RAS question
Nitrogen fixation in soil is primarily done by:
Correct answer: (A) Rhizobium bacteria in legume root nodules.
Nitrogen fixation in soil is primarily done by Rhizobium bacteria living in the root nodules of leguminous plants such as peas and beans.
Explanation
Rhizobium is the primary biological agent of nitrogen fixation in soil. The NCERT chapter hosted on SATHEE states that Rhizobium is involved in nitrogen fixation in leguminous plants and lives in their root nodules, including plants such as beans and peas, in a symbiotic relationship. These bacteria convert atmospheric N2 into ammonia. This matters because atmospheric nitrogen cannot be taken directly by plants and animals; it has to be fixed into usable nitrogen compounds before plants can absorb it through their roots. Free-living bacteria such as Azotobacter and blue-green algae also fix nitrogen, but Rhizobium in legume root nodules is the precise agent for soil nitrogen fixation in legumes.
Why the other options are wrong
- (B) Earthworms improve soil structure and fertility indirectly, but they lack the nitrogen-fixing role carried out by Rhizobium, certain free-living bacteria, and blue-green algae.
- (C) Fungi act on nitrogenous wastes in the nitrogen cycle, but the fixation of atmospheric nitrogen is attributed to certain bacteria, blue-green algae, and specifically Rhizobium in legumes.
- (D) Insects can carry disease-causing microbes, but they do not fix atmospheric nitrogen in soil.
Concept
The nitrogen cycle and useful microorganisms in agriculture are central to basic ecology, soil fertility, and applied science in RAS. These areas often turn on the difference between decomposers, soil improvers, and true nitrogen fixers.