Potassium (K) is an essential mineral nutrient for plants (Vasileva et al., 2022; Thriveni et al., 2024). It participates in activating enzymes, osmosis, protein synthesis, cation-anion balance and supporting plants to overcome abiotic stresses (Marschner, 2012). In particular, K⁺ can activate more than 60 enzymes during glycolysis and Krebs cycle within plants (Nieves-Cordones et al., 2016). Long-term intensive agricultural soil results in low K availability in  soil (Das et al., 2021). Furthermore, sustainable agriculture trends reduce chemical fertilizer use, leading to lower and lower soil exchangeable K (exK) (Mouhamad et al., 2016). Approximately 98% of soil K exists in mineral forms, including muscovite, biotite and feldspar (Mouhamad et al., 2016). Alluvial soil is usually deposited with alluvial materials from  rivers, leading to great nutrient content in soil (Tong, 2017). However, in some regions, dykes were constructed to prevent floods and protect agricultural land, leading to restriction of alluvial deposition into the soil and eventually lower nutrient content compared with regions without dykes (Khuong et al., 2023a; Tuan et al., 2007).  
       
K-solubilizing (K-sol) bacteria are potent in exploiting insoluble K minerals in soil (Etesami et al., 2017). Because of low exK and high cost of chemical fertilizer, the role of K-sol bacteria becomes increasingly important (Singh et al., 2023). Normally, these K-sol bacteria produce organic acids, such as oxalic acid, gluconic acid, malic acid, lactic acid,  and succinic acid (Naraian and Kumari, 2017). These acids release H⁺ to directly solubilize K into soluble forms (Reddy et al., 2023). Among K-sol bacteria, purple non-sulfur bacteria (PNSB) belonging to α and β-proteobacteria (Imhoff et al., 2005) feature flexible metabolism, i.e. they can perform as a photoautotroph, photoheterotroph, chemoautotroph or chemoheterotroph (Madigan and Jung, 2009). Thus, PNSB are highly potent in solubilizing K from soil. Thus, the current study was conducted to assess the K-sol. capacity of K-sol PNSB Cereibacter sphaeroides M-Sl-09, Rhodopseudomonas thermotolerans M-So-11 and R. palustris M-So-14 in undeposited alluvial soil under in vitro conditions.

Materials
 
The origin of bacteria
 
The K-sol PNSB used in the study included Cereibacter sphaeroides M-Sl-09 (PP814768), Rhodopseudomonas thermotolerans M-So-11 ( PP814769) and R. palustris M-So-14 (PP814770). These K-sol. PNSB strains were isolated from un-deposited alluvial soil for maize cultivation and kept in the Faculty of Crop Science, College of Agriculture, Can Tho University (Thu et al., 2024).
       
Un-deposited alluvial soil belonged to the dyke system for maize cultivation in An Phu district, An Giang province.
       
The basic isolation medium (BIM) included 1.0 g (NH₄)₂SO₄, 0.5 g K₂HPO₄, 0.2 g MgSO₄.H₂O, 2.0 g NaCl, 5.0 g NaHCO₃, 1.5 g yeast extract, 1.5 g glycerol and 0.03 g L-cysteine and distilled water making up to 1 L of solution.
 
Methods
 
Bacteria preparation: The K-sol PNSB strains were propagated in BIM under a light intensity of 3,000 lux for 72 h. The PNSB culture was diluted to reach an OD₆₆₀ of 0.5 before being used.
       
The experiment was conducted in a completely randomized with 6 treatments and 3 replications in the Faculty of Crop Science, College of Agriculture, Can Tho University from September 2025 to December  2025. Each replication was a centrifugal tube containing 25 g of dry soil, 0.25 g of insoluble K minerals (K_nNa₁₂-n[(AlO₂)₁₂(SiO₂)₁₂]•xH₂O), 2.0 mL of bacteria culture, 1.0 mL of sterilized distilled water as described in Table 1.


       
The amount of exK and cation exchange capacity (CEC) were determined on days 2, 4, 6, 8 and 10 after PNSB inoculation. The exK was measured by extracting soil with BaCl₂ 0.1 M and using an atomic absorption spectrometer at the 766.5 nm wavelength. The CEC in soil was determined by extracting soil with MgSO₄ 0.02 M and titrating extracted solution with EDTA 0.01 M (Sparks et al., 2020). The PNSB density was measured 10 days after PNSB inoculation. In brief, 1 g of soil of each treatment was added into BIM, illuminated at 3,000 lux for 5 days and finally measured for optical destiny (OD₆₆₀) to determine the relative density.
 
Statistical analysis
 
The SPSS version 13.0 was used to examine the differences among treatments according to Duncan’s test at 5% significance.

Changes in exchangeable K content in soil by the potassium-solubilizing PNSB
 
Table 2 shows that at day 2 after PNSB inoculation, the exK in treatment with mixed PNSB strains was greatest (3.31 meq 100 g^-1) and lowest was under the treatment without soil nor PNSB (0.085 meq 100 g^-1). Likewise, on days 4, 6, 8 and 10, treatments with single or mixture of K-sol PNSB out-performed the ones without bacteria, with 2.48-2.78>0.085-2.26 meq 100 g^-1 (day 4), 2.29-2.62>0.088-1.42 meq 100 g^-1 (day 6), 2.37-2.47>0.093-1.55 meq 100 g^-1 (day 8) and 2.19-2.42>0.093-1.67 meq 100 g^-1 (day 10).


 
Changes in soil CEC by potassium-solubilizing PNSB
 
On days 2, 4, 8 and 10 days after PNSB inoculation, treatments with had equivalent CEC to each other and greater than those of treatments without PNSB, with 13.2-14.1>11.9 meq 100 g^-1, 13.4-14.6>12.1 meq 100 g^-1, 13.2-13.5>12.2 meq 100 g^-1 and 13.6-14.3>12.1 meq 100 g^-1. Furthermore, supplying single M-Sl-09 strain had greater CEC on day 6 than supplying single M-So-14 strain or the mixed PNSB, with 14.6>13.8-13.9 meq 100 g^-1, respectively (Table 3).


 
Changes in soil bacteria density by potassium-solubilizing PNSB
 
As per Fig 1, on 10 after PNSB inoculation, PNSB density in soil varied significantly at 5%. In particular, supplying single M-Sl-09 strain had an equivalent OD₆₆₀ mixed PNSB, roughly 2.32-2.44. Moreover, treatments with a single strain of M-So-11 or M-So-14 had equivalent OD₆₆₀ to each other and were greater than those without bacteria, with 2.14-2.27>1.02, respectively.


       
Supplying K-sol. PNSB strains C. sphaeroides M-Sl-09, R. thermotolerans M-So-11 and R. palustris M-So-14 solubilized insoluble K minerals (K_nNa₁₂-n [(A lO₂)₁₂ (SiO₂)₁₂]•xH₂O) into available forms (Table 2). Bangun et al. (2023) have successfully selected strains of Burkholderia paludis KSB.TB5, B. cepacia KSB.TB6 and Paraburkholderia phymatum KSB.PB1 can solubilize insoluble K from weathered materials of mountains in Bahorok, Langkat province, Indonesia. In addition, K-sol bacteria KSB-40, KSB-17, KSB-31, KSB-94 and KSB-105 can release K⁺ from biotite minerals at the amounts of 3.50-5.00, 4.40-6.30 and 5.30-7.20 µg mL^-1 after 7, 14 and 21 days of incubation, respectively (Nath et al., 2017). Khuong et al. (2023b) found that PNSB Rhodopseudomonas pentothenatexigens TT07.4, AN05.1 and AC04.1 isolated from acid sulfate soil in paddy fields can solubilize K at 11.8-17.7 mg L^-1 under microaerobic light and 16.4-24.7 mg L^-1 aerobic dark conditions. Furthermore, strains of Pantoea agglomerans P5, Pseudomonas putida P13, P. fluorescens Chao, P. putida Tabriz, P. fluorescens Tabriz,  Azotobacter sp. and Bacillus megaterium JK3 can perform K solubilization from minerals of muscovite at 3.50-5.38 mg g^-1 and biotite at 5.44-7.25 mg g^-1 (Sarikhani, 2016). K appears in soil as components of minerals such as muscovite, biotite and feldspar (Paramisparam et al., 2021). Therefore, the strains of C. sphaeroides M-Sl-09, R. thermotolerans M-So-11 and R. palustris M-So-14 successfully performed K solubilization from minerals, which is potent to reduce the use of chemical fertilizers for sustainable agriculture.
       
CEC is an important soil feature indicating the nutrient availability for plants (Manrique et al., 1991). The increase in exK when supplying C. sphaeroides M-Sl-09, R. thermotolerans M-So-11 and R. palustris M-So-14 enhanced soil CEC (Table 3). Cations hold onto anions on the surface of organic matter or shale. These exchange positions are the main sources of exchangeable cations for plants. Cations loosely attached to these surfaces are taken by plants and resupplied to soil solution. Thus, current study focused on facilitating plants to take these cations more easily, especially K.

Supplying K-sol. PNSB C. sphaeroides M-Sl-09, R. thermotolerans M-So-11 and R. palustris M-So-14 increased exK and CEC in soil compared with no bacteria supplementation. Furthermore, PNSB density in with PNSB was greater than ones without PNSB. Further research should be carried to evaluate the potency of current K-sol PNSB strains under greenhouse conditions and field trials.

This work was supported by Can Tho University [Grant number CTCS2024-14-01].

The authors declare no conflicts of interest.