Nitrogen gas from the atmosphere can be converted into a useable form of nitrogen in the soil by

Fixation

Explanation:

The majority of nitrogen is fixed by bacteria which are symbiotic with plants which are converted to biologically useful forms by specialized bacteria. This occurs in two steps:

bacteria convert ammonia in to (nitrites) NO₂, bacteria species convert it to NO₃ (nitrate).

Nitriates are a form of nitrogen that is usable by plants. It is assimilated into plant tissue as protein. The nitrogen is passed through the food chain by animals that consume the plants, and then released into the soil by decomposer bacteria when they die.

It can be converted by fixation