All plants require nitrogen for the manufacture of proteins.
However plants cannot make use of the nitrogen in the air.
The nitrogen in the air must first be converted to nitrates
before the plant can absorb it.
This can be done in two ways:
Nitrogen Fixing Bacteria
Nitrogen fixing bacteria present in the root nodule of some plants,
change atmospheric nitrogen to nitrates.
Lightning can also change atmospheric nitrogen to nitrates.
The steps are as follows:
N2 + 02 2NO
2NO + O2 2NO2
2NO2 + H2O HNO3 + HNO2
The fixed nitrogen is taken up by the plants and changed to proteins.
Animals will obtain their protein either directly or in directly from these plants.
Upon death of the plants and animals all the protein is converted to humus.
Humus along with egesta and excreta is changed by nitrifying bacteria to ammonium compounds
The ammonium compounds are changed first to nitrite by bacteria
in a process called nitrification.
The effect of all of this is to return nitrates to the soil.
However, also present in the soil are harmful denitrifying bacteria which change valuable nitrates into free nitrogen (N2),
which cannot be used by the plant.
Denitrifying bacteria thrive best in waterlogged soils.
The conversion of nitrogen in the air into a usable form is called Nitrogen Fixation.
In nature fixation is carried out by bacteria.
In industry fixation is carried out in the Haber process,
the product being ammonia.
Industrial fixation now exceeds natural fixation.