Biofertilizers are formulations containing living microorganisms, such as bacteria, fungi, or algae, applied to seeds, soil, or plant surfaces to enhance the supply or availability of primary nutrients for the host plant. These microbial inoculants colonize the rhizosphere and leverage natural biological processes to improve plant nutrition. They represent an alternative to conventional chemical inputs in modern agriculture, working to build a self-renewing nutrient system. This approach supports plant growth while fostering a healthier, more biologically active soil environment.
Categorization by Function
Biofertilizers are categorized based on their primary biological function in the soil. One prominent group is the nitrogen-fixing organisms, which includes symbiotic bacteria like Rhizobium that form specialized nodules on the roots of legumes. Other nitrogen fixers, such as Azotobacter and Azospirillum, are free-living or associative, fixing atmospheric nitrogen outside of a dedicated plant structure. These microbes convert inert nitrogen gas into biologically useful forms, specifically ammonia.
Another major category comprises phosphate-solubilizing and mobilizing organisms, which address phosphorus locked in insoluble mineral forms unavailable to plants. Bacteria from the Bacillus and Pseudomonas genera, along with certain fungi, release organic acids and enzymes to free this fixed nutrient. Arbuscular Mycorrhizal Fungi (AMF) are mobilizing organisms that extend thread-like hyphae far into the soil, acting as an extension of the root system to transport immobile nutrients like phosphorus and zinc back to the plant. The third functional group is Plant Growth Promoting Rhizobacteria (PGPR), which are diverse microbes that enhance plant health through various mechanisms beyond simple nutrient delivery, focusing on stimulating overall plant growth.
Mechanisms of Plant Growth Promotion
The beneficial effects of biofertilizers stem from precise biochemical processes carried out by the microorganisms. Nitrogen fixation is facilitated by the nitrogenase enzyme complex, which reduces dinitrogen gas ($\text{N}_2$) into ammonia ($\text{NH}_3$), a form the plant can readily assimilate. Because the nitrogenase enzyme is highly sensitive to oxygen, symbiotic fixers like Rhizobium rely on the plant-produced protein leghemoglobin within the root nodules. This protein scavenges oxygen and maintains the necessary anaerobic environment for the reaction to occur.
Phosphate solubilization occurs when microbes secrete organic acids, which chelate the cations binding the phosphate in soil minerals. This chemical action releases the phosphate ion into the soil solution, making it available for root uptake. PGPR strains promote plant growth by synthesizing and releasing various phytohormones. Indole-3-acetic acid (IAA), a type of auxin, is commonly produced and stimulates cell elongation and division, leading to increased lateral root formation and overall root surface area. Other hormones, like gibberellins and cytokinins, also contribute by promoting shoot growth and delaying leaf senescence.
Application and Storage
Applying biofertilizers requires specific techniques to ensure the survival and effective colonization of the microorganisms. One common method is seed treatment, where seeds are coated with a slurry made by mixing the microbial inoculant with a sticker agent, such as a sugar solution, before planting. For transplanted crops, the seedling root dip method is employed, where the roots are immersed in a biofertilizer suspension to allow the microbes to adhere to the root surface. Biofertilizers can also be applied directly to the soil, often by mixing the granular or liquid formulation with compost or farmyard manure and then broadcasting or applying it in furrows.
Since biofertilizers contain live cultures, their efficacy is highly dependent on proper handling and storage. They must be stored in cool, dry, and dark conditions, generally within a temperature range of $4^\circ\text{C}$ to $20^\circ\text{C}$. Exposure to high temperatures (above $35^\circ\text{C}$) or direct ultraviolet (UV) light can quickly kill the beneficial microbes. The shelf life is limited, typically lasting around six months for carrier-based powder formulations, although liquid formulations often contain cell protectants that can extend the viability of the organisms up to two years.
Comparison to Synthetic Fertilizers
Biofertilizers differ significantly from conventional synthetic inputs, primarily in their mode of action and long-term consequences for the environment and soil health. Synthetic fertilizers deliver readily available nutrients directly to the plant, offering a rapid but temporary boost. Biofertilizers work indirectly by building up the soil’s natural capacity to supply nutrients over time. A major environmental distinction lies in the air and water pollution associated with synthetic nitrogen fertilizers, which can result in nutrient runoff and leaching into waterways, contributing to eutrophication. The production and application of these chemicals also release nitrous oxide ($\text{N}_2\text{O}$), a potent greenhouse gas, into the atmosphere.
In contrast, biofertilizers promote sustainability by reducing the dependency on synthetic chemicals. Their continuous use improves the physical and biological properties of the soil, enhancing soil structure, increasing water-holding capacity, and improving soil aeration. The addition of beneficial microbes also increases the overall microbial diversity of the soil, fostering a more resilient and balanced agro-ecosystem. While the immediate yield increase may be less dramatic than from synthetic inputs, the long-term benefits of enhanced soil structure and reduced need for costly chemical applications represent a more enduring cost-benefit proposition.