Question

How do plants in the fields get nitrogen present in the air?​

Answer 1

First of all let me tell you that plants does not need nitrogen and if they need it they will absorb it using stomata present on their leaves as they absorb carbon dioxide

Answer 2

the plants can't use the nitrogen present in the air directly.

the roof of leguminous plant like peas, gram, and other pulses contain rhizobium bacteria , which converts nitrogen to ammonium and mix in the soil . the ammonium can be easily used by the plants . also the soil gets nitrogen.