Nitrogen are essential for growth of plants and for preparing proteins. leguminous plants have root nodules where certain bacterias named rhizobium lives. they convert atmospheric nitrogen into soluble form which plants absorb from soil with water. so no need of adding fertilizers is needed. after this crops cultivation the farmers don't need to add fertilizers because the soil become rich in nitrogen. thus they can save  money.
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Becoz these plants are already rich in nitrogen. Instead of taking from the soil they provide the nitrogen to plant. Hence, farmers grow these types of plants for the enrichment of nitrogen contents in the soil which are used up by different other crops.

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