Green manuring :advantage, disadvantages, and limitations of green manuring, recommendation and Use.

 

Green manuring :advantage, disadvantages, and limitations of green manuring, recommendation and Use. 

Definition: Green manuring consists of ploughing in green, not woody plants or plant parts. The plant material can come from a crop that was grown after or between the main crop, or from a weed that grew during a fallow period. It can also come from a shade plant or tree whose cuttings or fallen leaves are suitable for ploughing into the soil. The goals of green manuring are to: 

• make nutrients available for the main crop; 
• improve the soil structure; 
• increase or retain the level of organic matter in the soil; 
•  increase the ability of the soil to retain moisture; 
•  protect the soil against rain and wind erosion, dehydration and extreme temperature fluctuations at a time when no other crops are present; and, when using leguminous plants as green manure, to: 
•  fix extra nitrogen out of the air, which becomes available to the main crop after the manure has been ploughed into the soil. 

Advantages of green manures 

• During their growth period, green manures provide the same bene-fits as mulch. They are therefore sometimes called ëliving mulchí. 

• Their advantage over mulch is that they absorb nutrients, so these cannot be leached during a period in which no main crops are grown. 

• After the green manures are ploughed under, these nutrients become available via decomposition. 

• Green manures also have a positive effect on the soil structure, be-cause of the penetration of their root systems, they add organic matter, and they stimulate the growth of soil organisms. 

• Organic matter nourishes the soil organisms, which also benefit from the higher moisture content and the limiting of extreme temperatures during the day.

Disadvantages and limitations of green manures 

• If farmers are not accustomed to growing green manures, they may not readily accept the method. While the farmers have to invest their time and labour, they receive no obvious benefit, such as cash or food. 

• The direct advantage in the form of increased production is not always immediately noticeable. Moreover, ploughing under a green manure is hard work, especially if done by hand. An alternative that is easier to introduce is intercropping with a green manure.

• The green manure is then grown in combination with the main crop. To prevent competition for nutrients, the green manure plant is sown later than the main crop. 

• This is possible even in a short season, because the green manure plant does not have to mature fully. One plant that has been used quite successfully for this purpose is mucuna under corn.

Method and recommendations 

 It is important to choose a plant species that quickly covers the ground and produces a deep and extensive root system, so that the nutrients from the deep soil layers can be transported to the surface. 

• A fast groundcover also prevents the growth of weeds, because it shades them. 
• However, the green manure may not grow so quickly and easily that it expands to other fields where a different crop is being grown. And it may not be so resilient that it continues to grow after it has been ploughed under. 
• A few species that are often used as green manures are: Crotolaria juncia (sun hemp), Sesbania aculeata (daincha), Vigna unguiculata(cowpea), Vigna mungo (black gram), and Vigna radiata (green gram). If these species are not available, other species that grow well in the area can be used, as long as they satisfy the requirements listed above. 

The green manures are usually ploughed under when they are still young and succulent. The material is then broken down quickly by the soil organisms, whereby the nutrients become available. Within a few months the material is completely decomposed. Thus, little addition is made to the level of organic matter in the soil. Young and succulent material should be ploughed under at least two months before the new crop is sown, because in the initial period of decomposition, sub-stances are released that can damage the young sprouted plants or can make the root ends sensitive to damage by pathogens. 

If the material is ploughed under when it is older and tougher it will be broken down much slower. In that case it does add to the level of organic matter in the soil. Since the nutrients are slowly made available, their effect in the first season is less than with young and succulent material. However, the effect is noticeable for several seasons. If the soil has a low organic content, it is better to let the green manure get old and tough, so that an addition is made to the level of organic matter in the soil. The level of organic matter in the soil is after all themost important indicator of soil fertility. Material that is old and tough generally is difficult to decompose. Many soil organisms are needed to do this. Before the soil organisms can start to digest the organic matter they have to grow themselves. To grow the organisms use nitrogen like plants do (this is also called nitrogen immobilisation). This means that if plants grow at the same time as the organisms the plants will lack nitrogen. Therefore it is better to first allow the soil organisms to grow and decompose the organic matter before the crop is sown. 

So the green manure must be ploughed under 5-6 weeks before the main crop is sown. 

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