Microbes as Biofertilisers: NEET MCQs with Answers

Explanation: Crotalaria juncea (sunn hemp) and Alhagi camelorum are used as green manures in India. Sunn hemp is a fast-growing leguminous plant that fixes nitrogen into the soil and is well adapted to sandy soils, while Alhagi improves the soil by adding organic matter.

Explanation: Anabaena is a free-living nitrogen-fixing cyanobacterium which can also form a symbiotic association with the water fern Azolla, living in the cavities of its leaves.

Explanation: Eutrophication, the nutrient enrichment of a water body, is caused by run-off from fertilised fields, sub-urban lawns, feed lots and detergent-rich sewage. Azotobacter is free-living while Rhizobium is symbiotic; Anabaena and Nostoc are nitrogen-fixers rather than phosphate mobilisers; and maize can be grown with organic manures and compost.

Explanation: Snails are not used in organic farming because they damage crops by eating leaves and stems, which reduces yield. Earthworms, Glomus and Oscillatoria are beneficial organisms that improve soil fertility.

Explanation: Azospirillum is an associative nitrogen-fixing bacterium of the root zone of cereals such as rice; it is the common nitrogen fixer and biofertiliser of paddy fields.

Explanation: Rhodospirillum is a free-living, anaerobic nitrogen-fixer. Beijerinckia and Azotobacter are free-living but aerobic nitrogen-fixing microbes, and Rhizobium is a symbiotic nitrogen-fixer.

Explanation: Agrobacterium is not a biofertiliser. It is a Gram-negative bacterium that causes tumours in plants; A. tumefaciens carries the Ti-plasmid and causes crown-gall disease, and it is used as an important tool for gene transfer in genetic engineering.

Explanation: Rhizobium leguminosarum is a symbiotic bacterium found in the root nodules of legumes and carries the nitrogen-fixing nif gene. Since soyabean is a legume, Rhizobium is used as its biofertiliser.

Explanation: Anabaena azollae is a free-living nitrogen-fixing cyanobacterium which may also live symbiotically in the leaf cavities of Azolla, a free-floating aquatic fern. Because it possesses nitrogenase it can fix nitrogen, and its growth in paddy fields gives a remarkable increase in yield.

Explanation: Several species of Glomus, including G. aggregatum, are cultured and sold as mycorrhizal inoculants for agricultural soils. Being endomycorrhiza, they help the plants in the absorption of nutrients, especially phosphorus, from the soil.

Explanation: Rhodospirillum is a free-living nitrogen-fixing bacterium and has nothing to do with mycorrhiza. Mycorrhiza is a symbiotic association between fungi and roots of higher plants, its commonest fungal partners being Glomus species.

Explanation: Biofertilisers are organisms that enrich the nutrient quality of the soil. Cyanobacteria fix atmospheric nitrogen in free-living or symbiotic form and Rhizobium fixes nitrogen in root nodules of legumes; Aspergillus and Rhizopus are not used as biofertilisers.

Explanation: Snails are garden pests that damage crops rather than aid them. Oscillatoria fixes nitrogen, Glomus improves phosphorus uptake, and earthworms aerate the soil and enrich it with castings, so all three are valued in organic practice.

Explanation: Mycorrhizal fungi of the genus Glomus greatly extend the absorbing surface of the root and supply phosphorus and other minerals and water, receiving sugars in exchange. Plants so colonised also show greater resistance to root pathogens and to drought.

Explanation: Soybean is a legume, and Rhizobium invades its root hairs and forms nodules within which it fixes atmospheric nitrogen, receiving carbohydrate from the host in return. Seed is therefore inoculated with the appropriate strain before sowing.