What are the forms of nitrogen containing compounds in the soil?

Ammonium (NH4) and nitrate (NO3) are the main inorganic forms of nitrogen in the soil. Ammonia is present in both exchangeable and non exchangeable forms.

Nitrate (NO2) and nitrous oxide are present in soil in lesser quantities. Normally, plants use nitrogen in only the ammonium and nitrate forms. Nitrite is actually toxic to plants.

In the nitrogen cycle, the process begins with nitrogen in its simplest stable form known as dinitrogen (N2). The next steps involve fixation, mineralization, nitrification, leaching, plant assimilation, ammonia volatilization, denitrification and immobilization.