Of all the mineral nutrients in the soil, nitrogen is generally the most difficult for plants to acquire. Even though nitrogen is the most abundant element in the atmosphere, plants alone, do not have the ability to obtain (fix) it. Only plants, that form a symbiotic relationship with nitrogen fixing bacteria i.e. rhizobia, gram-negative bacilli that invaded the roots of plants, can fix atmospheric nitrogen.

Biological nitrogen fixation is an important factor in any sustainable agriculture program. Biological nitrogen fixation is a process in which nitrogen is taken from its relatively inert molecular form (N₂) in the atmosphere and converted into nitrogen compounds useful for building plant protein.

The best known nitrogen fixing plants are legumes i.e. clover which contain symbiotic rhizobia bacteria within nodules in their root systems, producing nitrogen compounds that help to fertilize the soil. The great majority of legumes have this association, but a few genera (e.g. Styphnolobium) do not.

A few dollars worth of rhizobia inoculant can replace hundreds of dollars worth of  nitrogen fertilizer and significantly improve soil and crop health.