It is present in the form of inorganic and organic compounds
in the soil. Nitrogen is present in the inorganic from as
nitrates and nitrites id calcium and potassium, and in the
form  of proteins as organic compounds. The nitrogen-present
in the soil is used by the green plants. The ammonia
converted by nitrifying bacteria into nitrates and this
process is known as Nitrification. There are  denitrifying
bacteria which disintegrates some amount of ammonia into
free nitrogen which escapes into atmosphere. This process is
known as Denitrification

               By discharge of electricity in the atmosphere
and by activity of symbiotic and saprophytic bacteria the
free nitrogen of air is fixed.

              Saprophytic bacteria like Azotobacter and
Clostrium helps in fixing the Nitrogen of air

              Roots of leguminous plants some swelling
called nodules. In these nodules are present some of the
bacteria which help in fixing the nitrogen, leguminous
plants supply the  plants nitrogenous food. So these two are
mutually benefited to each other. This relationship is known
as Symbiosis.

Nitrogen Cycle:-There is regular circulation of nitrogen
through the air, soil and plants and animals. Free nitrogen
of atmosphere is brought down by certain natural devices
into the soil as ultimate products of nitrate and nitrite.
The plants absorb these nitrates and in their body these
nitrates are changed to proteins. Animals take these plant
proteins and after the death and decay of plants and animals
these proteins are again turned back to nitrates and
nitrites and again plants absorb these. At the same time
some of the ammonium compunds present in the soil are
disintegrated by denitrifying bacteria into free Nitrogen
which escapes into air