Phosphate-solubilizing bacteria-mediated rock phosphate utilization with poultry manure enhances soil nutrient dynamics and maize growth in semi-arid soil

1 Introduction

Sustainable food production is dependent on the fertility and health of soil. As the global population continues to expand, the imperative of ensuring sustainable food production takes center stage [1]. A vital aspect of this challenge is optimizing nutrient availability in soils to support the growth of crops like maize. In many regions, nutrient deficiency or imbalances in agricultural soils, particularly phosphorus (P), nitrogen (N), and potassium (K), have emerged as a primary constraint to achieving optimal crop yields [2–4]. Addressing the challenge of P deficiency has led to the global utilization of various P fertilizers [5]. Yet, a significant limitation of inorganic P fertilizer application lies in its efficiency, and only 25–30% is available for plant uptake, while the remaining converts into insoluble P forms [6]. Consequently, enhancing P efficiency within soil systems necessitates strategies such as organic fertilizer use, targeted P fertilizers, and cultivation of low-P requirement plants [7–9]. However, these approaches have inherent limitations in maximizing P utilization. Alternatively, introducing beneficial microbes to augment P solubility and use efficiency offers a promising avenue, potentially reducing reliance on synthetic fertilizers while positively impacting plant growth, a crucial factor in agricultural ecosystems that can mitigate soil and water pollution [10,11].

Similarly, N is one of the primary nutrients, and its soil deficiency restricts crop production. In the last half-century, crop yields have increased due to the widespread use of nitrogen-based fertilizers [12]. However, it is worth highlighting that, during the growing season, the average recovery rate of N in crops only hovers around 30–40% [13]. Any excess N not taken up by the crops can become fixed within the soil, leached into deeper soil layers, or released into the atmosphere as ammonia or nitrogen oxides [5]. Such N losses contribute to heightened agricultural production costs and hasten the degradation of the environment [13]. The optimization of N fertilizer application techniques presents an avenue to enhance N fertilizer utilization, resulting in more efficient usage. Developing novel crop cultivars with heightened N efficiency could achieve elevated yields with minimal N input, a pathway that encourages the sustainable advancement of agriculture [14,15]. After P and N, K is a vital macronutrient for sustainable crop production. Nevertheless, much K is lost or builds up in plant tissues. Consequently, the agricultural system faces a significant challenge in effectively recycling the K that accumulates within plant tissues and minimizing K losses from the soil [16].

Considering these factors, the importance of beneficial soil microorganisms, such as phosphate-solubilizing bacteria (PSB), thriving in the rhizosphere of most plants has gained significant attention for their positive effects in nutrient-depleted soils [17,18]. Given that a majority of soils in the arid and semi-arid regions of Pakistan are alkaline and carbonate-dominated, which obstructs the accessibility of essential nutrients such as P, N, and K for plant growth and uptake [19]. In such challenging conditions, PSB can be advantageous for plants through mechanisms such as the secretion of phenolic compounds, the release of protons [20], and the production of mineral acids [21] within the soil. These actions culminate in soil acidification [22], consequently aiding the liberation of P from compounds such as Ca₃(PO₄)₂. Additionally, PSB can effectively chelate cations such as Ca²⁺, Al³⁺, and Fe³⁺ by exuding organic acids, thereby augmenting the bioavailability of P [23]. Utilizing PSB to enhance inorganic P solubilization and aid in the availability of other soil nutrients has shown remarkable improvement. PSB primarily lowers the pH of the rhizosphere by releasing organic acids, which can eventually increase the amount of inorganic P in the soil [24]. However, rhizobacteria can mineralize organic soil P by secreting phosphatase enzymes [25]. The conversion of organic P into inorganic ( HPO 4 2 − and H 2 PO 4 − ) forms, which are easily absorbed by plants, depends on soil phosphatases. Furthermore, inoculation with PSBs has been shown to significantly influence the composition, diversity, and abundance of soil microbial communities, thereby enhancing microbial interactions. These changes contribute to improved degradation of organic matter and overall enhancement of soil nutrient status [26]. PSBs produce enzymes such as phytase and phosphatase, which facilitate the solubilization of phosphate from both organic and inorganic sources. This process is mediated through mechanisms including chelation, the production of siderophores, and the secretion of extracellular polysaccharides [27,28]. Therefore, the microbiological tools for increasing P availability, particularly from organic sources for plant uptake, are the secretion of the phosphatase enzyme for organic P mineralization and the formation of organic acid for lowering soil pH and, consequently, greater P solubilization [29]. Moreover, PSB is also involved in the synthesis and secretion of phytohormones that promote plant growth [30], such as gluconic acid (GA) and indole acetic acid, as well as other enzymes such as phosphatases and organic acids such as carbonic acid [31].

Similarly, incorporating organic amendments such as poultry manure (PM) yields advantageous effects on soil fertility and physical attributes [32,33]. The application of PM mitigates the reliance on chemical fertilizers to a certain extent while fostering the production of polysaccharides by microorganisms, thereby enhancing soil conditions [34,35]. Furthermore, as an economically viable and ecologically friendly substitute for commercial P fertilizers, naturally occurring low-grade rock phosphate (RP) presents itself as an option [36]. The northwestern region of Pakistan boasts substantial recoverable RP deposits, offering a potentially economical source of P fertilizers to bolster the nation’s overall grain production. However, the direct utilization of low-grade RP as a P fertilizer necessitates specific amendments, particularly in alkaline soils, to amplify microbial RP solubilization [37]. Soil microorganisms play a pivotal role in rendering insoluble nutrients, including RP, bioavailable, thereby functioning as key facilitators in the biogeochemical cycling of nutrients, notably N and P [38,39].

Despite numerous laboratory and field-based studies exploring the individual effects of biofertilizers (e.g., PSB, AMF, cyanobacteria), organic amendments (e.g., PM, FYM), and RP, limited field research has investigated their synergistic impact on nutrient dynamics, soil health, and crop performance under semi-arid conditions. While the sole application of these amendments has been well documented, few studies have integrated them into a unified, practical strategy for improving nutrient availability and crop productivity. This study introduces a new field-based approach combining PSB, PM, and locally available low-grade RP to enhance nutrient availability and crop yield in nutrient-deficient semi-arid soils. The new aspect of this research lies in assessing the synergistic effects of these components on inorganic and organic phosphorus mobilization, soil chemical properties (pH, EC, soil organic matter [SOM]), and plant nutrient uptake – particularly under realistic field conditions. The main objectives of this study were to (i) evaluate the individual and combined effects of RP, PM, and PSB on key soil properties (pH, EC, SOM); (ii) assess their impact on the availability and uptake of N, P, and K nutrients in maize; and (iii) determine their influence on maize productivity under semi-arid field conditions.

2 Methodology

2.1 Experimental setup

During the summer of 2022, a field experiment was conducted at the New Developmental Farm of the University of Agriculture, Peshawar, Pakistan. The monthly meteorological data of the experimental site were obtained from the Pakistan Meteorological Department (https://cdpc.pmd.gov.pk/), which is graphically presented in Figure 1. From the obtained data, the monthly mean temperature for the month of March was 22.9°C, with a very low total precipitation of 5.2 millimeters (mm) and a vapor pressure of 9.4 millibars (mb). The experiment followed a three-factor factorial randomized complete block design with three replications. The main plot treatments included various sources of phosphorus sources, such as RP, PM, and their mixtures, along with PSB inoculation (inoculated vs. non-inoculated). The inoculums of PSB were obtained from the National Agriculture and Research Council, Islamabad, Pakistan. The PSB inoculums used in the experiment were analyzed for bacterial composition according to Bergey’s Manual of Systematic Bacteriology [40] and Bergey’s Manual of Determinative Bacteriology [41]. PSB was applied through seed treatment. The average population density of PSB in the maize inoculum was 1.73 × 10⁸ CFU mL⁻¹. Further details regarding the characteristics, population, and composition of the applied PSB in this research study can be found in a prior report by Adnan et al. [23]. Maize seeds (obtained from University Research Farm) were inoculated with a maize-specific PSB inoculum containing 1.44 × 10⁷ CFU g⁻¹ (wet weight). Seeds were soaked in a 10% sugar solution for 2 h to enhance adhesion, then treated with 8 g of PSB inoculum per 50 g of seeds, corresponding to a field application rate of 2 kg PSB inoculum per 25 kg of seeds per hectare, following the method of Alagawadi and Gaur [42]. Control seeds were soaked in distilled water and not inoculated. Seeds from both treatments were sown using line sowing, with a row spacing of 60–70 cm and a plant spacing of 20–25 cm. In addition, RP with a P content of 167 g P kg⁻¹ was procured from the Nuclear Institute for Food and Agriculture, Peshawar, and analyzed for Olsen P. PM was sourced from the University of Agriculture’s poultry farm, properly dried, crushed, sieved, and analyzed for NPK contents before application (Table 1). RP was applied at the rates of 0, 60, and 90 kg ha⁻¹ and PM at 0-, 4-, and 6 tons ha⁻¹, both with and without the inoculation of PSB. The details of the specific treatments administered to the experimental plots can be found in Table 2. RP and PM were applied at rates sufficient to meet crop P requirements and to reduce P fixation commonly observed in the semi-arid soils of the study area. The selection of these amendments and their application rates was based on the need to address both residual and anticipated P deficiencies under the prevailing alkaline soil conditions, which limit P availability. RP served as a slow-release P source, while PM provided additional nutrients and organic matter to improve soil structure, microbial activity, and nutrient availability.

Figure 1

Monthly mean weather data for the year 2022.

Table 1

NPK concentration in PM analyzed before application

N (%)	P (%)	K (%)
2.17	1.05	1.21

Table 2

Details of the treatments along with their respective dosage applied during the experiment

Treatments	RP P2O5 (kg ha−1)	PM (tons ha−1)	PSB
T1	RP 0	PM 0	Without PSB
T2	RP 0	PM 0	With PSB 1
T3	RP 0	PM 4	Without PSB
T4	RP 0	PM 4	With PSB
T5	RP 0	PM 6	Without PSB
T6	RP 0	PM 6	With PSB
T7	RP 60	PM 0	Without PSB
T8	RP 60	PM 0	With PSB
T9	RP 60	PM 4	Without PSB
T10	RP 60	PM 4	With PSB
T11	RP 60	PM 6	Without PSB
T12	RP 60	PM 6	With PSB
T13	RP 90	PM 0	Without PSB
T14	RP 90	PM 0	With PSB
T15	RP 90	PM 4	Without PSB
T16	RP 90	PM 4	With PSB
T17	RP 90	PM 6	Without PSB
T18	RP 90	PM 6	With PSB

Before initiating the experiment, a representative soil sample was obtained and subjected to laboratory analysis to assess the physical and chemical attributes of the soil within the experimental site. The results indicated that the soil had alkaline properties with a pH of 7.74 and was not saline, having an EC of 0.16 dS m⁻¹. It had a silty clay loam texture. The soil contained a relatively low amount of organic matter (0.83%) and displayed deficiencies in extractable P (5.54 mg kg⁻¹), K (78.8 mg kg⁻¹), and N (0.077 mg kg⁻¹), as outlined in Table 3. Following the experiment, soil samples were collected from 0–30 cm depth in the field, placed into sampling bags, and transported to the laboratory for further analysis. The soil samples underwent a drying process in the laboratory and were subsequently passed through a 2 mm sieve.

Table 3

Description of the soil physicochemical properties in the experimental area

Property	Unit	Values
Sand	%	23.5
Silt	%	54.7
Clay	%	21.8
Textural class	—	Silty clay loam
EC	d Sm−1	0.016
pH	—	7.74
SOM content	%	0.83
Soil total N content	mg kg−1	0.077
AB-DTPA extractable P content	mg kg−1	5.54
AB-DTPA extractable K content	mg kg−1	78.8

3 Results

3.1 Effect of treatments on post-harvest soil pH, EC, and SOM

The highest ECe is shown in Figure 2, where the 90-RP + 6-PM + PSB exhibited the highest ECe (0.24 dS m⁻¹), followed closely by 90-RP + 4-PM + PSB, surpassing the control. Although other treatments also increased ECe, their differences were not statistically significant. Conversely, 90-RP + 6-PM + PSB reduced soil pH from 7.74 (control) to 7.49, with 90-RP + 4-PM + PSB decreasing it more. Other treatments also resulted in a reduction in pH, although without statistically significant differences (Figure 2). The SOM was highest in the 90-RP + 6-PM + PSB treatment (0.83%), followed by 90-RP + 4-PM + PSB, both exceeding the control, as shown in Figure 2.

Figure 2

Effect of treatments on soil pH, EC, and organic matter. Bars on each column are the mean values of three replicates and contain a standard error of mean (n = 3). In each panel, bars with different letters differ significantly from each other at p < 0.05.

3.2 Effect of treatments on N, P, and K contents in the post-harvest soil

90-RP + 6-PM + PSB resulted in the highest increase in soil TN content (0.077%), followed by 90-RP + 4-PM + PSB, both significantly higher than the control (Figure 3). Similarly, the highest soil P content was observed in the 90-RP + 6-PM + PSB treatment (5.54 mg kg⁻¹), closely followed by 90-RP + 4-PM + PSB, compared to the control treatments (Figure 3). The trend persisted for soil K, where the 90-RP + 6-PM + PSB treatment showed the highest value (78.8 mg kg⁻¹), followed by 90-RP + 4-PM + PSB, both exceeding the control, as shown in Figure 3.

Figure 3

Effect of treatments on soil total N, extractable P and extractable K. Bars on each column are the mean values of three replicates and contain a standard error of mean (n = 3). In each panel, bars with different letters differ significantly from each other at p < 0.05.

3.3 Effect on plant TN, P, and K contents

The 90-RP + 6-PM + PSB treatment resulted in the highest plant TN content (1.28%), followed by 90-RP + 4-PM + PSB, both significantly higher than the control (Figure 4). Similarly, the highest P content was observed in the 90-RP + 6-PM + PSB treatment (0.92%), while the 90-RP + 4-PM without PSB inoculation recorded 0.85%, both exceeding the control (Figure 4). For K content, the 90-RP + 6-PM + PSB inoculation showed the maximum value (1.66%), followed closely by 60-RP + 6-PM + PSB, both surpassing the control, as can be seen in Figure 4. Although other treatments also enhanced plant TN, P, and K contents, the differences were not significantly different.

Figure 4

Effect of treatments on plant N, P, and K concentration. Bars on each column are the mean values of three replicates and contain a standard error of mean (n = 3). In each panel, bars with different letters differ significantly from each other at p < 0.05.

3.4 Effect of treatments on N, P, and K uptake by maize plants

In Figure 5, the 90-RP + 6-PM + PSB treatment exhibited the highest N uptake by maize plants (85.6 kg ha⁻¹), followed by the 90-RP + 4-PM + PSB inoculation treatment, both exceeding the control. As shown in Figure 5, the highest P uptake (17.50 kg ha⁻¹) was observed in the 90-RP + 6-PM + PSB treatment, followed closely by 90-RP + 4-PM + PSB. Both treatments exhibited higher uptake than the control. Similarly, in Figure 5, the 90-RP + 6-PM + PSB treatment recorded the maximum K uptake (69.92 kg ha⁻¹), followed by the 90-RP + 4-PM + PSB treatment, exceeding the control.

Figure 5

Effect of treatments on N uptake, P uptake, and K uptake. Bars on each column are the mean values of three replicates and contain a standard error of mean (n = 3). In each panel, bars with different letters differ significantly at p < 0.05.

3.5 Effect of treatments on growth parameters of maize

In Figure 6, the 90-RP + 6-PM + PSB inoculation produced the highest grain yield (4,141 kg ha⁻¹), followed by 90-RP + 4-PM + PSB treatment, both surpassing the control. As shown in Figure 6, the highest biological yield (9,939 kg ha⁻¹) was also recorded in the 90-RP + 6-PM + PSB treatment, followed by the same treatment without PSB, both exceeding the control. According to Figure 6, the tallest plants (194 cm) were observed in the 90-RP + 4-PM + PSB treatment, followed by the 90-RP + 6-PM treatment without PSB inoculation. Both values were significantly higher than that in the control. Figure 6 shows that the highest thousand-grain weight (312 g) was obtained in the 90-RP + 6-PM + PSB treatment, followed closely by the 90-RP + 4-PM + PSB treatment, both exceeding the control treatment.

Figure 6

Effect of treatments on grain yield, biological yield, plant height, and 1,000-grain weight. Bars on each column are the mean values of three replicates and contain a standard error of mean (n = 3). In each panel, bars with different letters differ significantly from each other at p < 0.05.

3.6 Relationship between the studied variables

The relationship between soil properties (EC, pH, and SOM), NPK concentrations in both soil and plants, and their influence on NPK uptake by maize plants and maize growth was measured using Pearson’s correlation and PCA. The results are presented in Table 4 and Figure 7. Notably, soil pH exhibits a weak negative correlation with all the variables, indicating that the treatments significantly affect it. Soil pH is crucial in regulating macronutrient (N, P, and K) dynamics in the soil–plant system. Likewise, a strong positive correlation existed between soil and plant macronutrient concentrations, their uptake, and the morphological attributes of the maize crop. The PCA results confirmed that soil pH, macronutrient dynamics, and wheat productivity attributes were negatively associated. These results provide additional evidence that the concentration of macronutrients and their uptake by maize plants strongly correlate with their availability in the soil. The first principal components (PC1) and PC2 explained 81.90 and 2.81% of the total variation, respectively.

Table 4

Details regarding the correlation analysis between different soil and plant parameters

Variables	Soil pH	Soil EC	SOM	Soil N	Soil P	Soil K	Plant N	Plant P	Plant K	N uptake	P uptake	K uptake	Ph	TGW	GY	BY
Soil pH	1.00
Soil EC	−0.39	1.00
SOM	−0.55	0.49	1.00
Soil N	−0.64	0.58	0.49	1.00
Soil P	−0.64	0.60	0.58	0.92	1.00
Soil K	−0.60	0.63	0.60	0.93	0.92	1.00
Plant N	−0.64	0.60	0.63	0.87	0.89	0.87	1.00
Plant P	−0.65	0.59	0.60	0.77	0.87	0.82	0.75	1.00
Plant K	−0.67	0.58	0.59	0.91	0.95	0.86	0.84	0.88	1.00
N uptake	−0.55	0.53	0.58	0.97	0.92	0.93	0.82	0.78	0.91	1.00
P uptake	−0.49	0.59	0.53	0.91	0.92	0.88	0.89	0.82	0.93	0.92	1.00
K uptake	−0.58	0.58	0.59	0.92	0.90	0.90	0.87	0.62	0.88	0.91	0.88	1.00
Ph	−0.31	0.59	0.58	0.82	0.87	0.84	0.74	0.84	0.88	0.88	0.92	0.78	1.00
TGW	−0.39	0.60	0.59	0.89	0.92	0.90	0.88	0.79	0.87	0.91	0.95	0.89	0.93	1.00
GY	−0.42	0.69	0.60	0.86	0.88	0.83	0.78	0.78	0.87	0.85	0.91	0.84	0.92	0.90	1.00
BY	−0.47	0.61	0.69	0.84	0.85	0.83	0.76	0.89	0.88	0.83	0.88	0.78	0.92	0.88	0.92	1.00

Two-tailed test of significance. TWG: 1,000-grain weight; GY: Grain yield; BY: Biological yield; ph: Plant height.

Figure 7

PCA among the soil and plant variables.

4 Discussion

The widespread and unregulated application of synthetic fertilizers has detrimental effects on soil quality, health, and agricultural productivity. To enhance crop production while reducing dependence on synthetic inputs, several sustainable approaches can be adopted, including the use of organic soil amendments and beneficial microorganisms such as PSB. Organic amendments improve not only the physical and chemical properties of soil but also stimulate microbial activity, which plays a crucial role in enhancing nutrient availability and crop yields [23]. In the present research, the inoculation of PSB with RP and PM resulted in a reduction in soil pH, an elevation in SOM content, and an increase in ECe in the soil. These results are in line with the previously conducted experiment where the soil pH was significantly reduced by the combined application of PSB and PM by H⁺ ions released in the soil, which also enhanced the availability of P in soil pools [51].

PSB is known to secrete various low molecular weight organic acids, such as gluconic, 2-keto gluconic, lactic, malic, acetic, oxalic, and succinic acids, which release H⁺ ions, thereby acidifying the soil environment and contributing to the reduction in pH [52,53]. These organic acids also facilitate the solubilization of mineral nutrients, particularly from RP, by chelating and releasing essential cations such as Ca²⁺ and Mg²⁺, contributing to an increase in soil EC [54,55]. While PSB alone may not significantly increase SOM, their integration with PM provides an additional source of organic carbon, which is vital for improving SOM content. This enhancement in SOM further stimulates microbial activity and supports the mineralization of essential nutrients [56]. Additionally, using PM and bio-slurry together has been proven to boost microbial activity and root colonization, contributing to increased soil organic carbon and improved soil fertility [57]. Besides interaction with organic and inorganic sources and their role in the solubilization of mineral P and other nutrients, PSB can also help in solubilizing mineral P already present in the soil. This was evidenced by Song and coworkers while studying insoluble phosphorus dissolution by Burkholderia cepacia DA23 [58]. Another study showed that Pseudomonas aeruginosa KR270346 also solubilizes phosphorus by secreting GA by direct oxidation pathway as the dominant acid for phosphorus solubilization [59].

Furthermore, the inoculation of PSB along with RP and PM has been shown to significantly improve post-harvest soil levels and plant concentrations of P, N, and K through synergistic interactions. PSB facilitates the solubilization of insoluble phosphorus from RP and native soil minerals by secreting organic acids that lower soil pH and mobilize bound P for plant uptake [53,60]. In the Khyber Pakhtunkhwa Province of Pakistan, huge reservoirs of RP with around 6.9 million tons as resource and 5.58 million tons PR serving as reserves are present [61,62]. However, scientists discourage the direct sole application of RP to the soil. Therefore, we find a new approach to applying RP with organic and bio-fertilizers to increase its solubility and P recovery in nutrient-poor alkaline soils. This claim is supported by the literature where the use of organic amendments (FYM, press-mud, PM, compost, etc.), acid treatment of PR (partially), use of phosphorus-solubilizing microorganisms (PSMs), etc. [63] can reduce the use of chemical fertilizers. Moreover, this approach can be viable, economical, and environmentally friendly in the agricultural context of Pakistan. In parallel, PM serves as a rich source of organic matter, enhancing microbial activity and contributing to P availability through mineralization processes [64]. During the study, the applied PM + RP sources along with PSB inoculation were assessed quantitatively. The results showed that the combination of these three amendments to alkaline calcareous soils released P levels similar to synthetic sources. Based on previous studies, similar results have been recorded by the application of bio-organic-based amendments on P availability and recovery from RP [65,66]. Several other studies have demonstrated that integrating bio-based amendments with RP can substantially improve macronutrient dynamics in soil, particularly P [67,68]. For example, Gangwal et al. [69] reported that PSB seed inoculation stimulates the release of root exudates and residues, further enriching the soil nutrient pool. Adeli et al. [70] observed increased residual NO₃-N in soils treated with PM compared to those receiving only inorganic fertilizers. PSB inoculation also led to higher total nitrogen content in soil compared to untreated controls. Enhanced total nitrogen in plant tissues can also be attributed to the nutrient-rich nature of PM, which supports improved plant growth. Akanbi et al. [71] confirmed that the elevated nitrogen levels in PM-amended soils promote the vegetative development of various crops. The combined application of organic and inorganic fertilizers has been found to increase extractable post-harvest K levels compared to the sole use of inorganic fertilizers. This is primarily due to the high K content in PM, which makes it a suitable alternative to synthetic K fertilizers [72]. Moreover, high levels of plant-available potassium and phosphorus in PM-treated plots have been validated by multiple studies, which report greater concentrations of these nutrients compared to untreated soils [73]. Awaad et al. [74] also demonstrated that combining inorganic fertilizers with organic sources, such as farmyard manure, significantly enhances N, P, and K uptake in canola. Additionally, Emami et al. [75] found that co-application of PSB and endophytes improved phosphorus-use efficiency across different wheat cultivars. In sugarcane systems, even the sole application of PSB has been effective in enhancing phosphorus availability and uptake [76].

Soil moisture plays a critical role in sustaining viability and functional activity of PSB, especially under arid and semi-arid climatic conditions. Water availability governs microbial processes in three principal ways: serving as a vital resources, functioning as a solvent for biochemical reactions, and facilitating the transport of solutes and microorganisms within the soil matrix [77]. Enhancements in soil moisture and thermal regimes can stimulate microbial proliferation and enzymatic activity, thereby influencing long-term soil fertility and nutrient cycling [78]. Elevated water content creates a more favorable microenvironment for microbial survival during periods of water stress, which in turn promotes microbial metabolism, respiration, and biomass accumulation [79]. Our experimental findings indicate that co-application of PSB with RP and PM markedly increases nitrogen (N), phosphorus (P), and potassium (K) uptake in maize plants (Figure 4). The most pronounced N uptake occurred in treatments receiving both PM and RP, collaborating the results of Awaad et al. [74]. Similarly, the synergistic application of RP and PM led to substantial improvements in plant P acquisition, which is consistent with observations reported by Aisha and Taalab [80], who demonstrated enhanced N and P uptake with the integrated use of organic amendments and RP. Potassium uptake was also significantly augmented in plots treated with PSB in combination with organic and inorganic inputs. This effect may be attributed to the secretion of organic acids by PSBs, which not only mobilize phosphorus but also facilitate potassium release within the rhizosphere. Notably, amino acids exuded by the roots of crops such as sugar beet have been implicated in enhancing K solubilization from soil minerals [81]. Moreover, both PSB and PM contribute to lowering soil pH through the release of organic acids, thereby improving the solubility and bioavailability of both native and supplementary phosphorus forms [82,83]. Supporting this, previous studies have shown that combined inoculation of PSB strains (e.g., Pseudomonas spp., Azospirillum spp., and Agrobacterium spp.) with RP and PM significantly boosts phosphorus uptake and plant growth in chili, achieving comparable outcomes to those obtained using diammonium phosphate [66].

In addition to improvements in soil properties and nutrient uptake, the application of PSB with RP and PM also led to significant enhancements in various maize growth parameters, including grain yield, biological yield, 1,000-grain weight, and plant height. These findings align with the study by Han and Lee [84], which reported improved root development, shoot biomass, and overall crop yield following PSB inoculation. Further supporting our results, Sharma et al. [85] highlighted that PSB – classified as plant growth-promoting bacteria – stimulates plant growth through multiple mechanisms, notably the production of phytohormones, antibiotics, and siderophores. The release of phytohormones [86] and organic acids [87] by PSB plays a crucial role in enhancing phosphorus solubility, thereby promoting plant development and yield performance.

5 Conclusions

The integrated application of low-grade RP and PM, combined with PSB, significantly improved maize yield, plant growth attributes, and the availability and uptake of N, P, and K in the soil, especially under nutrient-deficient semi-arid conditions. The RP + PSB combination outperformed the sole use of RP, highlighting the potential of microbial inoculation in enhancing P solubility. Based on our findings, we recommend the use of PSB in conjunction with locally available PM and RP as a cost-effective and eco-friendly alternative to commercial P fertilizers. This integrated approach not only boosts crop productivity but also promotes long-term soil health, offering a scalable and sustainable solution for farmers. This approach can be validated as a sustainable soil health improvement in the following subsequent cropping years. Considering the large reserves of RP and substantial PM production in Pakistan, this strategy presents a viable pathway for policymakers and stakeholders aiming to enhance food security and environmental sustainability. We also encourage future studies across different agroecological zones and cropping systems to validate and optimize this approach.