Nitrogen is an essential element for plants and is found in abundance as atmospheric nitrogen. But most plants are unable to use it. Why is it so and in what form do plants utilize them?

Explanation

Most plants cannot utilize nitrogen as the atmospheric nitrogen is inert in its nature.
Nitrogen combines with oxygen by the atmospheric activities and is brought down by rain to the soil. Highly specialised organisms, nitrogen fixers, occur in the soil and they convert nitrogen gas `(N_2)` to anionic forms such as nitrates `(Na^(3-))` or nitrite `(NO_2)` or a reduced cationic form such as ammonium `(NH^(4+))`. These compounds enter the plants as nutrient through the root and are assimilated as organic nitrogen.