Question

Most plants obtain their nitrogen from the soil in the form of

• A. nitric acid
• B. free nitrogen gas
• C. nitrates
• D. nitrite

Answer 1

Answer: (C)

nitrates

Answer 2

Free nitrogen gas, i.e., molecular nitrogen cannot be directly utilised by plants. It has to be fixed into nitrogenous compounds prior to utilisation. Most plants absorb nitrogen in the form of $NO^{-}_{3}$ (nitrate) or $NH^{+}_{4}$ (ammonium ion). Nitrate is the most important source of nitrogen to the plants. It can accumulate in the cell sap of several plants and take part in producing osmotic potential. However, it cannot be used as such by the plants It is first reduced to nitrite $(NO^{-}_{2})$ and then to ammonia $(NH_{3})$ Ammonia then combines with some organic acids to produce amino acids, which then form various types of nitrogenous compounds.