Green Manuring on crops

I. Introduction

 The practice of green manuring involves incorporation of plant material into the soil with a view to augmenting soil fertility. The plant material is mostly of leguminous plants. They are either grown in the same field where they are intended to be turned down or collected from outside and used in the stipulated field.

Sometimes, the effect of green manure crop may not be recognized in the first instance. But its effect is definitely noticed on succeeding crops in crop rotation. It supplies plant nutrients and humus to the soil and helps conserving native plant nutrient. Humus keeps the soil particles knitted together thereby resisting loss of soil erosion.

As green manure possess a narrow C:N (carbon & nitrogen) ratio, great care must be taken in the planting of the main crop. the main crop has to be transplanted (e.g. paddy) or sown (eg. cereals) soon after the application of green manure crop. An interval of less than a week under assured water supply will be the optimum for the planting of the crop. Otherwise loss of nitrogen as ammonia will take place during the initial stages of decomposition.

It is often complained that a farmer loses a valuable season by growing the green manure crop. This problem can be easily solved by suggesting to grow green manure as a mixed crop of green manure and main crop is not feasible, the growing of green manure on bund or field boundaries is recommended. To supplement this, the green leaves from suitable plants can be collected and incorporated into the field directly. Thus the fanner need not sacrifice a cropping season for growing a green manure crop. By making suitable adjustment in cropping technique and selecting the right green manure crop he can be sure of reaping rich dividends from the soil. In short, green manuring is a good practice to the soil as it stimulates crop growth.

II. Importance of Green Manuring

The importance of green manuring can be understood from the points given below.

1. Green manuring contributes 40 to 80 kg nitrogen per hectare to the field. Green manuring of high yielding rice variety with dhaincha increases the rice yield from 20- 30% depending on the soil type and other conditions.

2. Besides supplying nitrogen, green manure also prevents loss of nitrogen by leaching and erosion.

3. Vigorous root system of green manure keeps the soil particles bound together. Dhaincha is found to mobilize soil phosphorus and potassium and oilier trace elements likely to be deficient in surface layers and leaves them there in a readily available form.

4. Green manuring reclaims saline and alkali soils. Decomposing manure releases a large amount of organic acid which neutralizes the soil reaction.

5. Certain green leaf manure crops serve the dual purpose of nutrient supply and fodder supply. Green leaves of perennial legume plants can be used for green leaf manuring. In the off season, green leaves can be fed to the cattIe.

6. The growth of green manure crops (especially, dhaincha and sunnhemp) is very fast. Within 40-50 days they accumulate 6-10 tonnes of biomass per hectare. However, decomposition is also very fast due to richness of nitrogen.

III. Crops for Green Manuring

Green manuring is of two types (1) green manuring in situ and (2) green leaf manuring. As is obvious from the name, green manuring in situ is carried out by growing green manure crops in the field itself where their use is intended. In the latter type, green succulent leaves of leguminous or non-leguminous plants are collected, carted to the field and incorporated by ploughing. The crops which are grown for green manuring are also, therefore, grouped into two categories according to the type of green manuring. The following are the commonly grown green leaf and green manure crops.

 A. Green leaf manuring crops

Many types of leguminous and non-leguminous plants are grown on bunds or wastelands for the purpose of utilizing their foliage as green manure.

1. Gliricidia maculata

This plant thrives well in a variety of soils. Under certain climate it grows to a height of about 4.5 meter. During a year two cuttings can be taken; first at the beginning of the monsoon and the second in December. The fresh weight contains 0.49% nitrogen and 8.5% carbon. The yield of leafy material per plant according to the various observations, is about 22.5 kg after 10 years of planting. Leaves can be obtained for several years. The drawback with this plant is that it very often gets infested with mealy bugs.

2. Pongamia glabra

The average yield of green matter per tree about 130 kg. On fresh weight basis it contains 81.5% moisture, 7.2% carbon and 0.57% nitrogen.

3. Calotropis gigantia

This plant grows wild on fallow lands and road sides under different soil and climatic conditions of the country. The leaves are quite succulent and the yield of the green matter per plant is about 5 kg.

4. Tephrosia perpuria

It is a freely branching and spreading annual shrub found growing in fallow lands. It grows well on well drained loamy soils. The average yield of green-matter per plant is about 0.7 kg.

5. Tephrosia candida

This is an eruct shrub, resistant to pruning and is used extensively for green manuring and as a hedge. It is a shady plant and grows largely in tea gardens. The average yield of leafy matter per plant is 2.5 kg.

6. Sesbania speciosa

The practice of growing this plant around the borders of the rice fields, when the crop is standing, was experimented for three years at Cuttack. On an average it produces about 3450 kg of green matter per hectare within four months to be incorporated into the soil for the use of the second crop of paddy in January.

8. Ipomea carnea

This is another important plant which can be grown under varied soil and climatic conditions for green manuring. In the heavy rainfall areas its loopings can directly be used for green leaf manuring, whereas in the dry areas it can be composed before use.

9. Cassia tora

One hectare crop of this plant yield about 4000 kg of green leaves and twigs. Most of the species of Cassia yield even greater amount of manurial material. Cassia is largely used in paddy fields.

 B. Green manuring crops.

Following are the crops which can be grown in the field for in situ turning.

1. Crotalaria juncea

It is used as a green manure in practically all the states of the country except the areas prone to water logging. It is sown with the break of the monsoon. It grows very fast and attains a height of 1-2 meter. It grows well even in poor soils. The plant contains a large proportion of herbage and does not become woody soon hence it is rapidly decomposable. About 20000 to 25000 kg of green matter can be obtained from a hectare in duration of eight weeks. In addition to being a valuable green manure crop, it provides strong fibre for rope making.

2. Crotalaria mucronata

It is a useful green manure crop for coconut gardens. It grows wild in waste and uncultivated lands in almost all the major coconut tracts. It is hardy and herbaceous and grows to the height of 1.5 meters or more under favourable conditions. It thrives in a variety of soils including slightly saline ones. When it is well established it can withstand extremes of rainfall and drought It bushes out well under coconut shades. It is not relished by cattle and even goats.

 3. Crotalaria anagyroids

This plant has come from Sri Lanka. The plant is a branching shrub, attaining a height of 2.3 m. In Maharashtra, it is cultivated as cover crop and as green manure in coconut estates.

4. Sesbania aculeata

As a green manure crop sesbania or dhaincha is a widely used in Bangladesh. It can tolerate water logging and alkalinity to a fair extent. It can also tolerate drought if germination has been sound. It is an ideal green manure crop for rice growing soils. In waterlogged fields it grows to a height of 1.5 to 1.8 meters in a very short time. The resistance of the crop to water logging could be appreciated by the fact that the dhaincha crop survives even after being submerged to a depth of about 60 cm for a week. This crop also grows well in alkaline soil and, therefore, can be used for the reclamation of saline and alkaline soil.

5. Sesbania speciosa

This is commonly used in paddy. It is similar to dhaincha in appearance and performance. The crop is raised by broadcasting 17 kg of seeds per hectare. Sometimes when it is not possible to raise a full crop of sesbania, the seeds are sown in small plot and the seedlings are then transplanted on bunds at a distance of about 1 m between plants. Seeds are available at the end of the fifth month when the crop has matured. During ! the first crop itself, the seedlings can be planted at an interval of 5 to 8 cm between plants. It yields 2200 to 3200 kg of green leaf per hectare. The one redeeming feature of Sesbania speciosa is that it is not relished by cattle. Sesbania rostrata is yet another highly promising species which contains nodules on stem which also fixes atmospheric nitrogen. It grows well in waterlogged soil.

6. Phaseolus mungo

This is an annual herbaceous plant cultivated mainly for edible seeds. Mung T -} is a short duration crop maturing within 60-70 days. If sown with the start of monsoon i.e. first week of July, it matures by the fIrst week of September enabling two pickings of the ripe pods and yields on an average 3-4 quintal per hectare. The entire crop after picking the pods, is succulent enough to be turned under as a green manure for wheat. unlike other green manure crop it does not require the use of heavy soil turning plough. An average farmer with bullocks can easily turn it in.

7. Phaseolus trilobus

This is also known as pillipesara. This is comparatively a new green manure crop but is now widely used in Tamil Nadu and Andhra Pradesh. It is also an important pulse crop, because of the creeping and spreading habit. Pillipesara affords a good cover and thus prevents weeds from coming up. It also decomposes quickly. It can be grown either in a standing rice crop in autumn or winter or after ploughing the soil in summer. Pillipesara leaves are relished by cattle and hence can be grown as fodder-cum-green manure crop.

Among the other kharif pulses belonging to the genus phaseolus, moth (Phaseolus aconitifolius) and urd (Phaseolus mungo) are used as green manure to a limited extent.

8. Melilo/us spp

Melilotus indica or sweet clover is a commonly grown green manure crop in the United States for sugarcane crop.

9. Trifolium alexandrium

Berseem is a popular fodder crop. However, it can be grown as green manure crop also. In south, it is used as a poultry feed. It is very rich in protein and, therefore, used as poultry and cattle feed. It belongs to clover family. It was introduced in the beginning of twentieth century form Egypt. It yields about 25000 to 37000 kg of green fodder in four to six cuttings per acre. Besides being a good fodder and poultry feed, it builds up soil fertility by increasing the nitrogen content of the soil.

10. Cyamopsis tetragonoloba

Cyamopsis or guar is used as a green manure crop in the dry and low rainfall areas. This plant grows well even on poor soils and make very good fodder and manure.

Besides these crops, certain other crops are also used as green manure crops in one region or the other. Some varieties of Vigna  grow very fast, establish quickly in both the seasons i.e. rabi and kharif and make a very good green manure.. It is annual and mostly grown in winter I and establishes itself even when sown on standing rice before harvest. Desmodium is sown as both cover and green manure crop. Cowpea  which is mostly grown as a fodder crop, is a promising green manure crop. Other leguminous plants such as kulthi (Dolichos biflorus) and Lupinus spp are used to certain extent as green manures.

 IV. Technique of Green Manuring

 All the operations involved from sowing till turning in of green manures depends on the kind of crop. Green manuring will yield rich dividends only when the cultural practices involved in the process are properly understood and practised. Sowing time of the green manure crop, as it differs, depending on the season and crop type, should be so arranged as to bury it at a time when it furnishes its nutrients to the subsequent crop and the time the latter needs it. This, would also help checking the loss of nutrients from the soil.

The unique ability of dhaincha (Sesbania aculeata) to flourish in a variety of soil and climate has made it adaptable throughout  the country. Therefore, the technique described is biased towards dhaincha. However, the basic principles involved are applicable other green manure crops after suitable modification.

1. Soil

Green manure crops can be grown on any type of soils, provided there is sufficient rainfall or alternative irrigation facility. Poor sandy soils, in particular, benefit most by these practices. The heavy soils are also opened up with the incorporation of the organic matter. Dhaincha can tolerate alkalinity to a level of pH 9.5. It can grow in soils having salt concentration of more than 1 % which is hazardous for all other commonly grown crops. In case where pH and salt content are higher than 9.5 and 1 % respectively, leaching of the soil should be done before sowing the seeds. Dhaincha gives out lateral i roots from the stem to have a firm grip in the soil so as to stand against strong winds.

The land is given one or two ploughings for its preparation. In case if green manuring is followed by application of gypsum in alkaline soils, leveling of the field is necessary.

2. Sowing or the seeds

Sowing may be undertaken from early June-July. In these regions, wheat is harvested in March and the field is ploughed, harrowed and leveled in the first week of May. Fine soil tilt is required due to the small size of seed. The advantage of establishing the crop earlier than the onset of monsoon is to resist damage from the subsequent heavy rains.

In case of dhaincha, it should be sown 45-50 days before the stipulated date of rice transplanting.

The amount of seeds required for one hectare vary form 45 to 60 kg. Higher dose is required for saline and alkaline soils in the wake of lower germination due to excess salts.

Before sowing, seeds of dhaincha should be soaked overnight in water. For seeds, intended to be grown in alkaline soil, a little salt should be added in the water. However, the treatment of seed with Rhizobium culture has been found to give a good growth of crop. The culture for the various green manure crops is available with local agriculture universities. The process of treatment involves dissolving 125 g of jaggery in 500 ml of water and boiling the solution for half an hour. After cooling, the contents of the packet of Rhizobium culture are transferred and mixed into slurry. To this slurry are added the seeds required for one acre and then the contents are mixed well. The treated seeds are then dried in shade on a sheet of paper. The treated seeds should not be dried in the sun on any account. The treated, dry seeds are sown either immediately or later, but the storage period should not exceed 48 hours.

The addition of jaggery solution causes an effective stickiness of the Rhizobium on the seeds, and the sugar content of the jaggery serves as an excellent medium for the multiplication of the Rhizobia so that the load of their population on seeds and rhizosphere of soil is in requisite number. Such treatment has been found to increase the yield of green manures by 10-25 per cent.

3. Fertilizer use

As most of the green manure crops are legumes, they have got nodules in their root system. In these nodules Rhizobium bacteria, exist which fix atmospheric nitrogen. Therefore, green manure crops do not require any nitrogenous fertilizer as the nature itself has gifted them with the capacity to utilize atmospheric nitrogen. However, when the plant is young, root nodules are not so active. Therefore, application of 10-15 kg of nitrogen per hectare helps the crop to grow faster.

Application of phosphorus is essential for sound growth of green manure crops. As the applied phosphorus is not completely utilized and as considerable portion of it is fixed in the soil, several attempts have been made to assess the best form in which phosphates can be applied to the crops. The placement of phosphate at the root zone has been tried. All the dose of phosphatic fertilizer intended for main crop was applied to dhaincha and no phosphorus was given to the main crop of paddy. A good crop of dhaincha was obtained which increased the rice yield significantly. Therefore, the phosphorous applied to dhaincha is not lost, rather meets the phosphorus requirement of both green manure and paddy.

Certain impurities mixed with phosphatic fertilizer, serve as the source of micronutrients which are needed for the growth of crop. Dhaincha often does not need any additional dose of potassic fertilizer as its extensively grown root system absorbs enough potash from the soil to meet its requirement.

4. Irrigation

Usually 5-6 irrigations are required for raising a 45 days old crop in summer. During germination, sufficient moisture in the soil at 5-10 cm depth is required. If possible one presowing  irrigation is desired for good germination. The first irrigation after sowing is given to one week old crop subsequent irrigations are given as and when needed depending on rainfall. Where annual precipitation is less than 65 cm, alternative arrangement of irrigation is necessary.

 Incorporation of the Green Manure Crop

The best results are achieved when a dhaincha crop of 45- 50 days old is incorporated into the soil just before transplantation of paddy. At this age dhaincha is in its pre-flowering stage and quite succulent A 50-day old crop furnishes about 20-24 tonnes of biomass per hectare and supplies 95-115 kg of nitrogen in the same area. The crop should be ploughed in with the help of soil turning or disc plough. Ploughing buries the crop in 15-20 cm deep surface soil.

Dhaincha for green manuring purpose should not be allowed to grow for more than 50 days or else the crop becomes fibrous. It takes longer time for a fibrous crop to decompose and liberate nutrients timely because of its high lignin content. A thumb rule for turning the crop of green manure is at the time when flowers have just started to emerge.

 Decomposition

Countless microorganisms participate in the decomposition of organic matter. The factors conducive to complete decomposition are the stage of maturity of the crop and the moisture level of the soil. Desired results will follow if moisture content is high in the beginning, producing semi-aerobic conditions and low afterwards for inducing aerobic condition under which nitrification can take place. Decomposition, besides depending on moisture content of the soil, is also dependent on the composition of the green matter and the presence of available inorganic nutrients. In light soils, crop should be buried deeper than in heavy soils.

There is no need to allow any time for the decomposition of the green matter. After incorporation, the field is flooded and transplanted with 30-40 days old rice seedlings. Being succulent and having narrow C:N ratio (25:1), it decomposes easily and quickly on incorporation into the soil. The standing water in rice field, hastens the decay rather than hampering without causing any harm to the rice crop. About 50% of the nitrogen becomes available within 4- 6 days of incorporation, the rest, within 20 days of application under the temperate conditions prevailing in the main rice growing fields. Ammonia is released during decomposition which is directly utilized by the paddy roots.

In the normal well-drained soil, the end products are mainly carbon dioxide, nitrate, sulphate and other resistant residues. The decomposition of green manure under waterlogged conditions, as in rice field, takes an entirely different course. It differs from the well drained aerated soil where it is slower and produce different end products. The principal gas produced by green manure decomposition under water logging condition is methane. Small amount of carbon dioxide, hydrogen and nitrogen are also formed. However, the major portion of methane is again convened by the microorganisms on soil surface into carbon dioxide. Blue green algae, which is very often found floating on the water of paddy fields absorbs this carbon dioxide and in return liberate oxygen. This oxygen is dissolved in water and ensures adequate oxygen supply to the paddy roots. Thus, green manuring stimulates the growth of the algae and the production of oxygen in the rice soils resulting in greater aeration or oxygenation of the roots.

Green manure also produces some growth hormones and other bio-chemicals which stimulates the growth of paddy.

A growth period of 45 days without much time for decomposition results in good yield and saving of longer growth period renders the plant hardy resistant to decomposition. The time saved thereby can be adjusted in favor of grain crop in the rotation.

 VI. Place of Green Manure in Crop Rotation

 Having seen the response of crops to green manuring, we are now in a position to discuss its place in rotations of wheat, sugarcane, paddy and other crops. Green manure-wheal/sugarcane/paddy rotations have been more remunerative than fallow wheat-sugarcane- paddy rotation, but utilization of a leguminous crop for fodder on grain and moderate fertilization of subsequent crop with inorganic nitrogen has been more remunerative than rotations having green manure. With the availability of short duration, high yielding varieties of cereals, farmers are now adopting multiple cropping rotations where green manure does not have any place, because the cost of growing green manure in wheat rotation comes very high. With high prices prevailing in the country, more and more farmers are adopting double and even triple cropping in a year. After the harvest of main crop, short duration catch crops of legumes like Lobia could be grown which could be utilized for green manuring. Even here, farmers prefer to feed a part of the crop to their cattle.

The main drawback existing with the green manure in situ, which is more beneficial than green-leaf manuring is that, the green manure crop occupies the cultivable field for at least 50-55 days, at the cost of cereal crop. Increased food grain requirement due to the increase in population, leads the farmers to keep his field occupied by cereals and cash crops. On the other hand, the expansion of fertilizers use technology, gave another setback to green manuring. Yet the increasing costs of the inorganic sources of nitrogen, their undesirable impacts on the physical condition of the soil and the popularization of the concept of 'sustainable agriculture' which is taking place in the country today to keep up their pivotal position in sustaining soil fertility.

To get significant response from green manuring, sufficient dose of green manuring should be given to the crop. It is better reduce the area required for adequate green manuring rather than spreading a little green manure in a extensive field. Certain doses which can be recommended for different crops are shown in Table-

Table: Dosage of green manure or green leaf for different crops and the type of green manure recommended.

Sl.No	Name of crop recommended	Green manure	Doses in kg/ha
1	Paddy	Sesbania (dhaincha) or sunnhemp	2300
2	Sugarcane	Sunnhemp	5000
3	Cotton	-	Green manuring not recommended
4	Wheat	Sunnhemp	7000
5	Banana	Glyricidia	25 kg per pit
6	Grape	Green leaf manuring	100 kg per pit before onset of monsoon
7	Potato	Lupin or buck wheat	1000

VII. Effect of Green Manuring on Soil

Green manuring exerts multiple beneficial impact on the soil which can be discussed under the following headings.

1. Soil fertility

The literal meaning of the word 'fertility of soil' is the ability of a soil to supply plant nutrients in and available form. The availability of nutrients greatly depends upon the total content of nutrients present in the soil.

Green manures supply nitrogen to the soil and consequently make it available to the plants. As most of the green manure crops are leguminous, they fix atmospheric nitrogen in their root nodules. ft is also observed that some fixed form of nitrogen diffuses out from the nodules to the soil, thereby, benefiting other plants growing along with the Legumes. On the other hand when a green manure crop is turned in, all the nitrogen fixed in the plant body is liberated , in the soil in the form of ammonium. To sum up, leguminous green manures convert unavailable nitrogen of the atmosphere into available ammonium of the soil. Ammonium form of nitrogen can be easily converted into nitrate form by microorganisms.

The over dry weight of leguminous plants contain on an average 2.5 to 3.0 % nitrogen. The roots of annual legumes may contain 5 to 20% of the total nitrogen found in the plant. Table-5 gives an idea about the amount of nitrogen fixed by some important leguminous green manures.

 Table 5: Amount of nitrogen supplied by selected crops

Sl.No	Crop	Nitrogen supplied (kg/ha)
1	Senji	80
2	Guar	60
3	Libia	55
4	Dhaincha	75
5	Mung	40
6	pillipesara	55

At the decomposition of green manure begins immediately after turning in, nitrogen of the soil is utili7.cd by the micro-organisms, and thus are saved from leaching losses. Decomposing organic matter has a solubilizing effect on phosphorus, potassium and trace elements. There are several mechanisms through which phosphorus becomes available to the plants. The increased solubility of phosphorus might be owing to the production of a large quantity of carbon dioxide during decomposition of organic matter, resulting in lowering the pH of the soil solution. Carbon dioxide dissolved in water, helps in the conversion of mineral phosphate into organic phosphate which becomes available to the subsequent crops. Some of the phosphorus which is strictly fixed with iron and aluminum is also liberated for the crops' use.

2. Soil structure

The effect of green manuring on soil structure has been controversial. In some of the experiments conducted in this respect it is found that, tinder tropical conditions which are existing in our country, the green manures added is rapidly oxidized before it can be dispersed into the soil colloids.

Materials that are resistant to decomposition have little, effect upon the size distribution of soil aggregates but are effective over a longer period by mechanically loosening the soil. In sandy soils, where the amount of silt and clay in proportion to the sand is often very small to produce other than single grain structure, the application of organic material will have little effect on the aggregation of the  soil.

3. Organic matter

Green manure in most parts of the world has been applied more successfully for increasing the available nitrogen supply than the humus content of the soil. As a matter of fact, green manuring can either increase the humus content or the supply of available nitrogen in the soil, but it can rarely replenish both at the same time. There they differ from the effect of well decomposed farmyard manure fairly resistant to decomposition resistant plant material which is If typically low in nitrogen. The available nitrogen supply is only increased if readily decomposable material, high in nitrogen content such as young plants -are added to the soil.

4. Reclamation of saline and alkali soils

Green manures play an effective role in the reclamation of saline and alkali soils. During decomposition of these manures, considerable amount of organic acid is liberated which brings down the pH of the soil, besides forming number of salts with the sodium of the exchangeable complex. This reduces the sodium content of the clay of the soil. Dhaincha has been found to be an ideal green manure crop for reclaiming saline and alkali soils. Saline soils mend themselves if copiously irrigated with good quality water which washes down the salts to the lower horizons of the soils profile. i Growing a green manure crop in saline soil brings out a rapid transformation of the soil to a cultivable land.

In any scheme of reclamation of alkali soil, gypsum and dhaincha are necessarily involved. In the process of reclamation, gypsum is applied followed by leaching of the treated soil. The cultivation of any crop in the leached land begins with dhaincha. An experiment, involving the use of 10 tonnes per acre of gypsum, with frequent flooding and draining, followed by green manuring with dhaincha, resulting in the reclamation of clayey alkali soil and gave higher yields of crops thereafter.

Green manuring also imparts buffering capacity to the soil which helps in keeping down the harmful effects of excessive salt concentration. To get the maximum benefit from green-manuring, the crop should be ploughed at a time when the moisture content of soil is low, so that the initial desiccating reactions may fix the organic complexes, and a highly buffered humified organic residues will help in reducing the concentration of sodium on clay colloids.

oids.