CBSE Class 10 Answered

Nitrogen. Plants cannot fix nitrogen on their own, but need it in one form or another to make amino acids and proteins.

Rhizobia converts nitrogen into forms usable by living organisms. Legumes form nodules on their roots in a symbiotic relationship with the soil bacteria known as rhizobia, to fix nitrogen into a form usable by plants (and animals). Rhizobia (e.g., Rhizobium, Mesorhizobium, Sinorhizobium) fix atmospheric nitrogen or dinitrogen, N₂, into inorganic nitrogen compounds, such as ammonium, NH₄⁺, which is then incorporated into amino acids, which can be utilized by the plant. 

Because legumes form nodules with rhizobia, they have high levels of nitrogen available to them. Their abundance of nitrogen is beneficial not only to the legumes themselves, but also to the plants around them. There are other sources of nitrogen in the soil, but these are not always available at the levels required by plants, making the symbiotic relationship between legumes and rhizobia highly beneficial.