Effects of solid biofertilizer containing potassium-solubilizing purple nonsulfur bacteria on potassium dynamics, growth, and yield of hybrid maize grown in dyked alluvial soils

Vo Yen Ngoc; Le Thi My Thu; Nguyen Duc Trong; Tran Trong Khoi Nguyen; Le Thanh Quang; Tran Loc Thuy; Tran Chi Nhan; Ly Ngoc Thanh Xuan; Nguyen Quoc Khuong

doi:10.1515/opag-2025-0487

Article Open Access

Effects of solid biofertilizer containing potassium-solubilizing purple nonsulfur bacteria on potassium dynamics, growth, and yield of hybrid maize grown in dyked alluvial soils

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Published/Copyright: January 19, 2026

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From the journal Open Agriculture Volume 11 Issue 1

Abstract

This study was conducted to evaluate the effects of solid biofertilizer (SBF) containing potassium-solubilizing purple nonsulfur bacteria (K-PNSB), including Cereibacter sphaeroides M-Sl-09, Rhodopseudomonas thermotolerans M-So-11, and Rhodopseudomonas palustris M-So-14, on soil properties, K uptake, growth, and yield of hybrid maize cultivated in dyked alluvial soil collected from an Phu, An Giang. A two-factor experiment was arranged in a completely randomized design with four replications. The first factor was K fertilizer levels (0, 25, 50, 75, and 100 % of the recommended rate), and the second was SBF types (sterile water only, single strains M-Sl-09, M-So-11, M-So-14, or a mixture of all three strains). The application of SBF containing either single or mixed K-PNSB strains significantly enhanced soil exchangeable K, total K uptake, plant growth, and grain yield compared with the uninoculated control. The highest yield and total K uptake were recorded under 100 % K fertilizer combined with the three-strain SBF (106.5 g plant⁻¹), demonstrating a strong synergistic interaction between biofertilizer and K application. These findings suggest that K-PNSB-based biofertilizer can improve potassium availability and maize productivity in nutrient-depleted dyked alluvial soils.

Keywords: hybrid maize; purple nonsulfur bacteria; solid biofertilizer; yield; Zea mays L

1 Introduction

Maize (Zea mays L.) is a high-value cereal crop, ranking third globally in cultivated area after wheat and rice [1]. In 2023, the global maize cultivation area reached 208,234,140 ha with an average yield of 5.9 tons/ha, while in Vietnam, it was 880,209 ha with an average yield of 5.04 tons/ha [2]. Maize is nutritionally valuable and well adapted to harsh environmental conditions [3]. Apart from human consumption, hybrid maize is also used for animal feed and bioethanol production [4]. However, to meet global food demands by 2050 [5], it is essential to increase crop yields while minimizing environmental impacts.

In the downstream provinces of the Mekong River, flood prevention dykes have been built to enable intensive cropping and avoid abnormal floods in the Mekong Delta (MD) [6]. However, these long-term dykes have adversely impacted intensive agriculture in the MD, particularly in An Giang province [7]. Because dykes prevent annual sediment deposition, natural alluvium is depleted, leading to increased fertilizer costs and usage compared to non-dyked areas [7], 8]. The absence of annual alluvial replenishment results in nutrient-depleted soils, especially potassium, which is crucial for plant growth [9].

Chemical fertilizers are key factors influencing plant growth and yield [10]. Hybrid maize, in particular, demands high nutrient inputs during vegetative and reproductive stages [11]. However, excessive use of chemical fertilizers beyond recommended levels harms the environment and human health. Farmers often apply more than necessary in an attempt to increase yield, but only a fraction benefits the crop while the rest causes soil degradation and water pollution [12]. Additionally, monocropping practices facilitate the buildup of pests and diseases, reducing productivity and economic efficiency [13]. Flood-control dykes built to prevent seasonal inundation have allowed intensive cultivation but also restricted the annual deposition of nutrient-rich sediment [14]. As a result, soils in dyked fields, especially in An Giang province, have become increasingly depleted in macronutrients, notably potassium (K) [7]. Preliminary analysis of the experimental soil revealed low exchangeable K (0.27 meq 100 g⁻¹) and low total K availability, confirming its K-deficient condition. Potassium plays a vital role in enzyme activation, osmotic regulation, and assimilate translocation; hence, its depletion significantly reduces crop growth and yield [15]. Therefore, identifying biofertilizers to replace chemical K fertilizer is crucial for sustainable K supply [16]. Biofertilizers containing K-solubilizing bacteria (KSB) can enhance available K in soil and improve plant K uptake [17].

Among various microbial inoculants, potassium-solubilizing bacteria (KSB) are capable of mobilizing insoluble K from minerals such as mica, feldspar, and illite by releasing organic acids (e.g., citric, oxalic, gluconic acids), exopolysaccharides, and siderophores that chelate cations and free K⁺ ions into the soil solution [18]. These microorganisms can also improve soil structure, nutrient cycling, and plant growth through phytohormone synthesis. K-solubilizing bacteria are a promising approach to enhance K availability in K-deficient soils without adverse environmental effects [19].

Purple nonsulfur bacteria (PNSB) are capable of N-fixation, phosphate and potassium solubilization, and can promote plant growth and yield [20]. Their application in agriculture can also reduce the need for chemical fertilizers [21]. In the MD, several PNSB strains have been identified with K-solubilizing abilities, including Cereibacter sphaeroides M-Sl-09, Rhodopseudomonas thermotolerans M-So-11, and Rhodopseudomonas palustris M-So-14 [22]. They have been successfully isolated from alluvial soils of the Mekong Delta, showing strong K-solubilizing potential under both microaerobic light and aerobic dark conditions [22]. To survive and proliferate in soil, PNSB strains require carriers and substrates, thus making solid biofertilizer (SBF) a suitable form to maintain viable populations. When incorporated into compost-based carriers, PNSB can remain viable during storage and contribute to improved nutrient availability and plant performance after application. Various materials have been tested to prolong microbial shelf life [23], 24]. Despite these advantages, the interactive effects between PNSB-based solid biofertilizers (SBF) and different K fertilizer levels on soil K dynamics and maize yield have not been fully elucidated, particularly in nutrient-depleted dyked alluvial soils. Therefore, this study aimed to evaluate the effects of SBF containing K-solubilizing PNSB (Cereibacter and Rhodopseudomonas spp., K-PNSB) on soil properties, K uptake, growth, and yield of hybrid maize grown in dyked alluvial soil collected from An Phu, An Giang.

2 Materials and methods

2.1 Materials

Planting pots: The pots measured 30 × 25 × 27 cm (top diameter × bottom diameter × height) and had seven drainage holes at the base.

Planting soil: Surface soil (0–20 cm) was collected from dyked alluvial maize fields in An Phu district, An Giang province. The soil was air-dried and large organic debris (roots, leaves) removed. 10 kilograms of soil were weighed for each pot for greenhouse experimentation. The study was conducted from January to July 2025 in Greenhouse No. 5, College of Agriculture, Can Tho University. Initial soil properties were as follows: pH_H2O = 5.44 ± 0.06, pH_KCl = 4.82 ± 0.05, EC = 0.26 ± 0.03 mS cm⁻¹, CEC = 12.8 ± 1.77 meq 100 g⁻¹, total N = 0.059 ± 0.04 %, available nitrogen (NH₄ ⁺) = 15.6 ± 2.44 mg kg⁻¹, total P = 0.095 ± 0.07, soluble P = 47.3 ± 2.11 mg kg⁻¹, and exchangeable K = 0.27 ± 0.03 meq 100 g⁻¹. These data confirm the low-K availability of the experimental soil.

Maize variety: Hybrid maize seeds of cultivar DK6919S were provided by Bayer Company. The actual germination rate of the current variety was 90 %.

Fertilizers and pesticides: Urea (46 % N; Phu My company, Vietnam), superphosphate (16 % P₂O₅; Long Thanh company, Vietnam), and muriate of potash (60 % K₂O; Phu My company, Vietnam) were used.

2.2 Preparation of solid biofertilizer (SBF)

Preparation of carrier material: Maize husks, leaves, and stems were crushed and sieved through a 2-mm mesh. These materials, along with rice husk ash, were packed in bags, sterilized at 121 °C for 30 min, and oven-dried at 65 °C to serve as carrier and substrate.

Bacterial strains: C. sphaeroides M-Sl-09, R. thermotolerans M-So-11, and R. palustris M-So-14, K-solubilizing PNSB strains, were isolated from maize fields in An Phu, An Giang, and stored at −80 °C at the College of Agriculture, Can Tho University [22]. Each strain was cultured separately in basic isolation medium [25]. Cultivation was conducted under microaerobic light (3,000 lux.) at 30 ± 2 °C for 5 days until the cell density reached 10⁸ CFU mL⁻¹. Strains were propagated individually, then mixed at equal cell densities before incorporation into the solid substrate for the three-strain treatment.

2.3 Methods

Experimental design: A two-factor randomized complete block design was used. Factor 1 was K application rates at five levels: 0 %, 25 %, 50 %, 75 %, and 100 % of the recommended rate (200 N–90 P₂O₅ – 80 K₂O (kg ha⁻¹). Factor 2 was SBF applications with five levels: the control (SBF inoculated with no bacteria), SBF with single strain M-Sl-09, SBF with single strain M-So-11, SBF with single strain M-So-14, and SBF with a mixture of these three strains. A total of 25 treatment combinations were tested, each with four replicates, and one pot (10 kg soil) per replicate.

Composting process: Materials were sterilized at 121 °C for 30 min to eliminate harmful microbes, then dried at 65 °C. Under sterile conditions, materials were mixed in a 1:1:3 ratio (rice husk: stem: leaf) and homogenized. PNSB cultivated under light condition for 48 h, after that the cells were used in materials. In a laminar flow hood, 50 mL of bacterial suspension was added to the mix to reach a final density of 0.33 × 10⁶ CFU/g per bag (or sterile water for the control). Moisture content was adjusted to 60 % with sterile water and homogenized. The mixture was incubated in a well-ventilated and shaded area for one month. Mature compost was dark brown, porous, free of foul odors, and used as SBF.

Fertilizer application: Bacteria were inoculated onto maize seeds before sowing according to Thu et al. [22]. In addition, 10 g of SBF was applied per pot at 10, 25, 40, and 55 days after sowing (DAS) in accordance with treatment protocols. PNSB were applied on the soil surface and subsequently covered by thin soil layer. All mineral fertilizers (urea, superphosphate, and muriate of potash) were applied simultaneously with SBF at each scheduled dose. Phosphorus was fertilized before sowing. Nitrogen was split into three applications at the rates of 30, 30, 40 % on 10, 20, 45 after sowing (DAS). Potassium fertilizer was split into two equal applications at 10 and 45 DAS.

2.3.1 Growth parameters

Plant growth measurements included: Plant height (cm) measured from the base at soil level to the tip of the highest leaf; stem diameter (cm) measured at three points (top, middle, bottom), and the average was taken; leaf length (cm) measured from the leaf collar to the tip of the uppermost fully developed leaf; leaf width (cm) measured at the widest point of the uppermost fully developed leaf; number of leaves counted for all true leaves on each plant; cob diameter (cm) measured after removing grains. All growth parameters were recorded at harvest (90 days after sowing).

Yield and yield components included: Cob length (cm) measured from the base to the tip of the cob; cob diameter (cm) measured at three positions on the cob and averaged; number of rows per cob counted in each cob; number of grains per row counted on a randomly chosen row; weight of 100 grains (g) randomly selected and weighed using an electronic balance (three decimal places); and grain yield per pot (g) harvested and weighed, while moisture content was measured and grain yield was standardized to 15.5 % moisture. Some maize plants could produeced two ears. However, the second ear (lower ear) was eliminated after two-day appearance to ensure uniformity, because some plants produced only one ear.

Dry matter biomass: Cobs were dried naturally, grains separated, placed in labeled paper bags, and oven-dried at 70 °C for 72 h until constant weight. Other parts (stem, leaves, roots, tassels, cobs, husks) were also dried, weighed separately, and recorded.

2.3.2 Soil analysis

Soil sampling: One composite sample before planting and two core samples per pot at harvest were collected and analyzed at Laboratory D204, Faculty of Crop Science, College of Agriculture, Can Tho University. The soil was air-dried for 7 days, crushed, and sieved through 2 mm and 0.5 mm mesh.

Analytical procedures were conducted following Sparks et al. [26]:

Actual acidity (pH_H2O): 10 g of air-dried soil via a sieve (2.0 mm × 0.5 mm) was added with 25 mL of distilled water. The suspension was centrifuged at 2,000 rpm for 10 min using a Hettich Universal 320 R centrifuge (Swing-out rotor, 6-place 1,619, radius: 151 mm), corresponding to approximately 1,999×g, filtered via filter paper (Whatman #2, Cytiva, USA) and measured by a pH meter. Potential acidity (pH_KCl) was done the same but distilled water was replaced with KCl (1.0 M). Electrical conductivity (EC) was done the same as pH_H2O measurement but measured by an EC meter.

Exchangeable K⁺: 2.5 g of sieved soil and 30 mL BaCl₂ 0.1 M was centrifuged thrice. The extract was diluted to 100 mL by BaCl₂ 0.1 M and measured by atomic absorption spectroscopy (AAS) (AA7000 Shimadzu, Japan) at 766.5 nm. Cation exchange capacity (CEC): Soil from the above extraction was shaken overnight with 30 mL 0.025 N BaCl₂, then centrifuged at 4,000 rpm for 10 min, and removed from water. After that, 30 mL MgSO₄ 0.02 M was added, shaken for 2 h, centrifuged for 10 min, and filtered for extract. The extract was made up to 50 mL with MgSO₄ 0.02 M, then added with 20 mL of distilled water, 1.0 mL of pH 10, and four drops of color indicator. The extract was titrated with 0.01 M EDTA under the sample turned from red to blue.

Total nitrogen (N): 1.0 g of air-dried soil (sieved through 0.5 mm mesh) was weighed and digested using a concentrated H₂SO₄ and salicylic acid mixture. Total N content was then determined using the Kjeldahl distillation method. NH₄ ⁺ was determined by weighing 2.0 g of air-dried soil (0.5 mm particle size), adding 50 mL of 2.0 M KCl solution, shaking for 1 h, then centrifuging and filtering. The extracted solution was analyzed for NH₄ ⁺ content using a spectrophotometer (UV/Vis UV1900 Shimadzu, Japan) at 640 nm wavelength.

Total phosphorus (P): 5.0 mL concentrated H₂SO₄ and 1.0 mL HClO₄ was added to 1.0 g of soil (sieved through 0.5 mm mesh) to digest the sample. The resulting solution was measured using a spectrophotometer at 880 nm. Ca–P content was determined by weighing 0.5 g of soil (0.5 mm mesh) into a centrifuge tube. 20 mL sodium citrate (Na₃C₆H₅O₇) and 2.5 mL sodium bicarbonate (NaHCO₃) was added, followed by 0.5 g sodium dithionite (Na₂S₂O₄). The suspension was heated in a water bath at 85 °C for 15 min, then shaken, and centrifuged to collect the supernatant. 25 mL H₂SO₄ was added to the soil residue, shaken for 1 h, and centrifuged for 10 min to collect the supernatant. The soil residue was washed twice with 25 mL saturated NaCl, shaken, centrifuged, and combined. The phosphorus content in the combined extract is measured at 880 nm using a spectrophotometer. For the soluble phosphorus, 2.0 g of soil (0.5 mm mesh) was weighed into a centrifuge tube, added with 10 mL of extraction solution, shaken for 1 min, and filtered through filter paper. 1 mL of the filtrate was added with 4 mL of 2.4 % boric acid. Then, 1.0 mL of reagent A (containing 2.5 M sulfuric acid, ammonium molybdate, 0.1 M ascorbic acid, and potassium antimonyl tartrate) was added, shaken well, and let react for at least 20 min before measuring with a spectrophotometer at 880 nm.

PNSB population: The density of PNSB from harvest soil was enumerated by most probable number technique as described by Harada et al. [27] and modified by Kantachote et al. [28]. In brief, the number of viable microorganisms in a sample was statistically estimated under culture-based technique via serial dilution and observation of growth presence or absence. This allows for quantitative estimation of viable purple non-sulfur bacteria populations in soil.

2.3.3 Plant analysis

Sample preparation: After harvest, maize parts including grain, stem, leaf, root, husk, tassel, and cob were oven-dried and then finely ground using a grinder. Samples were analyzed according to Houba et al. [29].

Potassium content analysis: The K content in grain, stem, leaf, root, husk, and cob was analyzed as follows: Weigh 0.3 g of the finely ground sample (grain, stem, leaf, husk, root, tassel, or cob) into a digestion flask; digest the sample using a mixture of concentrated H₂SO₄ and 30 % H₂O₂ at 180 °C; add 3.3 mL of digestion solution (containing H₂SO₄ and salicylic acid), shake well, and let the sample sit for 24 h; then heat the mixture until it becomes completely colorless. The digest was diluted to a final volume of 50 mL and used for K measurement. Potassium concentration was determined using an atomic absorption spectrophotometer at a wavelength of 766.5 nm.

2.3.4 Statistical analysis

All data were analyzed using SPSS version 13.0. Treatment means were compared using Duncan’s multiple range test at a 5 % significance level.

3 Results

3.1 Soil chemical properties before and after treatment

The experimental soil before planting was acidic and low in exchangeable K (0.27 meq 100 g⁻¹), confirming its K-deficient nature. After maize cultivation, both K fertilizer and SBF treatments significantly affected soil properties.

The pH_H2O values in treatments with SBF containing bacteria were higher than those without bacteria, ranging from 5.16 to 5.21 compared to 5.07, respectively. However, the pH_KCl values in the control SBF treatments were similar to those with bacterial inoculation, ranging from 4.40 to 4.62. Additionally, electrical conductivity (EC) values in the treatments with single strains M-So-11, M-So-14, and the three-strain mixture (M-Sl-09, M-So-11, and M-So-14) were lower than the non-inoculated SBF control, with values of 0.331–0.353 compared to 0.410 mS cm⁻¹. Furthermore, the highest values of exchangeable K, NH₄ ⁺, and soluble P were recorded in the treatments receiving 100 % K fertilizer combined with the three-strain SBF mixture, reaching 0.461 meq 100 g⁻¹, 18.6 mg kg⁻¹, and 48.0 mg kg⁻¹, respectively. In contrast, the SBF-inoculated treatments had lower levels of insoluble P compared to the non-inoculated SBF control with Al–P at 280.7–329.0 < 349.9 mg kg⁻¹, Fe–P at 86.2–92.7 < 114.0 mg kg⁻¹, and Ca–P: 253.4–281.9 < 305.1 mg kg⁻¹. Moreover, across the different K application levels, there were no significant differences in pH_H2O, pH_KCl, CEC, total P, total N, Al–P, Fe–P, or Ca–P. Similarly, among the SBF types, CEC, total P, and total N contents did not differ significantly (Table 1). Moreover, the SBF applicatiom improved the PNSB density as compared to no added SBF, with 4.841–4.871 and 3.872–3.894 MPN g DSW⁻¹, respectively (Figure 1).

Table 1:

Effects of potassium application rates and solid biofertilizer containing potassium-solubilizing purple nonsulfur bacteria strains on the fertility of dyked alluvial soil for hybrid maize cultivation.

Factor		pH_H2O	EC	CEC	Exchangeable K	Total P	Total N
Factor		–	mS cm⁻¹	meq 100g⁻¹		%
Potassium rate (A)	100 %	5.14	0.385^a	17.0	0.443^a	0.044	0.099
	75 %	5.11	0.358^ab	16.6	0.425^b	0.043	0.100
	50 %	5.14	0.387^a	16.8	0.414^c	0.047	0.103
	25 %	5.20	0.363^ab	17.3	0.401^d	0.044	0.099
	0 %	5.20	0.328^b	17.6	0.404^d	0.044	0.101
SBF (B)	No-SBF	5.07^b	0.410^a	16.7	0.372^e	0.044	0.099
	M-Sl-09	5.17^a	0.383^ab	16.8	0.435^b	0.049	0.099
	M-So-11	5.16^a	0.342^c	16.8	0.401^d	0.043	0.014
	M-So-14	5.21^a	0.353^bc	17.5	0.417^c	0.044	0.099
	MTS	5.18^a	0.331^c	17.5	0.461^a	0.042	0.101
Significance (A)		ns	*	ns	*	ns	ns
Significance (B)		*	*	ns	*	ns	ns
Significance (A × B)		ns	*	ns	*	ns	ns
CV (%)		2.45	15.1	10.0	3.34	20.0	13.9

Factor		pH_KCl	NH₄ ⁺	Soluble P	Al–P	Fe–P	Ca–P
Factor		–	mg kg⁻¹
Potassium rate (A)	100 %	4.48	15.3^a	41.0	316.5	93.0	275.7
	75 %	4.55	14.8^b	41.3	313.1	92.3	272.0
	50 %	4.49	14.1^c	40.8	314.4	93.4	273.4
	25 %	4.52	13.8^cd	40.9	312.7	92.7	273.2
	0 %	4.54	13.5^d	40.5	312.2	94.8	280.4
SBF (B)	No-SBF	4.53^ab	7.11^e	32.1^e	349.9^a	114.0^a	305.1^a
	M-Sl-09	4.52^ab	13.2^d	42.3^c	329.0^b	86.3^c	258.9^c
	M-So-11	4.50^ab	14.5^c	36.9^d	316.9^c	92.7^b	281.9^b
	M-So-14	4.40^b	18.0^b	45.1^b	280.7^e	86.4^c	275.4^b
	MTS	4.62^a	18.6^a	48.0^a	292.6^d	86.2^c	253.4^c
Significance (A)		ns	*	ns	ns	ns	ns
Significance (B)		*	*	*	*	*	*
Significance (A × B)		ns	ns	ns	*	ns	ns
CV (%)		4.95	4.71		3.19	6.89	5.09

Within the same column, values followed by different letters (a, b, c, d, and e) are significantly different at the 5 % level (*); ns, not significantly different; SBF, solid biofertilizer; No-SBF, no solid biofertilizer applied; M-Sl-09, Solid biofertilizer containing the strain Cereibacter sphaeroides M-Sl-09; M-So-11, Solid biofertilizer containing the strain Rhodopseudomonas thermotolerans M-So-11; M-So-14, Solid biofertilizer containing the strain Rhodopseudomonas palustris M-So-14; MTS, the mixture of three strains.

Figure 1:

Interaction effects of potassium application rates and solid biofertilizer containing potassium-solubilizing purple nonsulfur bacteria strains on purple nonsulfur bacteria density. Note: Values followed by different letters (a, b, c, d, e, and f) are significantly different at the 5 % level (*); SBF: Solid biofertilizer; No-SBF: No solid biofertilizer applied; M-Sl-09: Solid biofertilizer containing the strain Cereibacter sphaeroides M-Sl-09; M-So-11: Solid biofertilizer containing the strain Rhodopseudomonas thermotolerans M-So-11; M-So-14: Solid biofertilizer containing the strain Rhodopseudomonas palustris M-So-14; MTS: The mixture of three strains.

Figure 2. demonstrates that both K fertilizer rate and biofertilizer inoculation significantly affected soil K availability, with the three-strain mixture performing best at full K fertilization and the treatment with M-So-11 at 0 % K. At higher K application rates, treatments with M-Sl-09, M-So-11, and the no bacteria control showed decreasing exchangeable K, whereas M-So-14 and the MTS maintained relatively stable values, except for a notable increase at 100 % K in the MTS treatment. However, not all inoculated treatments differed significantly from their respective K-only controls. For example, all M-So-11 treatments, except for the unusually high value at 0 % K, were statistically similar to their corresponding controls.

Figure 2:

Interaction effects of potassium application rates and solid biofertilizer containing potassium-solubilizing purple nonsulfur bacteria strains on soil exchangeable potassium. Note: Values followed by different letters (a, b, c, d, e, f, g, h, and i) are significantly different at the 5 % level (*); SBF: Solid biofertilizer; No-SBF: No solid biofertilizer applied; M-Sl-09: Solid biofertilizer containing the strain Cereibacter sphaeroides M-Sl-09; M-So-11: Solid biofertilizer containing the strain Rhodopseudomonas thermotolerans M-So-11; M-So-14: Solid biofertilizer containing the strain Rhodopseudomonas palustris M-So-14; MTS: The mixture of three strains.

3.2 Effects of potassium fertilizer rates and solid biofertilizers containing potassium-solubilizing purple nonsulfur bacteria on biomass and potassium uptake in hybrid maize

3.2.1 Effects on biomass of maize plant parts

At different K application levels, the root, stem, leaf, tassel, husk, grain, and cob biomass were highest under the 100 % K treatment, reaching 14.2, 24.8, 34.2, 2.05, 10.9, and 85.1 g plant⁻¹, respectively, except for husk biomass at 25–75 % K and cob biomass at 75 % K. Similarly, SBF containing the three-strain mixture (M-Sl-09, M-So-11, and M-So-14) produced the highest biomass for roots, stems, leaves, tassels, and husks, with respective values of 19.1, 24.2, 34.2, 2.18, and 11.6 g plant⁻¹ (Table 2).

Table 2:

Effects of potassium application rates and solid biofertilizer containing potassium-solubilizing purple nonsulfur bacteria strains on the biomass of hybrid maize grown on dyked alluvial soil.

Factor		Biomass (g plant⁻¹)
Factor		Root	Stem	Leaf	Tassel	Husk	Grain	Cob
Potassium rate (A)	100 %	14.2^a	24.8^a	34.2^a	2.05^a	10.9^a	85.1^a	13.7^a
	75 %	13.8^b	23.6^b	33.5^b	1.95^b	10.7^a	83.7^b	13.3^ab
	50 %	13.2^c	22.6^c	31.8^c	1.94^b	10.5^a	81.7^c	13.1^b
	25 %	12.7^c	22.1^c	31.5^c	1.87^c	10.5^a	80.5^d	12.9^bc
	0 %	12.2^d	21.1^d	30.9^d	1.81^d	9.52^b	73.5^e	12.6^c
SBF (B)	No-SBF	10.3^d	21.6^d	31.1^d	1.74^d	9.49^c	73.0^c	11.9^d
	M-Sl-09	11.6^c	22.8^bc	32.0^c	1.84^c	10.3^b	83.3^a	14.0^a
	M-So-11	11.5^c	22.2^cd	31.9^c	1.87^c	10.3^b	81.4^b	12.6^c
	M-So-14	13.6^b	23.4^b	32.6^b	1.99^b	10.5^b	83.1^a	13.2^b
	MTS	19.1^a	24.2^a	34.2^a	2.18^a	11.6^a	83.7^a	14.0^a
Significance (A)		*	*	*	*	*	*	*
Significance (B)		*	*	*	*	*	*	*
Significance (A × B)		*	ns	*	ns	ns	*	ns
CV (%)		5.26	4.92	2.23	4.05	6.05	1.81	5.23

Within the same column, values followed by different letters (a, b, c, and d) are significantly different at the 5 % level (*); ns, not significantly different; SBF, Solid biofertilizer; No-SBF, No solid biofertilizer applied; M-Sl-09, Solid biofertilizer containing the strain Cereibacter sphaeroides M-Sl-09; M-So-11, Solid biofertilizer containing the strain Rhodopseudomonas thermotolerans M-So-11; M-So-14, Solid biofertilizer containing the strain Rhodopseudomonas palustris M-So-14; MTS, the mixture of three strains.

Husk biomass across the 25–100 % K treatments was statistically similar and higher than that of the 0 % K treatment, ranging from 10.5–10.9 > 9.52 g plant⁻¹. In addition, grain biomass in the treatments with single-strain SBF (M-Sl-09 or M-So-14) or the three-strain mixture was statistically similar and higher than the treatments with M-So-11 or the non-inoculated control, with values of 83.1–83.7 > 73.0–81.4 g plant⁻¹, respectively. Cob biomass under the 100 % K treatment was comparable to the 75 % K treatment and higher than 0–50 % K treatments, with 13.3–13.7 > 12.6–13.1 g plant⁻¹. Similarly, cob biomass in the treatments with single-strain M-Sl-09 or the three-strain mixture was equivalent and higher than the other treatments, with both reaching 14.0 g plant⁻¹ compared to 11.9–13.2 g plant⁻¹ (Table 2).

3.2.2 Effects on potassium content in maize plant parts

Table 3 shows that the 100 % K fertilizer treatments resulted in the highest K content in roots, stems, tassels, husks, and grains, with values of 0.577 %, 1.26 %, 2.11 %, 1.34 %, and 0.787 %, respectively. Additionally, K content in leaves and cobs under the 75 % and 100 % K treatments was statistically similar and higher than that of the other treatments, ranging from 1.26–1.29 % versus 1.07–1.21 % for leaves, and 0.824–0.863 % versus 0.497–0.766 % for cobs. The lowest K content was recorded in the 0 % K treatment: 0.408 % (root), 0.862 % (stem), 1.07 % (leaf), 0.575 % (grain), and 0.497 % (cob).

Table 3:

Effects of potassium application rates and solid biofertilizer containing potassium-solubilizing purple nonsulfur bacteria strains on potassium content in hybrid maize grown on dyked alluvial soil.

Factor		Potassium content (%)
Factor		Root	Stem	Leaf	Tassel	Husk	Grain	Cob
Potassium rate (A)	100 %	0.577^a	1.26^a	1.29^a	2.11^a	1.34a	0.787^a	0.863^a
	75 %	0.539^b	1.11^b	1.26^a	2.00^b	1.25b	0.752^b	0.824^a
	50 %	0.482^c	0.984^c	1.21^b	1.98^b	1.27b	0.683^c	0.766^b
	25 %	0.440^d	0.894^d	1.14^c	1.81^c	1.23bc	0.625^d	0.667^c
	0 %	0.408^e	0.862^e	1.07^d	1.75^c	1.19c	0.575^e	0.497^d
SBF (B)	No-SBF	0.400^d	0.908^c	1.06^d	1.56^e	1.11b	0.587^d	0.557^e
	M-Sl-09	0.460^c	1.07^a	1.35^a	2.12^d	1.28a	0.760^a	0.779^b
	M-So-11	0.505^c	1.02^b	1.11^c	1.85^c	1.32a	0.671^b	0.655^c
	M-So-14	0.473^b	1.04^b	1.15^c	1.77^b	1.30a	0.636^c	0.613^d
	MTS	0.607^a	1.07^a	1.29^b	2.35^a	1.29a	0.766^a	1.01^a
Significance (A)		*	*	*	*	*	*	*
Significance (B)		*	*	*	*	*	*	*
Significance (A × B)		*	*	*	*	*	*	*
CV (%)		4.88	4.83	5.71	4.52	5.32	4.63	8.91

Within the same column, values followed by different letters (a, b, c, d, and e) are significantly different at the 5 % level (*); ns: not significantly different; SBF, solid biofertilizer; No-SBF, no solid biofertilizer applied; M-Sl-09, solid biofertilizer containing the strain cereibacter sphaeroides M-Sl-09; M-So-11, solid biofertilizer containing the strain Rhodopseudomonas thermotolerans M-So-11; M-So-14, solid biofertilizer containing the strain Rhodopseudomonas palustris M-So-14; MTS, the mixture of three strains.

Among the biofertilizer treatments, the mixture of three PNSB strains (M-Sl-09, M-So-11, and M-So-14) led to the highest K content in roots (0.607 %), tassels (2.35 %), and cobs (1.01 %). For stems and grains, the SBFs with either M-Sl-09 alone or the three-strain mixture gave statistically similar and higher K content than other treatments, with 1.07 % and 0.760–0.766 % compared to 0.908–1.04 % and 0.587–0.671 %, respectively. Moreover, leaf K content was highest in the treatment with SBF containing M-Sl-09 (1.35 %). K content in husks was statistically similar among all SBF treatments but was significantly higher than in the non-inoculated control (1.28–1.32 % > 1.11 %). Additionally, there was a significant interaction between K fertilizer rates and SBF types on K content in all maize plant parts (Table 3).

3.2.3 Effects on total potassium uptake by hybrid maize

Potassium uptake in roots, stems, leaves, tassels, husks, grains, and cobs was highest in the 100 % K fertilizer treatment, with values of 83.1, 312.0, 440.0, 43.5, 146.3, 671.9, and 120.6 mg plant⁻¹, respectively. Conversely, the lowest uptake was observed in the 0 % K treatment, with values of 51.6, 182.1, 330.5, 32.0, 114.2, 424.0, and 63.5 mg plant⁻¹, respectively. Similarly, K uptake in roots, stems, tassels, husks, and cobs was highest in the treatment with SBF containing the three-strain mixture (M-Sl-09, M-So-11, and M-So-14), with respective values of 116.0, 260.5, 51.2, 149.5, and 142.7 mg plant⁻¹. In contrast, the SBF treatment without bacteria showed the lowest K uptake in all plant parts: 42.1 (root), 196.4 (stem), 331.5 (leaf), 27.3 (tassel), 105.4 (husk), 431.2 (grain), and 66.4 (cob) mg plant⁻¹. Additionally, K uptake in leaves and cobs was significantly higher in treatments with SBF containing M-Sl-09 or the three-strain mixture compared to other treatments with 432.2–443.4 mg plant⁻¹ vs. 331.5–373.8 mg plant⁻¹ and 636.8–644.6 mg plant⁻¹ vs. 431.2–548.3 mg plant⁻¹, respectively. Furthermore, there were significant interactions between K fertilizer levels and SBF types on K uptake in roots, stems, leaves, tassels, grains, and cobs (Table 4).

Table 4:

Effects of potassium application rates and solid biofertilizer containing potassium-solubilizing purple nonsulfur bacteria strains on potassium uptake in hybrid maize grown on dyked alluvial soil.

Factor		Potassium uptake (mg plant⁻¹)							Total potassium uptake (mg plant⁻¹)
Factor		Root	Stem	Leaf	Tassel	Husk	Grain	Cob	Total potassium uptake (mg plant⁻¹)
Potassium rate (A)	100 %	83.1^a	312.0^a	440.0^a	43.5^a	146.3^a	671.9^a	120.6^a	1,817.4^a
	75 %	75.9^b	263.7^b	421.2^b	39.3^b	134.8^b	631.9^b	111.6^b	1,678.3^b
	50 %	65.7^c	222.0^c	386.6^c	38.7^b	133.6^b	560.8^c	101.3^c	1,508.5^c
	25 %	58.4^d	198.0^d	359.4^d	34.2^c	129.9^b	504.7^d	86.0^d	1,370.6^d
	0 %	51.6^e	182.1^e	330.5^e	32.0^d	114.2^c	424.0^e	63.5^e	1,198.1^e
SBF (B)	No-SBF	42.1^e	196.4^d	331.5^d	27.3^d	105.4^c	431.2^c	66.4^d	1,200.3^d
	M-Sl-09	53.7^d	247.2^b	432.2^a	39.1^b	131.8^b	636.8^a	110.1^b	1,651.0^b
	M-So-11	58.5^c	230.3^c	356.6^c	34.7^c	136.4^b	548.3^b	82.6^c	1,447.3^c
	M-So-14	64.5^b	243.4^b	373.8^b	35.4^c	135.8^b	532.6^b	81.2^c	1,466.6^c
	MTS	116.0^a	260.5^a	443.4^a	51.2^a	149.5^a	644.6^a	142.7^a	1,807.9^a
Significance (A)		*	*	*	*	*	*	*	*
Significance (B)		*	*	*	*	*	*	*	*
Significance (A × B)		*	*	*	*	ns	*	*	*
CV (%)		7.01	6.60	5.50	5.87	8.88	5.19	10.1	2.94

Within the same column, values followed by different letters (a, b, c, d, and e) are significantly different at the 5 % level (*); ns: not significantly different; SBF, solid biofertilizer; No-SBF, no solid biofertilizer applied; M-Sl-09, solid biofertilizer containing the strain cereibacter sphaeroides M-Sl-09; M-So-11, solid biofertilizer containing the strain Rhodopseudomonas thermotolerans M-So-11; M-So-14, Solid biofertilizer containing the strain Rhodopseudomonas palustris M-So-14; MTS, the mixture of three strains.

As shown in Table 4, total K uptake was highest at the 100 % K level (1817.4 mg plant⁻¹) and lowest with no K application (1,198.1 mg plant⁻¹). The 25–75 % K treatments had total uptake ranging from 1,370.6 to 1,678.3 mg plant⁻¹. Similarly, among the SBF types, the three-strain mixture treatment resulted in the highest total K uptake (1,807.9 mg plant⁻¹), followed by the M-Sl-09 single strain (1,651.0 mg plant⁻¹), M-So-11 and M-So-14 (1,447.3–1,466.6 mg plant⁻¹), and the lowest was in the non-inoculated control (1,200.2 mg plant⁻¹).

There was significant interactions between K application rates and SBF types for total K uptake. The combination of 100 % K and the three-strain mixture yielded the highest uptake (2,141.0 mg plant⁻¹). Notably, 75 % K combined with SBF containing M-Sl-09 achieved greater uptake (1,861.0 mg plant⁻¹) than 100 % K fertilizer alone (1,440.6 mg plant⁻¹) (Figure 3).

Figure 3:

Interaction effects of potassium application rates and solid biofertilizer containing potassium-solubilizing purple nonsulfur bacteria strains on total potassium uptake in hybrid maize grown on dyked alluvial soil. Note: Values followed by different letters (a, b, c, d, e, f, g, h, i, j, k, and l) are significantly different at the 5 % level (*); SBF: Solid biofertilizer; No-SBF: No solid biofertilizer applied; M-Sl-09: Solid biofertilizer containing the strain Cereibacter sphaeroides M-Sl-09; M-So-11: Solid biofertilizer containing the strain Rhodopseudomonas thermotolerans M-So-11; M-So-14: Solid biofertilizer containing the strain Rhodopseudomonas palustris M-So-14; MTS: The mixture of three strains.

3.3 Effects of potassium fertilizer rates and solid biofertilizers containing potassium-solubilizing purple nonsulfur bacteria on the growth of hybrid maize

Table 5 shows that plant height, ear height, and leaf length were significantly affected (at the 5 % level) by both K fertilizer rates and types of SBF. Specifically, treatments receiving 75–100 % K fertilizer produced greater plant height, ear height, and leaf length than the other treatments, with values of 250.7–252.1 > 235.8–246.3 cm, 117.4–118.5 > 102.8–114.8 cm, and 108.0–109.1 > 104.3–106.3 cm, respectively. Similarly, application of SBF containing the three-strain PNSB mixture resulted in the highest plant height (254.6 cm), ear height (119.6 cm), and leaf width (7.78 cm). Moreover, treatments with the three-strain SBF and the single-strain M-So-14 SBF had statistically similar leaf lengths (109.8–110.4 cm), both significantly higher than the remaining treatments (101.1–107.6 cm). Meanwhile, stem diameter, leaf position bearing the cob, leaf width, and number of spikelets on the tassel were not significantly different among K application levels. Likewise, the leaf position bearing the cob and number of spikelets on the tassel did not differ significantly among SBF treatments.

Table 5:

Effects of potassium application rates and solid biofertilizer containing potassium-solubilizing purple nonsulfur bacteria strains on the growth of hybrid maize grown on dyked alluvial soil.

Factor		Plant height	Ear set height	Stem diameter	Ear set leaf position	Leaf length	Leaf width	Number of spikelets per tassel
Factor		cm			Leaf	cm		Spikelets
Potassium rate (A)	100 %	252.1^a	118.5^a	0.952	7.00	109.1^a	7.48	11.7
	75 %	250.7^a	117.4^a	0.947	7.25	108.0^a	7.52	12.2
	50 %	249.5^a	114.8^b	0.939	7.20	106.3^b	7.43	11.9
	25 %	246.3^b	111.1^c	0.938	6.99	105.9^b	7.45	11.9
	0 %	235.8^c	102.8^d	0.951	7.00	104.3c	7.57	12.4
SBF (B)	No-SBF	232.4^c	101.0^d	0.800^b	6.85	101.1^d	7.40^b	11.8
	M-Sl-09	248.7^b	112.9^c	0.977^a	7.10	104.5^c	7.41^b	11.8
	M-So-11	249.8^b	114.6^bc	0.990^a	7.30	107.6^b	7.36^b	12.1
	M-So-14	248.8^b	116.4^b	0.978^a	7.19	109.8^a	7.51^b	12.0
	MTS	254.6^a	119.7^a	0.982^a	7.00	110.4^a	7.78^a	12.5
Significance (A)		*	*	ns	ns	*	ns	ns
Significance (B)		*	*	*	ns	*	*	ns
Significance (A × B)		*	ns	ns	ns	ns	ns	ns
CV (%)		1.59	2.68	5.35	8.59	2.37	3.59	20.0

Within the same column, values followed by different letters (a, b, c, and d) are significantly different at the 5 % level (*); ns, not significantly different; SBF, solid biofertilizer; No-SBF, no solid biofertilizer applied; M-Sl-09, solid biofertilizer containing the strain cereibacter sphaeroides M-Sl-09; M-So-11, solid biofertilizer containing the strain Rhodopseudomonas thermotolerans M-So-11; M-So-14, solid biofertilizer containing the strain Rhodopseudomonas palustris M-So-14; MTS, the mixture of three strains.

3.4 Effects of potassium fertilizer rates and solid biofertilizers containing potassium-solubilizing purple nonsulfur bacteria on yield and yield components of hybrid maize

Table 6 shows that cob diameter, number of grain rows per ear, number of grains per row, and weight of 100 fresh grains were statistically similar among the different K application levels and SBF types. In addition, the SBF-inoculated treatments recorded higher cob length and ear length than the treatments without bacterial inoculation, with values of 18.2–18.5 > 16.9 cm and 18.5–19.5 > 18.0 cm, respectively. Moreover, the 75–100 % K treatments produced statistically similar ear lengths, both higher than the other K levels, at 19.2–19.3 > 18.3–18.9 cm. Furthermore, the 50–100 % K treatments resulted in longer cob lengths (18.2–18.4 > 17.4 cm), and the 25–100 % K treatments produced larger ear diameters (3.89–3.96 > 3.77 cm) compared to the 0 % K treatment.

Table 6:

Effects of potassium application rates and solid biofertilizer containing purple nonsulfur bacteria strains on yield components and grain yield of hybrid maize grown on dyked alluvial soil.

Factor		Cob length	Cob diameter	Ear length	Ear diameter	Number of rows per ear	Number grains per row	100-Fresh-grain weight	Grain yield
Factor		cm				Rows	Grains	g	g plant⁻¹
Potassium rate (A)	100 %	18.4^a	2.06	19.3^a	3.96^a	13.6	34.6	30.1	99.7^a
	75 %	18.4^a	2.05	19.2^a	3.96^a	13.6	35.4	28.5	96.4^b
	50 %	18.2^a	2.08	18.9^b	3.95^a	13.0	34.6	29.2	95.1^c
	25 %	17.8^ab	2.07	18.8^b	3.89^a	13.4	34.3	29.4	93.2^d
	0 %	17.4^b	2.01	18.3^c	3.77^b	13.8	34.8	28.9	88.0^e
SBF (B)	No-SBF	16.9^b	1.98	18.0^d	3.82	13.6	34.4	29.3	86.3^d
	M-Sl-09	18.3^a	2.05	19.1^b	3.94	12.8	33.7	30.2	97.5^b
	M-So-11	18.2^a	2.07	19.4^ab	3.92	13.2	34.3	28.7	92.7^c
	M-So-14	18.3^a	2.09	18.5^c	3.92	14.0	35.2	29.0	96.6^b
	MTS	18.5^a	2.09	19.5^a	3.93	13.9	36.2	29.0	99.4^a
Significance (A)		*	ns	*	*	ns	ns	ns	*
Significance (B)		*	ns	*	ns	ns	ns	ns	*
Significance (A × B)		ns	ns	ns	ns	ns	ns	ns	*
CV (%)		5.18	4.89	2.42	3.30	12.7	8.59	7.89	1.88

Within the same column, values followed by different letters (a, b, c, d, and e) are significantly different at the 5 % level (*); ns, not significantly different; SBF, solid biofertilizer; No-SBF, no solid biofertilizer applied; M-Sl-09, solid biofertilizer containing the strain cereibacter sphaeroides M-Sl-09; M-So-11, solid biofertilizer containing the strain Rhodopseudomonas thermotolerans M-So-11; M-So-14, solid biofertilizer containing the strain Rhodopseudomonas palustris M-So-14; MTS, the mixture of three strains.

Hybrid maize grain yield was significantly affected (at the 5 % level) by both K application rates and types of SBF. The highest yield was observed in the 100 % K treatment (99.7 g plant⁻¹), while the lowest was in the 0 % K treatment (88.0 g plant⁻¹). Among the biofertilizer treatments, the SBF containing the three-strain PNSB mixture (M-Sl-09, M-So-11, and M-So-14) resulted in the highest yield (99.4 g plant⁻¹). This was followed by the single-strain SBFs containing M-Sl-09 and M-So-14, which gave statistically similar yields ranging from 96.6–97.5 g plant⁻¹. The treatment with M-So-11 alone yielded 92.7 g plant⁻¹, while the lowest yield was recorded in the uninoculated SBF control (86.3 g plant⁻¹) (Table 6). In addition, there was a significant interaction between K application rates and SBF types on hybrid maize yield. The combination of 100 % K and the three-strain mixture resulted in the highest grain yield (106.5 g plant⁻¹). Treatments combining 25–75 % K with the three-strain SBF produced higher yields (96.3–101.0 g plant⁻¹) than the 100 % K-only treatment (89.1 g plant⁻¹), respectively. Furthermore, the combination of 75 % K with any single-strain SBF (M-Sl-09, M-So-11, or M-So-14) yielded 95.5–99.7 g plant⁻¹, which was higher than the 75 % K-only treatment (88.5 g plant⁻¹) (Figure 4).

Figure 4:

Interaction effects of potassium application rates and solid biofertilizer containing potassium-solubilizing purple nonsulfur bacteria strains on the grain yield of hybrid maize grown on dyked alluvial soil. Note: Values followed by different letters (a, b, c, d, e, f, g, h, i, j, k, l, m, and n) are significantly different at the 5 % level (*); SBF: Solid biofertilizer; No-SBF: No solid biofertilizer applied; M-Sl-09: Solid biofertilizer containing the strain Cereibacter sphaeroides M-Sl-09; M-So-11: Solid biofertilizer containing the strain Rhodopseudomonas thermotolerans M-So-11; M-So-14: Solid biofertilizer containing the strain Rhodopseudomonas palustris M-So-14; MTS: The mixture of three strains.

4 Discussion

4.1 Effects of solid biofertilizer on the properties of dyked alluvial soil cultivated with hybrid maize

The application of solid biofertilizer (SBF) containing K-solubilizing purple nonsulfur bacteria (K-PNSB) contributed to improving soil fertility by increasing the exchangeable K content in the soil cultivated with hybrid maize. Among these, the mixture of three K-solubilizing strains, C. sphaeroides M-Sl-09, R. thermotolerans M-So-11, and R. palustris M-So-14, improved exchangeable K content by 23.9 %, the highest value, followed by strain M-Sl-09 (16.9 %), M-So-14 (12.1 %), and M-So-11 (7.80 %) compared to the control (Table 1). These results confirm that the inoculated PNSB strains actively mobilized unavailable K and P forms through organic acid production, proton extrusion, and complexation of cations such as Al³⁺ and Fe³⁺. This agrees with previous reports by Etesami et al. [30] and Lodi et al. [31], who demonstrated that organic acids (e.g., citric, oxalic, gluconic acids) released by KSB enhance mineral weathering and release K⁺ from mica and feldspar. According to Table 1, the application of SBF also increased NH₄ ⁺ content by 85.7–161.6 % compared to the control treatments. This result agrees with the conclusion of Sakarika et al. [32] that PNSB provide a source of available nitrogen through their nitrogen-fixing function. Similarly, the application of SBF reduced Al–P by 5.97–19.8 %, Fe–P by 18.7–24.4 %, and Ca–P by 7.60–16.9 % compared to SBF without bacterial inoculation (Table 1). The observed reduction of Al–P, Fe–P, and Ca–P indicates active phosphate solubilization, possibly mediated by siderophores production and chelation of Fe³⁺ ions. This mechanism is consistent with the findings of Sakarika et al. [32], who reported that siderophores-producing phototrophic bacteria enhance P solubility through Fe complexation and reduction [32], 33] resulting in lower insoluble P fractions in soil. These improvements suggest that PNSB may mobilize insoluble K and P through mechanisms such as organic acid secretion, proton extrusion, or cation complexation; however, these processes were not directly measured in this study and therefore hypotheses rather than confirmed mechanisms. Thus, future studies hould quantify organic acid profiles, pH shifts, siderophore production, and mineral weathering rates under controlled conditions to validate the specific biochemical pathways responsible for PNSB-mediated K and P mobilization.

However, the use of chemical fertilizers at rates higher than recommended can negatively affect soil fertility and indigenous microbial communities [34]. Continuous application of chemical fertilizers alters soil pH, leading to a reduction in beneficial microorganisms [35]. In this study, however, the use of SBF improved soil pH, specifically increasing pH_H2O from 5.07 to 5.21. Several PNSB strains are capable of producing 5-aminolevulinic acid (ALA), which indirectly stabilize rhizosphere pH by stimulating root exudation and enhancing microbial buffering capacity [36]. In addition, the application of SBF containing either the single strains M-So-11 or M-So-14, or the three-strain mixture, reduced EC compared to the control, with values of 0.331–0.353 vs. 0.410 mS cm⁻¹, respectively (Table 1).

In Figure 1, the PNSB mixture consistently showed higher population density than single-strain inoculants at low K application rates, whereas the differences became smaller at higher K doses. This pattern indicates that the mixture maintained stronger growth under K-limited conditions. One possible explanation is that mixed strains benefit from functional complementarity, allowing broader substrate use and mutual support, which enhances survival and propagation when mineral K is scarce. In contrast, higher K fertilization rates may reduce the selective advantage and propagation of K-solubilizing bacteria, because abundant readily available K diminishes the need for microbial K solubilization. Consequently, PNSB activity and proliferation may be less stimulated at high K levels. However, this relationship between soil K availability and the population dynamics of K-solubilizing PNSB warrants further investigation, particularly to determine whether excessive mineral K inputs suppress the ecological competitiveness or activity of these beneficial strains.

While the compost carrier could have supported other heterotrophic microbes, the treatments inoculated with specific PNSB strains exhibited higher exchangeable K and soluble P than the sterile control, confirming that PNSB activity, not spontaneous compost decomposition, was the dominant contributor to nutrient mobilization.

4.2 Effects of solid biofertilizer on potassium uptake in hybrid maize

The biomass of plant parts such as roots, stems, leaves, tassels, grains, and cobs differed significantly (at the 5 % level) among K application rates and between treatments with and without bacterial inoculation in SBF. This result is consistent with the findings of Römheld & Kirkby [37], who stated that K plays an important role in regulating water balance and sugar transport, thereby promoting the development of storage organs such as grains. This result agrees with the findings of Ahmed et al. [38], who reported that bacterial mixtures improve biomass and maize yield by enhancing assimilation and nutrient uptake capacity.

The potassium content in plant parts varied according to K application rates and the use of SBF. This demonstrates that K content is influenced by the amount of fertilizer applied. According to Marschner [39], K is an essential mineral element that regulates ion exchange and electrical balance in plants. Compared with SBF without bacterial inoculation, K content in grains increased from 0.587 % to 0.766 %. This improvement reflects the role of PNSB in increasing plant-available K in the soil (Table 3). According to Takata et al. [40], PNSB created a more favorable rhizosphere environment for K uptake through the secretion of bioactive compounds.

The total K uptake of maize increased with higher K application rates and the use of SBF. This result is consistent with the findings of Hafeez et al. [41], who reported that adequate K supply increases the efficiency of assimilate conversion into storage material, thereby contributing to greater total K uptake. The application of SBF containing the three-strain bacterial mixture achieved the highest total K uptake (1807.9 mg plant⁻¹) compared to that of the 100 % K treatment without SBF. Specifically, K uptake in grains increased from 431.2 mg plant⁻¹ (without SBF) to 644.6 mg plant⁻¹ (with the three-strain SBF), and in roots from 42.1 to 116.0 mg plant⁻¹ (Table 4). This demonstrates that applying the three-strain SBF can partially substitute for chemical K fertilizer. Xu et al. [42] also reported increased K uptake when chemical fertilizers were combined with microorganisms, due to improvements in the rhizosphere environment and K ion transformation capacity. Moreover, bacteria enhance nutrient absorption area by improving root systems and capillary networks [43]. The enhancement of K uptake in maize was closely associated with both K fertilizer rate and PNSB inoculation. The mixed inoculum (C. sphaeroides, R. thermotolerans, and R. palustris) recorded the highest total K uptake (2,141 mg plant⁻¹) and improved K concentration in all plant parts. This synergistic effect may result from functional complementarity among strains. For instance, R. palustris contributes strong organic acid secretion for K solubilization [44], while C. sphaeroides exhibits efficient N ₂ fixation and phytohormone production [45], and R. thermotolerans may enhance root vigor under stress conditions. Their co-inoculation thus promotes multiple nutrient cycles simultaneously. The interaction between mineral K fertilizer and biofertilizer application was significant for both K uptake and yield, indicating that PNSB inoculation enhances fertilizer-use efficiency rather than replacing mineral K completely. At 75 % K plus the M-Sl-09 or mixed inoculum, total K uptake and yield were comparable to 100 % K alone, demonstrating that integrating PNSB can reduce chemical K input by 25 % without compromising productivity.

4.3 Effects of solid biofertilizer on growth, yield components, and yield of hybrid maize

Treatments with 25–100 % K application increased plant height (4.45–6.94 %), leaf length (1.53–4.60 %), and ear height (8.07–15.3 %), thereby contributing to improved hybrid maize grain yield (Table 6). Adequate K fertilization promotes plant height development because K participates in regulating osmotic pressure, nutrient transport, and enzyme activities [46]. This, in turn, improves hybrid maize health, resulting in better nutrient uptake. Therefore, on dyked alluvial soil, hybrid maize responds positively in yield to K nutrition.

The application of SBF containing the three-strain bacterial mixture increased grain yield by 15.2 % (Table 6). These improvements originate from the biological activities of PNSB, such as the production of plant hormones, the urease enzyme, nitrogen fixation, phosphate solubilization, and the solubilization of insoluble K [32]. These results demonstrate the role of K fertilizer and SBF in ear formation and development in hybrid maize. According to Gao et al. [16], biofertilizers combined with chemical fertilizers enhance dry matter accumulation, increase ear size, and improve grain yield. Moreover, among SBF treatments, the three-strain mixture showed the greatest yield increase (15–23 %) compared with uninoculated control. The synergistic effects among PNSB strains likely improved nutrient cycling and microbial stability in the rhizosphere, leading to prolonged nutrient release and stronger plant vigor. This observation aligns with Ahmed et al. [38], who reported that bacterial consortia exhibit higher resilience and metabolic diversity than single inoculants.

At lower K doses, yield declined slightly even with SBF inoculation, suggesting that bacterial K-solubilization efficiency depends on adequate mineral K reserves in soil. Therefore, the combined application of mineral K and PNSB-based biofertilizer provides the most balanced and sustainable strategy for nutrient management. From an agronomic perspective, integrating K-PNSB biofertilizer with 75–100 % of the recommended K fertilizer could maintain high maize yields while reducing chemical K inputs. The solid compost carrier ensures longer bacterial viability and allows farmers to use locally available crop residues for production, lowering costs and waste.

However, further field-scale verification is needed to confirm these results under variable environmental conditions and to evaluate the economic return and microbial persistence in long-term soil ecosystems. Future research should also quantify the contribution of individual PNSB functions, such as K solubilization, N ₂ fixation, and phytohormone synthesis to the overall plant response.

5 Conclusions

The present study demonstrated that solid biofertilizer (SBF) containing potassium-solubilizing purple nonsulfur bacteria (K-PNSB) effectively enhanced soil fertility, potassium uptake, growth, and yield of hybrid maize cultivated in dyked alluvial soil. Inoculation with C. sphaeroides, R. thermotolerans, and R. palustris, either individually or in combination, increased soil exchangeable K and PNSB population density while reducing insoluble P fractions. Among the treatments, the three-strain mixture combined with 100 % of the recommended K fertilizer produced the highest total K uptake and grain yield, indicating a strong synergistic interaction between mineral K and biofertilizer application. Importantly, maize treated with 75 % K plus biofertilizer showed equivalent yield to the full K control without K-PNSB, suggesting that K-PNSB can reduce mineral K fertilizer requirements by approximately 25 %. However, only maize treated with 75 % K plus biofertilizer especially R. thermotolerans achieved comparable yield to the full K control with K-PNSB. Overall, integrating K-PNSB-based biofertilizer with moderate mineral K fertilization represents a sustainable nutrient management approach for improving crop productivity and soil health in nutrient-depleted dyked alluvial soils of the Mekong Delta. Further field trials and cost–benefit analyses are recommended to assess the economic feasibility, microbial persistence, and long-term impacts of PNSB-based biofertilizer use under farmers’ field conditions.

Corresponding author: Nguyen Quoc Khuong, Faculty of Crop Science, College of Agriculture, Can Tho University, Can Tho, 94000, Vietnam, E-mail: nqkhuong@ctu.edu.vn

Funding information: This work was supported by Can Tho University (Grant number CTCS2024-14-03).
Author contributions: VYN- Conceptualization, Formal Analysis, Investigation, Methodology, Writing – Original Draft Preparation; LTMT – Conceptualization, Formal Analysis, Investigation, Methodology; NDT – Conceptualization, Formal Analysis, Investigation; TTKN – Conceptualization, Formal Analysis, Investigation; LTQ – Conceptualization, Formal Analysis, Writing – Review & Editing; TLT – Conceptualization, Formal Analysis, Investigation; TCN – Conceptualization, Formal Analysis, Investigation; LNTX – Conceptualization, Formal Analysis, Investigation; NQK- Conceptualization, Formal Analysis, Investigation, Methodology, Funding acquisition, Project administration, Supervision, Writing – Review & Editing.
Conflict of interest: Authors state no conflict of interest.
Data availability statement: The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

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Received: 2025-09-07

Accepted: 2025-12-11

Published Online: 2026-01-19

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/opag-2025-0487

Keywords for this article

hybrid maize; purple nonsulfur bacteria; solid biofertilizer; yield; Zea mays L

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