Afarmer grows and plows under of alfalfa (a legume) to enrich the soil with

Nitrogen

Explanation:

Nitrogen is an important nutrient required for the proper growth and development of plants. Farmers usually plow and grow under alfalfa - a leguminous crop, to enhance the fertility of soil. This is because alfalfa adds more amount of Nitrogen in the soil than any other crop. Studies suggest that around 50 lbs. of nitrogen per acre can be added in soil each year  by growing alfalfa in the crop fields. This has immense potential to overcome the problems that crops face due to lack of fertilizers.

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