The amount of nitrogen fixed electrochemically and photochemically is:

Nitrogen is present in the soil in the form of
$I.$ Nitrates $II.$ Ammonium salts
$III.$ Nitrite $IV.$ None of these

$A$: Ammonia is oxidized first to nitrite and then to nitrate by the action of $Nitrosomonas$ and $Nitrobacter$ respectively.
$R$: Nitrite thus formed is absorbed by leaves.

Write the nutrition method of nitrifying bacteria.

List and describe the main phases of the nitrogen cycle.

$NO_2^- \rightarrow NO_3^-$
The above mentioned reaction occurs in the presence of .....