Impact of Coated Phosphorus Fertilizers and Application Methods on Soil Fertility, Yield, and Ionic Regulation of Common Beans (Phaseolus vulgaris L.) Grown in Saline Soil

Abstract

Salinity is a major limitation on common bean productivity, while phosphorus in many soils is often immobilized, limiting its availability to plants. This study investigated the effects of coated and uncoated superphosphate fertilizers, applied at different rates and using distinct methods, on soil properties, plant growth, and ion regulation in common beans grown in saline soil over two seasons (2023–2024). Treatments combined two fertilizer types (coated with potassium sulfate and uncoated), two P rates (360 and 480 kg/ha), and two application methods: (1) conventional application, broadcasting followed by plowing to 30 cm depth during soil preparation; (2) surface application, broadcasting without incorporation. Six treatments were applied: T₁: 360 kg/ha of uncoated superphosphate (conventional method); T₂: 480 kg/ha of uncoated superphosphate (conventional method); T₃: 360 kg/ha of coated superphosphate (conventional method); T₄: 480 kg/ha of coated superphosphate (conventional method); T₅: 360 kg/ha of coated superphosphate (surface method); and T₆: 480 kg/ha of coated superphosphate (surface method). The results demonstrated that soil pH was unaffected across treatments. However, T₄ and T₆ significantly improved nutrient availability (N, P, and K), biomass, grain yield, and seed nutritional quality (protein, P, K, and Ca). Despite increased soil EC, these treatments enhanced ionic balance (higher K/Na and Ca/Na ratios) indicating improved stress tolerance. Importantly, T₃ (360 kg/ha coated) performed comparably to T₂ (480 kg/ha uncoated), suggesting that coated superphosphate at lower rates can reduce input costs without compromising yield. These results demonstrate the agronomic and environmental benefits of coated superphosphate, particularly under saline conditions, through enhanced nutrient use efficiency and improved crop performance.

1. Introduction

The common bean (Phaseolus vulgaris L.) is a highly valuable leguminous crop, recognized for its rich content of essential minerals, dietary fiber, protein, and bioactive compounds [1]. Beyond its nutritional benefits, it is considered environmentally sustainable due to its short growing season, low carbon footprint and ability to fix atmospheric nitrogen, which reduces dependence on synthetic fertilizers [2]. When included in crop rotations, common beans contribute significantly to soil fertility and nutrient cycling, particularly by enhancing phosphorus availability through root exudates and improved rhizosphere interactions [3].

Soil salinity is a major constraint on agricultural productivity, affecting large areas of cultivated land worldwide [4]. This challenge is particularly acute in regions such as Egypt’s Nile Delta, where salinization poses a significant threat to crop production [5,6]. High salinity levels negatively impact plant physiology by reducing water and nutrient uptake, disrupting metabolic processes, and inhibiting overall growth and development [7]. Common bean is especially sensitive to abiotic stressors such as salinity and drought, which can lead to oxidative stress, diminished photosynthetic efficiency, and impairments in metabolic and enzymatic activity [8,9]. Salinity not only promotes the excessive production of reactive oxygen species (ROS), exacerbating oxidative damage, but also interferes with phosphorus uptake, further limiting plant growth and productivity [10].

Phosphorus plays a critical role in numerous plant physiological processes, including the synthesis of proteins, phospholipids, and nucleic acids [11]. The application of phosphorus fertilizers has been shown to alleviate some of the detrimental effects of salinity by improving phosphorus availability and enhancing plant antioxidant defense mechanisms [12]. Under saline conditions, phosphorus supplementation has been associated with increased biomass production and improved nutrient uptake [13,14]. In common bean cultivation, phosphorus deficiency is already a widespread concern, and even moderate salinity levels can significantly reduce dry matter accumulation in both shoots and roots [15]. Mohamed et al. [16] found that applying phosphorus in the form of superphosphate or urea phosphate (35 kg P ha⁻¹) to Phaseolus vulgaris improved key physiological traits, including soluble sugar content, proline accumulation, and chlorophyll levels, all of which contribute to enhanced stress tolerance. Phosphorus fertilization also promoted better growth performance, as indicated by increased plant height, leaf area, and root and shoot dry weights.

Phosphorus (P) deficiency affects over 43% of global agricultural soils, primarily due to chemical immobilization, even when total phosphorus levels are adequate [17]. This issue is particularly pronounced in saline and alkaline soils, which are common in arid and semi-arid regions. In alkaline soils, elevated pH levels promote the formation of insoluble calcium phosphate compounds (e.g., tricalcium phosphate), which are unavailable for plant uptake [18]. Additionally, excessive Na ions disrupt soil structure and compete with essential cations, thereby reducing phosphorus solubility and absorption. Although Cl ions do not directly contribute to phosphorus fixation, they exacerbate osmotic stress, impairing root function and further limiting nutrient uptake [17]. In Egyptian soils, for example, where pH typically ranges from 7.5 to 8.2 [19], phosphorus fixation is intensified by both alkaline conditions and salinity stress [20]. As a result, plants generally absorb only 10–25% of applied phosphorus fertilizers, with the remainder becoming immobilized or lost through leaching or runoff. This inefficiency presents serious challenges for crop productivity, environmental sustainability, and fertilizer management.

Enhancing phosphorus use efficiency (PUE) has become increasingly important, particularly in saline and alkaline soils where conventional phosphorus fertilizers are often ineffective due to strong fixation and limited mobility, despite their agronomic potential. To overcome these limitations, various technologies are being explored to improve phosphorus bioavailability under adverse soil conditions. These approaches include foliar phosphorus applications [21], nanofertilizers [22], and microbial inoculants [23]. Among these innovations, controlled-release fertilizers (CRFs), which gradually release nutrients in synchronization with plant demand, have emerged as a promising and sustainable alternative [24,25].

Compared to conventional phosphate fertilizers, controlled-release fertilizers (CRFs) have been shown to reduce environmental losses and enhance nutrient use efficiency across various cropping systems [26]. Research has demonstrated that CRFs enhance phosphorus uptake and increase yields in crops such as wheat [27] and maize [28].

However, the effectiveness of phosphorus-coated fertilizers, a type of CRF, in salt-affected soils remains underexplored, particularly for salt-sensitive crops like common beans. The comparative benefits of coated versus uncoated phosphorus fertilizers in saline environments are not yet well understood, and existing studies on this topic are limited.

To address this knowledge gap, the present study investigates the effects of phosphorus-coated fertilizers on common bean yield, nutrient uptake, and physiological performance under saline soil conditions. Specifically, it compares the performance of coated and uncoated phosphorus applied at different rates and using distinct application methods. The aim is to identify more effective and sustainable phosphorus fertilization strategies for cropping systems vulnerable to salinity. The findings are expected to provide valuable insights into phosphorus management in legume crops and offer practical recommendations for improving soil fertility and crop productivity under saline conditions.

2. Materials and Methods

2.1. The Site of the Experiment

Field experiments were conducted during the 2023 and 2024 growing seasons at the Experimental Farm of the Faculty of Agriculture, Mansoura University, Dakahliya Governorate, Egypt (31°22′59.88″ N, 31°05′31.38″ E). The region has a semi-arid climate, characterized by hot summers and mild winters. Key agroclimatological parameters, such as precipitation, relative humidity, and temperature, recorded during the experimental period, are summarized in

Table 1. Agroclimatology data recorded during the field experiments.

Season	Month	Temperature Max. (°C) 1	Temperature Min. (°C) 1	Relative Humidity (%) 1	Precipitation (mm/Day)
2023	October	37.53	17.74	60.72	0.02
	November	35.42	11.64	60.79	0.15
	December	28.96	9.28	73.3	1.12
	January	24.45	7.4	72.89	0.64
2024	October	37.47	15.83	56.63	0.00
	November	29.94	12.21	61.06	0.14
	December	25.61	6.27	65.54	0.03
	January	27.36	5.35	67.07	0.35

¹ Temperature Maximum and Minimum; Wind Speed and Relative Humidity measured at 2 m above ground level during the 2023 and 2024 growing seasons (October to January).

.

Prior to planting, soil samples were collected from 0 to 30 cm depth for physicochemical analysis. The results of this analysis are presented in

Table 2. Selected physicochemical properties of the soil (0–30 cm depth) prior to cultivation.

Season	pH 1	EC (dS/m) 1	O.M (%) 1	Ava. N (mg/kg)	Ava. P (mg/kg)	Ava. K (mg/kg)
2023	8.16	7.22	1.30	17.60	9.50	243.9
2024	8.20	7.07	1.40	22.50	9.70	269.4
Season	Coarse sand (%)	Fine sand (%)	Silt (%)	Clay (%)	Texture 1
2023	3.5	15.80	31.00	49.00	Clayey
2024	3.52	15.28	32.55	48.65	Clayey

¹ pH in 1:2.5 soil suspensions; EC in soil past extract; O.M: soil organic matter; the soil texture triangle is used to determine the soil texture.

.

2.2. Superphosphate Coating Process

Superphosphate (containing 15% P₂O₅) was coated with potassium sulfate (48% K₂O) at the Fertilizer Development Unit of the Talkha Fertilizer Factory. Superphosphate coating was performed using a BB fertilizer mixer, which allows a maximum coating ratio of up to 20%. Before coating, the superphosphate was sieved through a 2 mm mesh to ensure uniform particle size. Once uniformity was confirmed, formaldehyde was applied as an adhesive. The fertilizer mixture was then transported on a conveyor belt for coating. Each batch consisted of approximately 10 kg of superphosphate, with potassium sulfate added incrementally. A coating ratio of 17% (equivalent to 1.7 kg potassium sulfate per batch) was determined to be the most effective for superphosphate coating.

2.3. Design of Experiments

Six treatments were established by combining two types of phosphorus fertilizer (coated and uncoated superphosphate), two application rates (360 and 480 kg/ha), and two broadcasting methods. The two application methods were defined as follows:

• Conventional application: Fertilizer was broadcast and then incorporated into the soil by plowing to a depth of 30 cm during field preparation.
• Surface application: Fertilizer was broadcast without incorporation into the soil.

The experiment was arranged in a randomized complete block design (RCBD) with six treatments and three replications. The treatment details are as follows:

-

T₁: 360 kg/ha of uncoated superphosphate (conventional method);

-

T₂: 480 kg/ha of uncoated superphosphate (conventional method) as a control;

-

T₃: 360 kg/ha of coated superphosphate (conventional method);

-

T₄: 480 kg/ha of coated superphosphate (conventional method);

-

T₅: 360 kg/ha of coated superphosphate (surface method);

-

T₆: 480 kg/ha of coated superphosphate (surface method).

2.4. Field Preparation and Planting

To improve soil aeration and ensure optimal seedbed conditions, the experimental field was plowed twice in perpendicular directions. Phosphorus fertilizers were uniformly incorporated during soil preparation to ensure even distribution across all plots.

Each plot consisted of 12 rows, each 5 m in length and spaced 15 cm apart. Seeds of Phaseolus vulgaris L. cv. Giza 6, were obtained from the Sakha Agriculture Research Station in Kafr El-Sheikh, Egypt, and sown at the recommended seed rate of 95 kg/ha.

Prior to planting, seeds were inoculated with Rhizobium biofertilizer at a rate of 1.36 kg/ha, with a sugar solution applied as an adhesive agent. The inoculated seeds were then air-dried in the shade for one hour. This biological inoculation was intended to enhance biological nitrogen fixation (BNF), thereby reducing the dependence on synthetic nitrogen fertilizers.

Seeds were sown on 3 October in both 2023 and 2024 seasons, at a depth of 4 cm, using two seeds per hole with 30 cm between plants. After germination, seedlings were thinned to one plant per hill before the first irrigation to ensure uniform plant density.

Fertilization practices followed standard Egyptian guidelines for common bean cultivation. The following inputs were applied:

-

Pre-planting, 60 kg/ha of magnesium sulfate, 120 kg/ha of sulfur, and 50 kg/ha of potassium sulfate were applied.

-

Vegetative stage (21 days after seeding), 50 kg/ha of ammonium sulfate was applied.

-

Flowering stage (60 days after seeding), 75 kg/ha of potassium sulfate was applied.

No additional potassium fertilization was applied to the coated superphosphate treatments (T₃ to T₆), as the potassium sulfate coating (81.6 kg/ha) was deemed sufficient to meet the crop’s potassium requirements.

2.5. Soil Testing and Sampling

Soil samples were collected from three randomly selected locations within the field using a 1 m-diameter soil auger at a depth of 0–30 cm, both before sowing and after harvest. The samples were analyzed using standard procedures to determine the following parameters: soil electrical conductivity (EC) using an EC meter, soil pH using a pH meter, available nitrogen measured by extraction (KCl) followed by colorimetric method, available phosphorus (extracted using a sodium bicarbonate solution 0.5 M, pH 8.5) and determined colorimetrically using a spectrophotometer at 880 nm, available potassium (extracted by ammonium acetate) and measured by flame photometer, and organic matter content by the Walkley and Black method [29]. Particle size distribution was determined using the pipette method [30].

2.6. Harvesting and Ionic Analysis

At full pod maturity (110 days after sowing), plants were harvested, and weighed. Shoot, root, and seed yields were recorded and converted to tons per hectare (ton/ha). Seed samples were oven-dried, finely ground, and subjected to wet digestion for chemical analysis.

Nutrient concentrations of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and sodium (Na), as well as seed protein content, were determined following standard procedures described by Motsara and Roy [31]. The K/Na and Ca/Na ratios were calculated for both shoot and root tissues to assess ionic balance.

Selective transport (ST) was calculated to evaluate ionic regulation under saline conditions, using the formula proposed by Wang et al. [32]:

$$\mathrm{S}\mathrm{T}={\displaystyle \frac{\mathrm{K}:\mathrm{N}\mathrm{a} \mathrm{i}\mathrm{n} \mathrm{s}\mathrm{h}\mathrm{o}\mathrm{o}\mathrm{t}}{\mathrm{K}:\mathrm{N}\mathrm{a} \mathrm{i}\mathrm{n} \mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t}}}$$

2.7. Statistical Analysis

Analysis of variance (ANOVA) was performed using CoStat software (version 6.303, 1998–2004). A one-way randomized block design was applied to the data. Analysis of variance (ANOVA) was performed, and treatment means were compared using Tukey’s Honestly Significant Difference (HSD) test at a significance level of p ≤ 0.05.

3. Results

3.1. Soil Chemical Properties

Analysis of soil pH showed no statistically significant differences among treatments across both growing seasons (

Table 3. Some soil chemical properties were affected by treatments in 2023 and 2024.

Treatments *	pH	ECe (dS/m)	Ava. N (mg/kg)	Ava. P (mg/kg)	Ava. K (mg/kg)
	2023
T1	8.26 a *	7.03 b	24.70 d	09.27 d	253.95 c
T2	8.29 a	7.14 ab	31.57 bc	13.53 abc	310.14 ab
T3	8.27 a	7.08 ab	31.14 bc	12.97 bc	294.93 b
T4	8.30 a	7.18 a	34.95 a	16.04 a	322.44 a
T5	8.27 a	7.06 b	30.06 c	12.32 c	290.25 b
T6	8.30 a	7.16 a	33.69 ab	15.64 abc	318.13 a
p value	ns	*	**	**	*
CV (%)	292.26	90.6	8.67	5.26	12.01
Treatments *	2024
T1	8.29 a	6.84 b	26.08 d	09.56 c	275.29 c
T2	8.32 a	6.97 a	34.87 bc	13.69 b	325.41 ab
T3	8.30 a	6.90 ab	34.57 bc	13.48 b	318.20 b
T4	8.34 a	7.03 a	37.57 a	16.74 a	333.71 a
T5	8.29 a	6.87 b	34.25 c	12.97 b	312.15 b
T6	8.33 a	7.00 a	36.82 ab	16.02 a	328.73 a
p value	ns	*	**	**	*
CV (%)	268.84	72.65	8.32	5.38	14.87

* T₁: 360 kg/ha of uncoated superphosphate (conventional method); T₂: 480 kg/ha of uncoated superphosphate (conventional method) as a control; T₃: 360 kg/ha of coated superphosphate (conventional method); T₄: 480 kg/ha of coated superphosphate (conventional method); T₅: 360 kg/ha of coated superphosphate (surface method); T₆: 480 kg/ha of coated superphosphate (surface method). Statistical significance is indicated as p ≤ 0.01 (**), p ≤ 0.05 (*), and not significant (ns) in p value row. The same letter indicates that an insignificant difference was observed.

). However, a slight increase in pH values was observed during the second season, with all values remaining within the alkaline range.

In contrast, soil salinity (EC) was significantly affected by the treatments (p ≤ 0.05). The highest EC values were recorded in treatment T₄ (7.18 and 7.03 dS/m in 2023 and 2024, respectively), although these values were not significantly different from those of treatments T₂ and T₆ (Table 3). Higher application rates (480 kg/ha) generally resulted in increased EC compared to the lower rate (360 kg/ha), except for treatments T₂ and T₃. For instance, EC in T₄ was 2.1% higher in 2023 and 2.8% higher in 2024 compared to T₁.

Treatments involving coated superphosphate (T₄ and T₆) significantly improved soil nutrient content, particularly nitrogen, phosphorus, and potassium (Table 3). T₄ recorded the highest nutrient levels, followed by T₆ and then T₂, while the lowest values were observed in T₁. When comparing the initial soil values (Table 2), T₄ increased nitrogen content by 98.6% in 2023 but decreased to 67.0% in 2024. Phosphorus increased by 68.8% and 72.5%, and potassium by 32.2% and 23.9% in 2023 and 2024, respectively.

3.2. Yield Performance

The impact of different fertilizer treatments on the yield and biomass of Phaseolus vulgaris L. is summarized in

Table 4. Yield and biomass of Phaseolus vulgaris L. were affected by treatments in 2023 and 2024.

Treatments *	Shoot Dry Weight (ton/ha)	Root Dry Weight (ton/ha)	Seed Yield (ton/ha)
	2023
T1	0.641 c *	0.078 c	1.120 c
T2	0.832 b	0.096 b	1.273 b
T3	0.810 b	0.095 b	1.262 b
T4	1.051 a	0.116 a	1.447 a
T5	0.794 b	0.097 b	1.233 b
T6	1.006 a	0.112 a	1.400 a
p value *	**	**	**
CV (%)	5.73	7.38	10.81
Treatments *	2024
T1	0.718 c	0.087 d	1.143 d
T2	0.887 b	0.104 bc	1.370 bc
T3	0.860 b	0.102 bcd	1.287 c
T4	1.107 a	0.121 a	1.557 a
T5	0.842 b	0.100 cd	1.133 d
T6	1.043 a	0.117 ab	1.450 ab
p value *	**	**	**
CV (%)	6.48	8.32	7.96

* T₁: 360 kg/ha of uncoated superphosphate (conventional method); T₂: 480 kg/ha of uncoated superphosphate (conventional method) as a control; T₃: 360 kg/ha of coated superphosphate (conventional method); T₄: 480 kg/ha of coated superphosphate (conventional method); T₅: 360 kg/ha of coated superphosphate (surface method); T₆: 480 kg/ha of coated superphosphate (surface method). Statistical significance is indicated as p ≤ 0.01 (**). The same letter indicates that an insignificant difference was observed.

. All measured parameters, shoot dry weight, root dry weight, and grain yield were significantly influenced by treatment (p ≤ 0.01).

The highest values were observed in treatments T₄ and T₆ (480 kg/ha coated superphosphate), with no significant difference between them. T₁ consistently produced the lowest biomass and yield in both seasons. Compared to the control (T₂), T₄ increased shoot dry weight by 25.6%, root dry weight by 18.5%, and grain yield by 13.6%. Similarly, T₆ improved these parameters by 19.3%, 14.1%, and 7.9%, respectively.

Notably, treatments T₃ and T₅, which used a lower rate (360 kg/ha) of coated fertilizer, showed no significant differences from the control (T₂), indicating their potential as cost-effective alternatives.

3.3. Seed Nutritional Quality

Analysis of variance revealed highly significant differences (p ≤ 0.01) among treatments for seed protein, phosphorus (P), potassium (K), calcium (Ca), and sodium (Na) content in both seasons (

Table 5. Some nutrient content in common beans were affected by treatments in 2023 and 2024.

Treatments *	Protein %	P%	K%	Ca%	Na%
	2023
T1	20.23 c	0.65 c	1.29 c	1.64 c	0.099 a
T2	22.56 b	0.76 b	1.42 b	1.79 b	0.080 b
T3	22.33 b	0.74 b	1.40 b	1.76 b	0.083 b
T4	24.04 a	0.84 a	1.55 a	2.08 a	0.062 c
T5	22.10 b	0.73 b	1.39 b	1.74 b	0.085 a
T6	23.81 a	0.81 a	1.53 a	2.06 a	0.065 c
p value *	**	**	**	**	**
CV (%)	16.70	10.31	15.07	10.08	5.70
Treatments *	2024
T1	19.94 c	0.68 d	1.32 c	1.67 c	0.097 a
T2	22.19 b	0.81 bc	1.45 b	1.80 b	0.075 b
T3	22.10 b	0.80 bc	1.43 b	1.78 b	0.080 b
T4	23.42 a	0.91 a	1.66 a	2.13 a	0.053 c
T5	21.85 b	0.78 c	1.41 b	1.77 b	0.082 a
T6	23.23 a	0.89 ab	1.61 a	2.09 a	0.054 c
p value *	**	**	**	**	**
CV (%)	18.10	9.42	12.04	9.90	4.35

* T₁: 360 kg/ha of uncoated superphosphate (conventional method); T₂: 480 kg/ha of uncoated superphosphate (conventional method) as a control; T₃: 360 kg/ha of coated superphosphate (conventional method); T₄: 480 kg/ha of coated superphosphate (conventional method); T₅: 360 kg/ha of coated superphosphate (surface method); T₆: 480 kg/ha of coated superphosphate (surface method). Statistical significance is indicated as p ≤ 0.01 (**). The same letter indicates that an insignificant difference was observed.

).

The highest protein content was recorded in treatment T₄ (24.04% in 2023 and 24.33% in 2024), while the lowest was found in T₁. Similar trends were observed for P, K, and Ca concentrations. In contrast, seed sodium content decreased significantly with higher application rates. The highest Na% was observed in T₁ and T₅, while the lowest values were in T₄ and T₆.

No significant differences were found between T₂ (control) and the lower-rate coated treatments T₃ and T₅ for any nutrient, further supporting their potential as efficient alternatives.

3.4. Ion Regulation

The K/Na and Ca/Na ratios in shoot and root tissues differed significantly among treatments (Figure 1). The highest shoot K/Na and Ca/Na ratios were observed in T₄ and T₆ in both seasons, with no significant difference between them. The lowest ratios were consistently recorded in T₁.

In roots, similar trends were observed. T₄ exhibited the highest root K/Na ratios (7.84 in 2023 and 8.15 in 2024), while T₁ had the lowest values (5.86 and 6.07). Ca/Na ratios followed a similar pattern. Treatments T₃ and T₅, despite using the lower application rate, showed comparable ionic balance to T₂.

Selective transport (ST) ratios, representing ionic regulation efficiency, also varied significantly between treatments (p ≤ 0.01). T₄ had the lowest ST values (4.04 in 2023 and 3.92 in 2024), indicating efficient translocation of beneficial ions under saline conditions. T₆ showed similar values, with no significant difference from T₄ (Figure 2). The highest ST values were recorded in T₁, confirming its lower stress tolerance.

4. Discussion

This study highlights the potential of potassium sulfate–coated superphosphate as an effective strategy to enhance Phaseolus vulgaris L. performance under saline soil conditions. While the application of different treatments had no significant effect on soil pH, notable improvements in soil nutrient availability, plant growth, and nutrient uptake were observed, even in the presence of elevated soil salinity.

4.1. Soil Chemical Properties and Nutrient Availability

Soil pH remained statistically unaffected (p > 0.05) across treatments and seasons, confirming the stability of alkaline conditions, likely due to the inherent buffering capacity of the soil. This observation is consistent with prior findings by Huang et al. [33] and Fertahi et al. [34], who reported minimal influence of phosphorus fertilizers—coated or uncoated, on soil pH. Interestingly, pH responses were weaker in coated treatments, potentially due to reduced interaction between fertilizer components and soil particles.

In contrast, electrical conductivity (EC) increased significantly (p ≤ 0.05) with higher phosphorus application rates, regardless of coating. The increased EC is not directly attributed to phosphorus, which has low mobility in soil, but rather to accompanying ions (e.g., Ca, SO₄, K) from the fertilizer formulation, particularly the potassium sulfate coating. These ions dissolve readily and contribute to total soil salinity. Although the increases in EC were statistically significant, values remained within acceptable limits for common bean growth; the increase highlights the importance of considering nutrient interactions and salinity dynamics linked to the composite fertilizer input. This supports findings by Işik et al. [35], who noted similar EC trends without adverse effects on plant development. Future research should consider detailed soil solution analysis to better understand ion interactions and movement following coated fertilizer application.

Enhanced nutrient availability, particularly nitrogen and potassium, may result from improved microbial activity stimulated by coated fertilizer treatments. This aligns with results from Zhang et al. [36] and Mehnaz et al. [37], who found that coated phosphorus fertilizers improve soil nutrient dynamics by promoting microbial mineralization and retention processes. Additionally, the potassium sulfate coating contributes a significant source of potassium, which can further stimulate microbial activity and improve nutrient cycling [38]. Coated formulations also mitigate phosphorus fixation in alkaline soils, maintaining phosphorus in a plant-available form [39].

4.2. Plant Growth and Yield Response

Coated superphosphate, especially at the higher application rate (T₄ and T₆), significantly enhanced biomass accumulation and seed yield compared to uncoated fertilizers. These results demonstrate the agronomic value of coating phosphorus sources with potassium sulfate, which not only improves phosphorus bioavailability but also contributes additional potassium, an essential for osmotic regulation, enzyme activation and mitigation under saline conditions.

The controlled release of nutrients from coated fertilizers ensures a gradual and sustained phosphorus supply throughout the growth cycle. This aligns with findings by Mohamed et al. [16] and Chen et al. [39], who observed improved root and shoot biomass in legumes following the application of coated phosphorus fertilizers under saline conditions. The improved vegetative growth observed in this study likely results from enhanced photosynthetic efficiency, increased root exploration, and reduced salinity-induced nutrient competition, particularly with sodium ions.

Furthermore, treatments T₃ and T₅, which involved lower fertilizer rates (360 kg/ha) of coated superphosphate, produced comparable outcomes to the uncoated control (T₂ at 480 kg/ha). This highlights the efficiency and cost-effectiveness of coated fertilizers in reducing application rates without compromising productivity.

4.3. Nutritional Quality and Ion Regulation

Improved seed nutritional quality in terms of protein, phosphorus, potassium, and calcium concentrations was clearly associated with coated fertilizer treatments, particularly T₄ and T₆. This enhancement reflects improved nutrient uptake and translocation to reproductive organs, a critical factor in ensuring food quality and nutritional security.

Importantly, the potassium sulfate coating acts as a deliberate potassium source, contributing to the significantly higher potassium content found in both soils and bean tissues treated with coated superphosphate. The application of coated fertilizers significantly reduced seed sodium content, which is crucial in saline soils where sodium often competes with essential cations such as K and Ca for uptake. This improved ionic balance is evident from the increased K/Na and Ca/Na ratios in both shoot and root tissues of treated plants. These results are consistent with the findings of Bargaz et al. [15] and Mohamed et al. [16], who demonstrated that phosphorus application improves ionic regulation and salinity tolerance by reducing sodium uptake and promoting the accumulation of beneficial nutrients.

The selective transport (ST) index further confirmed the advantage of coated fertilizers in maintaining higher K/Na ratios in shoots relative to roots, indicating effective exclusion of sodium and preferential translocation of potassium. The potassium provided by the coating material played a central role in enhancing membrane transport selectivity, stabilizing osmotic conditions, and protecting plant tissues under salt stress.

The observed changes in grain calcium content (notably in treatments T₄ and T₆) also suggest a relevant input of calcium from the phosphorus source, which should be accounted for alongside potassium and sulfate ions when interpreting nutritional and physiological responses. While these accompanying ions (K, SO₄, and Ca) have been acknowledged, their quantitative contributions and impacts on nutrient dynamics and plant uptake require explicit consideration.

These findings are also supported by Bouras et al. [40] and Sun et al. [41], who reported that phosphorus fertilization under salinity stress conditions improves nutrient availability and enhances mineral balance by suppressing sodium uptake.

5. Conclusions

This study demonstrated that superphosphate treatments significantly influenced soil nutrient availability, plant growth, yield performance, and ion regulation in common bean plants cultivated under saline soil conditions. Among the treatments, potassium sulfate-coated superphosphate applied at 480 kg/ha resulted in the highest improvements in biomass, grain yield, and seed nutrient content. The application of 360 kg/ha coated superphosphate produced similar results to 480 kg/ha uncoated superphosphate, indicating that it can serve as a cost-effective alternative with comparable agronomic outcomes.

Overall, coated superphosphate treatments enhanced nutrient uptake and ion selectivity, improving K/Na and Ca/Na ratios in plant tissues. These results confirm the effectiveness of potassium sulfate-coated superphosphate in mitigating salinity stress and improving productivity in common bean cultivation.