Nitrification

Nitrification is the conversation of ammonia (NH₃) to nitrite (NO₂^-), and then nitrate (NO₃^-). This process is carried out by bacteria in the soil, with different species carrying out different reactions. Ammonia and nitrates are reactive compounds which are able to be used by plants in a process known as assimilation.

NH₃ → NO₂^- → NO₃^-

Ammonia in the soil is converted to nitrites by nitrifying bacteria. A different group of nitrifying bacteria convert nitrites to nitrates.
Figure 1: An overview of nitrification

Referred from:

Article

The Nitrogen Cycle