Microbes as Biofertilisers

The Big Idea: Why Biofertilisers Matter

Chemical fertilisers have boosted crop yields for decades, but they come with a heavy price — they degrade soil over time, kill beneficial soil organisms, and pollute water bodies. Biofertilisers offer a natural, sustainable alternative. They are living microorganisms that, when applied to seeds, plant surfaces, or soil, colonise the plant's environment and promote growth by increasing the supply of essential nutrients. The key insight is that these microbes do not replace fertilisers entirely; they supplement them by making nutrients more available to the crop.

What Are Biofertilisers?

Biofertilisers are preparations containing live or latent cells of efficient strains of nitrogen-fixing, phosphate-solubilising, or cellulolytic microorganisms. They are used for application to seeds, soil, or composting areas with the objective of increasing the number of such microorganisms and accelerating those microbial processes that augment the availability of nutrients that plants can easily assimilate.

The Main Types of Biofertilisers

The NCERT text focuses on three major categories, each with a distinct mechanism of action.

1. Symbiotic Nitrogen-Fixing Bacteria: Rhizobium

Rhizobium is the classic example. These bacteria live in the root nodules of leguminous plants (peas, beans, groundnut, etc.). They form a mutually beneficial (symbiotic) relationship: the plant provides the bacteria with shelter and food (carbohydrates), and the bacteria convert atmospheric nitrogen (N2) into ammonia (NH3), which the plant can use to make proteins and other nitrogenous compounds.

2. Free-Living Nitrogen-Fixing Bacteria: Azospirillum and Azotobacter

Not all nitrogen-fixing bacteria need a plant host. Some live freely in the soil.

• Azospirillum: This bacterium lives in close association with the roots of many non-leguminous plants (like cereals — wheat, maize, rice). It is called a "rhizosphere bacterium" because it colonises the root surface and the soil immediately surrounding the roots. It fixes atmospheric nitrogen and also produces growth-promoting substances (phytohormones) that stimulate root development.
• Azotobacter: This is another free-living, aerobic nitrogen-fixer found in neutral to alkaline soils. It is less efficient than Rhizobium but can still contribute significant amounts of nitrogen to crops like cotton, sugarcane, and vegetables.

3. Cyanobacteria (Blue-Green Algae)

These are photosynthetic prokaryotes that can fix atmospheric nitrogen. They are especially important in paddy (rice) fields.

• Examples: Anabaena, Nostoc, Oscillatoria.
• How they work: They grow naturally in the waterlogged conditions of rice paddies. They fix nitrogen and also add organic matter to the soil through their biomass after they die and decompose.
• Practical use: In many parts of India, farmers deliberately inoculate their rice fields with a mixture of cyanobacteria (often called "algal biofertiliser").

Mycorrhiza: The Fungal Partner

Mycorrhiza is a symbiotic association between certain fungi and the roots of most higher plants. The fungi help the plant absorb phosphorus and other minerals from the soil, especially from poor or dry soils. In return, the plant supplies the fungus with carbohydrates.

• Types: The most common type for agricultural use is Vesicular-Arbuscular Mycorrhiza (VAM) . The fungus penetrates the root cells and forms tree-like structures (arbuscules) and storage vesicles.
• Benefits:
  • Increases the plant's uptake of phosphorus, zinc, copper, and other nutrients.
  • Improves the plant's resistance to root-borne pathogens.
  • Enhances the plant's tolerance to drought and salinity.
  • Reduces the need for chemical phosphate fertilisers.

Advantages of Using Biofertilisers

The textbook implicitly highlights several key advantages over chemical fertilisers:

• Environmentally friendly: They do not cause pollution or degrade soil health.
• Cost-effective: They are cheaper to produce and apply than chemical fertilisers.
• Sustainable: They improve soil structure, organic matter content, and the overall biological activity of the soil.
• Renewable: They are living organisms that can multiply and sustain themselves.
• Reduce dependency: They lessen a farmer's reliance on expensive, imported chemical fertilisers.

A Quick Reference Table