why is nitrogenous fertilizers not added in soil in which leguminous plants are grown?

Nitrogen is present in abundant quantities in air; however, plants are unable to take in nitrogen in the gaseous state. Hence, soil has certain nitrogen-fixing bacteria which convert gaseous nitrogen into a more usable form and release it in the soil. These bacteria are present in the root nodules. One such bacterium is Rhizobium. It is usually found in the roots of leguminous plants such as gram, peas, moong, etc. The plants provide food and shelter to bacteria, and in return, bacteria fix nitrogen for the plants. Therefore, the nitrogen deficiency of the soil is fulfilled by the nitrogen fixed by the nitrogen-fixing bacteria present in the root nodules of leguminous plants. Hence, external supply of nitrogenous fertilizers is not required to be added in soil in which leguminous plants are grown.