Most of the plants absorb nitrogen in the form of nitrates and nitrites.Plants take nitrogen from the soil by absorption through their roots as amino acids, nitrate ions, nitrite ions, or ammonium ions. Plants do not get their nitrogen directly from the air.
It is taken into the soil by bacteria, some algae, lightning, and other means. Nitrate is the form of nitrogen most used by plants for growth and development. Nitrate is the form that can most easily be lost to groundwater. Ammonium taken in by plants is used directly in proteins.