Microbes as Biofertilizers — Revision Notes

⚡ 30-Second Revision

Biofertilizers: — Living microorganisms enhancing plant nutrient availability.
Nitrogen Fixers:

* Symbiotic: *Rhizobium* (legumes, root nodules), *Frankia* (non-legumes). * Free-living Aerobic: *Azotobacter*, *Beijerinckia*. * Free-living Anaerobic: *Clostridium*. * Associative Symbiotic: *Azospirillum* (grasses, cereals). * Cyanobacteria (BGA): *Anabaena*, *Nostoc* (paddy fields, aquatic). * Azolla: Aquatic fern with symbiotic *Anabaena azollae* (paddy fields).

Phosphorus Solubilizers (PSB/PSF): — *Bacillus*, *Pseudomonas*, *Aspergillus*, *Penicillium* (convert insoluble P to soluble P).
Mycorrhiza (VAM): — Fungi (*Glomus* spp.) symbiotic with roots, enhance water and phosphorus uptake.
Benefits: — Eco-friendly, reduce chemical fertilizer use, improve soil health, sustainable agriculture.

2-Minute Revision

Biofertilizers are microbial inoculants that boost plant growth by enhancing nutrient availability through natural processes. They are a sustainable alternative to chemical fertilizers. The main types include nitrogen-fixing microbes, phosphorus-solubilizing microbes, and mycorrhizal fungi.

Nitrogen fixers, like symbiotic *Rhizobium* in legumes or free-living *Azotobacter* and *Azospirillum* for non-legumes, convert atmospheric nitrogen into usable forms. Cyanobacteria are particularly effective nitrogen fixers in paddy fields.

Phosphorus solubilizers, such as *Bacillus* and *Aspergillus*, make insoluble soil phosphorus available to plants. Mycorrhizal fungi, especially VAM, form symbiotic associations with plant roots, significantly improving water and phosphorus absorption.

These microbes contribute to better soil structure, increased crop yield, and reduced environmental pollution, making them crucial for eco-friendly farming practices.

5-Minute Revision

Biofertilizers are living microbial formulations that, when applied to soil, seeds, or plant surfaces, colonize the rhizosphere and enhance nutrient availability to plants. They are a cornerstone of sustainable agriculture, offering an environmentally benign alternative to synthetic chemical fertilizers.

Key Mechanisms:

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Nitrogen Fixation: — Microbes convert atmospheric nitrogen ($N_2$) into ammonia ($NH_3$).

* Symbiotic: *Rhizobium* (forms root nodules in legumes like peas, beans), *Frankia* (non-leguminous plants like *Alnus*). * Free-living: *Azotobacter* (aerobic, for non-legumes), *Azospirillum* (associative symbiotic, for grasses/cereals), Cyanobacteria (*Anabaena*, *Nostoc* - especially in paddy fields).

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Phosphorus Solubilization: — Microbes (e.g., *Bacillus*, *Pseudomonas*, *Aspergillus*) release organic acids to convert insoluble soil phosphorus into soluble forms ($H_2PO_4^-$).
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Mycorrhizal Association: — Fungi (e.g., VAM like *Glomus*) form a symbiotic relationship with plant roots, extending hyphae to increase surface area for water and nutrient (primarily phosphorus) absorption.

Major Types and Examples:

Rhizobium: — For leguminous crops (pulses, groundnut).
Azotobacter/Azospirillum: — For non-leguminous crops (wheat, rice, maize, cotton).
Cyanobacteria (Blue-green algae): — For paddy fields (e.g., *Anabaena*, *Nostoc*).
Azolla: — An aquatic fern with symbiotic cyanobacteria, used in rice cultivation.
Vesicular Arbuscular Mycorrhiza (VAM): — For most crops, enhancing phosphorus uptake.

Advantages:

Reduce reliance on chemical fertilizers, lowering input costs.
Minimize environmental pollution (eutrophication, greenhouse gas emissions).
Improve soil health, structure, and water-holding capacity.
Enhance crop yield and quality sustainably.
Promote plant growth by producing growth-promoting substances.

NEET Focus: Remember specific examples, their primary functions, and the types of crops they benefit. Understand the distinction between symbiotic and free-living forms. Recognize the environmental benefits and their role in sustainable farming.

Prelims Revision Notes

Microbes as Biofertilizers: Key Points for NEET UG

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Definition: — Biofertilizers are living microbial inoculants that enrich soil nutrient quality by enhancing nutrient availability to plants. They are eco-friendly and sustainable.
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Primary Nutrients Targeted: — Nitrogen (N) and Phosphorus (P) are the most common, followed by Potassium (K) and micronutrients.
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Nitrogen Fixers: — Convert atmospheric $N_2$ to usable forms ($NH_3$).

* Symbiotic: * *Rhizobium*: Associated with leguminous plants (e.g., peas, beans, groundnut). Forms root nodules. Requires leghemoglobin for anaerobic conditions. * *Frankia*: Symbiotic with non-leguminous plants (e.

g., *Alnus*). * Free-living: * *Azotobacter*: Aerobic, found in soil, for non-leguminous crops (wheat, maize, cotton). * *Azospirillum*: Associative symbiotic, found in rhizosphere of grasses and cereals (e.

g., maize, sugarcane). * *Clostridium*: Anaerobic, free-living. * Cyanobacteria (Blue-green algae/BGA): Photosynthetic, free-living, fix nitrogen in aquatic environments (e.g., *Anabaena*, *Nostoc*).

Highly effective in paddy fields. Also add organic matter. * Azolla: An aquatic fern that harbors symbiotic *Anabaena azollae* in its leaves, used as a biofertilizer in rice cultivation.

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Phosphorus Solubilizing Microorganisms (PSM): — Convert insoluble phosphate compounds into soluble orthophosphates ($H_2PO_4^-$ and $HPO_4^{2-}$).

* Bacteria: *Bacillus*, *Pseudomonas*. * Fungi: *Aspergillus*, *Penicillium*. * Mechanism: Secrete organic acids (e.g., gluconic acid) and phosphatases.

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Mycorrhiza: — Symbiotic association between fungi and plant roots.

* Vesicular Arbuscular Mycorrhiza (VAM): Most common type. Fungi (e.g., *Glomus* species) form extensive hyphal networks. * Function: Greatly increase surface area for water and phosphorus absorption. Also provide protection against root pathogens.

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Benefits of Biofertilizers:

* Reduce reliance on chemical fertilizers. * Environmentally friendly (reduce pollution, eutrophication, greenhouse gas emissions). * Improve soil structure, fertility, and water-holding capacity. * Enhance plant growth and yield. * Cost-effective in the long run. * Promote sustainable agriculture.

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Key Distinction: — Biofertilizers facilitate nutrient availability; they do not directly supply bulk nutrients like chemical fertilizers. Their action is gradual.

Vyyuha Quick Recall

Nice Plants Make Really Awesome Crops!

Nice: Nitrogen Fixers (Rhizobium, Azotobacter, Azospirillum, Cyanobacteria)
Plants: Phosphorus Solubilizers (Bacillus, Aspergillus)
Make: Mycorrhiza (VAM)
Really: Rhizobium (for Legumes)
Awesome: Azotobacter / Azospirillum (for non-Legumes)
Crops: Cyanobacteria (for Paddy Crops)