Legumes are desirable during crop rotation. give reasons

Legumes put nitrogen back into the soil after it's been depleted by another crop.

Leguminous crops provide shelter for soil bacteria like rhizobium.

Rhizobium (plural: Rhizobia) is the nitrogen fixing bacteria present mainly in the soil. The Rhizobia chemically convert the nitrogen from the air to make it available for the plant. These bacteria live in symbiotic relationship with leguminous plants. The Rhizobia live in nodules in the roots of the plant. These bacteria convert atmospheric nitrogen to nitrates and nitrites which can be used by the plants directly. Nitrogen

Explanation:

igneous rocks are formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.  

the composition depends on the source magma. there are almost a thousand types of composition found till now.  

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