Phosphate Solubilizing Bacteria: The Underground Architects Unlocking Soil’s Hidden Nutrient Wealth

Introduction

After nitrogen, phosphorus is the second most important essential macro nutrient for plants.¹ Phosphorous is a central component of ATP, which stores and transfers energy for photosynthesis, respiration, and metabolic processes.² It is essential for early root development and lateral root growth, increasing the root surface area for water and nutrient.^3,4 It is a key constituent of DNA, RNA, and phospholipids (the building blocks of plant cell membranes.⁵ Adequate phosphate levels in plants, enhances plant resistance to diseases and helps them cope with abiotic stresses like cold, heat, and drought.⁶ Phosphorous promotes early, uniform maturity and enhances the quality of fruits, vegetables, and grains.^3-8 It increases the nitrogen-fixing capacity of legumes and also play important role in the regulation of many enzymes.⁹

Phosphorous fixation in soil limits the bioavailability of soil phosphorus (P), which is found in both organic (30–65%) and inorganic (35–70%) forms. Phosphorous fixes with Fe/Al oxides in acid soils (pH <5.5) and with Ca (calcium phosphates) in alkaline soils (pH >7), reducing phosphate solubility.¹⁰ Soil typically contains 0.04–0.1% (w/w) total Phosphorus (P), yet most is unavailable for direct plant uptake.¹¹ A significant amount of phosphate in soils is found in both insoluble non available organic and inorganic forms, such as phytate and nucleic acid, as well as inorganic forms, such as Ca₃(PO₄)_2.^12,13 Plants primarily assimilate soluble inorganic orthophosphates (H₂PO₄^–, HPO₄^2-). Extraneously added phosphorus (P), including inorganic (IP) and organic (OP) sources, becomes immobilized and insoluble in soil by binding with cations like Ca, Al, Fe, Mg, and Mn. This rapid, pH-dependent fixation process transforms soluble fertilizer P into insoluble, non-labile, or “fixed” P forms (e.g., Ca₃(PO₄)₂, ALPO₄, FePO₄), rendering them unavailable to plants. These insoluble forms are only slowly released back into the soil solution over time (dissolution) or require phosphate-solubilizing microbes (PSM) and organic acids (e.g., citrate, gluconic acid) to unlock them. Phosphate solubilizing bacteria (PSB) and fungi produce organic acids such as citric, gluconic and oxalic acids, that lower soil pH, dissolving inorganic phosphate minerals.^14,15 Especially in phosphorus-deficient soils, extracellular enzymes like phosphatase and phytase produced by bacteria and fungi mineralize organic phosphates (OP) into plant-available inorganic phosphates.¹⁶

Research Methodology

This review uses a systematic qualitative research methodology to analyse phosphate-solubilizing bacterial biofertilizers. This approach integrates a structured literature   review, comprehensive and balanced assessment of the topic. Relevant literature and data were collected from different literature sources, including peer-reviewed    journals, sourced    from    MDPI, Frontiers, Elsevier, Springer, Wiley, Taylor & Francis, PLOS, Nature, ISME, MDPI, Frontiers, Elsevier, Springer, Wiley, Taylor & Francis, PLOS, Nature, ISME and scientific reports from journals like “The Hindu”. Over 150 documents were primarily screened, of which 80 were selected and included in the final review based on the relevance, quality, and data richness etc.

Literature Review and Analysis

Key Bacterial Genera Involved in Phosphate Solubilization

Utilizing phosphorus-solubilizing microbes (PSM) that possess the ability to mobilize multiple Phosphorous sources offers a sustainable, eco-friendly approach to improving soil quality, boosting agricultural productivity, and reducing reliance on chemical fertilizers. These microorganisms, such as Pseudomonas, Bacillus, and Enterobacter, convert insoluble organic and inorganic phosphorus into plant-available orthophosphate, thereby enhancing nutrient cycling and increasing crop yields. Approximately 80% of soil P is immobile and unavailable to plants, PSMs bridge this gap by converting insoluble forms into bioavailable orthophosphate through distinct physiological and molecular mechanisms. Up to now, 36 genera and 89 species of phosphorus-solubilizing microorganisms have been identified, primarily comprising phosphate-solubilizing bacteria (PSB) and phosphate-solubilizing fungi (PSF) and actinomycetes that solubilize phosphate (PSA).¹⁷

PSB are naturally found in diverse ecosystems, including agricultural fields, forest soils, fresh water, sea water, sediments and extreme environments.^18,19 Because of the nutrient-rich root exudates, the rhizosphere, the soil around plant roots, usually has the highest densities of phosphate-solubilizing bacteria (PSB).^20,21Because of their strong organic acid production and rhizosphere colonization, Bacillus and Pseudomonas species continue to be the most common and efficient phosphate-solubilizing bacteria (PSB), according to recent studies. ^18,22,23Species of Enterobacteris frequently found in the root zones of legumes and cereals, often showing high phosphate solubilization indices.^24,25Other Prominent phosphate-solubilizing bacteria (PSB) include Rhizobium, Azotobacter, Burkholderia, Serratia, Acinetobacter, and Pantoea. Table 1 outlines the taxonomic profile of the isolated and characterized phosphate-solubilizing bacterial (PSB) strains.

Table 1: List of phosphate soluble bacteria (PSB)

Methodology for the identification and quantitative assessment of phosphate solubilizing microorganisms

The isolation of phosphate-solubilizing bacteria (PSB) begins with the collection of rhizosphere soil, a region dense with microbial activity fueled by root exudates. The process typically utilizes a serial dilution and spread plating method, where soil samples are suspended in sterile saline or phosphate-buffered solutions to reduce microbial density.^48,49 Recent protocols emphasize a step-wise enrichment process in liquid media prior to plating, which helps selectively grow high-efficiency strains capable of mobilizing phosphorus from poorly soluble sources.⁵⁰ Small aliquots from these dilutions are inoculated onto specialized solid media, such as Pikovskaya’s (PVK) or NBRIP agar, containing insoluble tricalcium phosphate as the sole phosphorus source.^51-54The primary indicator of phosphorus mobilization is the formation of a clear halo zone around the colonies, a result of the bacteria producing organic acids or enzymes like phosphatases.⁵⁴To quantify this efficiency, researchers calculate the Solubilization Index (SI), comparing the diameter of the halo to the colony size to identify the most potent isolates for agricultural use.^55,56

Following initial screening, isolates undergo a quantitative assay in liquid media to precisely measure phosphate dissolution, often via the molybdenum blue method.^57,58 This is accompanied by testing for the abundant production of organic acids, which acidify the rhizosphere to release bound phosphorus. Once pure cultures are obtained through repeated reinoculation, researchers conduct detailed studies of morphology, colony characteristics, and biochemical tests. The most potent strains are further validated through molecular characterization via 16S rRNA gene sequencing to ensure accurate genetic identification and safety.^55,56,59

The transition from lab to land involves rigorous testing of the best inoculants on model plants. Promising strains move into greenhouse trials to assess plant growth-promoting traits under semi-controlled conditions.⁶⁰ For the development of commercial products like Microphos PSB, successful candidates are subjected to extensive field trials to prove efficacy across diverse environments. Finally, the process concludes with standardization and quality control, ensuring high viable cell counts and stable formulations for use as a commercial biofertilizer.⁶¹The methodology and crucial steps involved in the identification and quantitative assessment of phosphate-solubilizing microorganisms are presented in Figure 1.

Figure 1: Protocol for isolation and development of effective inoculants of PSM based biofertilizer.   Click here to view Figure

The crucial steps involved in development of phosphate solubilizing bacteria (PSB) fertilizers include

Soil sample collection

Serial dilution

Inoculation on media (pour plate/streaking) having different sources of insoluble P, depending on soil type

Clear zone around colony growth indicates PSM activity

Additional test in liquid media to assay Phosphate dissolution

Test isolates for abundant production of organic acids

Pure cultures by reinoculation

Study of the morphology and colony characteristics and biochemical tests

Screening of best inoculants in terms of P solubilizing activity

Identification at genetic level (molecular characterization)

Test on a model plant

Development of microphos PSB

Green house trials

Field trials

Standardization (quality control)

Commercial biofertilizer

Status of Phosphate Biofertilizers in India

India is the world’s largest importer of phosphorus, as domestic production covers only 15–20% of its total requirement.⁶² PSB is a microbial inoculant that converts insoluble phosphorus in the soil into plant-available forms, effectively reducing the need for synthetic DAP (diammonium phosphate) by up to 25–30%.⁶³India produced over 18,000 metric tons of PSB in 2023.⁶⁴The market is transitioning from solid carrier-based forms to liquid biofertilizers, which offer a longer shelf life (up to 12 months to 2 years) and better survival in harsh soil conditions.⁶⁵ Major producers include cooperatives like Indian Farmers Fertiliser Cooperative Limited (IFFCO) and National Fertilizers Limited (NFL), alongside private players like International Panaacea Limited. Gujarat, Maharashtra, Tamil Nadu, and Karnataka are the top producers and consumers of phosphate solubilizing microorganisms. Gujarat alone accounts for a significant portion of both solid and liquid biofertilizer production.⁶⁶ PSB biofertilizers are primarily used for crops like paddy, wheat, maize, and pulses.¹⁸ In regions like Siddipet (Telangana), farmers using PSB in rice cultivation reported reducing input costs by approximately ₹400 per acre.⁶⁷Key phosphate-solubilizing bacteria (PSB) biofertilizer brands and microbial strains available in the Indian market are summarized in Table 2.

Table 2: Key PSB Biofertilizer Brands and Microbial Strains in the Indian Market

The market for PSB biofertilizer is moving toward focused applications and high-tech formulations. PSB was a key ingredient in more than 40% of recently released microbial fertilizers as of 2024.⁶⁴ The EU Green Deal, which intends to cut the usage of synthetic fertilizers by 30% by 2030, has resulted in a 22% increase in PSB demand in Europe.⁶⁴The top adopters in Asia-Pacific are China, Indonesia, and India. India produced over 20,000 metric tons of PSB in 2023. India ranks first globally in PSB adoption, with over 29 million hectares (approx. 71 million acres) treated, followed by China which has approximately 21 million hectares under similar treatment.⁶⁴

Mechanistic insights into bacterial phosphorus mobilization

Phosphate-solubilizing bacteria (PSB) play a key role in the global phosphorus cycle by transforming insoluble soil phosphorus compounds into plant-available orthophosphates (H2PO₄^–) and (HPO₄^2- ).¹⁸Despite the plenitude of total phosphorus in most soils, over 80% is typically immobile due to its complexation with metal cations like Ca²⁺, Fe³⁺, Al³⁺.⁶⁸ PSB overcome this through by following biochemical and physiological strategies (Figure 2).

Figure 2: Overview of Mechanisms: Schematic representation of inorganic phosphate solubilization   Click here to view Figure

The Direct Oxidation “Overdrive” (Inorganic phosphaterelease)

The Direct Oxidation “overdrive” pathway is a key inorganic phosphorus (P) liberation strategy in many soils and rhizosphere bacteria. In this mechanism, organisms such as Pseudomonas and Pantoea deploy a periplasmic, pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase (GDH) system to oxidize glucose directly to gluconic acid (GA) at the cell surface, creating an acidic microenvironment that promotes mineral dissolution.⁶⁹ The resulting proton flux displaces Ca²⁺ from hydroxyapatite, while the gluconate anion acts as a multidentate ligand that chelates released metal cations and thereby limits phosphate re-precipitation, enhancing net phosphate solubilization.⁷⁰ Genomic and transcriptomic studies indicate that the capacity and regulation of this extracellular metabolic shunt are tightly controlled by the PqqABCDE operon, which functions as a molecular switch responsive to external phosphate levels and other environmental cues.^71,72Together, enzymatic GA production and operon-level regulation enable bacteria to efficiently mobilize mineral phosphate under nutrient-limiting conditions and shape phosphate availability in soils and plant-associated niches.

The Enzymatic “Scissoring” Toolkit (Organic P Mineralization)

In soils where phosphate is sequestered in organic vaults like phytate (inositol hexa phosphate), PSB deploy a suite of biocatalytic hydrolases. These include nonspecificacid phosphatases (NSAPs) and specialized phytases. Unlike plants, which have limited ability to access phytate-P, bacteria use beta-propeller phytases (BPPs) to perform a sequential “de-phosphorylation” of the inositol ring, systematically stripping phosphate groups one by one.⁷²This Enzymatic scissoring is often a community effort, where certain “pioneer” bacteria break down complex polyphosphates into simpler monoesters for the wider microbial and plant community.⁷³ Beta‑propeller phytases (BPPs) initiate sequential removal of phosphate groups from the inositol ring, lowering charge density and weakening metal chelation, while nonspecific acid phosphatases (NSAPs) hydrolyse the resulting shorter phosphomonoesters to release orthophosphate.⁷¹

Siderophore-Mediated “Metal-Stripping” (Indirect P Release)

In iron-rich (ferruginous) soils, phosphate is often “occluded” or trapped within iron oxide lattices. PSB employ high-affinity siderophores—small, iron-binding peptides—to execute a “metal-theft” strategy. By stripping Fe³⁺ from iron-phosphate complexes to satisfy their own metabolic needs, the bacteria inadvertently cause the structural collapse of the mineral matrix, releasing the “trapped” phosphate into the soil solution.⁷⁴This synergistic effect between siderophore secretion and acidification creates a dual-threat mechanism that is particularly effective in P-fixing tropical soils.⁷⁵

The “Electro-Chemical Shift”: Proton Extrusion via Ammonium Assimilation

Another important mechanism is the transmembrane proton flux. When phosphate-solubilizing bacteria such as Bacillus or Pseudomonas assimilate ammonium (NH₄⁺), they must preserve intracellular charge balance. To do so, they can export protons through membrane-associated H⁺-ATPase activity, creating a localized acidification zone that may enhance mineral phosphate dissolution. This mechanism can operate independently of organic acid secretion and may be especially relevant in nitrogen-rich microhabitats.⁷⁶

The “Biopolymer Shield”: EPS-Mediated Sequestration

Bacteria produce Exopolysaccharides (EPS), which form a viscous matrix around the cells and create a localized microenvironment for mineral interactions. This EPS matrix serves two critical functions:

Ligand concentration: It traps secreted organic acids and enzymes, preventing them from diffusing away into the bulk soil and concentrating the “chemical attack” on mineral surfaces.⁷⁷

Cation trapping: The negatively charged functional groups (carboxyl and hydroxyl) within the EPS act as a “cation sink,” binding to Ca²⁺ and Al³⁺ ions as soon as they are released, which shifts the chemical equilibrium to flavor more phosphorus dissolution.⁷⁸

The “Inorganic Volatilization”: Hydrogen Cyanide (HCN) and Volatiles

Recent research has highlighted the role of bacterialvolatile organic compounds (VOCs) and HCN and contribute indirectly to phosphate solubilization.While primarily known for biocontrol, HCN can act as a metal-complexing agent. In certain rhizospheric conditions, HCN facilitates the “displacement” of phosphate from iron and aluminum oxides by forming stable metal-cyanide complexes, a process often described as “volatile-mediated nutrient mining”.⁷⁹

The “Redox-Active” Transformation: Iron and Manganese Reduction

In anaerobic or waterlogged soils, certain PSB act as dissimilatory metal Reducers. By using Fe ³⁺ and Mn ⁴⁺ as alternative electron acceptors during respiration, they reduce these metals to their more soluble forms (Fe ²⁺ and Mn²⁺). Since phosphate is often “adsorbed” onto the surface of iron/manganese oxides, the reductive dissolution of these minerals can release the bound phosphate into the soil solution—a mechanism termed “reductive phosphorus mobilization”.⁸⁰

Discussion

Phosphate solubilizing bacteria (PSB) and phosphorus availability

Phosphate-solubilizing bacteria (PSB) have emerged as an important biological tool for improving phosphorus availability in agricultural soils, where a large fraction of total phosphorus is present in insoluble or fixed forms. The evidence reviewed in this article shows that PSB can mobilize phosphorus through several complementary mechanisms, including the production of organic acids, secretion of phosphatases and phytases, siderophore-mediated chelation, proton extrusion, exopolysaccharide production, and interactions with soil redox chemistry. Together, these mechanisms support the view that PSB are not merely supplementary inoculants but functional components of nutrient cycling in the rhizosphere.

Mechanistic diversity of phosphate solubilization

A major strength of PSB lies in their mechanistic diversity. Organic acid production remains the most widely reported pathway, especially in Bacillus, Pseudomonas, and Enterobacter species, where acidification of the microenvironment can dissolve calcium (Ca²⁺), iron (Fe²⁺), and aluminum (Al³⁺) bound phosphates. In parallel, enzymatic mineralization of organic phosphorus compounds expands the pool of plant-available phosphate by releasing orthophosphate from phytate and other organic substrates. This dual capacity is particularly valuable in soils with mixed phosphorus pools, because the microbial pathways can act on both inorganic and organic reservoirs. In this sense, PSB function as biochemical translators, converting inaccessible phosphorus into a form that plants can readily absorb.

Rhizosphere context and environmental controls

The rhizosphere is the most favourable habitat for PSB activity because root exudates supply readily available carbon sources and create a chemically dynamic niche. However, PSB performance is strongly influenced by soil pH, texture, moisture, temperature, and the resident microbial community. This explains why laboratory screening often shows strong phosphate-solubilizing indices, whereas field performance may be more variable. In controlled media, halo formation provides a useful preliminary indicator, but this does not always reflect the complexity of real soils. Therefore, strain selection should not rely only on in vitro phosphate solubilization, but also on root colonization ability, persistence, compatibility with crop species, and performance under site-specific field conditions. 

Integration into nutrient management and agronomic value

Another important point is that PSB should be viewed within the broader framework of integrated nutrient management. Their value is not limited to replacing chemical fertilizers entirely, but rather to improving fertilizer-use efficiency and reducing unnecessary phosphorus inputs. This is especially relevant in regions where fertilizer costs are high and phosphorus fixation is severe. By helping to mobilize legacy phosphorus already present in the soil, PSB can reduce dependence on synthetic fertilizers while supporting crop growth, soil health, and long-term sustainability. Their use is therefore aligned with both economic and environmental goals.

Formulation, delivery and quality control

The commercial success of PSB biofertilizers will depend heavily on formulation quality and delivery systems. Liquid formulations have clear advantages over traditional carrier-based products because they generally provide better shelf life, easier handling, and improved survival of viable cells. However, product effectiveness still depends on maintaining strain identity, viable cell counts, contamination-free production, and stable performance during storage and transport. Without strong quality control, the practical benefits of PSB may remain inconsistent, which can limit farmer confidence and adoption. This is especially important in large agricultural markets where biofertilizer performance must be reliable across different agroclimatic zones.

Consortia-based Inoculants: Opportunities and Challenges

Consortia-based inoculants are another promising direction. Because phosphorus mobilization is influenced by several interacting processes, combinations of compatible microbes may outperform single-strain products. For example, one organism may excel in organic acid secretion, while another may contribute phytase activity, rhizosphere colonization, or disease suppression. Such functional complementarity can improve resilience and broaden the effectiveness of inoculants under variable field conditions. At the same time, consortium design requires careful testing to avoid antagonism among strains and to ensure consistent ecological behavior after application.

Limitations, Knowledge Gaps and Research Needs

Despite their promise, several limitations must be acknowledged. The extent of phosphorus solubilization in the field is often lower than in laboratory assays, and long-term persistence of introduced strains is not always guaranteed. In addition, microbial efficacy may decline in soils with extreme pH, low moisture, high salinity, or poor organic carbon availability. There is also a need for greater understanding of how introduced PSB interact with native microbial communities and whether repeated application leads to stable colonization or only transient effects. These issues highlight the need for more rigorous multi-location trials and long-term agronomic studies.

Translational Example: Indian Biofertilizer Market and Adoption Considerations

The Indian biofertilizer market provides a useful example of the translational potential of PSB. Widespread interest in liquid biofertilizers and the growing number of commercial PSB products indicate that microbial phosphorus management is moving beyond the laboratory stage. Yet wider adoption will require better standardization, farmer education, and region-specific recommendations. Products should be matched to crop type, soil condition, and management practice rather than marketed as universal solutions. A more targeted strategy is likely to produce stronger agronomic outcomes and greater adoption success.

Conclusion

Phosphorus remains a fundamental paradox in agriculture; it is abundantly present in the soil yet remains the primary limiting factor for crop productivity due to its rapid immobilization. As highlighted, the transition from soluble orthophosphates to insoluble complexes with calcium, iron, or aluminium creates a “fixed” phosphorus pool that conventional plants cannot access. The utilization of phosphate solubilizing Bacteria (PSB), particularly dominant genera like Pseudomonas, Bacillus, and Enterobacter, represents a transformative shift toward sustainable intensification. By leveraging natural biochemical pathways—specifically the secretion of organic acids and the production of mineralization enzymes like phosphatases—these microbes effectively “unlock” soil phosphorus. This not only optimizes the efficiency of applied fertilizers but also taps into the vast reserves of legacy phosphorus already present in the earth. Ultimately, integrating PSB into modern agronomic practices offers a dual benefit. It reduces the environmental footprint and economic cost of synthetic chemical inputs while simultaneously enhancing soil health and crop resilience. By converting legacy soil phosphorus into bioavailable forms, these microorganisms reduce the necessity for chemical fertilizers, promoting a more sustainable and resilient agricultural framework. Future research and application should focus on optimizing the colonization of these beneficial bacteria in diverse soil types to ensure global food security in an eco-friendly manner.

Acknowledgement

I express my sincere gratitude to Prof. M. Praveena, Principal, Tara Government College (Autonomous), Sangareddy, Telangana, India, for her constant support and encouragement throughout the course of this study.

Funding Sources

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Conflict of interest

The authors do not have any conflict of interest.

Data Availability Statement

This statement does not apply to this article.

Ethics Statement

This research did not involve human participants, animal subjects, or any material that requires ethical approval.

Informed Consent Statement

This study did not involve human participants, and therefore, informed consent was not required.

Clinical Trial Registration

This research does not involve any clinical trials.

Permission to reproduce material from other sources

Not Applicable.

Author Contributions

The sole author was responsible for the conceptualization, methodology, data collection, analysis, writing, and final approval of the manuscript.

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Abbreviations

PSB: Phosphate-solubilizing bacteria

PSM:  Phosphate solubilizing microorganisms

PSA: Phosphate solubilizing Actinomycetes

P: Phosphorous/ phosphate

IP: Inorganic Phosphate,

OP: Organic phosphate

EU: European Union

ATP: Adenosine tri phosphate

DNA: Deoxy-ribo nucleic acid

RNA: Ribonucleic acid

pH: potential of hydrogen

SI-Solubilization index

PVK: Pikovskaya’s medium

EPS: Exopolysaccharides

VOCs: volatile organic compounds

NBRIP: National Botanical Research Institute’s Phosphate

GA: Gluconic acid

EPS: Exopolysaccharides