The review defines biofertilizers as products containing living microorganisms such as nitrogen-fixing bacteria, phosphorus-solubilizing bacteria, and mycorrhizal fungi. Unlike chemical fertilizers, which supply nutrients directly and can pollute water, air, and soil when overused, biofertilizers work through symbiotic relationships with plants. Microbial strains employ diverse mechanisms to optimize nutrient absorption: nitrogen fixation, potassium and phosphorus solubilization, secretion of phytohormones, synthesis of compounds that inhibit phytopathogens, protection against abiotic and biotic stresses, and even detoxification of subterranean pollutants. The authors note that most subsistence and commercial farmers are increasingly attracted to biofertilizers rather than depending solely on synthetic inputs.

The urgency behind the research is demographic. The world population is projected to reach roughly 9.7 billion by 2050, and conventional agriculture — reliant on extensive use of synthetic fertilizers and pesticides — is straining ecosystems through the pollution of water reservoirs, air, and soil. Biofertilizers, applied as seed or soil inoculants, multiply in the soil and actively contribute to the nutrient cycle, enhancing the growth of crops including rice, pulses, millets, cotton, sugarcane, and vegetables. The review frames them as a crucial element of organic farming that maintains a nutrient-rich soil environment while supporting long-term soil health.

Nitrogen is the primary limiting nutrient for plant growth, and plants cannot convert atmospheric N2 into usable forms on their own. The review details how nitrogen-fixing bacteria solve this problem through the nitrogenase enzyme complex: dinitrogenase reductase provides the electrons that dinitrogenase uses to convert atmospheric nitrogen into ammonia by reduction. Symbiotic fixers, members of the Rhizobiaceae family, form mutually beneficial associations with leguminous plants, while non-symbiotic microbes such as Cyanobacteria, Azospirillum, and Azotobacter exist in free-living and endophytic forms. Because the enzymes bond with oxygen and lose activity, the process requires careful oxygen regulation within the nodule environment.

Phosphorus presents a different challenge: its rock reserves are diminishing, with roughly 16 million metric tons extracted annually, primarily from North Africa, and projections suggest phosphate rock could be exhausted within 100 years. Phosphorus-solubilizing bacteria such as Pseudomonas, Bacillus, Rhizobium, and Burkholderia dissolve insoluble phosphates — di- and tri-calcium phosphates, hydroxyapatites, and rock phosphates — by producing organic acids that lower soil pH, disintegrating phosphate compounds and liberating plant-available phosphorus. The review reports that these bacteria can dissolve around 50–60% of bound phosphorus in soil. Potassium-solubilizing microorganisms, including Frateuria aurantia, similarly mobilize otherwise inaccessible potassium, which regulates stomatal function and photosynthesis. Notably, co-inoculation of Azospirillum brasilense and Rhizobium meliloti has been shown to improve grain production and nitrogen, phosphorus, and potassium concentrations in wheat.

On the physical side, biofertilizers improve soil structure by promoting stable soil aggregates that increase porosity, aeration, and water infiltration. Mycorrhizal fungi form symbiotic associations with plant roots, encouraging widespread root development that facilitates efficient water absorption and increases drought resistance. The review documents improvements in soil bulk density — as organic matter acts as a binding agent, pore spaces form and compaction decreases — along with soil moisture retention and soil temperature buffering, since humus created through microbial decomposition acts as an insulating layer that mitigates temperature extremes. Even soil color improves indirectly, as higher humus concentrations give soil a deep, dark hue.

Chemically, the review found that biofertilizer application caused a slight decrease in alkalinity and an increase in organic carbon content compared to untreated soil. Soil nitrogen content rises in the presence of Azospirillum and Rhizobia, which fix atmospheric nitrogen; four decades of research on Azospirillum-plant interactions have revealed multiple pathways through which the bacterium promotes growth. Mycorrhiza and phosphate-solubilizing bacteria enhance phosphorus availability, while arbuscular mycorrhizal fungi absorb and transport potassium from soil to plant roots. Applying Trichoderma to soil resulted in greater organic carbon increases than seed inoculation with phosphate-solubilizing bacteria in soybean and maize, underscoring the chemical transformation biofertilizers can drive.

Biologically, biofertilizers reshape the soil microbiome by introducing beneficial microorganisms that boost microbial diversity, promote disease suppression, and improve nutrient cycling. Populations of bacteria, fungi, and actinomycetes all respond: nitrogen-fixing and phosphate-solubilizing bacteria directly increase bacterial activity, mycorrhizal fungi foster favorable symbiotic connections with plant roots, and actinomycetes — particularly Streptomyces species — reduce soil-borne diseases by producing antibiotics. Microorganisms like Trichoderma and Bacillus stop the growth of soil-borne pathogens, while actively decomposing organic residues to release vital nutrients and enhance soil structure.

The review is candid about obstacles. Widespread adoption has been limited by the difficulty of replicating positive effects across environments with varying conditions. Key barriers include limited understanding of biofertilizers among farming communities, insufficient promotion by agricultural extension workers, scarcity of appropriate carrier materials, inadequate storage facilities that allow contamination, and extreme climatic conditions that cause inconsistent effectiveness. Poor labeling — missing expiry dates and unnamed microbial strains — further erodes credibility, and most biofertilizers exhibit selectivity in their effects, meaning performance varies by crop and soil type.

Still, the authors conclude that biofertilizers offer a sustainable solution for enhancing soil quality and promoting long-term soil sustainability across all three property domains. By improving microbial diversity, nutrient accessibility, and soil structure, they reduce the necessity for artificial fertilizers and lessen agriculture’s environmental footprint. Overcoming challenges such as crop-specific characteristics, limited shelf life, compatibility with existing farming procedures, and environmental sensitivity will require continued research and education — but the payoff, the review argues, is resilient, well-balanced agro-ecosystems capable of sustaining food production for decades to come.

Subject of Research: Biofertilizers and their effects on soil physical, chemical, and biological properties

Article Title: Biofertilizers: A sustainable strategy for enhancing physical, chemical, and biological properties of soil

Article References: Ghimirey, V., Chaurasia, J., Acharya, N., Dhungana, R., & Chaurasiya, S. (2024). Biofertilizers: A sustainable strategy for enhancing physical, chemical, and biological properties of soil. Innovations in Agriculture, 7, 1-11. https://doi.org/10.3897/ia.2024.128697

Image Credits: AI Generated

DOI: 10.3897/ia.2024.128697

Keywords: biofertilizers, soil health, nitrogen-fixing bacteria, phosphorus solubilization, potassium solubilization, mycorrhizal fungi, sustainable agriculture, soil microbiome, organic farming, soil organic carbon, plant growth-promoting bacteria, soil properties