Define nitrogen fixation, what are organisms are capable of nitrogen fixation? Why do farmers rotate their crops with legumes?

Nitrogen fixation is a process of fixation of atmospheric nitrogen into forms of nitrogen that can be directly utilized by the plants like nitrates and nitrites. This process is conducted by the soil bacteria.

The farmers rotate the crops with legumes because the roots of legumes shows symbiotic mutualistic association with nitrogen fixing soil bacteria. The soil get rich with the nitrogen element necessary for growth of plants.