C
CraftExam
medium difficulty6 min

Microbes as Biofertilisers

Biofertilisers are microorganisms that enrich soil nutrients or improve nutrient availability to plants through processes such as nitrogen fixation, phosphate absorption and symbiotic association.

In one line: Rhizobium gives nitrogen to legumes; mycorrhiza helps roots absorb minerals and water.

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Understand it

  • Microbial biofertilisers improve soil fertility by natural biological mechanisms.
  • Rhizobium forms nodules in roots of leguminous plants and fixes atmospheric nitrogen into forms usable by plants.
  • Free-living bacteria such as Azotobacter and Azospirillum also contribute to nitrogen enrichment.
  • Cyanobacteria like Anabaena and Nostoc are important in paddy fields because they add organic matter and fix nitrogen.
  • Mycorrhiza is a fungal association with roots that improves absorption of phosphorus and water and can increase plant tolerance under stressful soil conditions.
  • Exams test microbe-crop association, nutrient improved and ecological advantage.

Example

Rhizobium in root nodules of pea or other legumes fixes atmospheric nitrogen. Anabaena in association with Azolla is a commonly cited example linked with nitrogen enrichment in rice fields.

Rule to remember

Key biological rule: biofertilisers increase nutrient availability through living microbial activity, especially nitrogen fixation and improved mineral absorption.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1Question: Name two biofertilisers and their roles.
  2. 2Answer: Rhizobium fixes atmospheric nitrogen in legume root nodules, while mycorrhiza improves phosphorus and water absorption by plant roots.

Method you can reuse

  1. 1Name the microbial group.
  2. 2State whether it is symbiotic or free-living.
  3. 3Mention the nutrient or absorption benefit.
  4. 4Give a crop or plant association.
  5. 5Add the ecological advantage if the question asks for importance.

Visual tip

No compulsory diagram is required, but a root nodule or mycorrhizal root sketch can show the structure-function relation between microbe and plant root.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

How does mycorrhiza help plants?

Think first, then show answer
Mycorrhiza helps plants because the fungal partner increases the root's absorbing surface and improves uptake of phosphorus and water. In return, the fungus receives food from the plant.

Common wrong answer

Mycorrhiza fixes atmospheric nitrogen in legume root nodules.

Don't mix this up

Students often write that biofertilisers directly act like chemical fertilisers. Biofertilisers are living organisms that improve nutrient availability through biological activity.

Exam tip

Always connect the biofertiliser with the nutrient benefit: Rhizobium for nitrogen in legumes, cyanobacteria for nitrogen in paddy fields, and mycorrhiza for phosphorus absorption.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as organism-role matching, advantages over chemical fertilisers, short notes on cyanobacteria in paddy fields, or assertion-reason on symbiosis.

Answer plan

Define biofertiliser, give named examples, state the plant association and nutrient role, then add that these microbes help maintain soil fertility through biological processes.
  1. 1

    Enough for 1 mark

    Biofertilisers are microorganisms that enrich soil nutrients or improve nutrient availability to plants through processes such as nitrogen fixation, phosphate absorption and symbiotic association.

  2. 2

    Add for 2 marks

    Example: Rhizobium in root nodules of pea or other legumes fixes atmospheric nitrogen. Anabaena in association with Azolla is a commonly cited example linked with nitrogen enrichment in rice fields.

  3. 3

    Add for 3 marks

    Microbial biofertilisers improve soil fertility by natural biological mechanisms.

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