Question

Which of the four most abundant elements in most plants (C, H, O and N), does a terrestrial green plant procure mainly through its roots from the soil ?

• A. H and O
• B. H and N
• C. C and O
• D. O and N

Answer 1

Answer: (B)

Hydrogen $(H)$ is mainly obtained from soil water by roots. Oxygen $(O)$ is either derived from the air or from the water. Carbon $(C)$ is derived from atmosphere as $C{O}_2$. The plants are unable to use atmospheric nitrogen. It is inert and plants cannot make use of it directly. By the process of fixation of atmospheric $N_2$, plants obtain it from soil in the form of nitrites $(N{O}_2^{-})$, nitrates $(N{O}_3^{-})$, ammonium ion $(N{H}_4^{+})$, etc. These nitrogenous compounds enter plants as nutrients through the roots and are assimilated as organic nitrogen.