Question

why are leguminous crops grown after growing cereal crops

Answer 1

This system of growing crops of a certain type after a different type is referred to as crop rotation. 

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In specific; the kind of rotation where cereals are followed up by legumes is for the following reason:

- Legumes are plants that have nodules that contain rhizobia bacteria. These bacteria have the ability to fix/ incorporate free atmospheric nitrogen into the soil. This they do by converting the nitrogen in the atmosphere into nitrates and nitrites that can be used up as plant nutrients. The legumes are thus very important plants in the field in providing nitrogen nutrient in the soil. 
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The cereals on the other hand do not have this ability of fixing nitrogen into the soil. Therefore, what happens is that the cereals use up whatever nitrogen they find in the soil while they are grown. This therefore leads to a depletion of the existing nitrogen nutrient in the soil. 

For this reason, farmers plant legumes in the same field where cereals were planted that the legumes may replenish the nitrogen nutrient that has been used up by the cereals.

 

Answer 2

Crop Rotation :
Growing of different kinds of crops on a piece of land is called as crop rotation.
In crop rotation, Leguminous crops like pulses, groundnut, beans etc are sown in between the cereals crops like Wheat, Maize etc to restore the fertility of soil.
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1.Cereals crops when grown in the soil, they have the tendency to absorb lot of nitrogenous salts from the soil and
2. If another crop of cereal is grown it makes soil deficient of nitrogen.
3. Hence leguminous crop are grown by rotation.
4.Leguminous plants have a bacteria called as Rhizobium in their roots, which has the ability to fix the atmospheric nitrogen into compounds of nitrogen which crops can use for their growth.
5.Nitrogen compounds go deep into the roots and makes the soil fertile.
6.Thus planting a leguminous crop results in between cereal crop results in:
-----> increases soil of fertility.
-----> helps in increasing crop production.
------>helps in saving nitrogen fertilizers .
---->helps in controlling weed and pest.
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