Microbes as Biofertilisers — Revision Notes

⚡ 30-Second Revision

Biofertilisers: — Living microbes enhancing nutrient availability.
Nitrogen Fixers:

- Symbiotic: *Rhizobium* (legumes, root nodules), *Frankia* (non-legumes, *Alnus*). - Free-living: *Azotobacter* (aerobic), *Clostridium* (anaerobic), *Azospirillum* (associative), Cyanobacteria (*Anabaena*, *Nostoc* - paddy fields).

Phosphorus Solubilisers (PSB): — *Bacillus*, *Pseudomonas* (secrete organic acids).
Mycorrhizae: — Symbiotic fungi-root association.

- VAM (Endomycorrhizae): Fungal hyphae penetrate root cells (arbuscules, vesicles); enhance P, Zn, Cu uptake. - Ectomycorrhizae: Fungal sheath around root, between cells.

Benefits: — Reduce chemical fertiliser use, improve soil health, enhance plant growth, eco-friendly.

2-Minute Revision

Biofertilisers are microbial inoculants that boost plant growth and soil fertility naturally. They are categorized mainly by their function: nitrogen fixation, phosphorus solubilisation, and mycorrhizal associations.

Nitrogen fixers convert atmospheric nitrogen into usable forms. *Rhizobium* forms symbiotic nodules with legumes, while *Azotobacter* and *Azospirillum* are free-living or associative for non-legumes.

Cyanobacteria like *Anabaena* are crucial for paddy fields. Phosphorus Solubilising Bacteria (PSB) such as *Bacillus* and *Pseudomonas* make insoluble phosphorus available to plants by secreting organic acids.

Mycorrhizal fungi, especially Vesicular Arbuscular Mycorrhizae (VAM), form symbiotic relationships with plant roots, vastly increasing the surface area for water and nutrient (especially phosphorus) absorption.

These biological agents are vital for sustainable agriculture, reducing reliance on chemical fertilisers, improving soil structure, and enhancing overall plant health.

5-Minute Revision

Biofertilisers are a cornerstone of sustainable agriculture, utilizing beneficial microorganisms to enhance nutrient availability for plants. They are a greener alternative to chemical fertilisers, mitigating environmental pollution and improving soil health. The primary mechanisms involve nitrogen fixation, phosphorus solubilisation, and enhanced nutrient uptake through symbiotic associations.

Nitrogen Fixation: Atmospheric nitrogen ($N_2$) is abundant but unusable by plants. Nitrogen-fixing microbes convert it to ammonia ($NH_3$).

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Symbiotic: — *Rhizobium* forms root nodules in legumes (e.g., peas, beans), exchanging fixed nitrogen for plant carbohydrates. *Frankia* does this for non-legumes like *Alnus*.
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Free-living: — *Azotobacter* (aerobic) and *Clostridium* (anaerobic) live independently in soil. *Azospirillum* is associative with grass roots. Cyanobacteria (*Anabaena*, *Nostoc*) are vital in waterlogged paddy fields.

Phosphorus Solubilisation: Soil often contains insoluble phosphorus. Phosphorus Solubilising Bacteria (PSB) like *Bacillus* and *Pseudomonas* secrete organic acids (e.g., gluconic acid) that convert insoluble phosphorus into soluble forms, making it accessible to plants.

Mycorrhizae: These are symbiotic associations between fungi and plant roots. The fungal hyphae extend far into the soil, increasing the root's absorptive surface area, particularly for immobile nutrients like phosphorus, zinc, and copper. The plant provides carbohydrates to the fungus.

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Ectomycorrhizae: — Fungi form a sheath around the root and penetrate between cortical cells.
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Endomycorrhizae (VAM): — Fungi penetrate *into* cortical cells, forming arbuscules (nutrient exchange) and vesicles (storage). VAM are widespread in agricultural crops.

Benefits: Biofertilisers reduce the need for synthetic fertilisers, improve soil structure, enhance water retention, increase crop yield, and can boost plant resistance to diseases. They are crucial for organic farming and ecological balance. Remember to differentiate between the specific roles and target crops/environments for each type of biofertiliser for NEET questions.

Prelims Revision Notes

Microbes as Biofertilisers: NEET Quick Recall

1. Definition: Living microorganisms that enrich nutrient quality of soil and promote plant growth.

2. Types based on Nutrient Mobilization:

* Nitrogen-fixing Biofertilisers: Convert atmospheric $N_2$ to $NH_3$. * Symbiotic: * *Rhizobium*: Associated with leguminous plants (e.g., pulses, soybean, groundnut). Forms root nodules. Requires leghemoglobin for anaerobic conditions.

* *Frankia*: Associated with non-leguminous plants (e.g., *Alnus*). Forms root nodules. * Free-living/Non-symbiotic: * Aerobic: *Azotobacter*, *Beijerinckia*. * Anaerobic: *Clostridium*, *Rhodospirillum*.

* Associative Symbiotic: *Azospirillum* (with roots of grasses like maize, wheat). * Cyanobacteria (Blue-Green Algae): *Anabaena*, *Nostoc*, *Oscillatoria*. Important in paddy fields (waterlogged conditions).

Photosynthetic, fix N2. * Phosphorus Solubilising Biofertilisers (PSB): Convert insoluble soil phosphorus into soluble forms. * Bacteria: *Bacillus*, *Pseudomonas*, *Micrococcus*. * Fungi: *Aspergillus*, *Penicillium*.

* Mechanism: Secrete organic acids (e.g., gluconic acid, lactic acid) and phosphatases. * Potassium Mobilising Biofertilisers (KMB): Mobilise fixed potassium. * Bacteria: *Bacillus mucilaginosus*, *Acidothiobacillus ferrooxidans*.

3. Mycorrhizae (Fungus-Root Association): Symbiotic association between fungi and plant roots. * Role: Enhance absorption of water and mineral nutrients, especially phosphorus, zinc, copper.

* Types: * Ectomycorrhizae: Fungal hyphae form a dense sheath around root surface and penetrate *between* cortical cells (Hartig net). Common in forest trees (e.g., pine, oak). * Endomycorrhizae (Vesicular Arbuscular Mycorrhizae - VAM): Fungal hyphae penetrate *into* cortical cells, forming arbuscules (nutrient exchange) and vesicles (storage).

Most common type, found in 80% of plant species, including many crops.

4. Benefits of Biofertilisers:

* Reduce reliance on chemical fertilisers (up to 25-30% reduction). * Improve soil structure, texture, and water-holding capacity. * Enhance nutrient uptake efficiency. * Increase crop yield and quality. * Promote plant growth by producing phytohormones. * Help in disease suppression (some PGPR). * Environmentally friendly, non-polluting.

5. Key Distinctions:

* Biofertilisers: Living, gradual action, eco-friendly, improve soil health. * Chemical Fertilisers: Synthetic, rapid action, potential pollution, can degrade soil health.

6. Application Methods: Seed treatment, soil application, seedling root dip.

Vyyuha Quick Recall

To remember key Nitrogen-fixing Biofertilisers: Really Awesome Algae Can Always Fix Nitrogen.

Rhizobium (Symbiotic, Legumes)
Azotobacter (Free-living, Aerobic)
Azospirillum (Associative, Grasses)
Cyanobacteria (e.g., Anabaena, Nostoc - Paddy fields)
Frankia (Symbiotic, Non-legumes)
Nitrogen (The element they fix!)