Which one of the following statements is correct?
- Cyanobacteria such as Anabaena and Nostoc are important mobilizers of phosphates and potassium for plant nutrition in soil
- Both Azotobacter and Rhizobium fix atmospheric nitrogen in root nodules of plants
- At present it is not possible to grow maize without chemical fertilizers
- Extensive use of chemical fertilizers may lead to eutrophication of nearby water bodies
Answer
(4) Extensive use of chemical fertilizers may lead to eutrophication of nearby water bodies
Solution
Option (1): As per NCERT, cyanobacteria such as Anabaena, Nostoc and Oscillatoria are autotrophic microbes that fix atmospheric nitrogen; phosphorus is absorbed from soil with the help of fungi (mycorrhiza). ⇒ Incorrect.
Option (2): Rhizobium fixes nitrogen in root nodules of legumes, but Azotobacter is free-living in soil and forms no nodules. ⇒ Incorrect.
Option (3): Maize can be grown using biofertilisers and manures (e.g., with Azospirillum / Azotobacter), so chemical fertilisers are not indispensable. ⇒ Incorrect.
Option (4): Nitrates and phosphates from excess fertilisers are washed into nearby lakes and ponds, act as plant nutrients and cause algal blooms, i.e., eutrophication. ⇒ Correct.
Answer: (4)