Research Article

Isolation and Functional Characterization of Indigenous Phosphate-Solubilizing Microbes from Shallot Rhizosphere in Andisol of Karo, Indonesia

Agustina E. Marpaung1,3*, Tengku Sabrina2, Abdul Rauf2, and Dwi N. Susilowati3

1Doctoral Student of Agriculture Faculty, Universitas Sumatera Utara, Medan 20155, Indonesia; 2Faculty of Agriculture, Universitas Sumatera Utara, Medan 20155, Indonesia; 3Research Center for Horticultural, Research Organization for Agriculture and Food, National Research and Innovation Agency (BRIN), Indonesia.

Received | February 03, 2026; Accepted | March 9, 2026; Published | August 22, 2026

*Correspondence | Tengku Sabrina, Faculty of Agriculture, Universitas Sumatera Utara, Medan 20155, Indonesia; Email: [email protected]

Citation | Marpaung, A.E., T. Sabrina, A. Rauf and D.N. Susilowati. 2026. Isolation and functional characterization of indigenous phosphate-solubilizing microbes from shallot rhizosphere in andisol of karo, Indonesia. Sarhad Journal of Agriculture, 42(4): 1452-1464.

DOI | https://dx.doi.org/10.17582/journal.sja/2026/42.4.1452.1464

Keywords | Allium cepa L, Andisol, Characterization, Identification, Phosphat solubilizing microbe, P solubilization

Copyright: 2026 by the authors. Licensee ResearchersLinks Ltd, England, UK.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Introduction

Low phosphorus availability is a common problem in tropical soils, especially those found in Andisol. With 127 active volcanoes, Indonesia is home to a sizable portion of Andisol, or volcanic ash soil (Saing et al. 2020). About 5.395 million hectares, or 2.9% of the total land area, are covered by this type of soil (Sukarman and Dariah, 2014) and used for agriculture (Anda et al., 2021). However, plants access to phosphorus is greatly diminished by Andisol’s high phosphorus retention capacity, which is mostly caused by reactive clay minerals like allophane (Marpaung et al., 2021). Only around 0.1% of the total phosphorus can be absorbed by plants since allophane can retain up to 97.8% of it (Zhu et al., 2018). Large amounts of soluble phosphorus fertilizers must be applied since the limited solubility of phosphorus in these soils frequently inhibits plant growth.

Additionally, phosphorus (P) is essential for controlling physiological reactions and improving resistance to abiotic stressors (Hawkesford et al., 2023); (Lambers 2022). P is essential for photosynthesis and the growth of roots and is the second primary nutrient after nitrogen (N) (Billah et al., 2019). A lack of phosphorus in the soil has a negative impact on root formation, vegetative growth, and fruit production, which eventually lowers agricultural yields (Jezek et al., 2023); (Lopez et al., 2023); (Abobatta and Abd Alla, 2023). The majority of phosphorus in soil is insoluble, thus even if it is abundant, its availability is poor.

A viable substitute for increasing the amount of phosphorus in soil is the application of phosphate-solubilizing microorganisms (PSMs) (Tian et al., 2021). Because they mineralize organic phosphorus, solubilize inorganic phosphorus minerals, and store significant amounts of phosphorus in their biomass, PSMs, which comprise a diverse group of soil microflora, play a critical role in the soil phosphorus cycle (Gross et al., 2020); (Liang et al., 2020). The processes of phosphate solubilization include the release of organic acids, phosphatase enzymes, lowering the pH of the soil, and boosting chelation activity, which creates additional phosphorus adsorption sites. According to Billah et al. (2019). these microorganisms convert insoluble phosphorus into forms that plants can readily absorb, such as orthophosphates (PO43-, HPO42–, dan H2PO4–).

The rhizosphere, the soil area close to plant roots, is home to the majority of phosphate-solubilizing microorganisms and provides a space for root exudation (Hassan et al., 2020). Bacterial species like Rhizobium, Arthrobacter, Burkholderia, and Rahnella aquatilis HX2 ((Liu et al., 2019); (Zhang et al., 2019)), Pseudomonas, Enterobacter, Bacillus (Biswas et al., 2018), Rhizobium, Arthrobacter, and Burkholderia, as well as fungal species like Penicillium brevicompactum and Aspergillus niger (Perea Rojas et al., 2019), are effective phosphate-solubilizing microorganisms. Sembiring and Sabrina (2022) research identified the bacteria Burkholderia cepacia, Bacillus subtilis, Burkholderia cenocepacia, Burkholderia seminalis which are able to dissolve phosphate from Andisol soil affected by the eruption of Mount Sinabung. An environmentally responsible and sustainable method of treating phosphorus deficit in agricultural soils is to use phosphate-solubilizing microorganisms (Kalayu, 2019).

This study seeks to identify and describe putative phosphate-solubilizing microorganisms in the rhizosphere of shallots cultivated in Andisol soil from Karo regency, Indonesia. By separating and screening microbial strains, evaluating their capacity to solubilize phosphate, and examining their morphological, physiological, and biochemical traits.

Materials and Methods

Sampling location determination for andisol soil in karo, indonesia

The research was conducted in May until December 2023 in Karo Regency, North Sumatra, Indonesia. To establish exact coordinates, the sampling locations for Andisol soil in Karo regency, Indonesia, were identified using the soil/geological maps (1:50.000). The color of soil using android-based Munsell soil color chart (Priandana et al., 2014). Samples of soil were taken from the shallot rhizosphere that were two months old. Five locations, each encompassing the rhizosphere of five plants, were used to collect composite soil samples at depths ranging from 0 to 15 cm. Coordinates the each samples from Karo Regency were taken from two points in Dolat Rayat (T) with coordinates 3o12’6,921”N 98o32’23,774”E (T1) and 3o11’41,562”N 98o32’33,379”E (T2); two points in Barusjahe (BJ) with coordinates 3o10’59”N 98o32’57”E (BJ1) and 3o10’59”N 98o32’54”E (BJ2); and one point in Simpang Empat with coordinates 3o9’46,659”N 98o29’7,608”E (SE).

Phosphate-solubilizing microorganisms potential from andisol soil in the karo regency of north sumatra

Isolation of phosphate-solubilizing microorganisms from andisol soil

The Microbiology Laboratory of Universitas Sumatera Utara (USU) processed Andisol soil samples from ten pre-selected locations. In a 250 mL Erlenmeyer flask, 10 g of dirt was weighed for each sample, and 90 mL of 0.85% saline solution was added. At room temperature, the mixture was shaken for half an hour at 120 rpm. To guarantee the isolation of phosphate-solubilizing fungus, serial dilutions (10-¹ to 10-7) were made, and suspensions from three dilutions were utilized. The spread plate method was used to isolate microorganisms on solid Pikovskaya’s media (Pikovskaya, 1948). For two to three days, plates were incubated at 28°C. Clear zones (halozones) encircling colonies were indicative of bacterial and fungal phosphate solubilization. Purified halozone-containing colonies were then incubated for three more days to verify their phosphate-solubilizing capabilities. On Pikovskaya’s medium, purified isolates were obtained following multiple streakings.

Qualitative assessment of phosphate-solubilizing activity

For qualitative phosphate-solubilization experiments, bacterial cultures cultured for 24 hours on nutritional broth (NB) and fungal cultures on potato dextrose agar (PDA) were employed. Pikovskaya’s medium was inoculated with 5 μL and let to sit at room temperature for 1–7 days. Measurements were made of the clear zones surrounding the colonies on Pikovskaya’s medium, which show that Ca2(PO₄)₂ has been phosphate solubilized. The following formula was used to determine the solubilization index (SI) (Paul and Sinha, 2017).

Molecular identification of phosphate-solubilizing microorganisms

The BRIN lab, the isolates with the highest phosphate solubilization activity underwent molecular identification. A genomic DNA Wizard kit (Promega) was used to extract DNA. For bacterial isolates, PCR amplification focused on the 16S rRNA gene; for fungal isolates, it targeted the ITS4 and ITS5 sections. Cultures of bacteria and fungi cultivated in liquid Luria Bertani broth were used to extract DNA. Agarose gel electrophoresis (0.8% gel) was used to analyze the PCR products, and nucleotide sequences were then ascertained by sequencing. The RDP-II database’s CLASSIFIER program was used to accomplish taxonomic classification with 95% confidence (Cole et al., 2014).

Characterization of potential phosphate-solubilizing microorganisms

Macroscopic and microscopic characterization

Nutrient agar (NA) was streaked with pure bacterial cultures, and they were cultured for three days. Along with cell shape and the results of Gram staining, colony morphology including size, shape, color, and edge was studied. To observe colony morphology, including hyphal development features, colony color, and colony shape, fungal cultures were cultivated on PDA for three days. The hyphal traits, branching patterns, and conidia features were examined under a microscope through microscopic observations.

Biochemical characterization

Organic Acid Production Test: High-Performance Liquid Chromatography (HPLC) was used to quantify organic acids. Chromatograms were examined using retention durations and peak regions after samples were injected.

Phosphatase Enzyme Activity Test: Using p-nitrophenyl phosphate as a substrate, phosphatase activity was measured. At 400 nm, the resultant p-nitrophenol was measured using spectrophotometry (Margesin, 1996).

IAA and Gibberellin Test: PLC was used to measure the amounts of gibberellin and indole-3-acetic acid (IAA) (Jimtha et al., 2014). Using a reverse-phase column with methanol as the mobile phase, extracts were made, filtered, and subjected to HPLC analysis.

Hemolysis Test: The hemolysis test is performed using blood agar. A clear zone formed around the colony on the medium indicates that the microbe is pathogenic (Zimbro et al., 2009).

Inhibition test

The dual plate method was used to assess interactions between fungi and bacteria that solubilize phosphate. In the middle of a Petri dish filled with a 1:1 combination of PDA and NA medium, a fungal mycelium plug (6 mm in diameter) was positioned (Goswami and Deka, 2020). After streaking bacteria on both sides of the plate, the plates were left to incubate for seven to fourteen days at room temperature. The ability of fungi and bacteria to coexist and form a single growth pattern was used to

 

Table 1: Chemical content of soil from shallot rhizosphere at 3 sample locations

Locations	Soil pH	Organic carbon (%)	Phosphate total (mg 100 g-1)	Available phosphate (ppm)
Dolat Rayat, Karo Barusjahe, Karo Simpang Empat, Karo	6.14 6.56 5.45	4.79 5.75 3.72	51.88 37.98 44.60	3.28 3.79 4.71

 

determine the good interaction activity.

Results and Discussion

Determination of sampling locations for acidic andisol soil in the karo regency of north sumatra using soil type maps and taxonomy

Soil samples were collected as composite samples from Andisol soils based on geological maps (Figure 1) and characterized by a black soil solum depth of 25–30 cm. Based on the Munsell Soil Chart, the soil colors from Dolat Rayat were identified as 10YR 3/2 (dark grey), 2.5Y 4/4 (bright brown) for Barusjahe, and 2.5Y 3/6 (bright brown) for Simpang Empat. These findings align with the range of Andisol colors in Indonesia, which vary from black (10YR 2/1) to dark reddish-brown (10YR 3/4) (Sukarman and Dariah, 2015).

Soil samples’ chemical analysis showed that the amount of phosphorus (P) varied depending on the location (Table 1). At the sample location in Dolat Rayat, it was found that the chemical properties of the soil were very low in available P content (3.28 ppm), high total P content (51.88 mg 100g-¹), high organic carbon content (4.79%), and slightly acidic pH (6.14). In Barusjahe, very low available P content was also obtained (3.79 ppm), moderate total P content (37.98 mg 100g-¹), very high organic carbon (5.75%), and acidic pH (6.56). In Simpang empat, the soil contained very low available P (6.36 ppm), high total P content (44.60 mg 100g-¹), high organic carbon content (3.72%), and slightly acidic pH (5.45).

Isolation and identification of potential phosphate-solubilizing microorganisms in andisol soil from the highlands of north sumatra

Isolation of phosphate-solubilizing microorganisms from andisol soil

Microbial colonies were measured for every sample location following four days of incubation (Table 2). The largest bacterial population was found in Barusjahe 1 (8.10 × 10⁶ CFU/g soil), whereas the highest fungal population was found in Dolat Rayat 1 (3.2 × 107 CFU/g soil). Eight isolates; two fungi and six bacteria were taken from Karo Regency’s Andisol soils.

 

Table 2: Number of phosphate-solubilizing microbial populations in andisol soil in karo

Soil Source	Bacteria (Cfu/g tanah)	Fungi (Cfu/g tanah)
Dolat Rayat 1	3.6 x 106 ± 0.02	3.2 x 107 ± 0.00
Dolat Rayat 2	4.3 x 106 ± 0.09	9.9 x 106 ± 1.05
Barusjahe 1	8.1 x 106 ± 0.26	2.2 x 107 ± 2.63
Barusjahe 2	5.7 x 106 ± 0.23	2.8 x 107 ± 0.79
Simpang Empat	6.8 x 106 ± 0.09	1.1 x 107 ± 0.07

 

Qualitative phosphate-solubilization test

The 8 isolates were subjected to qualitative phosphate-solubilization tests. All of isolates were found capable of forming clear zones around their colonies on Pikovskaya’s medium (Bacteria: T11, T21, BJ11, BJ12, BJ22, SE; Fungi: BJ21, BJ23). In general, the bacteria solubilising index is hingher than fungi solubilising index. The solubilization index (SI) showed that bacterial isolate T21 exhibited the highest SI among all isolates during 1-4 days after inoculation. However, from 5-7 days, the solubilization index of bacteria isolat is variation. In fungi it is obtained that isolate BJ23 had the highest SI than BJ21 (Figure 2). In general, the SI produced by bacteria increased from 1 to 6 days and decreased on the 7th day, except for bacterial isolates T21, BJ12, and BJ22, which remained elevated. Meanwhile, the SI of fungi decreased starting at 4 days.

 

Table 3: Identification of 8 isolates of phosphate-solubilizing microbes

Soil Source	Isolate	Identification	Solubilization indeks 3 DAI
Dolat rayat, Karo	T11	Bacillus thuringiensis	2.25
	T21	Bacillus cereus	2.27
Barusjahe, karo	BJ11	Bacillus substilis	2.15
	BJ12	Bacillus cereus	2.10
	BJ22	Burkholderia gladioli	2.04
	BJ21	Aspergillus pseudodeflectus	1.24
	BJ23	Aspergillus niger	1.35
Simpang empat, Karo	SE	Bacillus cereus	2.22

 

DAI: Days after inoculation

 

Identification of phosphate-solubilizing microorganisms

Molecular identification of eight phosphate solubilizing isolates showed that Bacillus thuringiensis and Bacillus cereus were found in Dolat Rayat; Bacillus substilis, Bacillus cereus, Burkholderia gladioli, Aspergillus niger, and Aspergillus pseudodeflectus were found in Barusjahe; and Bacillus cereus was found in Simpang empat (Table 3; Figure 3 and Figure 4). In general, bacterial isolates produce a higher solubility index than fungi. These demonstrate the isolates varying capacities for solubilization, as seen by their capacity to produce clear zones on Pikovskaya’s medium.

 

 

Characterization of Potential Phosphate-Solubilizing Microorganisms from Andisol Soil in the Karo Regency, North Sumatra

Macroscopic and microscopic characterization of phosphate-solubilizing microorganisms

The macroscopic characterization of phosphate-solubilizing Bacillus isolates revealed circular colonies with white to milky-white coloration, large size, and

 

 

 

undulate colony margins. Different strains exhibited varying macroscopic characteristics. Microscopically, all six bacterial isolates had rod-shaped cells. Gram staining revealed that Bacillus strains and Burkholderia were gram-positive (Table 4).

The macroscopic characterization of fungi isolates showed distinct features. A. niger BJ23 had circular colonies with fine filamentous structures, small spores, dark brown-black coloration on the upper colony, and yellowish-white coloration on the lower colony surface. Hyphae were septate. A. pseudodeflectus BJ21 formed circular colonies, no spore, whit coloration on the upper colony, and the lower colony surface. Microscopically, A. niger BJ23 fungal isolates exhibited septate hyphae with unbranched structures and conidia forming round vesicles (Table 5).

Biochemical characterization of phosphate-solubilizing microorganisms

Organic acid production test

All eight phosphate-solubilizing isolates could produce organic acids (Table 6), including citric, oxalic, malic, lactic, and acetic acids. The quality and quantity of organic acids produced varied among the isolates. The highest production of citric acids was observed

 

 

in B. cereus (T21) (0.453 mg L-¹), oxalic acids in B. substilis (BJ11) and B. cereus (SE) (1.393 and 1.301 mg L-¹ respectively), malic acid in B. substilis (BJ11) (9.94 mg L-¹), lactic acid in A. niger (BJ23) (12.076 mg L-¹), and acetic acid in B. cereus (T21) (6.768 mg L-¹). Notably, B. gladioli (BJ22) did not produce citric, oxalic and malic acid.

 

Table 6: Organic acid tests on several phosphate-solubilizing microbial isolates

Isolate Code	Consentration (mg L-1)
	Citric	Oxalic	Malic	Lactic	Acetic
Control bacteria	Nd	Nd	Nd	Nd	Nd
Control fungi	Nd	Nd	Nd	Nd	Nd
Bacteria
B. thuringiensis (T11)	0.199	0.611	7.829	Nd	Nd
B. cereus (T2 1)	0.453	0.565	9.672	11.031	6.768
B. substilis (BJ1 1)	0.386	1.393	9.940	11.613	5.502
B. cereus (BJ1 2)	0.399	0.858	9.622	7.701	2.147
B. gladioli (BJ2 2)	Nd	Nd	Nd	2.770	2.080
B. cereus (SE)	0.109	1.301	9.852	4.990	2.215
Fungi
A. pseudodeflectus (BJ21)	0.248	0.973	5.971	6.063	5.608
A. niger (BJ2 3)	0.274	0.847	8.129	12.076	6.062

 

Nd = not detected

 

Phosphatase enzyme activity test

All types of bacteria and fungi can produce phosphatase enzymes. The phosphatase content of each microbe varies based on its ability to produce the enzyme. The three highest-ranking isolates producing phosphatase enzymes are Bacillus substilis (BJ11) with the highest (1.594 mg L-¹), followed by Bacillus cereus (BJ12) (1.469 mg L-¹) dan Bacillus thuringiensis (T11) (1.297 mg L-¹). The lowest phosphatase activity was found in Aspergillus pseudodeflectus (BJ21) (0.277 mg L-¹) (Figure 5).

Phytohormone (IAA and Gibberellin) test

All eight isolates produced higher concentrations of IAA compared to gibberellin. Fungal isolates, Aspergillus pseudodeflectus (BJ21) and Aspergillus niger (BJ23), produced higher IAA (11.16 mg L-¹ and 8.84 mg L-¹) and gibberellin (3.53 mg L-¹ and 2.22 mg L-¹) than bacterial isolates (Figure 6).

Hemolysis test

The results of the hemolysis test on eight isolates revealed that one isolate underwent lysis in Bacillus substilis (BJ11) blood agar (Table 7 and Figure 7). This indicates that both isolates are pathogenic to humans and animals because they can lyse red blood cells in the medium, as indicated by the formation of a clear zone in the growth medium.

 

Table 7: Hemolysis test on several phosphate-solubilizing microbial isolates

Isolates	Hemolysis
Bacteria
B. thuringiensis (T1 1)	-
B. cereus (T2 1)	-
B. substilis (BJ1 1)	+
B. cereus (BJ1 2)	-
B. gladioli (BJ2 2)	-
B. cereus (SE)	-
Fungi
A. pseudodeflectus (BJ2 1)	-
A. niger (BJ2 3)	-

 

Note: + hemolysis, - No hemolysis

 

 

Inhibition test between phosphate-solubilizing bacteria and fungi

Inhibition test between bacterial and fungal isolates revealed that none of the bacterial isolates exhibited compatibility with either of the fungal isolates (Table 8). Bacillus thuringiensis (T11) and Bacillus substilis (BJ11) were tested with Aspergillus pseudodeflectus, fungal growth was dominant, suppressing bacterial growth. Aspergillus niger was test with Bacillus thuringiensis (T11), Bacillus cereus (BJ12) and Burkholderia gladioli (BJ2 2), produce the dominant growth of fungi than bacteria.

The research results showed that the chemical examination of soil samples from each location revealed that levels of organic carbon were high, total phosphate was high, and there were very low amounts of available P. This trait is consistent with Andisol soils’ characteristics, which include little accessible phosphorus because of significant allophane phosphorus fixation (Zhu et al., 2018). According to (Nanzyo et al., 1993), Andisol soils are also known for having a high organic carbon content, which is in line with the high to extremely high quantities of organic carbon found in the soil samples. Andisol is appropriate for agricultural usage because of these qualities (Anindita et al., 2023). The pH values of the soil samples, which ranged from 5.45 to 6.56, were classified as somewhat acidic. Most Andisol soils in Indonesia are acidic (pH 4.5 to 5.5), with a typical pH range of 3.4 to 6.7 (Sukarman and Dariah, 2015).

 

 

Among the eight phosphate-solubilizing isolates tested, all were capable of solubilizing phosphate on Pikovskaya’s medium, which was indicated by the formation of a clear zone around the isolate. These findings confirm the ability of phosphate-solubilizing bacteria to effectively solubilize phosphate in qualitative tests (Ulfiyati and Zulaika, 2015). One species of phosphate-solubilizing bacteria is Bacillus sp. (Kumar and Rai, 2020). The genus Bacillus is known for its phosphate-solubilizing capabilities (Prakash and Arora, 2019). Previous studies have also identified Bacillus and Burkholderia as effective phosphate solubilizers ((Biswas et al., 2018); (Liu et al., 2019)). Similarly, Aspergillus niger has demonstrated significant phosphate solubilization potential (Perea Rojas et al., 2019) on tricalcium phosphate media (Bakri, 2019). According to Sembiring and Sabrina (2022) research, the Burkholderia cepacia resulted in an increase in the available P of the soil of 83.37%, and Bacillus subtilis by 25.28% than the control.

Organic acid release is the main mechanism by which microorganisms solubilize phosphate. Microorganisms can access labile inorganic phosphate (Pi) fixed in iron (Fe) or aluminium (Al) complexes by secreting organic acids, which chelate Fe and Al ions and release phosphorus (Baumann et al., 2018). In addition, organic acid anions produced by phosphate-solubilizing microorganisms can increase soil phosphorus availability and reduce phosphate fixation by blocking or competing for soil adsorption sites on iron and aluminum oxides (Alori et al., 2017). Organic acids like citric, acetic, propionic, and lactic acids are produced by Bacillus species (do Carmo et al., 2019). A. niger also secretes organic acids, such as formic and oxalic acids (Li et al., 2016); (Tian et al., 2019), which reduce the medium’s pH and make it easier for phosphates to dissolve.

Enzymatic activities, including the formation of phosphatase and phytase, are involved in the mineralization of phosphorus in soil. (Billah et al., 2019) Many organisms have the ability to solubilize and mineralize phosphate, such as Bacillus, Cyanobacteria, and Pseudomonas. According to, (Neira-Vielma et al., 2018) Aspergillus niger is also known to create phytase, which facilitates the consumption of phosphorus from phytates. Paul and Sinha, 2017 state that the production of phosphatase enzymes by phosphate-solubilizing fungus or the secretion of polysaccharides are frequently cited as the causes of the halo zones that surround fungal colonies.

Phosphate-solubilizing microorganisms are generally classified as plant growth-promoting microorganisms (PGPM) due to their phytostimulatory capacities;. (Hassan, 2017; Compant et al., 2019). Phosphate-solubilizing microbes produce growth-promoting hormones such as auxins, cytokinins, and gibberellins that promote cell division, cell differentiation, shoot growth, root development, flowering, germination, and xylem differentiation (Puri et al., 2020). Among PGPMs, those capable of solubilizing phosphate demonstrate substantial benefits for plant growth and yield (Tang et al., 2020) (Fahad et al., 2015). For instance, Bacillus sp. STJP (Prakash and Arora, 2019) and Bacillus sp. CP h60 (Ditta et al., 2018) have been reported to produce the phytohormone indole-3-acetic acid (IAA). Additionally, Bacillus sp. and Pseudomonas sp. produce gibberellins (Setiawati et al., 2015) which are also linked to their phosphate-solubilizing abilities. Phosphate-solubilizing microbes, in addition to increasing P availability in the soil, can also improve plant growth indicators. This is consistent with the Sembiring et al. (2016) research results of that found that the addition of phosphate-solubilizing bacteria significantly increased shoot dry weight (50.07-113.73%) and P uptake (34.47-163.28%) of potato plants on Andisols.

Phosphate-solubilizing microbial strains produce, in addition to organic acids and phosphatase enzymes, hemolysis. Research shows that Bacillus subtilis produces lysis on blood agar media, which can be harmful to living organisms. Bacillus subtilis is pathogenic, producing β-type hemolysis (Gai et al., 2023).

In order to assess the compatibility of bacterial and fungal isolates a critical step in the development of biofertilizers based on microbial consortiums, it is imperative to test for inhibition between them. The efficiency of advantageous additives can be increased by adding two or more compatible microbial strains from various species; their synergism makes it easier to obtain several desired benefits (Louca et al., 2018). In this study, the results of the inhibition tests indicated that none of the bacterial isolates exhibited compatibility with either fungal isolate. When co-cultured on a single petri dish, a clear inhibition zone was observed between the bacterial and fungal colonies, suggesting the formation of an antagonistic zone. This inhibition likely prevents the two types of microorganisms from coexisting harmoniously.

Conclusions and Recommendations

Based on the results of the isolation of phosphate-solubilizing microbes in Andisol soil in Karo, Indonesia, 8 isolates (6 bacteria and 2 fungi) were obtained that were able to dissolve phosphate based on a qualitative P dissolution test. The results of the identification of the eight types of isolates from each sample location were B. thuringiensis (T1 1), B. cereus (T2 1), B. substilis (BJ1 1), B. cereus (BJ1 2), B. gladioli (BJ2 2), B. cereus (SE), A. pseudodeflectus (BJ2 1) and A. niger (BJ2 3). The eight isolates can produce organic acids, phosphatase enzymes, and phytohormones of different types and amounts, that can dissolve inorganic phosphat that bound in Andisol minerals and plant growth stimulation. The hemolysis results obtained that B. substilis (BJ1 1) were pathogenic to living things. Each bacteria could not be consorted with fungi based on the inhibition test. Further research should be focused on seven types of phosphate-solubilizing microbes, namely B. thuringiensis (T1 1), B. cereus (T2 1), B. substilis (BJ1 1), B. gladioli (BJ2 2), B. cereus (SE), A. pseudodeflectus (BJ2 1) and A. niger (BJ2 3), which should be tested on plants in the field to determine their potential in P solubilization and increasing yield.

Acknowledgements

The author expresses gratitude to the Research Organization for Agriculture and Food (BRIN), Indonesia, for funding the research by an in-house project of the Research Organization for Food and Agriculture 2023 under project number 9/III.11/HK/2023.

Novelty Statement

This research has obtained seven types of phosphate-solubilizing microbes, B. thuringiensis (T1 1), B. cereus (T2 1), B. substilis (BJ1 1), B. gladioli (BJ2 2), B. cereus (SE), A. pseudodeflectus (BJ2 1) and A. niger (BJ2 3, which are able to dissolve phosphate qualitatively. These microbes are also able to produce organic acids, phosphatase enzymes, and phytohormones, which can dissolve inorganic phosphate bound to Andisols minerals and stimulate plant growth.

Author’s Contribution

Agustina E. Marpaung: Conceptualization, supervision, methodology, software, validation, formal analysis, investigation, data curation, writing-original draft and funding acquisition.

T. Sabrina: Methodology, validation, formal analysis, writing-review & editing and Funding acquisition.

Abdul Rauf: Methodology, validation, formal analysis and funding acquisition.

Dwi N. Susilowati: Methodology, validation, formal analysis, data curation and writing-review. All co-authors examined the final draft and provided its approval for the manuscript prior to submission.

Generative AI or AI assisted technology statement

During the preparation of this work, the author used Artificial Intelligence (AI). After using this tool/service, the author reviewed and edited the content as needed and takes full responsibility for the content of the publication.

Conflict of interest

The authors indicate no conflict of interest in this work.

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