The process of conversion of nitrogen $\left({\mathrm{N}}_2\right)$ to ammonia is termed as nitrogen fixation. Most of the ammonia is converted to nitrate by soil bacteria, where ammonia is first oxidized to nitrite by the bacteria Nitrosomonas or Nitrococcus. The nitrite is further oxidized to nitrate with the help of Nitrobacter. These steps are called nitrification and the nitrifying bacteria are called chemoautotrophs.