Question

Most of the plants obtain or absorb nitrogen from the soil in the form of
(a) Free Nitrogen gas
(b) Nitric acid
(c) Nitrite
(d) Nitrates

Answer 1

Hint: This ion of nitrogen causes the dilation of blood vessels. It is a common component of fertilizers and explosives. It is also a water-soluble ion. It is one of the primary contaminants of cultural or accelerated eutrophication.

Complete answer:
The plants obtain or absorb nitrogen from the soil in the form of nitrates. Nitrogen is a vital component of chlorophyll, required for photosynthesis. Photosynthesis is a crucial process for plants to synthesize their food.
Most plants absorb nitrogenous compounds from the soil through their roots. This can only be done when nitrogen is available in the form of ammonium or nitrates. The process of conversion of nitrogen gas into nitrites and nitrates via atmospheric, biological, and industrial methods is called ‘nitrogen fixation’. Atmospheric nitrogen has to be processed or ‘fixed’ into a form like ammonium or nitrates so that it can be taken up by the plants for their usage.
- Some plants have a symbiotic relationship with rhizobia. In these plants, some amount of nitrogen is assimilated in the form of ammonium ions directly from the rhizobium nodules.
- These ammonium ions are then further converted into nitrites and nitrates as they cannot be used directly.
So, the correct answer is, ‘Nitrates’.

Note:
- Ammonia gas is believed to be toxic to the plants.
- The nitrogenous wastes present in animal urine are broken down by nitrifying bacteria in the soil. This is further utilized by plants.
- Nitrogen gas in the soil is inert and unavailable for plants to use.