Perspectives of potassium solubilizing microbes in sustainable food production system - A review

Introduction

Potassium (K) is a critical plant nutrient enhancing biotic/abiotic stress resistance.
Intensive farming with high-yield varieties has depleted soil K.
Most soil K (90-98%) is in insoluble mineral forms, necessitating efficient rhizospheric microbes (ERMs) to solubilize it.
Rhizobacteria (e.g., Bacillus), fungi (e.g., Aspergillus), and nitrogen-fixing rhizobacteria (NFR) are involved in K mineral solubilization.
Microbes solubilize K using organic acids, pH reduction, acidolysis, chelation, exchange reactions, and complexation.
ERMs also promote growth via hormone production, N fixation, P dissolution, root enlargement, and antibiotic production.
Potassium solubilizing microbes (KSMs) are commercialized as biofertilizers for eco-friendly, sustainable food production.

Potassium Importance in Plants

K is the third essential macronutrient, absorbed abundantly as a cation.
Vital for plant physiology, metabolism, photosynthesis, growth, and sugar accumulation.
Facilitates root growth, seed development, yield, and fiber quality.
Enhances water use efficiency (WUE), carbohydrate/organic acid/fat metabolism, nitrogenous compound use and protein synthesis.
Improves drought resistance, cold tolerance, and stomatal regulation.
Mediates resistance against biotic stresses (microbes, insect pests).

Potassium Availability and Soil Dynamics

K makes up about 2.1% of the earth's crust. Plant uptake is limited by low soluble K.
K mineral ores are abundant in soil systems (∼2.5% of lithosphere), but 80–90% is insoluble.
1–10% exists as interlayer K in non-expanded clay minerals (illite) and lattice K in K feldspars.
A third K form is released from a non-exchangeable pool due to crop removal, erosion, leaching, and runoff.
High-yielding cultivars consume over 10 Mt of K annually.
Exchangeable K is limited (∼150 mg kg−1) in some soils (e.g., Pakistani soils).
K management is often ignored; fertilization practices commonly exclude K, leading to soil depletion.
Farmers underuse potash fertilizers due to inconspicuous K deficiency symptoms and misperceptions.

Increasing Potassium Deficiency

Rapid nutrient depletion by intensive cropping and ineffective K replenishment cause K deficiencies.
FAO estimates a 3,400,000-ton increase in global K demand from 2014 to 2018.
Plants absorb K as cations, similar to NH_4^+.
Phyto-available K is often below plant requirements, necessitating K fertilizers.
Crop residues, a potential K source, are often not returned to the soil.
Synthetic fertilizers (muriate/sulfate of potash) are used to augment soil K levels.
Increased K demand raises fertilizer prices, affecting farmer profitability.
Alternative indigenous K sources are needed for optimum plant uptake.

Role of Soil Microbes

Efficient soil microbes play a key role in the K-cycle and solubilization of insoluble K minerals.
Using soil microbes to solubilize indigenous K minerals is more sustainable and eco-friendly than synthetic fertilizers.
These microbes enhance plant growth by releasing K from insoluble minerals.
Microbes use mechanisms like siderophore and organic acid production, influencing pH and redox reactions.
K solubilizing microbes (KSMs) form complexes with metal ions on mineral surfaces.
KSMs decompose alumino-silicate minerals, releasing K, aluminum (Al), and silicon (Si).
KSMs suppress pathogens, change nutrient status and soil structure, and secrete bioactive materials and hormones.
Many microbes are used in K biofertilizers.

Potassium Dynamics in Soils

K is the third most abundant nutrient in soil and the most abundant cation in plant cells after N.
Represents 2.6% of earth's crust by weight.
Igneous (syenites, granites, basalts, peridotites) and sedimentary rocks are rich K sources.
Mineral soil K concentration ranges from 0.04 to 3%; surface soils contain 3000-100,000 kg K ha−1.
~98% K is in non-exchangeable form (silicate minerals like mica and feldspar); ~2% is available for plant uptake.
Non-exchangeable K is held between tetrahedral layers of micas and clay minerals.
Release of K+ from this complex structure occurs when solution K levels decrease through plant removal or leaching.

Major K Pools

Primary K mineral or structural K (90–98% of total K): silicate minerals like biotite, orthoclase, muscovite, feldspar, mica, vermiculite, illite, and smectite.
Exchangeable and non-exchangeable K are small percentages of mineral K, slowly available for plant uptake.
Availability depends on: weathering of K minerals, K levels in other pools, nature/particle size of K-bearing minerals, soil environment (Eh, pH, ions, wetting/drying, temperature).
Key K minerals: muscovite (white mica) KAl2 (AlSi3O{10}) (F,OH)2, biotite (black mica) K(Mg,Fe)3Al2Si3O{10}(OH,F)2, and orthoclase (KAlSi3O_8).
Micas (biotite and muscovite) are of interest as sources of Mg, K, Mn, and Zn.
Approximately 57% of applied K is fixed in Pakistani soils, with complex K transformations due to high pH and presence of non-expanded clay minerals like vermiculite (Mg, Fe^{++}, Al)3 (Al, Si)4 O{10} (OH)2 (4H2O) and illite (K, H3O) (Al, Mg, Fe)2 (Si, Al)4 O{10} [(OH)2, (H_2O)].
Bio-availability order: solution > exchangeable > fixed > mineral.
Plants and microbes uptake K from solution (1–2%), subject to leaching/fixation.
After K removal, K from mineral and non-exchangeable pools is released to maintain equilibrium.

Solution K Depletion

Intensive agriculture drives continuous plant uptake, depleting solution K over time.
Low solution K concentration (0.1–0.2%) is insufficient for plant growth (∼5% of total crop demand).
Countries in Southeast Asia, Africa, and Oceania face limitations due to limited natural K mineral reserves.
Exchangeable K (EK) and slowly exchangeable K (SEK) make up 1–2% and 1–10% of the total soil K, respectively.
EK is held by negatively charged clay minerals and organic matter, released by mineral weathering and exchanged by other cations as K+ ions.

Potassium Solubilizing Microbes (KSMs)

Plant-soil-microbe interactions are important for global food security.
Microbial groups (bacteria/rhizobacteria, fungi, actinomycetes) thrive in the rhizosphere through associative, symbiotic, or parasitic relationships.
Plant growth-promoting rhizobacteria (PGPRs) aid in soil weathering, nutrient retention, soluble compound exudation, organic material decomposition, nutrient mineralization, siderophore production, phytohormone synthesis, nutrient cycling, K mineral solubilization, phosphate solubilization, nitrification, fixation, denitrification, and sulfur reduction.
Microbes can reduce synthetic fertilizer use, an eco-friendly approach; sole mineral application is ineffective for plant growth.

KSM Diversity

Rhizobacteria, fungal species, arbuscular mycorrhizae, yeast, and nitrogen-fixing bacteria possess K mineral solubilization capacity.
They solubilize K minerals (micas, illite, orthoclase) and convert them into soluble forms through organic acid release.
KSMs improve degraded agriculture soils but are underutilized due to lack of awareness.
Rhizobacteria (Bacillus, Acidothiobacillus, Paenibacillus, Pseudomonas, Burkholderia) and fungi (Aspergillus, Glomas, Penicillium) solubilize K minerals.
Microbe availability depends on soil structure, texture, organic matter, and related soil properties.
Silicate solubilizing bacteria (B. mucilaginosus sub sp. siliceus) release K from feldspar and aluminosilicate minerals and decompose organic matter/crop residues.
K solubilizing rhizobacteria (KSR) are isolated from crop roots grown in K and silicate-amended soil.
Silicate solubilizing microbes exist in both rhizosphere and non-rhizosphere soil.

Bacterial Species

A variety of bacterial species are involved in K solubilization:
- B. edaphicus
- B. mucilaginosus
- B. circulans
- Burkholderia sp.
- A. ferrooxidans
- Arthrobacter sp.
- Paenibacillus mucilaginosus
- Paenibacillus glucanolyticus
While these bacteria solubilize K to some extent, only a few strains, such as B. mucilaginosus and B. edaphicus, are highly efficient.

Fungi

Some strains of fungi like A. niger and A. terreus, which were isolated from K rich soil samples, can also solubilize insoluble K.
In addition to above mentioned microbes, some others including Paenibacillus mucilaginosus, Arthrobacter sp., Cladosporium, Paenibacillus glucanolyticus, Aminobacter, Penicillium frequentans, Burkholderia, Sphingomonas, Pseudomonas have the ability to solubilize both K and P.

Mechanisms used by Potassium Solubilization Microbes (KSMs)

Microflorae adopt several mechanisms to solubilize complex soil minerals thereby enhancing plant growth and development for higher crop production:
- Solubilization of K-minerals
- Plant growth promotion (PGP) activities

Solubilization of K-Minerals

There is little information available on K solubilization by soil microflora (bacteria, fungi, algae, protozoa, nematode etc.). However, available reports indicate that these microbes adopted specialized mechanisms for K rock mineralization that may include redox reactions by the production of chelating molecules and organic acids for K weathering and for its maximum bio-availability.

Mechanisms of K-Solubilization by Bacteria/Rhizobacteria

The KSMs include primarily some fungi and bacteria, but bacteria play the central role in solubilization of K minerals, and widely known as potassium dissolving bacteria (KDB) or potassium solubilizing bacteria (KSB).
The K solubilization by bacteria/rhizobacteria include:
- direct way of solubilization
- indirect way of solubilization
- polysaccharides secretion
- biofilm formation on mineral surfaces

Direct Method of Solubilization

Microbes help in K solubilization through:
- strong organic acid production
- acidolysis of the rhizosphere minerals
- carbonic acid based chemical weathering
These microbes excrete organic acids like oxalic, tartaric and citric acids and H^+ ions, which lowers the pH of surrounding soil. Exudation of organic acids is an important mechanism of K minerals (mica, biotite, muscovite, feldspar, illite and orthoclase) solubilization through microbial-mediated acidification and protonation into plant available form.
These organic acids associated protons lower the pH of rhizosphere, and enhance solubility of essential cations such as Fe, K, and Mg etc. Further, microbial respiration, degradation of particulate and dissolved organic carbon can elevate the concentration of carbonic acid at mineral surfaces, which react with minerals and lead to an increase in the rates of mineral weathering by a proton-promoted dissolution mechanisms.
Among these, tartaric acid is the most important and its use as an agent for solubilization of K minerals is going up gradually. Other organic acids like acetic, glycolic, lactic, propionic, malonic and fumaric acids etc., were also reported to be involved in solubilization of K-minerals.
Organic acid molecules influence mineral weathering in three separate but linked steps known as triple action:
- acids adhere to the mineral surface and extract nutrients from the mineral particles by electron transfer reaction
- break the oxygen links
- chelate ions present in solution through their carboxyl and hydroxyl groups.
The third mechanism indirectly accelerates the dissolution rate by creating gradient between cation and anion concentrations in the solution.
Synergistic impacts of KSMs inoculation on crops have been reported by many investigators. Lin et al. (2002) observed that carboxylic acids and capsular polysaccharide excreted B. mucilaginosus and B. edaphicus solubilized feldspar and significantly enhanced plant growth and yield
B. mucilaginosus releases some polymers, low molecular weight ligands and mixtures of these, which can improve K solubilization (feldspar, muscovite and illite) from ∼68 to 83% compared to control.

Indirect Method of Solubilization

Instead of acidolysis or lowering of pH, microbes also solubilize K minerals through indirect method of solubilization like:
- chelation of the cations bound to K silicate
- exchange reactions
- solubilization by direct attachment of KSMs on mineral surfaces
- metal complexing ligands
- release of phytohormones through microbes.
Chelating ability of the organic acids is an important mechanism of solubilizing K minerals. The KSMs form metal–organic complexes with both Al and Si ions associated with K-minerals, and as result, K ion is released in solution.
Metal complexing ligands are another potential way to solubilize K through microbes. The efficient microbes, in addition to organic acids, exude high-molecular-weight organic ligands and polymers such as guluronic acid, mannuronic acid, and alginates, which form complexes with ions on the mineral surface to weaken the metal–oxygen bonding.
On the other hand, ligands can form complexes with ions in the solution and directly affect the saturation state of solution. Further, these polymers can accelerate ion diffusion from the surface of mineral by producing slime layers containing polysaccharides and enzymes around the mineral surface. This process can result in increased contact between mineral surface and water and enhance the solubilization of a number of minerals. These microbe-produced polysaccharides contain functional groups (eCOOe) that form complex with mineral ions, changing the saturation state of solution and thus boost the mobilization.
In addition, these microbial exudates (low molecular weight organic compounds) contain by products of metabolic processes, organic ligands, chelates and extracellular enzymes, which help in dissolution of K minerals through pH manipulation of micro environment.

Polysaccharides Secretion

Capsular exopolysaccharides (EPS) is another potential way by which rhizobacteria can solubilize K minerals. These microbe-produced EPS are strongly adsorbed by organic acids, and thus enhanced attachment to mineral surface occurs resulting in high concentration of organic acids on or around the minerals.
Microbes such as Bacillus, Clostridium and Thiobacillus with the capacity to secrete metabolic intermediates or mucilaginous capsules containing of EPS showed higher biodegradation of feldspar and illite to release K^+.
Microbial EPS with high protein content stimulates the formation of bacteria-mineral complex where microbes excrete organic acids and resultant low pH enhances solubilization of K minerals and K bioavailability.
Besides, EPS also have strong ability to adsorb organic acids. Consequently, micro environment where higher amount of organic acids is released, additional dissolution of the K minerals take place.
Furthermore, EPS binds K^+ and SiO_2 that helps maintaining the equilibrium between soil solution and minerals eventually enhancing K^+ release and bioavailability.
Rhizobacteria attack minerals to use them as an energy source and a nutrient of interest. During the interactions, other nutrients may also come into solution due to bio-weathering processes.
Mineral solubilization is principally attributed to redox reaction, as in nutrient deficient conditions, bacteria transfer electron to metal groups on mineral surfaces resulting in destruction of metal. Bacteria usually contain many multiheme cytochromes in outer membrane surfaces of cells, which allow proteins to transfer electrons easily as and when contact with mineral oxides occur.

Biofilm Formation

Biofilm formation is a potential but least studied mechanism of K reserve mobilization. Biofilm is a very early step in plant–microbe interaction in which bacterial cells are stuck to abiotic/biotic surfaces.
In biofilm, cells are fixed within a matrix of self-produced extracellular polymers that are junk proteins, DNA and polysaccharides. Higher microbial population in biofilms offers the opportunity to achieve improved biochemical reactions compared to single and dual cultures.
Certain microbial strains form a biofilm on the rhizospheric mineral surfaces and release organic acids, metabolites, and drops the pH that help in K mineral solubilization and uptake by plants.
It is concentrated microbial community on the root-hypha-mineral interface, which is protected by self-produced extracellular polymers. These rhizobacterial communities have a tremendous phylogenetic and metabolic diversity for their ability to adapt and colonize in extreme environments due to exo-polysaccharide layer in which they extract inorganic nutrients and energy directly from the mineral surfaces and help in weathering of minerals.
Basically, these primary proteins, extracellular polymers, and polysaccharides exudate by the microbes serve as a catalyst and help in mobilization of nutrients from complex mineral structures. Further, biofilms provide shelter to microbes against unsafe environmental effects, protect from heavy metals, antibiotics, other cations, nutrients and pathogens.
A large body of evidence suggests that biofilms may result in mineral weathering and nutrient uptake by plants through root hairs

Solubilization of K by Fungi

Role of fungi in K solubilization is more pronounced through organic acid production and uptake through the root system to contribute to plant biomass enhancement.
Some fungi such as Aspergillus, Penicillium, Fusarium and Aspergillus niger play a pivotal role in K solubilization and uptake. Fungi produce organic acids, especially oxalic, citric and gluconic acid, similar to rhizobacteria, which leads to deterioration of clay silicates, mica and feldspar.
Oxalic acid caused dissolution of feldspar in growth media whereas tartaric and oxalic acids were involved in mobilizing
illite and gluconate, and promoting dissolution of albite, quartz and kaolinite. Besides, fungi mineralize K through chelating reaction of mineral elements, acid hydrolysis, and secretion of insoluble macromolecules and polymers that play significant role to release K from K minerals.
Furthermore, fungi exert direct bio-physical forces which can fracture K mineral to decrease particle sizes and create more reactive surfaces. Fungi add acids to rhizospheric soil and help with dissolving silicate rock powders by mycorrhizae, which release fixed nutrients in soil.
Instead of acid production, some rock eating ectomycorrhizal fungi excrete low molecular weight organic compounds through hyphal tips that forms microscopic tunnels within K minerals (feldspar and hornblende) and significantly enhance the rate of mineral weathering in soil.
Enhanced K solubilization in soil was observed from fungal strains significantly enhance the titratable acidity and decrease the pH.
Thermophilic fungus (Aspergillus fumigatus) enhanced K solubilization when inoculated onto K bearing minerals. Biofertilizer preparation by utilizing beneficial fungi is another positive and emerging aspect pertaining to sustainable agriculture.
In addition, yeast also has the ability to mobilize K from silicate minerals. However, despite the known ability of yeasts to produce organic acids, there are only a few studies on K solubilization through yeast.
Strong acids released from yeast (Torulaspora globosa) could solubilize the alkaline ultramafic rock and release as much as ∼38% of total K in the medium within fifteen days. Soil yeasts Pichia anomala and Rhodotorula glutinis had strong ability to solubilize K minerals as well as providing significant plant growth enhancements (~14% increment in roots and ~23% increment in shoots)

Plant Growth Promotion (PGP) Activities

The KSMs promote plant growth through various direct and indirect growth promotion mechanisms. In soil, inoculation with KSMs enhance nutrient uptake, promote K solubilization, organic matter decomposition and many other functions.
Through direct mechanisms, microbes play a vital role in N2-fixation, P-solubilization, production of plant growth hormones (auxin, cytokinins, ethylene, IAA and GA3), organic acid production and K solubilization
In indirect mechanisms, microbes enhance plant growth by production of siderophores, antibiotics, H_2S, antifungal compounds, starch hydrolysis, and cellulose degradation.
Through these beneficial mechanisms, KSMs in most cases also promote plant growth. The K solubilization from K mineral is the main characteristic of KSMs, which significantly enhance the fertility status of soil and eventually promote plant growth
Utilization of KSMs for growth promotion is a sound technique and has the ability to restore unproductive soils.
Inoculation of Bacillus mucilaginosus in nutrient limited soil showed positive response on growth and yield of eggplant. Furthermore, maximum K release and uptake was observed in soil amended with K mineral and B. mucilaginosus.
Frateuria aurantia belonging to the family Pseudomonaceae considerably enhanced K solubilization and plant growth upon inoculation. They reported that K solubilization and PGP activities in this study were due to secretion of organic acids and enzymes produced by KSMs
In addition, different types of amino acids, vitamins and plant growth promoting substances such as gibberellic acid (GA3) and indole-3-acetic acid (IAA) are released, which help plants attain better growth
High auxin production was observed in the presence of B. edaphicus NBT strain, which ultimately enhances plant growth. The strain NBT has high potential to solubilize K and was found to maximize growth of cotton and rape. Inoculation of K solubilizing strain (Penicilium oxalicum CBPS-3F- Tsa) resulted highest K solubilization, secretion of enzymes, hormones and organic acids in microbial suspension.
Inoculation with KSMs enhanced N and P uptake together with K partly because of hormones, enzymes and organic acid exudation in root interface. This occurred due to microbial action, and KSM colonized roots contributed to boosting growth.
Azotobacter chroococcum A-41 is another K-solubilizers, plant growth and yield promoting strain. Plant growth promotion by this strain is attributed to the N2-fixing and K solubilizing ability in addition with phyto-hormone production. Phyto-hormones released from KSMs directly enhance plant growth and yield through production of lateral roots and absorbent root hairs in the rhizosphere, which facilitate nutrient uptake from soil away from the plant
Other than direct effect, KSMs also enhance plant growth through indirect effect by releasing nutrients in soil solution from the mineralization of rocks, which are available to plants. Instead of a single inoculation of KSMs, few studies have documented the effect of co-inoculation or a consortium of KSMs and PGPMs.
A chroococcum A-41 with K solubilizing B. mucilaginosus enhanced growth and nutrient uptake of sudan grass compared to single inoculations. KSMs also enhance plant growth by indirect mechanism through disease suppression and providing resistance against insect pests and diseases. Through combined beneficial effects of making nutrients more available and improving plant vigor, these microbes strengthen plants and thus enable them to fight against diseases.
For example, some microbes release K, aluminum and silicon through weathering of minerals and secrete phyto-hormones, which strengthen the defense mechanism by providing resistance against diseases and other external stresses, enhance plant growth, plant nutrition and competitiveness.
Rhizobacteria like Azospirillum, Bacillus, Pseudomonas, Enterobacter were used in many studies for their beneficial effects on plant growth, yield and disease resistance.

Factors affecting the activity of potassium solubilizing microbes

Soil systems
A number of factors affect microbial survival in soil
- pH
- oxygen concentration
- carbon sources
- temperature
- bacterial strain, which are used to solubilize K mineral

Microbes need food and higher level of energy for their survival

They obtain their food from different carbon (C) sources.
Without a C source, rate of multiplication slows and ultimately, they show reduced activity or activity may even cease. In the case of KSMs, lacks of food sources reduce their rate of K solubilization. Decomposition of organic matters in soil is the major C source for these microbes.
In general, higher K solubilization and higher microbial diversity is observed in the presence of a labile food source
KSMs in soil require specific temperature for maximum K solubilization. Maximum K mineral solubilization (∼49 mg L−1) was reported as temperature reached 25–30 °C

Soil pH

another important factor, which affects survival and diversity of microbes in soil. Most suitable pH reported for microbial growth and survival was in the neutral range. Microbial growth was highest at pH 7, but pH > 7 caused decrease in solubilization of K due to fixation of K at high pH
However, optimum growth conditions required to release ∼35 mg L−1 K in 7 days at 28 °C in the pH range from 6.5 to 8.0.
A large body of literature also indicated that KSMs not only solubilize K- but are also capable of P solubilization (∼490 to 758 mg L−1) at pH range of 6.5–8.0, which is suitable for microbial survival in soil–plant system.
In addition to these soils and environmental factors, K solubilization was also affected by type of K mineral, structure and composition of minerals
Different types of K minerals are present in soil; each with specific characteristics, different structural arrangement and resistance against K solubilization. Illite showed highest solubilization as compared to feldspar when both were used as a K source

K Bioavailability

was also dependent upon soil type, plant type and soil and environmental factors This effect is likely due to structural variation of minerals as some minerals are easily solubilized and release K, while others are complex in structure and resist solubilization. In addition to specific minerals, a suitable microbial strain is also very important for highest microbial activity and K solubilization.
Availability of oxygen is also a very important limiting factor because KSMs are aerobic in nature so, presence of oxygen is necessary for maximum solubility and K availability

Potential of potassium solubilizing microbes for crop production

Inoculation of seeds with KSMs as well as seedling treatment significantly enhanced seedling vigor, germination percentage, K uptake by plants, plant growth and yield under both greenhouse and field conditions
The beneficial effect of KSMs inoculation has been observed on a range of crops including wheat, sorghum, cotton and rape, sudan grass, chili, pepper and cucumber, tomato and khella.
field experiments conducted on multiple crops such as maize, wheat, rice, groundnuts and forage crop have shown that KSMs could significantly decrease the use of both synthetic and organic fertilizers.
These microbes also take part in the synthesis of chlorophyll by taking part in various enzymatic activities. These beneficial microbes enhance plant growth not only directly but also indirectly through root enlargement, hormone production, P solubilization as well as through enhancing the total chlorophyll and carotenoid contents.
B. mucilaginosus, A. chroococcum, and Rhizobium sp. in a phytotron growth chamber significantly enhanced the K content, plant biomass, crude protein and chlorophyll content in both crops (maize and wheat). It was also reported that B. mucilaginosus showed significantly higher solubilization and uptake of K than A. chroococcum and Rhizobium sp

Some other microbes also contribute to solubilization of K minerals

For example, Si solubilizing microbes solubilize Si as well as K. Likewise, through inoculation of soil with silicate solubilizing bacteria B. cirulans (solubilize K in addition to Si). Furthermore, increments in wheat yield was documented in response to inoculation of KSMs in eroded soil as compared to normally eroded uninoculated soil.
Due to co-inoculation of K and P bearing minerals with KSMs significant enhancement of K (∼41, 93 and 79%), P (∼71, 110 and 116%) and dry matter yield (∼48, 65 and 58%) of sorghum plant in three different textured soils i.e. clay, sandy and calcareous soils, respectively. significantly enhanced oil content (∼35%) and dry matter (∼25%) of groundnut, and increased P and K contents from ∼7 to 9 mg kg−1 and ∼87 to 99 mg kg−1, respectively, in soil compared to an uninoculated control
- inoculated with B. edaphicus NBT, a well-known K solubilizer, solubilized insoluble K, enhanced root/shoot growth, influenced K mineral (illite) solubiliza- tion, bioavailability and uptake in cotton and rapeseeds. A significant increase (∼26–30%) of K content was observed when an insoluble K source (eliot) was inoculated with a bacterial strain NBT.
Different types of fungi were also very active in solubilization of K minerals. The arbuscular mycorrhizae (AM) released protons or CO_2 and many different types of organic acids or compounds, which helped in K mineral solubilization (Meena et al., 2014a). Instead of K, AM also significantly enhanced N, Ca and Fe concentration in plant leaves and fruits

Role of potassium solubilizing microbes in disease and stress resistance in crops

Plants are exposed to many abiotic and biotic stresses such as chilling, drought, high light intensity, nutrient limitations, heat, diseases, reactive oxygen species {(hydroxyl radical (OH^−) and superoxide radical (O_2 ^−)}
A large body of evidence suggests that improvement of K status of plants can greatly enhance plants’ resistance against stress, disease and insect attack.
It promotes root growth and enlargement, stem strength, and provide resistance against cold and water stress. Potassium is directly involved in plant strength, crop quality, and improves overall plant productivity.
Potassium also strengthens plant defense system, and it is important for the survival of crop plants under environmental stress conditions such as chilling, drought and high light intensity. Applications of KSMs significantly enhance K content in plants, nutrient uptake and disease resistance. Plants inoculated with KSMs (B. mucilaginosus) showed increased solubilization of K mineral, IAA, N, P uptake and photosynthesis, which eventually improved growth of plant.
The solubilization behavior, which is generally observed in microbes, is due to the variation in their capacity of producing enzymes and organic acids can reflect specific characteristics of KSMs. These efficient microbes in soils can improve nutrient uptake, plant growth, root growth and enlargement, plant competitiveness, and enhance resistance against external biotic and abiotic stresses. Some associative bacterial species such as Bacillus, Enterobacter, Pseudomonas and Azospirillum have also been used in a number of studies due to their involvement and benefit on plant growth mechanism

Commercialization of potassium solubilizing microbes

Microbes present in the rhizospheric soil have multiple functions such as N2-fixation, P and K solubilization, siderophore production and other PGP activities that promote plant growth and development.
isolation of these microbes followed by mass production for augmentative use as biofertilizers and biological inoculants has the potential to mitigate K deficiency in the future.
use of KSMs as biofertilizers can be a better strategy to improve crop production and can be used to reduce synthetic fertilizer demand and thereby supporting environment friendly agriculture These biofertilizers were used in tea for K solubilization and growth production. However, solubilization index showed that biofertilizer formulation performed better for MOP compared to K minerals. was also observed that glucose and ammonium nitrate supplementation supported MOP availability than other mineral K sources. Considering the economic crises of tea cultivators as a case study, it appears that use of KSB containing biofertilizers in combination with synthetic K fertilizers may be an economically feasible eco- friendly option.
Regardless of the original discovery of plant association of a microbe, these products can be used for other plants multiple microbes are currently present in market and those should provide better solution for crop growth and provide more benefit to agriculture. Consequently, it is important to search for more species of microbes and genes, which can solubilize K minerals, and make them available to growers as commercial products.

Concluding remarks and prospects

Rhizospheric microbes significantly contribute to solubilization of mineral K sources through their innate specialized mechanisms. inoculation of soils with KSMs had higher available K content in soil and plant, showed enhanced plant growth and resistance to biotic and abiotic stresses. In addition with K solubilization, KSMs also release siderophores, plant growth hormones, ammonia, solubilize P and fix N from the environment.
There is a critical need to design studies encompassing different soil types, crops treated with microbial and synthetic fertilizer sources to find the best combination of different fertilizers and microbes, which could be an appropriate substitute of conventional K fertilizers. There is also a need to carefully select silicate minerals on the basis of their dissolution rate and K contents in future studies.
Commercialization of KSMs in the form of inoculum or biofertilizer is urgently needed to make them available to the end users because indigenous soil microbial populations are not high enough to meet plant K requirement. research should focus on factors affecting survival of KSMs in different soil types with diverse mineral resources for maximum plant growth and yield. Inclusion of KSMs in the same group of biofertilizers such as biological nitrogen fixers or PGPR will ensure optimum K bioavailability for sustainable crop production.