The "fixing" of nitrogen in the soil means:

Jump to: navigation, search. Nitrogen fixation is a process by which nitrogen (N2) in the Earth's atmosphere is converted into ammonia (NH3) ... Nitrogen fixation occurs naturally in the soil by nitrogen fixing bacteria (for example, Azotobacter affiliated with some plants, especially legumes).

Ypu plant things like legumes that will set it right again u alternate with them every year for ex one year u might plant corn the next year u would plant legumes