Importance of Nitrogen
All plants need nitrogen for the manufacture of proteins.
However plants cannot make use of the nitrogen in the air.
This nitrogen must first be converted to nitrates before the plant can absorb it. This can be done in two ways:
Nitrogen fixing bacteria present in the root nodule of same plants (legumes) change atmospheric nitrogen to nitrates
Lightning can change atmospheric nitrogen to nitrates
N2 + O2 → 2NO
2NO + O2 → 2NO
2NO + H2O → HNO3 + HNO2
Once it is fixed nitrogen it is taken up by plants and changed to protein.
Animals will obtain their protein from these plants
Upon death of plants and animals all protein is converted to humus
Humus along with excreta is changed by nitrifying bacteria to ammonium compounds.
The ammonium compounds are changed first to nitrite by bacteria
Then nitrite is changed to nitrate by bacteria in a process called nitrification
2NH3 + 302 → 2H+ + H20 + ENERGY
2N0-2 + O2 → 2NO3 + ENERGY
The effect of all of this is to return nitrates to the soil.