1. Introduction
Soil fertility is central to agriculture. Historically, farmers used compost, manure and crop rotations to maintain productive fields. The chemical fertilizer era (20th century) dramatically raised yields but produced environmental costs: nutrient runoff, groundwater pollution, loss of soil organic matter and reduced microbial biodiversity. Biofertilizers are microbial preparations that restore and improve natural nutrient cycles by using living microorganisms to make nutrients available to plants in biologically usable forms. They are a keystone of sustainable and eco-friendly agriculture.
Learning objectives
- Define biofertilizers and list major groups.
- Explain mechanisms: nitrogen fixation, phosphate solubilization, mycorrhizal uptake.
- Describe application methods and real-world uses.
2. Definition of Biofertilizers
Biofertilizers are preparations containing living microorganisms which, when applied to seeds, plant surfaces or soil, colonize the rhizosphere or the interior of the plant and promote growth by increasing supply or availability of primary nutrients to the host plant.
Important note: biofertilizers do not supply nutrients directly in the same way chemical fertilizers do; instead they transform, fix or mobilize nutrients so plants can take them up.
3. Historical background
The discovery of biological nitrogen fixation dates back to the late 19th century (Hellriegel & Wilfarth, 1886), who showed that legumes harbor bacteria that convert atmospheric N2 into ammonia. Commercial production of rhizobial inoculants began in the early 20th century. In India and other rice-growing countries, research in the 1950s–1970s established the use of blue-green algae and Azolla as biofertilizers for paddy systems. The recent push towards organic and low-input agriculture has renewed farmer and researcher interest in biofertilizers.
4. Importance of Biofertilizers
Biofertilizers contribute to:
- Sustainable nutrient supply: long-term maintenance of soil fertility.
- Environmental protection: reduced chemical runoff and eutrophication.
- Cost savings: lower fertilizer bills for smallholders.
- Improved soil health: enhanced organic matter decomposition and microbial activity.
- Crop resilience: improved tolerance to drought, salinity and some diseases.
5. Types of Biofertilizers
5.1 Nitrogen-fixing biofertilizers
Nitrogen is often the most limiting nutrient. Microbes that fix atmospheric N2 to ammonia include:
- Symbiotic nitrogen-fixers (e.g., Rhizobium)
- These bacteria form specialized structures called nodules on the roots of legumes. Inside nodules nitrogenase converts N2 into NH3. Leghemoglobin—an oxygen-binding protein—maintains a low-oxygen environment needed for nitrogenase to function.
Contribution: 50–200 kg N/ha/year depending on species and management. - Free-living (non-symbiotic) nitrogen-fixers (e.g., Azotobacter, Clostridium)
- These species fix nitrogen independently in soil. Their contribution is lower than symbiotic fixers but useful in neutral-to-alkaline soils (Azotobacter) or anaerobic niches (Clostridium).
- Associative (plant-associated) fixers (e.g., Azospirillum)
- They do not form nodules but colonize root surfaces and rhizosphere, improving root growth and enhancing N uptake—especially in cereals.
- Cyanobacteria (blue-green algae) (e.g., Anabaena, Nostoc)
- Photosynthetic N-fixers that form heterocysts (specialized N-fixing cells). Important in flooded rice paddies where they fix substantial amounts of N and contribute to soil organic matter.
- Azolla-Anabaena association
- Azolla, a small water fern, maintains an internal symbiosis with Anabaena, fixing nitrogen and acting as green manure for rice; often used in paddy rotations.
5.2 Phosphate-solubilizing microorganisms (PSM)
Most soil P is present in insoluble forms (bound to Ca, Fe or Al). PSM (e.g., Bacillus megaterium, Pseudomonas, Aspergillus) secrete organic acids (gluconic, citric, oxalic) and phosphatases that free phosphate ions (HPO42–, H2PO4–) for plant uptake. Improved P availability promotes root branching and energy metabolism.
5.3 Mycorrhizal biofertilizers
Mycorrhizae are mutualistic associations between fungal hyphae and plant roots. Two main types:
- Ectomycorrhizae: Hyphal sheath around roots (trees such as pines, oaks).
- Endomycorrhizae / Arbuscular mycorrhizae (AM): Hyphae penetrate root cortical cells forming arbuscules (common in many crop plants).
Mycorrhizae increase the effective absorptive surface of roots via hyphal networks, improving phosphorus, micronutrient and water uptake and providing protection against some pathogens.
5.4 Other specialized biofertilizers
- Potassium-solubilizers: Release K+ from minerals (e.g., Bacillus mucilaginosus).
- Zinc-solubilizers: Make Zn available from insoluble pools.
- Sulphur-oxidizers: (e.g., Thiobacillus) convert elemental sulphur to sulphate.
6. Mode of action of biofertilizers
Biofertilizers improve plant nutrition by several biological processes:
- Nitrogen fixation: N2 → NH3 via nitrogenase (sensitive to oxygen). Symbiotic systems use nodules and leghemoglobin to create microaerobic conditions.
- Phosphate solubilization: Organic acid secretion lowers pH and chelates cations, releasing soluble P.
- Mineral mobilization: Microbial weathering of K-bearing minerals and release of micronutrients.
- Phytohormone production: Many PGPR (plant growth-promoting rhizobacteria) produce auxins (IAA), gibberellins and cytokinins—stimulating root elongation and branching.
- Biocontrol and competition: Production of antibiotics, siderophores and enzymes that suppress pathogens or outcompete them for niches and nutrients.
7. Methods of application
Correct application ensures survival and efficacy of microbes. Common methods:
- Seed treatment: Seeds are coated with a slurry of inoculant (e.g., peat-based Rhizobium) just before sowing.
- Seedling root dip: Seedlings (e.g., rice, vegetables) are dipped into microbial suspension prior to transplanting.
- Soil application: Carrier-based inoculants mixed into soil or added with organic amendments.
- Green manuring: Growing Azolla or legume cover crops that are incorporated to add N and improve soil structure.
Best practices for farmers
- Use fresh inoculant within the recommended shelf-life and store under cool, dry conditions.
- Avoid mixing inoculants with pesticides or chemical fertilizers that can kill microbes immediately after application.
- Follow recommended doses for seed and soil; overuse does not increase benefits.
8. Advantages of biofertilizers
- Promote sustainable and resource-efficient farming.
- Reduce requirement for synthetic nitrogen and phosphorus fertilizers.
- Improve soil structure, organic matter and microbial diversity.
- Lower input costs and environmental footprint.
9. Limitations and constraints
- Variable field performance dependent on soil pH, temperature, moisture and crop species.
- Short shelf-life and sensitivity to high temperatures and UV light; need proper carriers (peat, lignite) and storage.
- Require farmer training for correct handling and application.
10. Applications in agriculture (crop-wise examples)
| Crop | Recommended biofertilizer | Major benefit |
|---|---|---|
| Pulses (e.g., chickpea, pigeon pea) | Rhizobium inoculation | Enhanced nodulation and N-fixation; higher protein yield |
| Cereals (wheat, maize) | Azospirillum / Azotobacter | Improved root growth and N availability |
| Rice | Cyanobacteria (BGA), Azolla | Supplementary N and organic matter in puddled soils |
| Horticultural crops (fruits, vegetables) | Arbuscular mycorrhizae (AM) | Better P uptake, healthier seedlings, stress tolerance |
Exam tip
Remember specific microbial examples: Rhizobium for legumes, Azotobacter for free-living N fixation, Azospirillum for cereals, BGAs (Anabaena, Nostoc) for paddy, and AM fungi for horticulture.
11. Future prospects
Research directions include:
- Genetic improvement and strain selection for higher efficiency and stress tolerance.
- Development of multi-strain consortia (consortium inoculants) combining N-fixers, P-solubilizers and PGPR for broad-spectrum benefits.
- Formulation advances (encapsulation, nano-carriers) to increase shelf-life and field survival.
- Integration with precision agriculture and remote sensing for targeted application.
12. Conclusion
Biofertilizers are critical tools for the transition to sustainable agriculture. When used appropriately—alongside organic amendments and good agronomic practices—they can reduce chemical dependency, improve soil health and maintain long-term productivity. Understanding their biology, correct handling and crop-specific use is essential for maximizing their benefits.
13. Diagram prompts and figure placeholders
Figure 1. Types of biofertilizers (flowchart)
Figure 2. Symbiotic nitrogen fixation in legume root nodule
Figure 3. Azolla-Anabaena association in a paddy field
Figure 4. Mycorrhizal association with plant root
