass and nitrogen contribution and among the grain legumes, cowpea ranks first both in terms of grain yield and biomass addition.
The available green manure crops and utilization pattern are as follows:
3. Improved Soil Structure:
Green manures improve soil structure, letting more air into the soil and improving drainage. Green manures help sandy sol hold more water and not drain so quickly.
4. Prevention of Soil Erosion:
Green manures help to stop the soil being carried away by wind and rain. The roots penetrate the soil and hold it in place.
5. Weed Control:
Green manures help to control weeds. Bare soil can become quickly overgrown with weeds, which can be difficult to remove. Green manures cover the ground well and stop weeds growing beneath them, by competing for nutrients, space and light.
Farmers often observe the benefits of green manures but many do not use them because they do not know which species to use and how to include them in their own farming system. It is, therefore, important to plan in advance where and when they are to be grown. The adoption of green manuring depends upon the agro-climatic conditions.
Broadly, the following two types of green manuring can be thought of:
1. Green Manuring in Situ:
In this system, green manure crops are grown and buried in the same field which is to be green-manured, either as a pure crop or as an intercrop with the main crop. In India, various methods of growing green manure crops in situ are followed to suit local conditions. For the proper decomposition of the green manure, it is necessary that the green material should be succulent and there should be adequate moisture in the soil.
Plants at the flowering stage, contain the greatest bulk of succulent organic matter with a low carbon/nitrogen ratio. The incorporation of the green-manure crop into the soil at this stage allows a quick liberation of nitrogen in the available form.
Green manuring in situ is followed in Northern India while green leaf manuring is common in eastern and central India. Green manuring is a practice of ploughing or turning into the soil un-decomposed green plant material for improving the physical conditions of the soil or for adding nutrients. Any crop or plant (generally leguminous) grown and ploughed in situ is called a green manure, for example, dhaincha, sesbania, sunnhemp, wild indigo and pillipesara.
A leguminous crop producing 10-25 tonnes of green matter per ha will add about 60 to 90 kg N. green leaf manure refers to turning under of green leaves and tender green twigs collected from shrubs and trees grown on the bunds, wastelands and nearly forest areas. Common shrubs and trees useful for this purpose are karanj, glyricidia, neem, subabul, etc.
i. Dhaincha (Sesbania Aculeata):
It is suitable for loamy and clayey soils. Dhaincha is an ideal green-manure crop for rice based cropping system. It is fairly resistant to drought as well as stagnation of water. The green manure grows well even in alkaline soils and corrects alkalinity if grown repeatedly for 4-5 years. The roots have plenty of nodules. It yields about 10-15 tonnes of green manure per ha and requires a seed rate of 30-40 kg/ha.
ii. Sesbania Speciosa:
It is a valuable green manure for wetlands and can be grown in a wide range of soils. Seed production is prolific; however, pods are frequently attacked by insects. The green manure can be raised on the field borders along the bunds. Sesbania seedling (21 days) can be planted in a single line at 5-10 cm apart in the borders of the fields close to the bunds.
In about 90 days it produces about 2-4 tonnes of green manure per ha. It does not affect the rice yield by shading or root effect. If second rice crop is planted immediately after the first crop, the manure can be incorporated into the field. About 300-400 g of seeds are sufficient to raise nursery and plant the seedlings along the bunds of one hectare.
iii. Sesbania Rostrata:
One of the important features of this green manure is that in addition to the root nodules, it produces nodules in the stem. The stem nodulation is an adaptation for waterlogged situation since flooding limits growth of green manures and may reduce root nodulation. Under normal condition, both root and stem nodules are effective in N fixation.
It can produce about 40-50 tonnes of fresh biomass from one ha in about 50-60 days. It has higher N content of 3.56 % on dry weight basis. Biomass production is higher during summer (April – June) than in winter (Dec. – Jan.) season. This green manure can also be produced by raising seedlings (30 days old) and planted in the paddy field along the bunds or as intercrop with rice.
iv. Sunnhemp (Crotalaria Juncea):
It is a quick growing green manure crop and gets ready for incorporation in about 45 days after sowing. It does not withstand heavy irrigation leading to flooding. The crop is at times subject to complete damage by leaf eating caterpillars. The crop can produce about 8-12 tonnes of green biomass per ha. The seed requirement is 30 kg/ha.
v. Wild Indigo (Tephrosia Purpurea):
This is a slow growing green manure crop and cattle do not prefer to graze it. The green manure is suitable for light textured soils, particularly in single crop wetlands. It establishes itself as a self-sown crop and the seeds remain viable till the harvest of rice. On an average about 3-4 tonnes of green manure is obtained in one ha.
The seed rate is 30 kg/ha. The seeds have a waxy impermeable seed coat and hence scarification is required to induce germination. Soaking seeds in boiling water for 2-3 minutes is also equally effective in promoting germination.
vi. Indigo (Indigofera Tinctoria):
It resembles wild indigo and is a long duration crop with more leafy growth. It comes up well in clayey soils with one or two irrigations.
vii. Pillipesara (Pheseolus Trilobus):
This is a dual purpose crop yielding good fodder for the cattle and green manure. Pillipesara comes up well in hot season with sufficient soil moisture. Loamy or clayey soils are best suited. After taking one or two cuttings for fodder or light grazing by animals, the crop can be incorporated into the soil. About 5-8 tonnes of manure can be obtained from one ha.
2. Green Leaf Manuring:
Green-leaf manuring refers to turning into the soil green leaves and tender green twigs collected from shrubs and trees grown on bunds, waste lands and nearby forest areas. The common shrubs and trees used are Glyricidia (Glyricidia maculata), Sesbania speciosa, Karanj (Pongamia pinnata), etc.
A number of leguminous and non-leguminous plants are grown on bunds or wastelands with the prime objective of utilizing their foliage as green manure.
i. Glyricidia (Glyricidia Maculeata):
This is a shrubby plant that comes up well in moist situations. Under favourable conditions, it grows well like a tree. It can be easily grown in waste lands, farm road sides, field bunds, etc. The crop can be established by stem cuttings or seedlings planted in the field borders.
It can be pruned for its tender loppings and compound leaves for green leaf manuring at the time of puddling rice. On an average, a well- established plant yields 12-15 kg green matter. About 400 plants on the peripheral bunds yields 5-6 tonnes green manure/ha.
ii. Karanj (Pongamia Glabra):
It is a leguminous tree grown in wastelands. On an average, a tree can yield 100-120 kg of green matter. The leaves contain about 3.7 % N (on dry weight basis).
iii. Neem (Azadirachta Indica):
It is an indigenous tree with wide range of adaptability. Almost all parts of the tree have been used for several millennia for preparation of medicine and its leaves and leaf-extracts to repeal insect pests of crops and stored grains. The leaves are pruned at the time of planting rice and incorporated as green leaf manure.
iv. Calatropis (Calotropis Gigantea):
On roadsides and fallow lands, the plant grows wild under different soil and climatic conditions. The leaves are more succulent and a plant can produce about 4-5 kg of green matter. Besides it also helps in controlling soil born pests like termite.
The nutrient content of green manure and green leaf manure crops are given in the Table 6.2.
2. Place in Farming System:
i. Green Manures in Rotation:
Growing green manures as part of a crop rotation is an important part of an organic farming system. These help to build soil fertility and are particularly useful when grown before crops, which need a lot of nutrients.
ii. Green Manures can be Used in Rotation as:
(1) Whenever there is no crop in the ground, rather than leaving the land bare and allowing weeds to grow and nutrients to leach out of the soil, and
(2) As break crops, when there is only a short time between main crops.
Timing of sowing is important. The green manure must be ready to dig in before the crop next is sown. There should not be a long gap between digging in the green manure and planting the next crop. This is to prevent nutrients from the green manure leaching out of the soil, before being taken up by the next crop.
iii. Green Manures and Under-Sowing:
Under-sowing involves growing a green manure at the same time as a crop, among the crop plants. Sometimes they are sown with the crop or slightly later when the crops are already growing. This reduces competition between the green manure and the crop.
For example, under-sowing is sometimes used with maize crops where a green manure is sown under the young maize plants. The green manure seeds are broadcast sown when the second weeding of the maize is carried out. In this way when the maize is harvested the green manure is already established and ready to grow quickly. This method means that no extra time is spent preparing the land and sowing the green manure.
iv. Long Term Green Manures:
Green manures can be grown for more than one season and used in the following ways:
1. Long term green manures restore poor soil. Using them over a long time has a greater benefit on soil fertility and structure of poor soil.
2. Long term green manures can be used when new land is being prepared for use, especially to help control difficult perennial weeds.
3. Long term green manures are used where land is to have a long fallow period. They can be sown at the beginning of the fallow of bush-fallow systems. They help to quickly build up the fertility of the soil and reduce the length of time before the land can be used to grow crops again.
4. Long term manures provide green material which can be cut and carried to other fields. Green material can be harvested from perennial species such as alfalfa (Medicago sativa), for digging in, mulching, composting or feeding to livestock.
v. Green Manures as Mulch:
Green manure plants can be cut and left on the soil surface as a mulch.
Mulching releases nutrients slowly but has some advantages:
(1) Mulching helps to prevent weed growth,
(2) Mulching protects the soil from erosion, and
(3) Mulching keeps the soil moist by reducing evaporation.
3. Green Manures in Agroforestry
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Agroforestry is the practice of growing trees and/or shrubs together, with crops and/or animals. The trees/shrubs act as long term green manures and the leaves can be used for digging in or as a mulch.
The regular pruning of agroforestry trees such as Leucaena (Leucaena leucoephala), Mother of cocoa (Gliricidia sepium) and Calliandra (Calliandra calothyrsus) during the crop growing period provides large amounts of green material for digging into the soil and reduces competition with the main crop.
The material can also be used as a mulch. It is spread on the top soil, usually between crop rows or before a crop has been planted. As well as improving the soil in the ways, trees and shrubs also provide food, fodder, fuelwood, erosion control and other benefits.
4. Management of Green Manuring
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i. Time of Sowing and Seed Rate:
March is the best suited period for raising green manure. Studies at Coimbatore (Tamil Nadu) reveal that summer season is the ideal for seed production of S. rostrata. Pre-season showers can be very well used to raise a green manure crop in single crop wetlands (August-January). A seed rate of 40-50 kg/ha is required to get higher biomass of green manure. If the population is dense, the stem will be very thin and the crop will not seed early (e.g., dhaincha).
ii. Seed Treatment:
Germination is a problem for the green manure seeds, like Tephrosia purpurea because of their hard seed coat. Scarification of the seeds with sand and pre-soaking in hot water (50° C) for 5-minutes are found to improve seed germination upto 98%. Acid seed treatment is also effective.
The seeds can be treated with concentrated commercial grade sulphuric acid (400 ml/kg of seed) for 30 minutes. The seeds are washed thoroughly with water, shade-dried and used for sowing.
iii. Stem Cuttings:
Glyricidia is one of the green leaf manure tree crops propagated through seedlings and stem cuttings. A large percentage of the cuttings fail to root readily. The cut ends have been kept immersed in 0.005% IAA and 0.1% seradix A for one day and planted in small beds and irrigated for six days. The hormonal treatment aided root formation to a remarkable degree (90%).
The green manure crops can be established using cutting of soft wood (from freshly formed branches) and hard wood (leaving top and bottom 30 cm in the stem and older branches). Soft wood cuttings of S. aculeate and hard wood cuttings of S. rostrata are superior. The establishment of the cuttings can be improved by dipping in fresh cowdung (10% suspension).
iv. Mixed Cropping:
It has been found feasible to cultivate green manure mixed with rice fallow pulses. Green manure crops like indigo and pulses (black gram) can be mixed with the ration of 1:3. pillipesara can also be sown in the same proportion. By adopting such practice in rice, fallow pulses no yield reduction has been noticed. Growth of green manure crop is also not spoiled. By the time pulse is harvested, the green manure putforth growth only to a few cm. After the harvest of pulses, the green manure put on growth.
v. Inter Cropping:
Field investigations at Coimbatore reveal the possibility of intercropping green manure in transplanted rice. Seedlings of S. rostrata were planted at an interval of 1.5 m. On 30 days after planting, the green manure is cut at a height of 15 cm from the ground level and incorporated as manure. The ratoon is allowed and the refresh incorporated while planting second crop of rice.
vi. Border Planting:
Border planting of green manure, particularly Sesbania speciosa in the rice field bunds was advocated even during 1950s in Thanjavur (Tamil Nadu). The seedlings of the green manure were planted at 60-90 cm apart on the bunds at the time of planting kuruvai crop during June.
About 70% of the crop was used as green manure for the thaladi crop planted in October. About 1.5-2.0 tonnes of green leaf was obtained from the bund crops and incorporated. The remaining was left standing for producing seeds. It was also possible to produce in one ha, the seeds sufficient for raising green manure for 2.5 ha.
vii. Phosphorus Response:
Incorporation of green manure with single superphosphate or rock phosphate facilitates higher response to added phosphorus. Recommended dose of phosphorus for rice can be very well applied to green manure crop and incorporated for greater efficiency.
viii. Digging in Green Manures:
Before a crop is sown, the green manure is dug back into the soil. Here it decomposes and the nutrients held inside green manure plants are released.
The plants take a short time, usually about two weeks, to rot down into the soil before the next crop is sown.
Green manures should not be ploughed in as this buries the plants and the nutrients too deep. They should be turned in just under the soil surface.
Digging is easier if the plants have been chopped into small pieces before digging. This also helps to prevent the problem of regrowth, if this occurs.
Younger green manure plants are easier to dig into the soil then older ones and land can be ready to use more quickly after they have been dug in. Therefore, over a long period, two short term green manures may be better than one longer term green manure. However, this may involve more of time and effort.
For most green manure plants, the best time to dig in is just before flowering begins, but this is different for some species.
If plants become too old and tough, it is more difficult to dig them. Soil organisms will find it difficult to break down and decompose old, tough plants. If this happens, green manures can be cut and composted instead.
5. The Choice of Green Manure
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When choosing which green manure plant to use, you should consider the following points:
1. A green manure must suit the local climate, and the soil that it is to be sown in. This will help to keep the green manure healthy and to keep pests and diseases to a minimum.
2. Fast growing and leafy green manures are often preferred as they provide more nutrients when dug in.
3. Green manures should not be closely related to the following crop as they could attract pests and diseases which may affect the following crop.
4. It is important to know whether seed is easily available and affordable.
5. The length of time that land is free and how long the green manure will take to grow.
6. Plants which can be grown as a green manure include legumes and non-legumes.
Legumes have nodules on their roots which contain bacteria. These bacteria take nitrogen from the air. This is known as nitrogen-fixation. Plants use this to grow, but this extra nitrogen is also made available to future crops when the legumes are dug into the soil. The ability of legumes to fix nitrogen makes them very good green manures. However, they do have limitations and non-legumes can sometimes be more suitable.
Legumes will only fix nitrogen if the right type of bacteria, for example Rhizobium, are in the soil. This is especially relevant if the legume is not a local plant. A product containing these bacteria can be bought but it may be expensive. Even if the bacteria are provided, other non-legume plants can be better in some situations. They may produce more organic matter and have a better root system. They may also survive better and grow faster and may be able to tolerate extreme weather conditions or poor soils.