Microbes as biofertilisers

What are biofertilisers?

In plain words: instead of pouring factory-made nitrogen and phosphorus onto a field, a farmer can use living microbes that pull nitrogen out of the air or fetch phosphorus from the soil for the crop.

Environmental pollution is a major concern today, and the use of chemical fertilisers to meet the ever-increasing demand for agricultural produce has contributed significantly to it. Because of the problems of overusing chemical fertilisers, there is large pressure to switch to organic farming, the use of biofertilisers (pages 157–158).

Nitrogen-fixing bacteria

• The nodules on the roots of leguminous plants are formed by the symbiotic association of Rhizobium. These bacteria fix atmospheric nitrogen into organic forms, which the plant uses as nutrient (page 158). See the root nodules of legumes in Class 11.
• Other bacteria fix atmospheric nitrogen while free-living in the soil, for example Azospirillum and Azotobacter, and so enrich the nitrogen content of the soil (page 158).

Mycorrhiza: fungi as biofertilisers

• Fungi also form symbiotic associations with plants, called mycorrhiza. Many members of the genus Glomus form mycorrhiza (page 158).
• The fungal symbiont absorbs phosphorus from the soil and passes it to the plant (page 158).
• Plants with such associations show other benefits too: resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth and development (page 158).

NCERT asks what advantage the fungus gets from this association (page 158). NCERT answers it in its chapter on organisms and populations: in mycorrhizae the fungi help the plant absorb essential nutrients from the soil, and the plant in turn provides the fungi with energy-yielding carbohydrates (NCERT Class 12 Biology, Chapter 11, Section 11.1.4, page 201). Both partners gain: it is a mutualism.

Cyanobacteria in paddy fields

• Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments; many of them can fix atmospheric nitrogen, for example Anabaena, Nostoc and Oscillatoria (page 158). Class 11 notes that some, such as Nostoc and Anabaena, fix nitrogen in specialised cells called heterocysts (NCERT Class 11 Biology, Chapter 2, page 13).
• In paddy fields, cyanobacteria serve as an important biofertiliser (page 158).
• Blue-green algae, another name for cyanobacteria (NCERT Class 11 Biology, Chapter 2, page 13), also add organic matter to the soil and increase its fertility (page 158).
• A number of biofertilisers are now available commercially in India, and farmers use them regularly to replenish soil nutrients and reduce dependence on chemical fertilisers (page 158).

Solved examples

Common mistakes to avoid

Quick revision summary

• Biofertilisers: organisms that enrich the nutrient quality of the soil; main sources bacteria, fungi and cyanobacteria; an answer to the pollution caused by overusing chemical fertilisers.
• Bacteria: Rhizobium in legume root nodules (symbiotic) fixes nitrogen into organic forms; Azospirillum and Azotobacter fix nitrogen free-living.
• Mycorrhiza: fungi such as Glomus absorb phosphorus and pass it to plants; plants also gain resistance to root-borne pathogens, tolerance to salinity and drought, and better growth.
• Cyanobacteria: autotrophic; many fix nitrogen (Anabaena, Nostoc, Oscillatoria); important in paddy fields; add organic matter.

Questions students ask

What is the difference between biofertilisers and biopesticides?

Biofertilisers feed a crop and, beyond the NCERT text, biopesticides protect it: NCERT defines biofertilisers as organisms that enrich the nutrient quality of the soil (page 158), while biopesticides are pesticides “derived from such natural materials as animals, plants, bacteria, and certain minerals”, the most widely used microbial ones being strains of Bacillus thuringiensis (US EPA, public domain). NCERT’s microbial biocontrol agents are therefore biopesticides (our reading). Root-zone fungi fall on either side: mycorrhizal fungi are biofertilisers (page 158), while Trichoderma is a biocontrol agent (page 157).

What are the two types of mycorrhiza, and which one is VAM?

Beyond the NCERT text, which names no types, mycorrhizae are of two main kinds, ectomycorrhizae and endomycorrhizae (MeSH, public domain), and VAM, vesicular-arbuscular mycorrhiza, is an endomycorrhiza. In an ectomycorrhiza, common on trees such as pines, the hyphae often sheathe the root and grow between its cortical cells; in an arbuscular mycorrhiza they enter those cells and branch into tree-like arbuscules, and ‘AM’ is now preferred to ‘VAM’ as some of these fungi form no vesicles (D.M. Sylvia, Pennsylvania State University, facts in our words). NCERT’s mycorrhizal genus belongs to the Glomeromycota (NCBI Taxonomy), fungi that “form arbuscular mycorrhizae with plant roots” (MeSH).

What is the difference between a biofertiliser and an organic manure such as vermicompost?

A biofertiliser is made of living organisms, while an organic manure, beyond the NCERT text, is plant or animal matter, often already broken down by microbes: NCERT defines biofertilisers as organisms that enrich the nutrient quality of the soil (page 158), and India’s Fertiliser (Control) Order, 1985, defines a biofertiliser by its living microbes and the farm work they do, whether fixing nitrogen, making phosphorus soluble or mobilising nutrients, while it lists composts such as vermicompost in a different class, organic fertilisers (facts in our words). By our reading, a manure mainly brings its nutrients with it, while a biofertiliser brings organisms that keep working in the soil.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Name two free-living nitrogen-fixing bacteria.

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   Azospirillum and Azotobacter (NCERT page 158).

2. Q2. Besides phosphorus, what benefits do plants with mycorrhiza show?

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   Resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth and development (NCERT page 158).

3. Q3. Name three nitrogen-fixing cyanobacteria.

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   Anabaena, Nostoc and Oscillatoria (NCERT page 158).