Question

Plant cannot consume free nitrogens present in the air then how do plants get nitrogens for their growth? Explain.

Answer 1

Plants directly can not consume free nitrogen so plants get nitrogen by following methods:

1. During lightning nitrogen reacts with oxygen to form nitric oxide gas.

${\mathrm{N}}_2+{\mathrm{O}}_2\to 2\mathrm{NO}$

This nitric oxide reacts with oxygen present in the environment to form nitrogen dioxide.

$2\mathrm{NO}+{\mathrm{O}}_2\to 2{\mathrm{NO}}_2$

The nitrogen dioxide reacts further with more oxygen and water to form nitric acid.

.$4{\mathrm{NO}}_2+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{HNO}}_3$

During rain, the dilute nitric acid washes down to the earth, where it further reacts with soil carbonates to form soil nitrates.

${\mathrm{CaCO}}_3+2{\mathrm{HNO}}_3\to \mathrm{Ca}{\left({\mathrm{N}}_3\right)}_2+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$

This form of nitrogen can be absorbed by plant roots.

2. Some plants (Leguminous) have bacteria associated with their roots in the form of root nodules where they fix free atmospheric nitrogen in nitrite $\left({{\mathrm{NO}}_2}^{-}\right)$and nitrates form $\left({{\mathrm{NO}}_3}^{-}\right)$ which can be absorbed by plants.