Test: Microbes as Biofertilizers (NCERT) - NEET MCQ

Organic farming includes serval methods to enhance soil fertility. In such farming, methods of biological origin are used, e.g., biopesticides, biogertilisers, IPM (Integrated Pest Management) green manure, bioherbicides, keeping pess and pathogens under control, etc. chemical fertilisers are not used in organic farming.

Azolla pinnata is a small free floating freshwater fern which multiplies rapidly, doubling every 5-7 days. The fern can live with Anabaena symbiotically in rice field to fix atmospheric nitrogen. It does not interfere with their growth. In some South-East Asian countries, especially China, the rice fields are regularly provided with Azolla.

Symbiotic nitrogen fixing bacteria form a mutually beneficial association with the plants. The bacteria obtain food and shelter from plants. In return, they give a part of their fixied nitrogen to the plants. The most important of the symbiotic nitrogen fixing bacteria is Rhizobium. it forms nodule on the roots of legume plants. They develop the ability to fix nitrogen only when they are present inside the root nodules. in the nodule cells, bacteria (bacteroids) lie in groups surrounded by membrane of the host which is lined by a pink-red pigment called leghaemoglobin.

Biofertilisers are microorganisms which bring about nutrient enrichment of soil by enhancing the availability of nutrients to crops. Microorganisms which act as biofertilisers are bacteria, cyanobacteria (blue green algae) and mycorrhizal fungi. Bacteria and cyanobacteria have the property of nitrogen fixation while mycorrhizal fungi preferentially withdraw minerals from organic matter for the plant with which they are associated. They maximize ecological benefits and minimize environmental hazards.

Chemical fertilisers are being used in increasing amounts to increase output in high yielding varieties of crop plants. However, chemical fertilisers cause pollution of water bodies as well as groundwater, besides getting stored in crop plants. They are expensive and also require a lot of energy resources in their manufacture. 

Several fungi are known to form symbiotic association with plants, i.e., Mycorrhiza. The most common fungi partners of mycorrhiza are Glomus species. Mycorrhiza roots show a sparse or dense wooly growth of fungus hyphae on their surface. The fungus remains restricted to the cortex of the root. The vascular strands and growing parts are not affected. The fungal symbionts in these associations absorb phosphorus from soil and pass it to the plant. Plants having such associations show other resistance to root-borne pathogens and drought and an overall increase in plant growth and development.

Cyanobacteria are photosynthetic, Gram negative prokaryotes. They have chlorophyll a and are probably major primary producers in the world's oceans. Hence, they are autotrophs.

The incorrect statement is Glomus is not a genus of cyanobacteria but a type of fungus that forms symbiotic relationships (mycorrhiza) with plants, helping in the absorption of phosphorus. Cyanobacteria, on the other hand, like Anabaena and Nostoc, are known for nitrogen fixation, especially in environments like paddy fields.

Anabaena azollae, a cyanobacterium that lives in symbiotic association with the free-floating water fern Azolla. Anabaena azollae can grow Photo-autotrophically and fixes atmospheric nitrogen. The inoculation of cyanobacteria in rice crops significantly influences the growth of rice crop by secretion of ammonia in flood water.

Nostoc and Oscillatoria are nitrogen fixing cyanbacteria. They add organic matter as well as extra nitrogen to the soil. Cyanobacteria are very important and low-cost biofertilisers.

Bacteria, such as Azotobacter is free living  and can fix atmospheric nitrogen in the soil, thus enriching nitrogen content of the soil.

Mycorrhiza is a symbiotic association of a fungus with the roots of a higher plant. Ectomycorrhiza, which lie on surface of the root, help in direct absorption of minerals from the soil over a large area. Rhizobium is a Parasite and Azospirillum is a free living nitrogen fixing bacteria. Methanogenic bacteria help in production of biogas

• Mycorrhizae helps in phosphorus uptake from the soil.
• Bacillus thuringiensis is a bioinsecticide.
• Leghaemoglobin is present in the root nodules in leguminous plants.
• Azadiractin is a secondary metabolite present in the neem seeds and it acts as a biopesticide.

Frankia, a nitrogen fixing mycelial bacterium(actinomycete), is associated symbiotically with roots nodules of several non - legume plants like Alnus(Alder), Myrica, rubus etc.

Cotton is a dicotyledonous crop. The best biofertiliser for cotton is Bacillus cereus and Azotobacter chroococcum. These are free-living N₂ fixing bacteria and enhance the fertility of the soil.