Biology · Microbes in Human Welfare · NEET
Rhizobium is SYMBIOTIC. It cannot fix nitrogen well on its own in the soil. It must live inside the root nodules of leguminous plants. This is the most common NEET trap. Free-living nitrogen fixers are different bacteria like Azotobacter and Azospirillum.
Rhizobium enters the roots of legumes and forms swellings called root nodules. Inside these nodules, the bacteria convert atmospheric nitrogen (N2) into organic nitrogen forms. The plant uses this fixed nitrogen as a nutrient, and in return the plant gives the bacteria food and shelter. Both partners benefit, so it is called symbiosis.
A biofertiliser enriches the nutrient quality of the soil. Rhizobium adds nitrogen to the soil, so it is a biofertiliser. A biocontrol agent kills pests or pathogens (like Trichoderma or Bacillus thuringiensis). Do not confuse the two. In NEET, Rhizobium always belongs to the biofertiliser list.
Only leguminous plants (family Fabaceae) — examples are pea, gram, beans, groundnut and clover. These plants show root nodules. Non-leguminous plants do not host Rhizobium in this way.
No. Rhizobium is symbiotic. Azotobacter and Azospirillum fix nitrogen while free-living in soil. Cyanobacteria like Anabaena, Nostoc and Oscillatoria also fix nitrogen and are important biofertilisers in paddy fields. Mycorrhiza (Glomus) is a biofertiliser too, but it supplies phosphorus, not nitrogen.
Among the following pairs of microbes, which pair has both the microbes that can be used as biofertilizers?
Which of following organisms cannot fix nitrogen? A. Azotobacter B. Oscillatoria C. Anabaena D. Volvox E. Nostoc
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the conversion of atmospheric nitrogen (N2) into usable organic nitrogen by bacteria that live inside a plant. Rhizobium doing this inside legume root nodules is the classic example. Both the bacterium and the plant benefit.
Rhizobium lives inside the root nodules of leguminous plants such as pea, gram and beans. It does not fix nitrogen effectively when free in the soil.
Rhizobium is a bacterium. The main sources of biofertilisers are bacteria, fungi and cyanobacteria — Rhizobium falls under bacteria.
Rhizobium is a bacterium that fixes NITROGEN in legume root nodules. Mycorrhiza is a fungus-root association (e.g. Glomus) that supplies PHOSPHORUS to the plant. Different nutrient, different organism.
Overuse of chemical fertilisers causes soil and water pollution. Biofertilisers like Rhizobium naturally enrich soil with nitrogen, reduce dependence on chemicals, and support organic farming.
