Bio-fertilizers

Bio-fertilisers are natural substances that contain living microorganisms which promote the growth of plants by increasing the supply or availability of primary nutrients to them. Unlike chemical fertilisers, which add nutrients directly to the soil, bio-fertilisers enhance the soil’s biological activity and fertility by establishing symbiotic or associative relationships with plants. They are an essential component of sustainable agriculture, reducing dependence on synthetic inputs while maintaining long-term soil health and productivity.

Definition and Concept

A bio-fertiliser may be defined as a preparation containing beneficial living microorganisms that, when applied to seed, plant surfaces, or soil, colonise the rhizosphere or the interior of the plant and promote growth by increasing the availability of nutrients such as nitrogen, phosphorus, and other essential elements.
These microorganisms function through various processes such as nitrogen fixation, phosphate solubilisation, potassium mobilisation, and production of growth-promoting substances like vitamins and hormones.

Types of Bio-fertilisers

Bio-fertilisers are classified based on the type of microorganisms they contain and the nutrient they help supply to plants.

1. Nitrogen-fixing Bio-fertilisersThese micro-organisms fix atmospheric nitrogen into forms available to plants. They may be symbiotic (living in association with plants) or asymbiotic (free-living).
   • Rhizobium:
     • Symbiotic bacteria living in root nodules of leguminous plants (peas, beans, clover).
     • Converts atmospheric nitrogen into ammonia, enriching soil fertility.
     • Each Rhizobium strain is specific to a particular legume species.
   • Azotobacter:
     • Free-living bacteria found in neutral or alkaline soils.
     • Beneficial for crops such as wheat, maize, cotton, and vegetables.
   • Azospirillum:
     • Associated with roots of cereals and grasses such as maize, rice, and sugarcane.
     • Enhances nitrogen uptake and secretes growth-promoting hormones.
   • Blue-Green Algae (Cyanobacteria):
     • Photosynthetic microorganisms (e.g., Anabaena, Nostoc) that fix atmospheric nitrogen.
     • Commonly used in paddy fields.
   • Azolla–Anabaena Association:
     • Azolla is a small aquatic fern containing the nitrogen-fixing cyanobacterium Anabaena azollae in its leaf cavities.
     • Widely used in rice cultivation, especially in South and Southeast Asia.
2. Phosphate-solubilising Bio-fertilisers (PSB)
   • These bacteria and fungi convert insoluble phosphates into soluble forms accessible to plants.
   • Examples: Bacillus megaterium, Pseudomonas striata, Aspergillus awamori, Penicillium digitatum.
   • Improve phosphorus availability and promote better root growth and flowering.
3. Potassium and Zinc Mobilising Bio-fertilisers
   • Some microorganisms release organic acids that solubilise potassium and zinc from soil minerals.
   • Examples: Frateuria aurantia (for potassium), Bacillus subtilis (for zinc).
   • These improve nutrient balance and enhance plant stress tolerance.
4. Mycorrhizal Bio-fertilisers
   • These consist of beneficial fungi that form symbiotic associations with plant roots, known as mycorrhizae.
   • Types: Ectomycorrhiza (external association) and Endomycorrhiza (internal penetration of root cells).
   • Improve absorption of phosphorus, zinc, and water; also enhance resistance to pathogens and drought.
5. Organic Decomposer Bio-fertilisers
   • Contain microorganisms like Trichoderma, Aspergillus, and Bacillus that decompose organic matter and convert it into humus.
   • Increase the availability of micronutrients and improve soil structure.

Mode of Action

Bio-fertilisers improve soil fertility and plant growth through several mechanisms:

• Biological Nitrogen Fixation: Conversion of atmospheric nitrogen (N₂) into ammonia (NH₃), which plants can absorb.
• Phosphate Solubilisation: Transformation of insoluble phosphates into soluble forms through acid production.
• Mineralisation: Decomposition of organic matter to release nutrients.
• Production of Growth Hormones: Microorganisms secrete substances such as indole acetic acid (IAA), gibberellins, and cytokinins, which stimulate root development.
• Disease Suppression: Some bio-fertilisers inhibit harmful soil pathogens through competition or production of antibiotics.

Method of Application

Bio-fertilisers can be applied to crops in several ways depending on the farming system and crop type:

• Seed Treatment: Seeds are coated with bio-fertiliser slurry before sowing, allowing early root colonisation.
• Soil Application: Bio-fertilisers are mixed with compost or soil and applied to the field before planting.
• Seedling Root Dip: In transplanted crops (e.g., rice), seedlings are dipped in a bio-fertiliser suspension before transplantation.
• Foliar Spray: In some cases, bio-fertilisers are sprayed directly on plant leaves for additional nutrient support.

Proper storage and application conditions are essential, as living microorganisms are sensitive to heat, light, and moisture.

Advantages of Bio-fertilisers

Bio-fertilisers offer several ecological and economic benefits over chemical fertilisers:

• Eco-friendly: Reduce environmental pollution and soil degradation.
• Cost-effective: Lower input costs for farmers compared to synthetic fertilisers.
• Improves Soil Fertility: Enhance microbial activity, nutrient cycling, and organic matter content.
• Increases Crop Yield: Promote better root growth and nutrient uptake.
• Sustainable Agriculture: Maintain long-term productivity without harming soil health.
• Reduced Dependence on Non-renewable Resources: Decrease reliance on fossil-fuel-based chemical fertilisers.

Limitations of Bio-fertilisers

Despite their advantages, bio-fertilisers face certain challenges:

• Limited Shelf Life: Microorganisms may lose viability if not stored properly.
• Sensitivity to Environmental Conditions: High temperatures, drought, and poor soil conditions can affect microbial survival.
• Slow Initial Response: Their effects are gradual compared to quick-release chemical fertilisers.
• Specificity: Some strains are crop- or soil-specific and may not work universally.
• Lack of Awareness: Farmers may be unfamiliar with correct application techniques.

Continuous research and awareness programmes are therefore necessary to maximise their effectiveness.

Major Bio-fertiliser-Producing Microorganisms and Their Applications

Government Initiatives and Research in India

India has promoted bio-fertilisers through various agricultural and environmental programmes. The National Bio-fertiliser Development Centre (NBDC) at Ghaziabad and Regional Bio-fertiliser Development Centres (RBDCs) under the Department of Agriculture and Farmers Welfare coordinate research, production, and training.
The Indian Council of Agricultural Research (ICAR) and National Centre of Organic Farming (NCOF) have developed efficient microbial strains and guidelines for standardisation and quality control.

Future Prospects

With the increasing demand for eco-friendly agricultural inputs, bio-fertilisers represent the future of sustainable farming. Research is advancing towards:

• Development of multi-strain consortia combining nitrogen fixers, phosphate solubilisers, and plant growth promoters.
• Use of bio-encapsulation and carrier technologies for improved shelf life.
• Integration of bio-fertilisers with organic and precision farming systems.