Question

The atmosphere contains about 80%
nitrogen gas, but nitrogen in this form
cannot be used by plants. Which process converts atmospheric
nitrogen to a useful form for plants?

Answer 1

nitrogen fixation is the process 

Answer 2

Two processes make the element nitrogen useful to plants:

1) The heat that lightning makes as it moves through the atmosphere and

2) the biological action of a type of plant called legumes.  Examples of

legumes are peanuts and soybeans.