A biofertilizer is a product that contains living microorganisms that have positive effects on the growth and development of plants (Mushtaq et al. 2021). Microbial strains employ diverse mechanisms to optimize nutrient absorption, enhance soil fertility, and boost crop yields. These mechanisms include nitrogen fixation, potassium and phosphorus solubilization, secretion of phytohormones, synthesis of compounds that inhibit phytopathogens, protection against abiotic and biotic stresses, and detoxification of subterranean pollutants (Bhardwaj et al. 2014).

At present, the world population is continuing to grow, and it is projected that by 2050, there will be roughly 9.7 billion individuals inhabiting the planet (Ehrlich and Harte 2015). The rapid expansion is closely linked to the extensive development of industry, cities, and agriculture. Given the increasing global population, traditional agriculture is crucial to fulfilling the nutritional needs of humanity (Chaurasia et al. 2020). It is projected that by 2020, the demand for food grain will reach 321 million tons, and traditional agriculture is necessary to ensure countries can produce enough food to sustain themselves (Gizaki et al. 2015). Conventional agricultural methods primarily rely on the extensive use of synthetic fertilizers and pesticides to provide plant nourishment and combat diseases (Van Bruggen et al. 2016). Applying these chemical inputs carefully and thoughtfully has undeniable benefits, not only for enhancing plant growth, crop yield, and quality but also for increasing farmers’ income (Emmanuel et al. 2016). Regrettably, the escalated utilization of synthetic resources may present a significant risk to the ecological system by polluting water, air, and soil (Wassie 2020).

Biofertilizers are a crucial element of organic farming that help maintain a nutrient-rich soil environment (Sinha et al. 2010). When biofertilizers are used as seed or soil inoculants, they undergo multiplication and actively contribute to the nutrient cycle, ultimately enhancing crop productivity (Kumar et al. 2022). Biofertilizers enhance soil fertility through nitrogen fixation from the atmosphere and the solubilization of insoluble phosphates. Additionally, they generate chemicals that promote plant growth inside the soil (Mazid and Khan 2015). Biofertilizers are beneficial in enhancing the growth of several agricultural plants, including rice, pulses, millets, cotton, sugarcane, and vegetable crops (Rashid et al. 2016; Singh et al. 2014).

Nitrogen is the primary limiting nutrient for plant growth. Plants are unable to decrease atmospheric N₂, hence they need externally obtained nitrogen for their growth and development (Gupta et al. 2012). The capacity of nitrogen-fixing (NF) bacteria to establish a mutually beneficial connection with leguminous plants and convert atmospheric nitrogen into a usable form has been utilized in agriculture to fulfill the nitrogen needs of these plants as illustrated in Fig. 1 (Sharma et al. 2023). Nitrogen-fixing microorganisms employ an enzyme complex called Nitrogenase to transform atmospheric nitrogen into ammonia (Barney 2020). Nitrogen fixers can be categorized into two groups: symbiotic and non-symbiotic. The Rhizobiaceae family members that form symbiotic associations with leguminous plants are classified as symbiotic organisms (Díez-Méndez and Menéndez 2021). Conversely, non-symbiotic microbes, such as Cyanobacteria, Azospirillum, Azotobacter, etc., exist in both free-living and endophytic forms (Chatterjee et al. 2019). The process of N₂ fixation is carried out by the nitrogenase enzyme complex. Dinitrogenase reductase provides the electrons, which dinitrogenase utilizes to convert N₂ into NH₃ by reduction. Enzymes form a bond with oxygen, which causes them to lose their activity (Mahanty et al. 2017).

Figure 1. 

Schematic diagram representing the process of symbiotic nitrogen fixation. Source: Masson-Boivin and Sachs 2018.

Phosphorus is recognized as a crucial resource for the economy, and its reserves are diminishing (Cordell and White 2013). Annually, a total of 16 million metric tons of phosphorus is extracted, primarily from North Africa. This extraction process has challenges associated with transporting the resource across vast distances and dealing with its volatile pricing (Jupp et al. 2021). According to Darwish et al. (2017), it is projected that phosphate rock will be exhausted within 100 years. Simultaneously, the extensive utilization of phosphate fertilizers causes the dissemination of this element in the surroundings, potentially leading to the eutrophication of water reservoirs. Efficient phosphorus management should incorporate recycling, aligning with the principles of a circular economy.

Phosphorus solubilizing biofertilizer (PSB) is applied to enhance the availability and accessibility of phosphorus, hence stimulating the growth and development of plants. PSB, or phospho-bacterin, is a substance that aids in the solubilization of insoluble phosphates such as di- and tri-calcium phosphates, hydroxyapatites, and rock phosphates (Abbey et al. 2019). This process makes these phosphates more easily accessible to plants (Mahdi et al. 2010). The solubilization effect is accomplished through the creation of organic acids by phospho-bacterin, which results in a reduction in soil pH. This, in turn, causes the disintegration of phosphate compounds and the subsequent liberation of abundant phosphorus that may be utilized by plants (Sharma 2002; Glick 2012; Mahanty et al. 2017). The bacteria capable of solubilizing and mobilizing phosphorus (P) include Pseudomonas, Bacillus, Rhizobium, Burkholderia, Achromobacter, Agrobacterium, Microccoccus, Aerobacter, Flavobacterium, Aspergillus, Erwinia as shown in Fig. 2 (Mahdi et al. 2010; Mahanty et al. 2017).

Figure 2. 

Schematic diagram of soil phosphorous mobilization and immobilization by bacteria. Adapted and modified from: Khan et al. 2009.

Potassium is one of the three fundamental elements that plants require. Potassium (K) plays a crucial role in various biochemical and physiological processes in plants, such as the regulation of stomata (Wang et al. 2013; Hasanuzzaman et al. 2018). Stomata are responsible for controlling the opening and closing of pores in plants. The optimal operation of stomata is crucial for the process of photosynthesis (Falquetto-Gomes et al. 2023).

In the soil system, there are three simultaneous forms of Potassium: unavailable, slowly available, and quickly available forms (Yadav and Sidhu 2016). Applying biofertilizers can aid in remedying this condition by converting the insoluble potassium into soluble forms, making it accessible for plants to absorb as shown in Fig. 3 (Bahadur et al. 2014). For instance, bacteria such as Frateuria aurantia can mobilize and solubilize potassium (K) that is otherwise inaccessible, hence promoting plant growth as reported by Meena et al. (2016).

The Potassium-Solubilizing Microorganisms (KSM) are shown in Table 1. The simultaneous introduction of Azospirillum brasilense and Rhizobium meliloti has a positive impact on grain production and the concentration of nitrogen (N), phosphorus (P), and potassium (K) in Triticum aestivum (Mohammadi and Sohrabi 2012).

Figure 3. 

Direct and indirect mechanisms used by potassium-solubilizing bacteria. Source: Ahmad et al. 2016.

Biofertilizers are essential for improving the physical characteristics of soil and creating a more favorable and fruitful growing environment for plants (Fig. 4). Biofertilizers improve soil structure by encouraging the production of stable soil aggregates, which increase porosity, improve aeration, and increase water infiltration (Kumar et al. 2015). Moreover, the application of biofertilizers improved soil bulk density, soil temperature, and soil moisture (Sharma et al. 2012). Beneficial microbes are added to the soil to aid in the effective cycling of nutrients, increasing the availability of vital nutrients for plants and improving soil fertility in general (Altomare et al. 2011).

Figure 4. 

Schematic diagram representing the effect of biofertilizer on pore space of soil. Source: Chatterjee et al. 2017.

Stable soil aggregate development is aided by beneficial microorganisms found in biofertilizers, such as nitrogen-fixing bacteria and mycorrhizal fungus. By generating pore spaces in the soil, these aggregates increase porosity and decrease bulk density, this leads to a decrease in soil compaction, improving the flow of water and air through the soil profile (Singh et al. 2020). Additionally, the decomposition of organic matter is aided by the activities of these microbes, which raises the soil’s overall organic content (Bhattacharyya et al. 2022). By acting as a binding agent, this organic matter enhances soil structure and lowers bulk density. A progressive increase in soil bulk density is supported by the cumulative effect of using biofertilizers, even though these changes might not be apparent right away.

By forming stable aggregates and increasing the amount of organic matter in the soil, the use of biofertilizers improves soil structure and increases soil moisture retention. Mycorrhizal fungi, one of the beneficial microorganisms found in biofertilizers, form symbiotic associations with plant roots, encouraging widespread root development that facilitates effective water absorption (Hosseinzadeh et al. 2021). Furthermore, using biofertilizers promotes healthy plant growth by increasing nutrient availability, which helps plants better control water intake (Tahiri et al. 2022). A more favorable environment for long-term plant growth is produced overall by decreased water evaporation, higher water-holding capacity, and enhanced resilience to changes in moisture levels.

By encouraging stable soil aggregates, increasing aeration, and boosting heat transmission, biofertilizers have an indirect impact on soil temperature. Humus is created as a result of the breakdown of organic matter which is aided by biofertilizers (Wong et al. 2020). Biofertilizers help in the formation of humus which acts as an insulating layer to mitigate extremes in soil temperatures (Khalid et al. 2021). Better root systems, especially those containing mycorrhizal fungi, are encouraged by biofertilizers, which also improve water uptake and help control temperature (Ghorchiani et al. 2018). A more stable and favorable environment for plant growth is produced by the combined influence on soil structure and root development, even when the direct impact on soil temperature is minimal.

Since most elements that affect soil color are related to organic matter, drainage, and mineral content, using biofertilizers usually has little direct effect on soil color. However, through their impacts on microbial activity and organic matter content, biofertilizers indirectly contribute to improved soil color. Biofertilizers accelerate the breakdown of organic matter in the soil, which helps humus to develop. A higher humus concentration can give the soil a deep, dark color (Sumathi et al. 2012).

The impact of biofertilizers on soil microbes is complex (Sharma et al. 2012). Beneficial microorganisms like phosphate-solubilizing and nitrogen-fixing bacteria can be found in biofertilizers, which can directly increase their population and activity and increase the availability of nutrients for plants (Timofeeva et al. 2022). Furthermore, biofertilizers reduce harmful bacteria and promote a better soil ecology by increasing the diversity of soil microorganisms.

Using biofertilizers has various effects on soil fungi, depending on the particular micro-organisms included in the fertilizer. Biofertilizers can enhance nutrient intake by fostering favorable symbiotic connections with plant roots, particularly those that contain mycorrhizal fungus (Filho et al. 2017). Certain biofertilizers have the potential to impact soil fungal communities by augmenting the number of microbial populations and modifying the dynamics of resource competition (Trabelsi and Mhamdi 2013). Specific formulations containing antagonistic microorganisms aid in suppressing soil-borne diseases, whereas formulations high in organic matter might hasten decomposition and include several fungi in the cycling of nutrients (Pathma et al. 2019).

Because biofertilizers frequently contain strains or nutrients that favor the development of these filamentous bacteria, their use can greatly benefit soil actinomycetes through direct growth promotion (Li et al. 2022). Increased antagonistic activities may result from this, especially in biofertilizers containing antagonistic bacteria like Streptomyces species, which reduce soil-borne diseases by producing antibiotics (Roopa and Gadag 2019). Furthermore, organic matter-rich biofertilizers promote actinomycetes’ ability to break down complex organic compounds, which aids in the cycling of nutrients.

Despite the increasing popularity of using beneficial soil microorganisms in biofertilizers to improve plant productivity, widespread adoption on a large scale has been limited due to the challenge of replicating their positive impact on plants in natural environments with varying conditions (Ibáñez et al. 2023). The primary obstacles to the implementation of microbial biofertilizer include a limited understanding of the ecological significance of microbial biofertilizer among farming communities, insufficient efforts by agricultural extension workers to promote and motivate farmers to use biofertilizer products, a scarcity of appropriate carriers for biofertilizer formulation, inadequate storage facilities to prevent contamination of biofertilizer products, and the adverse effects of extreme climatic conditions, which result in inconsistent effectiveness of biofertilizers on plant productivity in natural environments (Gautam et al. 2021). Moreover, the application of biofertilizer products can lose credibility due to insufficient labeling, such as the absence of an expiry date and the omission of the names of the microorganisms utilized in the creation of the biofertilizer (Narayanasamy 2013). Additionally, it is important to note that most biofertilizers exhibit selectivity in their effects (Malusà et al. 2016).