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Green Manuring Selector

Green Manuring can bring many advantages to the farmer by adding organic matter to the soil, increasing biological activity, improving soil structure, reducing erosion, increasing the supply of nutrients available to plants (particularly by adding nitrogen to the system by fixation), reducing leaching, weed suppression and so on. There are some disadvantages and whilst these are few they should also be noted – lost opportunities for cash cropping, exacerbated pest and disease problems (green bridge effect), and the potential for green manures to become weeds in their own right. These problems can be overcome with thought and measured usage, and the benefits to future crops cannot be dismissed lightly.

A wide range of plants species can be used as green manures. Different crops bring different benefits and the final choice is influenced by many considerations. If the most is to be made of green manuring, it is important that they are carefully integrated into the crop rotation and proper attention paid to their husbandry.

Nitrogen (N) in legumes comes from uptake of soil N and the fixation of N from the atmosphere. The amount of N fixed by different legumes is determined by the inherent capacity of the crop/rhizobium symbiosis to fix N, modified by the crops’ management and length of time for which the crop is grown. Consequently, the influence of all these factors means that a wide range of values have been reported by different researchers. The presence of soil mineral N is generally thought to reduce fixation capacity. Factors that will increase the soil mineral N pool include manure application, cutting and mulching, and grazing. Fixation tends to decrease with legume age, mainly because the amount of soil N tends to increase. Where growth of legumes is affected by nutrient deficiency (or acidity) the potential for soil N build up is reduced. Phosphorus, Sulphur and some trace elements (e.g. molybdenum) are particularly important.

Where there are large off-takes of soil nutrients as in silage crops both P and K supplies need to be adequate for satisfactory legume growth.

Green Manuring can bring many advantages to the farmer by adding organic matter to the soil, increasing biological activity, improving soil structure, reducing erosion, increasing the supply of nutrients available to plants (particularly by adding nitrogen to the system by fixation), reducing leaching, weed suppression and so on.  There are some disadvantages and whilst these are few they should also be noted – lost opportunities for cash cropping, exacerbated pest and disease problems (green bridge effect), and the potential for green manures to become weeds in their own right.  These problems can be overcome with thought and measured usage, and the benefits to future crops cannot be dismissed lightly.

A wide range of plants species can be used as green manures.  Different crops bring different benefits and the final choice is influenced by many considerations.  If the most is to be made of green manuring, it is important that they are carefully integrated into the crop rotation and proper attention paid to their husbandry.

Nitrogen (N) in legumes comes from uptake of soil N and the fixation of N from the atmosphere.  The amount of N fixed by different legumes is determined by the inherent capacity of the crop/rhizobium symbiosis to fix N, modified by the crops’ management and length of time for which the crop is grown.  Consequently, the influence of all these factors means that a wide range of values have been reported by different researchers.  The presence of soil mineral N is generally thought to reduce fixation capacity.  Factors that will increase the soil mineral N pool include manure application, cutting and mulching, and grazing.  Fixation tends to decrease with legume age, mainly because the amount of soil N tends to increase.  Where growth of legumes is affected by nutrient deficiency (or acidity) the potential for soil N build up is reduced.  Phosphorus, Sulphur and some trace elements (e.g. molybdenum) are particularly important.

Where there are large off-takes of soil nutrients as in silage crops both P and K supplies need to be adequate for satisfactory legume growth. 

Incorporation of Green Manure

 
With all green manuring crops it is important to make sure that the plant is incorporated when it is in its most succulent state; in this way the amount of carbon within the plant structure is very low. Decomposition needs soil nitrogen to break down carbon, the woodier the plant the greater the carbon content and hence greater quantities of soil nitrogen are required to break it down. It should also be borne in mind that 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 unfavourable 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 it will best serve as fertiliser for the crop it is to benefit.
 

 


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