A sustainable pH responsive hydrogel-based phosphorus fertilizer using nano oxalic acid-modified rock phosphate and humic acid

Tharushi Perera; Sanduni Dabare; Imalka Munaweera; Kasun M Thambugala; Saranga Diyabalanage

doi:10.1039/d6ra03574d

. 2026 Aug 5;16(39):44788–44818. doi: 10.1039/d6ra03574d

A sustainable pH responsive hydrogel-based phosphorus fertilizer using nano oxalic acid-modified rock phosphate and humic acid

Tharushi Perera ^a,^†, Sanduni Dabare ^a,^†, Imalka Munaweera ^a,^✉, Kasun M Thambugala ^b,^c,^d, Saranga Diyabalanage ^e,^f

PMCID: PMC13439622 PMID: 42559034

Abstract

Phosphorus (P) fixation and precipitation severely limit fertilizer efficiency in alkaline soils, contributing to resource depletion and environmental losses. In this study, a sustainable, pH-responsive phosphorus nano fertilizer was developed by mechanochemically converting Eppawala rock phosphate (ERP) into nano oxalic acid-modified rock phosphate (nano-Ox-ERP) and encapsulating it within a biodegradable sodium alginate-carboxymethyl cellulose (SA-CMC) hydrogel matrix supplemented with humic acid (HA). Nano Ox-ERP particles (60–180 nm) were characterized using PXRD, SEM-EDX, and FTIR. Hydrogel beads with varying nano Ox-ERP (20–80%) and humic acid (1–3%) compositions were evaluated for encapsulation efficiency, swelling behavior, and phosphorus release across pH 3.5–8.5. Formulation B8 (80% nano Ox-ERP + 2% HA) achieved 91.5% phosphate encapsulation and sustained release following Korsmeyer–Peppas kinetics. Pot experiments using Capsicum annuum L. (banana pepper) grown in alkaline soil demonstrated that the optimized formulation significantly enhanced plant growth, yield, and phosphorus uptake compared with conventional phosphorus sources, including triple super phosphate (TSP), untreated rock phosphate, and oxalic acid-treated rock phosphate, while also increasing soil bacterial abundance. The integrated nano-hydrogel–humic acid system effectively improves phosphorus bioavailability, minimizes nutrient losses, and promotes soil microbial activity, offering a scalable and environmentally benign strategy for sustainable phosphorus management in alkaline agroecosystems.

Combining mechanochemical nano-modification and pH-responsive hydrogel encapsulation to achieve controlled phosphate release and enhanced soil fertility. graphic file with name d6ra03574d-ga.webp

1. Introduction

Phosphorus (P) is a vital macronutrient crucial for plant growth, energy metabolism, and global crop productivity. However, the world's phosphorus reserves are limited, and current extraction rates are rapidly depleting high-quality rock phosphate deposits, which poses a significant threat to long-term food security.¹ Conventional soluble phosphate fertilizers, primarily produced from phosphate rock through energy-intensive methods, present considerable agronomic and environmental challenges. Such fertilizers frequently suffer from rapid nutrient leaching in soils, resulting in poor fertilizer use efficiency, commonly reported to be only 10–25%.^1,2 This leads to excessive nutrient accumulation within agricultural systems and significant phosphorus losses through surface runoff and groundwater contamination.² Additionally, the production of concentrated phosphatic fertilizers significantly contributes to global carbon emissions, exacerbating the environmental challenges posed by climate change.^3,4

Rock phosphate (RP), although abundant and relatively cheap, contains phosphorus in forms that have limited bioavailability because of its low solubility in neutral to alkaline soils.⁵ Chemically modifying RP is a promising strategy to increase phosphorus solubility and plant availability without using energy-intensive industrial processes. Compared with commercially produced mineral acid-modified rock phosphates, organic acid-modified rock phosphate offers distinct advantages, including milder processing conditions, reduced environmental and corrosion-related hazards, lower secondary salt formation, and improved compatibility with soil biogeochemical processes.⁶ Organic acids further promote localized calcium chelation and pH-responsive phosphorus release, enhancing nutrient use efficiency while minimizing ecological impacts. Also, recent innovations, mechanochemical activation with organic acid has shown significant potential for improving the dissolution rates and bioavailability of RP. This mechanochemical method helps break down phosphate mineral structures through mechanical stress combined with chemical reactions, allowing nano-scale particles to form and increasing surface reactivity without excessive energy use or harmful chemical byproducts.⁷

Hydrogel-based delivery systems provide a revolutionary solution by offering slow-release mechanisms that keep nutrients available for longer periods, thereby reducing environmental losses and increasing fertilizer efficiency. Sodium alginate (SA) and carboxymethyl cellulose (CMC) are natural, biodegradable polymers that form stable three-dimensional hydrogel networks capable of absorbing nutrients and releasing them slowly in response to soil moisture.⁸ These biopolymers are non-toxic, cost-effective, and easily obtained from renewable sources, making them ideal for sustainable fertilizer products. Encapsulating nutrient-rich solid particles within alginate-CMC hydrogels creates a matrix that shields nutrients from leaching and supplies a steady release that matches plant uptake patterns.

Despite the existence of studies on hydrogel-based slow-release fertilizers, organic acid-modified phosphates, and humic acid-enhanced nutrient systems, the present research introduces a functionally integrated and mechanistically distinct platform that goes beyond incremental combination.

Recent studies have independently explored hydrogel-based controlled-release fertilizers, organic acid-modified rock phosphate, nano-sized phosphate materials, and humic acid-based soil amendments. Hydrogel systems mainly regulate nutrient release through diffusion and polymer swelling, whereas organic acid modification has primarily been employed to enhance phosphate dissolution. Likewise, humic acid has been incorporated into fertilizers to improve soil properties and nutrient utilization. However, studies integrating nano-sized organic acid-modified rock phosphate and humic acid within a biodegradable pH-responsive hydrogel matrix remain limited.^9–11

A pH-responsive hydrogel system may provide an effective strategy for synchronizing phosphorus release with changes in the soil microenvironment while simultaneously improving phosphorus availability and reducing nutrient losses. Incorporating nano-sized oxalic acid-modified rock phosphate can further increase the reactive surface area available for dissolution, whereas humic acid may improve soil physicochemical and biological properties. The combination of these components within a biodegradable hydrogel therefore represents a promising approach for developing environmentally sustainable phosphorus fertilizers.^11,12

Therefore, the objective of this study was to develop a sustainable pH-responsive hydrogel-based phosphorus fertilizer by encapsulating nano oxalic acid-modified rock phosphate and humic acid within a sodium alginate/carboxymethyl cellulose hydrogel matrix. The physicochemical characteristics, encapsulation efficiency, swelling behaviour, and phosphorus release performance of the developed hydrogel were investigated to evaluate its potential as an environmentally friendly controlled-release phosphorus fertilizer. Fig. 1 illustrated the research design.

2. Methodology

2.1. Materials

All analytical-grade reagents used in this study were purchased from Sigma-Aldrich (USA), including oxalic acid, sulfuric acid, ascorbic acid, potassium dihydrogen phosphate, antimony potassium tartrate, ammonium molybdate, sodium alginate, carboxymethylcellulose, sodium hydroxide, hydrochloric acid, nitric acid, sodium bicarbonate, humic acid, and calcium chloride. Eppawala rock phosphate (ERP) was obtained locally.

2.2. Preparation of nano oxalic acid-modified rock phosphate (nano-Ox-ERP)

Nano-Ox-ERP was prepared by ball milling ERP (12.00 g), oxalic acid (12.00 g), and distilled water (10.0 mL) at 500 rpm for 1 h using 124 stainless steel balls (5 mm). The product was dried at 60 °C. Nano-ERP was prepared under identical conditions without oxalic acid.

2.3. Characterization of ERP, nano-ERP, and nano-Ox-ERP

Phase composition was analyzed by PXRD (Rigaku Ultima IV, Japan; 40 kV, 30 mA; 2θ = 10–60°).¹³ Surface morphology was examined using SEM (ZEISS, Germany), while elemental composition was determined by EDX. FTIR spectra were recorded over 4000–400 cm⁻¹ using the KBr pellet method.¹⁴

2.4. Determination of total phosphate

Total phosphorus content of the nano-Ox-ERP, along with raw ERP and triple superphosphate (TSP), were determined after aqua regia digestion using the molybdenum blue method at 880 nm.¹⁵

The release behavior of phosphate in solutions of three different pH values was examined. Approximately 50.0 mg of nano-Ox-ERP was added to 10.0 mL of aqueous media adjusted to pH 3.5, 7.0, and 8.5 using standardized HCl and NaOH solutions at room temperature. The acidic and alkaline media were prepared using standardized HCl and NaOH solutions, respectively. Aliquots of 1.0 mL were withdrawn at predetermined time intervals (24 h, 48 h, 72 h, and subsequently every 24 h up to 15 days) using separate sample sets prepared for each interval to avoid disturbance of the system. The concentration of released phosphorus was quantified at 880 nm using the molybdenum blue method.¹⁴

2.5. Preparation of SA-CMC-based hydrogel beads loaded with nano-Ox-ERP and humic acid

SA (1.00 g) and CMC (1.00 g) were dissolved in 100.0 mL deionized water. Nano-Ox-ERP and HA were added according to Table 1 and stirred for 1 h. The suspension was dropped into saturated CaCl₂ solution to form hydrogel beads (SA-CMC- Ox-ERP-HA), cured for 15 min, washed with distilled water, dried at 60 °C and stored.

Table 1. Nano-Ox-ERP and HA compositional variations of the synthesized SA-CMC hydrogel beads loaded with Ox-ERP and humic acid.

Type of hydrogel bead	Composition of SA-CMC-Ox-ERP and SA-CMC- Ox-ERP-HA
Type of hydrogel bead	Nano-Ox-ERP (w/w) %	HA (w/w) %
B1	20	1
B2	20	2
B3	20	3
B4	40	1
B5	40	2
B6	40	3
B7	80	1
B8	80	2
B9	80	3
C1	20	—
C2	40	—
C3	80	—

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SA-CMC- Ox-ERP-HA hydrogel beads were characterized using FTIR, SEM and EDX.

2.6. Characterization of the prepared SA-CMC-Ox-ERP-HA hydrogel beads

The synthesized hydrogel beads were subjected to comprehensive physicochemical characterization. FTIR spectroscopy was employed to identify the functional groups present and to confirm the successful incorporation of nano-Ox-ERP and humic acid into the SA-CMC polymer matrix. Surface morphology and structural features of the beads were examined using SEM. Additionally, EDX was performed in conjunction with SEM to determine the elemental composition and to visualize the spatial distribution of key elements within the hydrogel beads.

2.7. Encapsulation efficiency of phosphate in SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel bead fertilizer

The encapsulation efficiency of phosphate within the hydrogel bead fertilizer was evaluated following acid digestion of the beads. After complete digestion, the released phosphorus content was quantified using a UV-Vis spectrophotometer at 880 nm. Encapsulation efficiency was calculated from the ratio of encapsulated phosphorus to the initial phosphorus content.¹⁴

2.8. Encapsulation efficiency of humic acid in SA-CMC-Ox-ERP-HA hydrogel bead fertilizer

Following bead dissolution and extraction, the resulting solution was diluted with distilled water and thoroughly shaken to ensure complete homogenization. The mixture was then filtered using dry, medium-speed qualitative filter paper to remove any undissolved residues. The concentration of humic acid in the filtrate was quantified using a UV-Vis spectrophotometer at 268 nm. The encapsulation efficiency was subsequently calculated based on the proportion of humic acid retained within the hydrogel beads relative to the initial amount incorporated during synthesis.¹⁶

2.9. Swelling capacity of the SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

Swelling capacity was determined by immersing 20.0 mg dried beads in 15.0 mL deionized water. Samples were weighed at predetermined intervals (1, 2, 3, 5, 7, 14, and 21 days) and swelling percentage was calculated using eqn (1).¹⁷

where W_D is the weight of the dried hydrogel and W_S is the weight of the swollen hydrogel. All measurements were performed in triplicate to ensure reproducibility and reliability of the data.

2.10. Release behavior of phosphate from SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads in solutions of different pH

Release studies were conducted in aqueous media adjusted to pH 3.5, 7.0, and 8.5 using standardized HCl and NaOH solutions. These systems were not buffered; therefore, minor pH variations during the release process cannot be excluded. The pH was initially adjusted using a calibrated pH meter. The selected pH conditions represent acidic, neutral, and alkaline environments relevant to soil systems. At predetermined time intervals (24, 48, 72 hours, and subsequently every 24 hours up to 30 days), 1.0 mL aliquots were withdrawn from separate sample sets prepared for each time point to prevent disturbance of the system. The phosphate concentration in the collected aliquots was quantified using UV-Vis spectrophotometry at 880 nm following the molybdenum blue method. All experiments were performed in triplicate to ensure accuracy and reproducibility.¹⁴

2.11. Biodegradability test of SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

The biodegradability of the synthesized hydrogel beads was evaluated using the soil burial method in agricultural soils with acidic, neutral, and alkaline pH. Approximately 20 mg of hydrogel beads were buried at a depth of 2–3 cm in soil-filled containers and maintained at room temperature under controlled moisture conditions to prevent water loss through evaporation.

At predetermined time intervals (1, 5, 10, 15, and 20 days), the beads were carefully retrieved and qualitatively assessed for signs of degradation, including discoloration, shrinkage, and disintegration. The extent of biodegradation was quantitatively determined by calculating the weight loss (W_{t loss}) using eqn (2).¹⁷

where W_ti is the initial weight of the hydrogel beads before burial and W_tf is the weight of the beads after the specified degradation period. Before weighing, retrieved beads were gently rinsed with distilled water to remove adhering soil particles. All measurements were performed in triplicate to ensure accuracy and reproducibility.

2.12. Germination study and plant bioassay

Root elongation and germination assays were carried out to assess the effects of different hydrogel bead formulations and conventional fertilizers on seedling growth in soils of varying pH. Based on the phosphorus release data, the hydrogel bead formulations SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP (B5, B7, B8, B9, C2, and C3) were selected for further evaluation, and the fertilizers (ERP, Ox-ERP, and TSP) were applied separately to neutral, acidic, and alkaline soils. For each treatment and soil type, three replicate pots were prepared, with six seeds sown per pot at uniform spacing. The pots were maintained under controlled environmental conditions suitable for seed germination and seedling growth. Seedlings were allowed to grow for four weeks. After two weeks, one set of seedlings was carefully uprooted, and root lengths were measured from the base of the stem to the root tip. Measurements were compared to control pots without fertilizer to evaluate potential phytotoxic effects. At the end of the four weeks, a second set of seedlings was harvested, and root lengths were measured to assess overall growth promotion using eqn (3)–(5). All data were statistically analyzed to determine significant differences in root elongation among the various treatments and soil types, providing insights into the relative performance of the hydrogel bead formulations compared to conventional fertilizer.

GI = RSG × RRG × 100

2.13. Plant trail

Pot trials were conducted using banana pepper (Capsicum annuum) to evaluate the effects of hydrogel bead formulations on plant growth and productivity. Six different hydrogel bead types SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP (B5, B7, B8, B9, C2, and C3) were tested alongside conventional phosphorus fertilizers (TSP, ERP), Ox-ERP, and a no-fertilizer control. Seedlings, 21 days old and uniform in growth, were obtained from a nursery and transplanted into pots containing a standardized potting medium. For each treatment, six seedlings were used, with one seedling per pot.

Fertilizer application rates followed standard agricultural recommendations and were scaled for pot experiments. Nitrogen and potassium were supplied equally to all treatments to eliminate nutrient bias. Nitrogen was applied in the form of urea at a rate of 0.369 g per pot, applied at two-week intervals, while potassium was supplied as muriate of potash (MOP) at a rate of 0.240 g per pot.

Phosphorus was supplied either through hydrogel bead formulations or conventional fertilizers, depending on the treatment. The hydrogel bead application rates were 2.1 g per pot for treatments B8, B9, and C3, and 4.2 g per pot for treatments B5 and C2. Conventional phosphorus treatments included TSP at 0.7 g per pot, ERP at 0.7 g per pot, and nano Ox-ERP at 1.68 g per pot. Hydrogel beads were applied one week after transplanting, corresponding to week 1 of the experimental period. Growth and yield parameters, including plant height, number of branches, flowers, and pods, as well as yield at the first harvest, were recorded to assess the effects of hydrogel bead formulations relative to conventional phosphorus fertilizers and the unfertilized control.¹⁷

2.14. Determination of soil microbial population

Culturable populations of soil microbes were determined using the method described by ref. 18. Soil samples were collected from alkaline soil after 30 days of fertilizer application. The treatments included B8, C3, TSP, and an unfertilized control. Treatment B8 consisted of oxalic acid modified ERP combined with humic acid, C3 consisted of oxalic acid modified ERP alone, TSP represented triple superphosphate fertilizer, and the control consisted of soil without fertilizer application. The media used, and the target microbial populations were as follows: Nutrient Agar (NA) medium for bacteria and Potato Dextrose Agar (PDA) medium for fungi. One gram of soil samples was transferred to a 15.0 mL sterilized centrifuge tube, to which sterile distilled water was added to a total volume of 10.0 mL, and then shaken vigorously. Ten-fold serial dilutions were prepared, and 40 µL aliquots from each tube were spread on the media. Each soil sample was assessed in triplicate. The inoculated plates were incubated at 28 °C for varying durations (48 h for bacteria and 5 d for fungi). The bacterial colonies that appeared on the NA media were counted and expressed as log CFU g⁻¹ dry soil, determined by weight after drying at 105 °C to a constant weight. Fungal colonies grown on PDA were presumptively identified based on macroscopic colony characteristics, including color, texture, and growth pattern, using standard mycological descriptions based on literature.

3. Results and discussion

3.1. Characterization of ERP, Nano-ERP and OX-ERP

3.1.1. SEM morphology characteristic of ERP, nano-ERP and nano-Ox-ERP

The SEM images of pristine ERP (Supplementary Fig. S1) reveal a heterogeneous and compact morphology, predominantly composed of irregularly shaped, coarse aggregates with rough and uneven surfaces. Well-defined hexagonal prismatic crystals are not evident, suggesting that apatite crystal boundaries are closely intergrown or obscured by fine mineral particles. Such irregular morphology is characteristic of natural apatite-rich RP, in which fluorapatite, chlorapatite, and hydroxyapatite coexist with minor impurities and secondary minerals formed during geological weathering.

The SEM micrographs of nano-ERP and Ox-ERP (Fig. 2) reveal pronounced differences in their structural and morphological features. The nano-ERP sample exhibits agglomerated particles with a heterogeneous morphology and a broad particle size distribution, ranging from approximately 200 to 450 nm, with a mean particle size centered around 280–300 nm. In contrast, the Ox-ERP sample shows a marked transformation in particle size characteristics. Oxalic acid–assisted mechanochemical treatment leads to the formation of more uniformly distributed, well-defined crystalline particles with substantially reduced dimensions, predominantly in the range of 60–180 nm and a narrower size distribution centered at approximately 100–120 nm. These observations are consistent with previous reports indicating that high-energy ball milling promotes particle size reduction via fracture-dominated mechanisms while simultaneously generating chemically reactive surface sites.¹⁹ The micron-sized, plate-like crystals in Fig. 2(C) are likely calcium oxalate precipitates formed during oxalic acid modification, as supported by the XRD analysis. Consequently, these structures were excluded from the nanoparticle size measurements, which targeted only the dispersed nanoparticles shown in Fig. 2(A).^20,21

This particle size distribution shifts closer to the PXRD-derived crystallite size scale, as discussed below, indicating that surface modification and enhanced grinding efficiency induced by oxalic acid treatment not only reduce the overall particle size but also diminish the extent of crystallite agglomeration. The narrowing of the particle size distribution and the closer convergence between SEM-observed particle sizes and PXRD-estimated crystallite dimensions in below further support that oxalic acid facilitates more efficient crystallite fragmentation while suppressing extensive re-agglomeration during the milling process.²² The mechanochemical treatment with oxalic acid is expected to induce several synergistic effects, including enhanced particle size reduction due to improved grinding efficiency, surface modification through chelation of calcium ions, and suppression of particle re-agglomeration via electrostatic stabilization. EDX analysis (SI Fig. S2) confirmed that nano-ERP consists predominantly of calcium (Ca), phosphorus (P), oxygen (O), and carbon (C), along with minor impurities such as iron (Fe) and aluminum (Al), in agreement with previous reports. Notably, the relative contents of O and C increased in nano-Ox-ERP, indicating the successful incorporation of oxalate species during the mechanochemical activation process.

3.1.2. PXRD analysis of raw ERP, nano ERP and nano-Ox-ERP

The presence of multiple apatite phases in the raw ERP (Fig. 3c) is characteristic of sedimentary phosphate rocks and reflects variations in the physicochemical conditions prevailing during their deposition and diagenetic transformation.²³ The identification of fluorapatite is primarily based on its characteristic XRD reflections. Fluorapatite is generally more resistant to weathering and chemical dissolution than other apatite forms, owing to the strong Ca–F bonding within its crystal lattice, which enhances structural stability.²⁴ In addition, the presence of quartz as a minor phase, evidenced by its diffraction peaks, is typical of natural phosphate deposits. The detection of hematite (Fe₂O₃) further indicates iron oxide contamination, which is commonly associated with sedimentary phosphate rocks.²³

The crystallite size of both the as-received ERP and the ball-milled nano ERP samples was determined using XRD patterns. The most intense diffraction peak corresponding to the (221) plane of the dominant hydroxyapatite/chlorapatite phase was selected for analysis, as it provided a clear and well-resolved reflection suitable for accurate evaluation of peak broadening. Crystallite sizes were calculated using the Scherrer equation: 3.1.2. where D is the crystallite size, K is the shape factor (taken as 0.9), λ is the X-ray wavelength (Cu K_α), β is the full width at half maximum (FWHM) of the selected peak (in radians), and θ is the Bragg angle of the peak.

For the unprocessed ERP sample, the calculated crystallite size was 756.76 nm, indicating that the as-received mineral possessed relatively large crystalline domains. In contrast, after ball milling, the crystallite size of the nano ERP sample was significantly reduced to 58.66 nm. This dramatic decrease in crystallite size suggests the effectiveness of the mechanical milling process in refining the crystal structure and inducing nano crystallinity. The observed peak broadening and reduction in coherent domain size after ball milling arise from increased lattice defects, strain, and fragmenting of larger crystallites into smaller domains. Such behaviour is consistent with established observations where extended milling or mechanical deformation causes peak broadening and corresponding decreases in crystallite size due to grain refinement and accumulation of internal strain.²⁵

Upon oxalic acid modification, particularly under mechanochemical treatment, apparent structural modifications are evident. As illustrated in Fig. 3, the emergence of new peaks attributed the formation of new crystalline phases corresponding predominantly to calcium oxalate monohydrate (whewellite, CaC₂O₄·H₂O; JCPDS PDF No 20-0231). This phase formation suggests that Ca²⁺ ions released from apatite during acid-mediated dissolution were efficiently scavenged by oxalate anions, leading to the precipitation of thermodynamically stable whewellite.²⁰

The formation of these calcium oxalate phases is initiated by proton-promoted dissolution of apatite minerals (fluorapatite, chlorapatite, and hydroxyapatite), which releases Ca²⁺ and phosphate species into solution. Using Chlorapatite as a representative phase, the dissolution reaction under acidic conditions can be expressed as:Ca₅(PO₄)₃Cl_(s) + 7H_(aq)⁺ → 5Ca_(aq)²⁺ + 3H₂PO_4(aq)⁻ + HCl_(aq)

Simultaneously, oxalic acid dissociates to generate oxalate anions capable of strongly complexing Ca²⁺:H₂C₂O_4(aq) ⇌ H_(aq)⁺ + HC₂O_4(aq)⁻HC₂O_4(aq)⁻ ⇌ H_(aq)⁺ + C₂O_4(aq)²⁻

The liberated Ca²⁺ ions rapidly react with oxalate anions, driving supersaturation and precipitation of calcium oxalate phases. Under oxalate-rich conditions, the dominant reaction pathway leads to whewellite formation:Ca_(aq)²⁺ + C₂O_4(aq)²⁻ + H₂O_(l) → CaC₂O₄·H₂O_(s)

Under locally elevated Ca²⁺ activity, transient formation of weddellite may occur: Ca_(aq)²⁺ + C₂O_4(aq)²⁻ + 2H₂O_(l) → CaC₂O₄·2H₂O_(s)

followed by its transformation to whewellite:CaC₂O₄·2H₂O_(s) → CaC₂O₄·H₂O_(s) + H₂O_(l)In parallel with calcium oxalate precipitation, partial reprecipitation of dissolved phosphate species occurs in the form of monetite (CaHPO₄; JCPDS PDF No. 01-071-1759), an anhydrous dicalcium phosphate that forms under acidic conditions:Ca_(aq)²⁺ + HPO_4(aq)²⁻ → CaHPO_{_4(s)}

Monetite is confirmed by its distinctive PXRD reflections (Fig. 3) at 2θ ≈ 26.84°, 29.12°, and 40.18°, corresponding to the (020), (2−1−1), and (−2−2−1) crystallographic planes. Despite oxalic acid treatment, diffraction peaks corresponding to fluorapatite, chlorapatite, and hydroxyapatite at 2θ 30°–35° remain discernible, although with reduced relative intensities, indicating incomplete reaction and the persistence of residual apatite phases.

The coexistence of residual apatite, monetite, and calcium oxalate phases therefore suggests a competitive dissolution-precipitation pathway governed by oxalate activity, solution acidity, and local Ca²⁺ availability. In contrast, quartz (SiO₂; JCPDS PDF No. 46-1045) and hematite (Fe₂O₃; JCPDS PDF No. 05-0586) remain essentially unchanged after modification, as expected due to their chemical stability and resistance to reaction with organic acids.

3.1.3. FTIR analysis of ERP, nano-ERP and nano-Ox-ERP

The FTIR spectra of the raw ERP (Fig. 4c) revealed characteristic absorption bands of phosphate PO₄³⁻ ions at 910 cm⁻¹, 1640 cm⁻¹, and the two split absorption bands of 566–610 cm⁻¹ and 1030 cm⁻¹. The broad absorption band at 3440 cm⁻¹ corresponded to O–H stretching vibrations, confirming the presence of hydroxyl groups, suggesting that there was hydroxyapatite (Ca₅(PO₄)₃OH) in the ERP.²⁶ Additionally, absorption bands associated with carbonate ions (CO₃²⁻) were observed in the1400-1450 cm⁻¹.²⁷ The band at 799 cm⁻¹ refers to the Si–O stretching band, which represents the quartz in the sample.

The absorbance of the phosphate vibration bands at 1050 cm⁻¹ (ν₃ asymmetric stretching of PO₄³⁻) and 1630 cm⁻¹ (H–O–H bending of adsorbed water) markedly changed following grinding (Fig. 4b). This behavior is indicative of mechanochemical activation, where particle size reduction induces increased surface disorder, disruption of long-range crystalline order, and partial amorphization of the phosphate lattice. Such structural modifications reduce dipole–dipole coherence and enhance surface reactivity, thereby attenuating the vibrational band intensities.²⁸ There is a significant reduction in the absorption band of carbonate radical in apatite mineral at 1460 cm⁻¹ after grinding, this could be explained by the partial breakdown of the carbonate as a result of the mill's mechanical action and the resulting mechanical and thermal effects. As a result, tricalcium phosphate Ca₃(PO₄)₂ is formed, and carbon dioxide is evolved and adsorbed on the particle surfaces.²⁸ The nano-Ox-ERP spectrum in Fig. 4a demonstrates notable peak broadening and shifts in the phosphate region compared to raw ERP, indicating structural modification of the phosphate matrix. This broadening suggests decreased crystallinity and increased structural disorder, consistent with mechanochemical activation effects reported in the literature.²⁶ A progressive broadening is observed within the asymmetric stretching region, with characteristic PO₄³⁻ bands appearing at 1110 cm⁻¹ for nano-Ox-ERP. This spectral broadening and slight band shifting reflect increased structural disorder in the phosphate lattice, possibly caused by the incorporation of oxalate anionic species into the apatite framework. Such substitutions perturb the PO₄³⁻ vibrational symmetry and induce local distortions within the apatite unit cell.²⁹

With noticeable broadening, the intense absorption band observed at 987 cm⁻¹ in nano-Ox-ERP is attributed to stretching vibrations within the phosphate matrix, supporting the formation of a monetite phase. Additional bands in the 513 cm⁻¹ region correspond to bending vibrations of O–P–O and P–O–Ca linkages, characteristic of phosphate frameworks.³⁰ The formation of calcium oxalate monohydrate (whewellite) is identified by the appearance of peaks around 780 cm⁻¹ and increased intensity at 1610 cm⁻¹.³¹ The broad, intense COO⁻ bands centered at 1610 cm⁻¹ in nano-Ox-ERP indicate coordination of Ca²⁺ ions through electrostatic attraction and/or ion–exchange interactions, forming surface complexes of the type (R–COO⁻···Ca²⁺) (Fig. 4a)³²

3.2. pH-dependent phosphorus release behavior of nano-Ox-ERP, nano-ERP, and TSP in aqueous systems

The most striking finding across all three ERP materials is the dramatic enhancement of cumulative P release under acidic conditions compared to neutral and alkaline treatments. In acidic pH environments, cumulative P release reaches approximately 190, 195, and 5 mg g⁻¹, for nano-Ox-ERP, TSP, and nano-ERP, respectively, at day 16, substantially exceeding release under neutral and alkaline conditions (Fig. 5). This pH dependence reflects fundamental aqueous chemistry principles governing phosphate solubility and apatite mineral dissolution. At lower pH values, phosphate ions undergo protonation, shifting the speciation equilibrium from the poorly soluble PO₄³⁻ form toward the more soluble HPO₄²⁻ and H₂PO₄⁻ species.³³ This shift in phosphate speciation dramatically increases the thermodynamic driving force for apatite dissolution, as the Gibbs free energy of the dissolution reaction becomes increasingly favorable at lower pH. Additionally, acidic conditions suppress the sorption and precipitation of phosphate onto secondary minerals, particularly aluminum and iron oxides, which exhibit maximum surface charge and P-binding capacity near neutral pH.³⁴

Fig. 5(A) demonstrates that nano-Ox-ERP exhibits the most pronounced and sustained cumulative P release over the 15 days experimental period, reaching approximately 190 mg g⁻¹ under acidic conditions. This superior performance compared to Nano-ERP reflects the multifaceted action of oxalic acid as both a pH modifier and a powerful chelating agent capable of dissolving apatite minerals through complexation mechanisms.³⁵ Oxalic acid functions through two primary mechanisms: first, it directly lowers solution pH through its dissociation, creating the thermodynamically favorable conditions described above; second, and perhaps more importantly, it forms stable complexes with calcium ions released from apatite dissolution, effectively removing Ca²⁺ from solution and pulling the dissolution equilibrium forward according to Le Chatelier's principle.³¹ This calcium-complexation mechanism is particularly significant in water systems where the primary constraint on P release is the backward reaction rate and the reprecipitation of dissolved P as secondary calcium-phosphate minerals. The sustained release trajectory visible in (Fig. 5(A)), with continued increase from approximately 140 mg g⁻¹ at day 4 to 190 mg g⁻¹ at day 16 under acidic conditions, indicates that the oxalic acid-modified material maintains favorable dissolution kinetics throughout the experimental period, suggesting the chelating effect continues to prevent calcium-phosphate precipitation from limiting dissolution.³⁶

Fig. 5(B) reveals that TSP achieves cumulative P release reaching approximately 195 mg g⁻¹ under acidic conditions at day 16, slightly exceeding the Ox-ERP. However, this direct comparison requires careful interpretation, as TSP represents a chemically processed, highly soluble P source fundamentally different in composition and structure from the ERP materials. TSP, produced through the reaction of phosphate rock with phosphoric acid, consists primarily of monocalcium phosphate monohydrate (Ca(H₂PO₄)₂·H₂O) and is inherently much more soluble in water than apatite minerals. The high absolute release values in Fig. 5(B) reflect this high inherent solubility rather than enhanced dissolution mechanisms; indeed, the curves show a characteristic plateau at approximately 180 mg g⁻¹ by day 8–10 under acidic conditions, suggesting depletion of the readily soluble P fraction. Under neutral and alkaline conditions, TSP release is substantially suppressed, indicating that even the enhanced solubility of TSP cannot overcome the thermodynamic limitations imposed by elevated pH and increased tendency for secondary mineral precipitation. This finding emphasizes that pH remains the dominant controlling variable even for highly soluble P sources, reinforcing the conclusion that effective P availability in alkaline systems requires either pH modification or use of acid-modified materials that prevent re-precipitation.⁵

Fig. 5(C) presents a striking contrast to nano-Ox-ERP and TSP, with cumulative P release from nano-ERP reaching only approximately 5 mg g⁻¹ under acidic conditions and less than 1.5 mg g⁻¹ under alkaline conditions at day 15.

A critical observation across all three materials is the severe suppression of cumulative P release under alkaline pH conditions, with absolute release values reaching only approximately 120, 105, and 1.5 mg g⁻¹, respectively, at day 16. This substantial reduction in P availability under alkaline conditions, corresponding a 30–75% decrease relative to acidic treatments, can be attributed to the formation of calcium-rich secondary phosphate phases at the ERP-solution interface. In alkaline environments characterized by elevated pH and increased dissolved calcium activity, phosphate ions released during initial apatite dissolution rapidly encounter conditions exceeding the solubility product (K_sp) of secondary calcium-phosphate phases, including poorly soluble apatite, octacalcium phosphate (OCP), and related calcium-phosphate minerals.³⁴ These secondary minerals precipitate directly onto the ERP particle surface, forming a diffusion barrier that prevents fresh dissolution medium from contacting the reactive apatite core and prevents dissolved phosphate ions from diffusing outward into solution.³⁷

The absence of a buffering system may influence phosphate release behavior due to potential pH drift during the experiment. However, the selected pH conditions (3.5, 7.0, and 8.5) were chosen to represent typical acidic, neutral, and alkaline environments, allowing for comparative evaluation of pH-responsive release behavior. Future studies will incorporate well-defined buffer systems to maintain constant pH and controlled ionic strength for more precise mechanistic evaluation.

The enhanced phosphorus release observed for nano-Ox-ERP can be interpreted based on established dissolution and complexation mechanisms. Under acidic conditions, increased proton activity promotes apatite dissolution and shifts phosphate speciation toward more soluble forms. In addition, oxalic acid may contribute to this process by forming stable complexes with Ca²⁺ ions released during dissolution, thereby reducing re-precipitation and favoring continued phosphate release. The sustained release behavior observed over time is therefore likely influenced by a combination of pH-dependent solubility effects and calcium complexation. However, it should be noted that these mechanisms are inferred based on observed trends and literature evidence, and were not independently quantified in the present study.

3.3. Characterization of SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

3.3.1. FTIR analysis

The FTIR spectra of the hydrogel beads (B1–B9 and C1–C3) (Fig. 6(A)) revealed characteristic absorption bands confirming the successful incorporation of Ox-ERP and HA within the calcium-crosslinked CMC and SA polymer matrix. A broad absorption band at 3440–3450 cm⁻¹ observed in all samples is attributed to O–H stretching vibrations from hydroxyl groups in the polysaccharide backbone, adsorbed water molecules, and phenolic groups of humic acid.³⁸ As the humic acid content increased in samples B1–B3, B4–B6, and B7–B9, the relative intensity and breadth of the O–H stretching band also increased. This enhancement is attributed to the higher concentration of phenolic and carboxylic hydroxyl groups present in the humic substances.³⁹ This interpretation is further supported by the samples C1–C3, which lacked humic acid and consequently exhibited relatively narrower O–H stretching bands in this 3440–3450 cm⁻¹ region. Additional overtone and carbonate-related features observed in some samples, particularly in the 2320–2420 cm⁻¹ region, are likely attributable to the formation of surface carbonates or interactions with atmospheric CO₂. Higher oxalic acid-modified ERP content increases the number of exposed surface sites, particularly calcium ions, which can interact with atmospheric CO₂ to form detectable carbonate groups. In the FTIR spectrum, these are observed as weak overtone or combination bands around 2110 cm⁻¹. Evidence from studies on calcium-rich materials shows that reactive calcium surface sites readily interact with CO₂ to form surface carbonate species detectable by FTIR. For example, CO₂ adsorption on calcium hydroxyapatite results in the formation of carbonate (CO₃²⁻) groups via reaction with surface hydroxyls, as indicated by characteristic IR bands in the carbonate region.⁴⁰ Similarly, studies on CaO surfaces demonstrate that CO₂ readily adsorbs at multiple basic Ca²⁺ sites, producing distinct IR signals attributable to carbonate species and confirming that increased surface basicity enhances carbonate formation.⁴¹ Additionally, mechanochemical activation of mineral surfaces has been reported to incorporate carbonate species from ambient CO₂ into activated surfaces, consistent with enhanced surface reactivity and defect density after milling.⁴² When HA is present, it adsorbs onto the ERP surface, effectively coating and masking the reactive sites and thereby inhibiting direct carbonate formation. Consequently, beads with low Ox-ERP content, where most active sites are covered by humic acid, do not exhibit this carbonate-related feature.

In contrast, C1 and C3 exhibit the weak 2110 cm⁻¹ feature, as they lack HA, leaving more calcium-rich surface sites available for interaction with atmospheric CO₂ and subsequent carbonate formation. This explains the absence of this peak in beads with both low Ox-ERP content and humic acid, such as B1, B2, and B3, where most reactive sites are effectively masked by the polymer-bound humic acid.

The strong absorption bands at 1620–1640 cm⁻¹ are attributed to asymmetric stretching vibrations of carboxylate groups (COO⁻) from both CMC and SA.³⁸ A notable shift in the carboxylate peak position was observed across different formulations. Samples with higher HA content (B3, B6, B9) showed bands at slightly lower wavenumbers (1620 cm⁻¹) compared to humic-free samples (C1–C3, appearing at 1640 cm⁻¹). This red shift suggests increased ionic interactions and hydrogen bonding between carboxylate groups and calcium ions or HA functional groups.⁴³ The broadening of phosphate bands in the encapsulated samples (B1–B9, C1–C3) compared to raw rock phosphate suggests that phosphate groups are experiencing diverse chemical environments due to interactions with carboxylate groups of the polysaccharide chains and calcium ions.⁴⁴ The band at 1110 cm⁻¹, which intensified with increasing Ox-RP content, is characteristic of P–O in hydrogen-bonded or coordinatively interacting phosphate groups, indicating enhanced interactions with the hydroxyl groups of CMC and alginate.⁴⁵ These observations suggest that oxalic acid treatment not only solubilizes phosphate but also activates it for improved interactions with the polymer matrix through surface modification.⁴⁵ Humic acid's impact on phosphate speciation can be directly demonstrated by comparing samples with the same Ox-ERP content but different humic acid levels (e.g., B7, B8, B9, or C3). Sharper, more distinct phosphate peaks were seen in the humic-free sample C3, suggesting comparatively homogeneous phosphate environments. B9, which had a high humic acid content but the same Ox-ERP content, on the other hand, displayed broader, more complex phosphate bands, indicating that humic acid produces heterogeneous microenvironments around phosphate groups. Because different phosphate species would have different solubility and release kinetics in soil conditions, this heterogeneity might be advantageous for controlled nutrient release.^44,46

3.3.2. SEM analysis

The SEM analysis was conducted on selected beads, and the resulting micrographs revealed distinct morphological differences among the composite beads formulated with varying humic acid concentrations. All beads exhibited roughly spherical geometries at low magnification (Fig. 6(A)), consistent with the ionic gelation mechanism typical of alginate-based systems.⁴⁷ However, surface topography and internal pore architecture varied significantly with humic acid loading, indicating complex interactions between the polysaccharide matrix and humic substances during bead formation.

The control sample (C3) displayed a remarkably smooth, dense surface with minimal visible porosity (Fig. 6(B)d). This non-porous morphology is characteristic of conventional alginate beads formed through simple calcium cross-linking, where polymer chains pack tightly to form a compact gel network.⁴⁸ The absence of significant pore structures in C3 suggest that humic acid plays a critical role as a pore-forming agent in this composite system, rather than the oxalic acid-modified ERP serving as the primary porogen.

The incorporation of humic acid dramatically altered bead microstructure, with pore size measurements revealing a non-linear relationship between humic acid content and porosity. B7 (1% HA) exhibited uniform micropores averaging 266 nm in diameter, distributed throughout a sponge-like matrix (Fig. 6(B)a). This relatively small, homogeneous pore structure suggests controlled phase separation during gelation, where humic acid molecules create localized regions of reduced polymer density.⁴⁹

At 2% humic acid loading (B8), the bead structure transformed dramatically, showing the largest average pore size of 459 nm (Fig. 6(B)b). The high-magnification images revealed well-developed, interconnected porous networks with distinct sheet-like or crystalline formations embedded within the matrix. This morphology indicates optimal interaction between humic acid, sodium alginate, and CMC at this concentration. Similar layered structures have been reported in alginate-based composites when anionic additives induce localized electrostatic repulsion, creating expanded gel networks.⁵⁰ The substantial increase in pore size from B7 to B8 (+72.9%) suggests that 2% humic acid represents a critical threshold where the polyelectrolyte nature of humic substances maximizes spatial organization within the polymer matrix.

Interestingly, further increasing humic acid to 3% (B9) resulted in decreased pore size to 397 nm, accompanied by more irregular, fragmented surface morphology (Fig. 6(B)c). This non-monotonic behavior suggests structural densification or pore collapse at excessive humic acid concentrations. Several mechanisms may contribute to this phenomenon. First, excess humic acid may saturate available binding sites on the alginate-CMC backbone, leading to aggregation rather than network expansion.⁵¹ Second, the high concentration of anionic functional groups from both polysaccharides and humic acid may induce charge screening effects, reducing electrostatic repulsion and allowing polymer chains to pack more closely.⁵² Third, competitive interactions for calcium cross-linking sites between alginate carboxyl groups and humic acid functional groups may compromise gel integrity, resulting in partial structural collapse during drying.

The development of porosity in alginate-based hydrogel beads has been attributed to several mechanisms, including gas evolution, phase separation, etc. In the present study, the systematic variation in pore architecture with humic acid content strongly suggests that pore formation primarily occurs through phase separation mechanisms rather than gas evolution. During the gelation process, these functional groups of humic acid interact with the alginate-CMC network through multiple mechanisms; The anionic nature of humic acid creates localized charge repulsion with negatively charged alginate and CMC chains, promoting network expansion. This effect would be concentration-dependent, explaining the maximum pore size at 2% HA where electrostatic spacing is optimized without excessive charge screening. As well, extensive hydroxyl and carboxyl groups on humic acid can form hydrogen bonds with polysaccharide chains, creating physical cross-links that influence network architecture.⁵³ At optimal concentrations, these interactions may stabilize expanded pore structures. The amphiphilic nature of humic acid, containing both hydrophobic aromatic domains and hydrophilic functional groups, may induce micro-phase separation within the predominantly hydrophilic polysaccharide matrix.⁵⁴ This could explain the crystalline or sheet-like structures observed in B8, representing humic acid-rich domains. The bulky, branched structure of humic macromolecules physically occupies space within the gel network, creating voids that become permanent pores upon drying.

The pore sizes observed in this study (266–459 nm) fall within the mesoporous to macro porous range and are comparable to or larger than those reported for similar alginate-based composite systems. Rajabi et al. (2018)⁵⁰ reported pore sizes of 50–200 nm for alginate-montmorillonite composite beads, while Shen et al. (2016)⁵⁵ observed pore diameters of 100–400 nm in alginate-graphene oxide composites. The relatively large pore sizes achieved with 2% HA suggest particularly effective pore formation by this natural polyelectrolyte.

The non-linear relationship between additive concentration and porosity has been documented in other alginate composite systems.³⁸ Maximum porosity at intermediate concentrations of carbon nanotubes in alginate beads, attributing this to a balance between network expansion and structural integrity. Similarly,⁵⁶ reported optimal pore development at moderate loadings of biochar in alginate composites, with excess filler causing pore blocking and densification, consistent with our observations at 3% HA.

The crystalline or layered structures observed in B8 are reminiscent of morphologies reported for alginate-clay nanocomposites, where intercalated mineral platelets create expanded, layered architectures.⁵⁷ The amphiphilic nature of humic acid, with both hydrophobic aromatic cores and hydrophilic functional groups, may promote similar organized structures within the polysaccharide matrix.

EDX analysis (Fig. S3) confirmed the presence of significant amounts of calcium (Ca), phosphorus (P), oxygen (O), and carbon (C), with minor impurities of iron (Fe) and aluminum (Al), in the hydrogel bead which confirms further encapsulation of Ox-ERP.

3.4. Phosphate and humic acid encapsulation in SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel bead fertilizer

The encapsulation efficiency of phosphate (Fig. 7(A)) and humic acid (Fig. 7(B)) in CMC-SA hydrogel beads highlights the influence of matrix composition on retention capacity. Incorporation of humic acid significantly enhanced phosphate encapsulation compared to control beads, while the encapsulation of humic acid itself exhibited concentration-dependent behavior. The phosphate encapsulation efficiency in control beads (C1–C3) ranged from 58.2% to 82.4% (Fig. 7(A)), demonstrating that the CMC-SA matrix alone provides moderate retention capacity through physical entrapment and ionic interactions with the calcium-crosslinked alginate network. However, the beads containing both nano-Ox-ERP and humic acid (B1– B9) generally showed superior performance, with formulations, with B8 (80% Ox-ERP + 2% HA) achieving the highest efficiency at approximately 91.5% (Fig. 7(A)).

This enhancement can be attributed to synergistic interactions between humic acid and phosphate groups within the hydrogel matrix. The carboxylic and phenolic functional groups of humic acid can form hydrogen bonds and electrostatic interactions with phosphate, creating additional retention sites beyond physical entrapment.⁵⁸ Furthermore, humic acid may contribute to network densification, reducing pore size and improving overall retention capacity of the hydrogel beads.⁵⁹

The relationship between nan-Ox-ERP loading and phosphate encapsulation efficiency provides important insights into matrix saturation and hydrogel performance. As nano-Ox-ERP content increased from 20% to 80%, the phosphate encapsulation generally improved, particularly in formulations with an optimal concentration of humic acid. This observation indicates that higher phosphate loading does not necessarily compromise encapsulation efficiency when the hydrogel matrix composition is appropriately balanced. The calcium-crosslinked alginate network, reinforced through interactions with CMC and HA, preserves structural integrity even at high phosphate loadings. The superior performance of B8, containing 80% Ox-ERP and 2% HA, suggests that this composition achieves an optimal balance between nutrient loading and matrix stability, thereby maximizing the functional capacity of the hydrogel bead delivery system.⁶⁰

The observed morphological differences in SEM images have direct implications for encapsulation performance. Sample B8, with its maximum pore size and open porous architecture, demonstrated superior encapsulation efficiency. This can be attributed to several factors, including the large pore volume (459 nm) and interconnected porous network, which provide extensive internal surface area and accessible void space for encapsulant incorporation.⁶¹ During the encapsulation process, the open structure facilitates deep penetration of active materials throughout the bead interior, rather than limiting loading to surface layers as observed in denser formulations. As well, the well-developed pore channels enable efficient diffusion of encapsulant solutions during the loading phase. Studies have shown that pore sizes in the range of 100–500 nm are optimal for encapsulation of biomolecules, proteins, and small organic compounds, balancing accessibility with structural integrity.^62,63 The higher humic acid content in B8 provides more carboxyl, phenolic, and hydroxyl groups for chemical interaction with encapsulants. These functional groups can participate in electrostatic attraction, hydrogen bonding, and van der Waals interactions, enhancing both physical entrapment and chemical adsorption of target compounds.⁶⁴

Formulations with 1% HA (B1, B4, B7) consistently exhibited higher HA retention (75–88%) compared to those with 2% or 3% HA (Fig. 7(B)). This inverse relationship between HA concentration and encapsulation efficiency can be explained by several interrelated phenomena. At higher concentrations, humic acid molecules may undergo self-aggregation or precipitation due to intermolecular interactions between their hydrophobic aromatic domains, reducing their effective distribution within the hydrogel matrix.⁶⁵ The hydrogel network likely has a finite number of binding sites capable of stably retaining humic acid through electrostatic and hydrogen bonding interactions. When HA concentration exceeds this binding capacity, excess molecules remain loosely associated and are lost during the crosslinking and washing steps.⁶⁶

Additionally, humic acid competes with alginate for calcium ions during the crosslinking process. At concentrations of 2–3%, this competition may disrupt optimal alginate gelation, creating a looser network structure with larger pores that allow both humic acid and phosphate to leach more easily. This effect is particularly evident in the poor performance of B5, which showed the lowest HA encapsulation at approximately 28%. This formulation appears to represent a critical instability point where the intermediate levels of both components create suboptimal crosslinking conditions. The calcium ions may be insufficiently distributed between alginate crosslinking and HA complexation, resulting in a weak matrix structure.⁶⁷ This hypothesis is supported by the recovery of performance in B6, where the higher nano-Ox-ERP content may provide additional structural support through particle–matrix interactions.

The notably poor phosphate encapsulation in B3 at approximately 47% and B9 at approximately 39% further suggest the detrimental effect of excess humic acid. In B3, the combination of low phosphate loading and high HA content creates an imbalanced system where HA aggregation disrupts the already limited matrix structure. The markedly poor performance of B9 is particularly revealing, as this formulation has the highest nano-Ox-ERP loading yet exhibits the lowest phosphate retention. These results suggest that 3% HA causes significant matrix disruption irrespective of phosphate loading, potentially through excessive calcium chelation that impedes adequate alginate crosslinking. Additionally, HA aggregation may create hydrophobic domains that repel the negatively charged phosphate ions, thereby limiting their stable incorporation into the hydrogel network.⁶⁸

The observed effects of HA on phosphate encapsulation in CMC-SA hydrogel beads are consistent with previous studies reporting the influence of HA on hydrogel structure, nutrient retention, and release behavior. HA can interact with the polymer network through hydrogen bonding, electrostatic interactions, and coordination with crosslinking ions such as Ca²⁺, thereby enhancing encapsulation and stabilizing the hydrogel matrix at moderate concentrations^69,70 However, excessive HA has been shown to disrupt network formation, reduce porosity, and cause aggregation, which can limit the stable incorporation of nutrients.^71,72 These findings support our results: formulations with moderate HA content (1–2%) showed improved phosphate retention and encapsulation efficiency, while higher HA concentrations (3%) in B3 and B9 led to aggregation and matrix disruption, reducing phosphate loading.

3.5. pH-dependent swelling behavior of the SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

The swelling capacity of the SA-CMC-Ox-ERP-HA hydrogel beads was compared with that of the SA-CMC-Ox-ERP hydrogel beads synthesized without adding humic acid (Fig. 8). Swelling capacity mainly depends on the hydrogel's structure, the polymerization process, the concentration of the cross-linking agent, and the particle size of the hydrogel. The concentration of the cross-linking agent has the greatest effect.^17,73

SA and CMC are polymers that have many carboxylic groups. This feature makes them hydrophilic, allowing them to absorb large amounts of water. When crosslinking occurs, Ca²⁺ ions attach to the carboxylate groups in SA and CMC through strong ionic interactions, creating a 3D network.¹⁷ Previous studies showed that humic acid and oxalic acid serve as crosslinking agent, the inclusion of humic acid and unreacted oxalic acid in this system also indicated its function as a crosslinking agent.^74,75 B1 showed the maximum swelling capacity in acidic (Fig. 8(A)), and alkaline (Fig. 8(E)) media, reaching its maximum on day 21 in all pH mediums with the reported swelling capacity of 292%, and 385% respectively. The hydrogel displayed the highest swelling capacity at alkaline pH, which aligns with the literature data.⁷³ At this pH, the carboxylic groups in polymer structures can be ionized and they primarily take the form of COO⁻. There could be an electrostatic repulsion between ionic groups, increasing the swelling and enlarging the network structure. Additionally, water molecules and COO⁻ groups may form hydrogen bonds, causing the hydrogel to swell when it absorbs water.⁷⁶ Hydrogel beads swelled less in acidic medium than neutral and alkaline due to the formation of carboxylic acid groups in the CMC, resulting in a tighter structure as the number of H-bonds rises.⁷⁶

An increase in the amount of encapsulated nano-Ox-ERP led to a decrease in swelling capacity because of a reduction in pore spaces,² as shown in this experiment. As an example, while B1 reached a maximum swelling of 278.36%, B5 showed 69.19%. B7 to B9 and C1 and C3 exhibited different behavior than B1 to B6, showing negative swelling. This reduction in swelling capacity can be described in two ways. It comes from the hydrogels' vulnerability to water pressure. This pressure causes partial rupture of polymer chains and leads to the collapse of voids within the hydrogel structure.⁷⁷ Although B7, B8, and B9 showed initial deswelling, they exhibited surprising behavior as swelling started after a few days. This delayed swelling phenomenon can be supported by previous research findings, which stated that ionically crosslinked alginate hydrogels weaken and disintegrate when exposed to monovalent ions, chelating agents/phosphates, and ion exchange processes.⁷⁸ In this study, slowly released phosphate ions can be bound with Ca²⁺ ions, which weakens the crosslinking density and promotes swelling.

C2 showed positive swelling capacity, while C1 and C2 showed negative swelling. This difference comes from the crosslinking density. C1 had a very low crosslinking density, which led to weak network formation and poor swelling. In contrast, C3 had a very high crosslinking density, which leads to low porosity and reduced swelling.

Sodium alginate is a linear anionic polysaccharide composed of β-d-mannuronic acid (M) and α-l-guluronic acid (G) residues bearing carboxyl groups with pK_a values in the range of ∼3.4–4.4. These carboxyl groups undergo pH-dependent protonation and deprotonation, which strongly influence the physicochemical behavior of alginate-based hydrogels.^79,80 Similarly, CMC contains carboxymethyl substituents (–CH₂–COOH) with comparable pKa values (∼4.0–4.5), enabling both polymers to exhibit pronounced pH-responsive swelling behavior.^81,82 The ionization equilibrium of the carboxyl groups (–COOH ⇌ –COO⁻ + H⁺) governs electrostatic interactions within the polymer network and, consequently, water uptake and matrix stability.⁴⁷

Under acidic conditions (pH < 4), the carboxyl groups of alginate and CMC remain predominantly protonated. This suppresses electrostatic repulsion between adjacent polymer chains, resulting in a compact network structure with reduced osmotic pressure for water absorption. In calcium-crosslinked systems, protonation can also partially disrupt Ca²⁺–carboxylate interactions due to competitive proton binding, leading to limited swelling or even matrix shrinkage. As a result, the hydrogel exhibits low permeability and restricted diffusion of encapsulated molecules in acidic environments.^80,83

At neutral pH values (approximately 6–8), partial to near-complete deprotonation of carboxyl groups occurs, generating negatively charged –COO⁻ moieties along the polymer backbone. This induces moderate electrostatic repulsion between polymer chains, promoting controlled swelling while preserving the integrity of calcium-mediated cross-links within the alginate “egg-box” structure. Consequently, the hydrogel network achieves an optimal balance between structural stability and permeability, making this pH range favourable for sustained and controlled release applications.^47,83

In alkaline conditions (pH > 8), extensive deprotonation of all acidic groups leads to strong electrostatic repulsion and a significant increase in osmotic swelling pressure, causing pronounced network expansion and high-water uptake. Simultaneously, calcium cross-links may progressively weaken due to reduced Ca²⁺ availability and possible formation of calcium hydroxide. At highly alkaline pH levels (pH > 10), alginate chains are susceptible to β-elimination reactions, resulting in polymer backbone degradation. Collectively, these effects produce extensive swelling, increased porosity, and eventual loss of mechanical integrity of the hydrogel matrix.^57,80

3.6. pH-dependent phosphorus release in aqueous solutions and kinetic analysis of SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

The cumulative phosphorus release profiles from SA-CMC hydrogel beads containing Ox-ERP and HA exhibited complex, composition-dependent behavior that varied significantly with both pH and bead formulation, as illustrated in Fig. 9. The release kinetics can be understood through the interplay of several factors: ERP loading, humic acid content, pore architecture as revealed by SEM analysis, and pH-dependent solubilization mechanisms.

Fig. 9 illustrated in phosphorus release in B1–B9 beads. Among the hydrogel beads labeled B1 to B9, beads B7, B8, and B9 exhibited significantly higher phosphate release across all three pH conditions tested. This increased release can be attributed to their higher fertilizer loading, containing 80% Ox-ERP from the total polymer mass. The release profiles of B7, B8, and B9 followed a similar overall trend across acidic, neutral, and alkaline solutions, with minor fluctuations observed among these three samples over the 42 days. Nutrient release occurs through multiple mechanisms, including diffusion, swelling, and degradation of the polymer. When the hydrogel is immersed in water, it absorbs water and gradually swells and forms a swollen gel. Initially, fertilizers adsorbed on the hydrogel's surface or outer layer dissolve rapidly, resulting in a rapid increase in the release rate. Subsequently, water molecules penetrate deeper into the hydrogel interior. At this stage, phosphate release is governed by diffusion driven by the concentration gradient between the fertilizer-loaded hydrogel matrix and the surrounding swelling medium.⁸⁴ The release ends when the once concentration equilibrium between the inner space of the matrix and the water solution is achieved. The nutrient release from hydrogel fertilizers is strongly influenced by the pH of the surrounding medium.⁸⁴ At higher pH, deprotonation of carboxyl groups in the polymer network increases electrostatic repulsion, expanding the hydrogel matrix and facilitating diffusion of encapsulated nutrients. Similar observations have been reported in alginate- and CMC-based hydrogels: curcumin release from alginate-based beads was faster at neutral/alkaline pH due to increased swelling.⁸⁵ and phosphate release from CMC-based hydrogels followed non-Fickian kinetics with higher release under basic conditions.⁸⁶ Moreover, urea-loaded hemicellulose hydrogels exhibited sustained but higher release in alkaline media, consistent with controlled, swelling-assisted diffusion.⁸⁷ These findings sugesst that pH-responsive swelling plays a key role in controlling nutrient release from hydrogel-based fertilizers. Ionic crosslinking and ionic interactions are directly impacted by solution pH, according to a prior study on anionic superabsorbent hydrogels like cellulose-g-P(AA-co-AM-co-AMPS)/MMT. Because of increased ionic repulsion between carboxylate and sulfonate groups, which expands the polymer network and decreases hydrogen bonding, the hydrogel showed its maximum swelling between pH 5 and 9. This result is consistent with the current investigation because both hydrogels are anionic, showing increased swelling and, as a result, higher rates of nutrient release in alkaline media.⁸⁸ Due to the lower nano-Ox-ERP content, which is 20% of the polymer mass, beads B1, B2, and B3 showed the least amount of phosphate release. Bead B1 exhibited a comparatively higher release rate among these in all 3 pH mediums, perhaps as a result of lower crosslinking density. (1% HA) and high encapsulation of the fertilizer.

Bead B6 showed the greatest phosphate release at all three pH levels out of beads B4, B5, and B6. B6 has 3% HA, even though the fertilizer content is constant within this group and represents a medium loading in comparison to the other two sets. Instead of integrating with the Ox-ERP matrix, high HA concentrations typically aggregate through hydrophobic interactions, π–π stacking, and hydrogen bonding.⁶⁵ HA solutions above optimal concentrations self-assemble into aggregates that range in size from nanometers to micrometers.⁵⁸ Similar to this, oversaturation causes phase separation and the formation of HA-rich domains. Because of the voids and heterogeneity, nutrients leak more quickly, which leads to the high release rate.

The release behavior of phosphate from the hydrogel system is governed by a combination of diffusion, swelling, and matrix relaxation processes. Upon immersion, the hydrogel absorbs water and swells, allowing phosphate species located near the surface to be released initially, followed by gradual diffusion from the interior of the matrix. The pH-responsive nature of the system can be attributed to the ionization behavior of carboxyl groups in SA and CMC, which influences hydrogel swelling and permeability. At higher pH, increased deprotonation leads to electrostatic repulsion and expansion of the network, facilitating nutrient diffusion. In addition to these physical processes, the presence of oxalic acid-modified ERP and humic acid may further influence release behavior. Oxalic acid is expected to enhance phosphate solubilization, while humic acid may modify the microenvironment within the hydrogel and potentially influence nutrient mobility.The overall release profile is therefore likely the result of combined physical and chemical effects, although the relative contribution of each mechanism has not been independently isolated in this study.

Among the control beads C1, C2, and C3, bead C3 exhibited the highest phosphate release in neutral acidic and alkaline mediums, which can be attributed to its higher nano-Ox-ERP content comprising 80% of the total polymer mass (g g⁻¹) (Fig. 10).

The release data were fitted (SI Table 01–03) to zero-order, first-order, second-order, Higuchi, Hixson corwell and Korsmeyer–Peppas models to further evaluate the release kinetics, among which the Korsmeyer–Peppas model provided the best fit for the present study.

The Korsmeyer–Peppas model provided an excellent description of phosphorus release kinetics from the hydrogel beads, yielding consistently high correlation coefficients (R² ≈ 0.99) (Fig. 11). This strong agreement suggests the suitability of the model for elucidating the transport mechanisms governing controlled nutrient delivery.

The model is expressed asM_t/M_∞ = ktⁿwhere M_t/M_∞ is the fractional release at time t, k is the release rate constant, and n is the release exponent indicative of the dominant transport mechanism.⁸⁹ For spherical matrices, n ≤ 0.43 corresponds to Fickian diffusion-controlled release, where molecular diffusion through the hydrated polymer network dominates. Values 0.43 < n < 0.85 of indicate anomalous (non-Fickian) transport involving a combination of diffusion and polymer chain relaxation, while n ≥ 0.85 signifies Case II transport, where polymer swelling and erosion govern release kinetics.⁹⁰ The rate constant k reflects release velocity and is influenced by polymer composition, crosslink density, filler loading, and environmental conditions.⁹¹

The Korsmeyer–Peppas parameters, including the release exponent (n) and the release rate constant (k), are presented in Table 2. At acidic pH, most formulations exhibited low n values (0.20–0.43), indicating predominantly Fickian diffusion-controlled release. Low nano-Ox-ERP SA-CMC-HA formulations (B1–B3) showed n = 0.20–0.37 with relatively higher k values (0.0417–0.0727), suggesting diffusion through compact but moderately hydrated polymer matrices. Medium nano-Ox-ERP SA-CMC-HA formulations (B4–B6) displayed similar n values (0.22–0.40) but lower k values (0.0242–0.0561), indicating that increased particle loading resulted in denser packing and reduced effective diffusion coefficients⁷⁹ High nano-Ox-ERP SA-CMC-HA formulations (B7–B9) exhibited n values approaching the anomalous regime (0.38–0.43) with the lowest k values (0.0166–0.0565), reflecting structural constraints imposed by high mineral content. These dense matrices create tortuous diffusion pathways that limit both molecular diffusion and matrix relaxation, consistent with previous reports on filler-rich polymer systems.⁹²

Table 2. Korsmeyer–Peppas parameters – release exponent (n) and release rate constant (k).

Sample	Release exponent (n)			Release rate constant (k)
Sample	Acidic (A)	Neutral (N)	Basic (B)	Acidic (A)	Neutral (N)	Basic (B)
B1	0.3216	0.2514	0.3735	0.0417	0.0417	0.0521
B2	0.2025	0.2208	0.2365	0.0439	1.0896	0.0331
B3	0.2636	0.2184	0.2003	0.0727	0.0702	0.0642
B4	0.3056	0.4322	0.2203	0.0249	0.0229	0.0242
B5	0.3603	0.3395	0.2200	0.0309	0.0345	0.0363
B6	0.3958	0.3515	0.2980	0.0417	0.0445	0.0561
B7	0.4282	0.3226	0.4669	0.0266	0.0381	0.0166
B8	0.4338	0.4143	0.5179	0.0259	0.0259	0.0198
B9	0.3812	0.4656	0.5073	0.0565	0.0434	0.0434

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At neutral pH, release mechanisms shifted toward anomalous transport. While formulation B2 retained a low n value (0.2208), it exhibited an exceptionally high k value (1.0896), indicating rapid phosphorus release. This behavior suggests optimal interactions between the polymer network and humic acid, leading to enhanced pore formation and efficient mass transport.⁹³ Medium to high nano-Ox-ERP formulations (B4–B9) showed increasing n values (0.33–0.52), with B8 and B9 exceeding 0.45, clearly indicating non-Fickian transport. At neutral pH, moderate ionization of carboxyl groups promotes balanced swelling while preserving matrix integrity, allowing diffusion and polymer relaxation to occur on comparable timescales.⁹⁴

At alkaline pH, all formulations exhibited elevated n values (0.20–0.52), with high Ox-ERP beads (B7–B9) firmly within the anomalous transport regime (n = 0.47–0.51). Formulation B9 (n = 0.5073) approached Case II transport, indicating that polymer chain relaxation increasingly governed release. However, despite higher swelling under alkaline conditions, k values were generally lower than those at neutral pH. This suggests that excessive swelling produced highly hydrated, loose networks with diminished concentration gradients, reducing the effective driving force for diffusion, a phenomenon well documented in polyelectrolyte hydrogels.^95,96

Under acidic conditions, extensive protonation strengthens hydrogen bonding within the polymer network, producing compact and mechanically robust matrices. Although some formulations exhibited high swelling, electrostatic interactions between the protonated polymer and phosphate species significantly inhibited phosphorus diffusion, resulting in low n values.⁹⁷ At neutral pH, partial ionization generated sufficient osmotic pressure to promote swelling without compromising structural integrity, yielding optimal release kinetics. In particular, B2 combined the highest swelling (307%) with the highest k value, reflecting ideal pore formation and sustained concentration gradients.⁹⁸ In contrast, at alkaline pH, full ionization caused extreme swelling and network dilution, reducing effective diffusion despite increased water uptake.⁴⁷

3.7. pH-dependent release behavior of humic acid from SA-CMC Ox–ERP-HA hydrogel beads in aqueous solutions

As illustrated in Fig. 12, across all formulations, the cumulative HA release follows the order of acidic > neutral > alkaline conditions. In acidic medium, formulations exhibited the highest HA release after 21 days, followed by a gradual sustained release phase extending to 70 days with final cumulative releases ranging from 0.7–1.0% for most formulations. In neutral conditions, the release patterns showed similar trends but with reduced initial burst, while alkaline conditions demonstrated the most restricted release profiles, with rapid decline after the initial phase and minimal sustained release beyond 30 days. Under alkaline conditions, both HA and alginate functional groups are extensively deprotonated^99,100 and HA molecules may aggregate under alkaline pH through hydrophobic interactions and hydrogen bonding between aromatic structures, further limiting their diffusion through the polymer matrix.¹⁰¹ HA loading and release are inversely correlated within each phosphate concentration group, according to a crucial finding from the release profiles. In particular, under all pH conditions, formulations with low humic acid content consistently show higher HA release than beads loaded with moderate and high humic acid. Of all the formulations, B7 had the highest cumulative release, whereas B3 had the lowest. This surprising pattern implies that matrix-humic acid interactions and humic acid self-association are key factors in regulating release kinetics. At high HA concentrations, extensive intermolecular associations occur through multiple mechanisms such as hydrophobic interactions between aromatic moieties, π-π stacking of conjugated ring structures, hydrogen bonding networks between carboxylic and phenolic groups, and cation bridging via residual calcium ions.^54,101 B7 demonstrated the maximum cumulative release of approximately 3.0–3.2% at day 21, substantially higher than all other formulations, which may be due to the highest encapsulation of HA in B7 than in other beads. The most important finding from this study was the consistent inverse relationship between humic acid loading and release efficiency observed across all phosphate concentrations and pH conditions. Within each phosphate group, increasing HA concentration systematically decreased cumulative release: B1 > B2 > B3, B4 > B5 > B6, and B7 > B8 > B9. This pattern may be HA undergoes concentration-dependent self-aggregation. At low HA concentrations, molecules remain dispersed with small hydrodynamic radii, enabling rapid diffusion through the polymer network. At high HA concentrations, extensive self-association occurs through π–π stacking, hydrophobic interactions, and calcium-mediated bridging, forming supramolecular aggregates with severely reduced mobility.⁸⁹

3.8. Biodegradability of SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

The hydrogel beads showed progressive degradation with notable weight loss within 20 days (Fig. 13), reflecting polymer network breakdown, suggesting the hydrogel matrix's biodegradability, aligning to develop ecofriendly hydrogel beads. The degradation was affected by the pH value of the soil. In alkaline soil, the rate of degradation slightly increased. Additionally, the highest swollen hydrogel was degraded faster than others in all pH conditions, which was B5, reaching 79.92% in neutral soil, 87.55% in acidic soil, and 62.94% in alkaline soil within 20 days.

This behavior is consistent with the previous study on the formation of ALG/CMC-1 hydrogels, which exhibited the same degradation pattern characterized by pH-dependent weight loss and faster erosion in the highly swellable formulation. Alginate/carboxymethylcellulose-based hydrogels as pH-sensitive drug delivery systems: facile production with enhanced degradation, thermal, and mechanical properties.⁷⁶ In alkaline pH conditions, anionic hydrogels like CMC and SA swell more due to the ionization of the acidic groups, which causes the repulsion between pendant groups on the polymer chain led to the swelling.¹⁰² This results in network expansion and loosening, disruption of hydrogen bonding, loss of physical crosslinking, and an increase in the degradation rate. Soil has more diverse microorganism including bacteria, fungi, and archaea, that enzymatically break down polymers compared to other environments.¹⁰³ Incorporation of humic acid can also stimulate the microbial population. Combined application of NPK and humic acid in sugarcane rhizosphere significantly increased catalase, dehydrogenase, phosphatase activities, and microbial biomass.⁷⁶ The study showed that enhanced soil enzyme activities and increased microbial biomass in the presence of humic acids may be due to greater carbon-rich substrate availability to microbes, thereby promoting heterotrophic growth.

3.9. Germination study and plant bioassay for SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel fertilizers

The germination test results demonstrated that SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads varied with the pH of the soil medium (Fig. 14). The highest number of germinated banana pepper seeds came from neutral soil conditions, showing high Germination indexes and root lengths, but it significantly decreased at acidic soil conditions. Previous studies have reported that the observed response is associated with hydrogen-ion–induced root injury under strongly acidic conditions (pH < 4.5). This injury results in changes to root membrane permeability, leading to membrane leakage, impaired absorption and translocation of nutrients, and increased loss of organic substrates (such as sugars and amino acids) and adsorbed cations, and reduces capacities for absorption of nutrients.¹⁰⁴ In contrast to low pH levels, a significant number of fine roots appeared at high pH levels. Where the roots were short and stubby. This phenomenon can be explained by insufficient calcium uptake, which has a detrimental effect on cell elongation and division, leading to a denser and shorter root system.¹⁰⁵ In their study they found that reduction in taproot length in soyabeans occurred at pH 4.7 due to the H⁺ toxicity. The pH of the solution had a greater impact on root growth than shoot growth. Several researchers have demonstrated that effect. As examples, Maharana et al. (2021)³⁷ found a 40% decrease in pea root dry mass without a decrease in shoot dry mass, Tan et al. (2003)³⁴ found an 80% reduction in root elongation of faba beans grown at pH 4.0 compared to those grown at pH 6.5. The concentration of rock phosphate in the hydrogel formulations exerted a significant influence on germination outcomes across all pH conditions. Treatments with high phosphate content (TSP, nano-Ox-ERP) generally exhibited reduced germination performance compared to slow-release hydrogel beads, particularly in alkaline soil conditions, showing significantly high GI index (475.23% for B7, 260.55% for B8). GI integrates germination percentage with germination speed, providing a more complete picture of seed vigor and treatment suitability. Treatments with high GI indicate not only high final germination but also rapid, uniform emergence, which is critical for field establishment and weed competition. In the present study, treatments combining HA with appropriate phosphate levels likely achieved GI >80% across pH conditions, while high-phosphate treatments without humic protection may have fallen below this threshold, particularly in alkaline soil. In contrast, RRG and RSG assess treatment effects relative to control conditions, helping identify growth-promoting versus growth-inhibiting formulations.¹⁰⁶ RRG values >100% indicate enhanced root development compared to the control, suggesting positive effects on nutrient uptake capacity and stress tolerance. Similarly, RSG >100% indicates germination stimulation beyond baseline levels. Among the fertilizer treatments, TSP represents highly soluble, immediately available phosphorus. While this ensures adequate phosphorus supply for germination, it also presents the highest risk of phosphate toxicity due to rapid dissolution and high local concentrations around seeds. Therefore, TSP may cause significant germination inhibition through salt effects and osmotic stress. The hydrogel encapsulated formulations, particularly B7, offer advantages in three types of soil mediums by reducing toxicity risk because of the gradual phosphorus release compared to TSP, and also improve nutrient supply compared to ERP due to the oxalic acid modification. An additional advantage lies in the protective effect of encapsulated HA, coupled with its capacity to stimulate plant growth. HA incorporation in the hydrogel formulations significantly enhanced germination across all pH conditions. Humic-containing treatments (B5, B7, B8, and B9) consistently outperformed non-humic controls (C2 and C3). Notably, B7 showed superior performance likely due to its higher humic acid release, as reported earlier in this study. Humic acid provides multiple protective functions that mitigate phosphate toxicity and environmental stress. HA possesses multiple functional groups that confer pH buffering properties.¹⁰⁷ This buffering action helps stabilize the pH microenvironment around germinating seeds, protecting them from extreme pH fluctuations that could impair enzyme activity and metabolic processes essential for germination. Çimrin et al. (2010)¹⁰⁸ reported that the combined application of phosphorus and humic acid alleviates salinity stress in pepper seedlings. Under saline conditions (50 mg NaCl/kg soil), humic acid application at 1000 mg kg⁻¹ significantly increased seed germination and seedling growth parameters compared to phosphorus-only treatments.

After 30 days of seedling growth in alkaline soil amended with various hydrogel bead treatments, significant differences were observed in both phosphorus (P) and calcium (Ca) uptake per plant. The highest P uptake (Fig. 15(A)) was recorded in treatments B8 (1.51 mg per plant) and B7 (0.76 mg per plant), representing approximately 2–3 fold increases compared to the control. Treatment B9 showed 1.04 mg per plant. Treatments without humic acid (C3) and the unamended control showed markedly reduced P uptake (0.26–0.30 mg per plant).

Treatment B8 dramatically enhanced Ca uptake (Fig. 15(B)) to 33.2 mg per plant, approximately 3 times higher than B7 (11.8 mg per plant) and B9 (10.7 mg per plant). The Ca uptake was substantially higher than P uptake across all treatments, with B8 showing the most pronounced effect. Treatments C3, ERP, TSP and the control resulted in very low Ca uptake (0.5–5.0 mg per plant).

The superior performance of treatment B8 in enhancing both P and Ca uptake sugesst a significant synergistic interaction between oxalic acid-modified rock phosphate and humic acid in alkaline soils. Humic acid likely enhanced nutrient availability through multiple mechanisms: (1) chelation of Ca²⁺ ions, increasing their mobility and reducing precipitation in alkaline conditions; (2) modification of soil structure within the hydrogel matrix, creating favorable conditions for root exploration; and (3) production of organic acids through microbial decomposition, further solubilizing soil minerals¹⁰⁹ The optimal HA concentration of 2% suggests that excess HA (3%) may have negatively affected nutrient release kinetics or hydrogel properties.

The exceptionally high Ca uptake in B8 (33.2 mg per plant) compared to other treatments is noteworthy. The observation that calcium uptake consistently surpassed phosphorus uptake across treatments aligns with plant nutrition trends in alkaline soils, where phosphorus is often deficient and calcium is abundant. Notably, the B8 treatment showed a significant difference (33.2 mg Ca vs. 1.51 mg P), indicating that while ox-ERP solubilized phosphate, oxalic acid also dissolved calcium minerals. Future formulations should explore the calcium-phosphorus uptake ratio and potential nutrient imbalances.

3.10. Plant growth assessment of banana pepper treated with nano-ERP, SA-CMC-Ox-ERP-HA, SA-CMC-Ox-ERP, and TSP

Based on the above findings, a banana pepper plant trial was conducted using selected formulations (Fig. 16), namely SA-CMC-Ox-ERP-HA (B5, B7, B8, and B9), SA-CMC-Ox-ERP (C2 and C3), along with TSP and nano-ERP as reference treatments, and an untreated control.

Treatment B8 demonstrated superior pod production, reaching approximately 7 pods per plant by week 10, closely followed by B5 (6 pods per plant). Early pod initiation was observed in B9 treatments (1 pods at week 6). By week 10, the ranking was: B8 > B5 > nano-Ox-ERP > TSP > Control. Treatments without HA and the control showed significantly reduced pod numbers. Notably, pod production plateaued between weeks 9–10 for most treatments (Table 3).

Table 3. Effect of treatments on pod number at reproductive stages (Mean ± SD, n = 6)^a.

Treatment	No pods per plant
Treatment	Week 8	Week 9	Week 10
Control	2.00 ± 0.82^fghi	2.25 ± 0.50^fghi	2.50 ± 0.58^efghi
B5	2.50 ± 0.58^efghi	5.75 ± 0.96^abc	6.50 ± 1.91 ^ab
B7	1.50 ± 1.00^hi	4.50 ± 0.58^bcdef	5.50 ± 0.58^abcd
B8	2.25 ± 0.96^fghi	6.00 ± 0.82^abc	7.25 ± 0.96^a
B9	4.25 ± 0.96^bcdefg	5.00 ± 1.41^abcde	6.50 ± 1.29 ^ab
C2	1.00 ± 0.00ⁱ	3.00 ± 1.41^defghi	3.75 ± 0.96^cdefgh
C3	2.00 ± 0.82^fghi	2.25 ± 0.96^fghi	2.50 ± 0.58^efghi
Nano-ERP	1.75 ± 0.50^fghi	2.00 ± 0.82^fghi	4.50 ± 1.00^bcdef
Nano-Ox-ERP	2.25 ± 0.96^fghi	2.50 ± 1.00^efghi	5.75 ± 0.96^abc
TSP	1.50 ± 0.58^ghi	3.00 ± 0.82^defghi	5.00 ± 0.82^abcde

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Different letters indicate statistically significant differences between groups (Tukey's HSD test, p < 0.05). (n = 6).

Plant height increased progressively throughout the trial. At week 10, B5 achieved the maximum height of 98 cm, followed by B7 (90 cm) and B9 (80 cm). Treatment B5 showed the steepest growth trajectory, gaining 83 cm over 8 weeks (15 to 98 cm). Early growth advantage was evident: by week 5, B5 plants reached 45 cm compared to 30 cm in the control. Treatment B8, despite excellent pod production, showed moderate height gains (81 cm), suggesting resource allocation favoring reproductive over vegetative growth. Control plants reached only 60 cm by week 10. Treatments ERP and TSP showed poor height development (67 and 69 cm, respectively) (Table 4).

Table 4. Effect of treatments on plant height (cm) at different growth stages (Mean ± SD, n = 6).

Treatment	Plant height (cm)
Treatment	Week 3	Week 4	Week 5	Week 6	Week 7	Week 8	Week 9	Week 10
Control	15.70 ± 0.48	20.85 ± 0.90	30.90 ± 0.90	41.65 ± 0.66	48.33 ± 1.22	55.90 ± 0.90	57.65 ± 0.39	60.95 ± 0.95
B5	15.88 ± 0.68	25.48 ± 0.46	45.33 ± 0.50	61.15 ± 0.93	78.58 ± 1.31	91.13 ± 0.65	93.30 ± 1.65	98.98 ± 0.92
B7	15.13 ± 0.55	24.10 ± 0.86	42.23 ± 2.63	54.95 ± 1.10	78.85 ± 0.93	87.98 ± 1.41	88.35 ± 1.06	90.58 ± 0.99
B8	14.65 ± 0.82	25.68 ± 0.72	43.18 ± 1.04	51.50 ± 1.47	70.85 ± 0.85	77.90 ± 0.58	78.50 ± 0.41	82.33 ± 0.79
B9	15.25 ± 1.04	25.43 ± 0.43	40.55 ± 0.44	51.00 ± 1.08	68.55 ± 0.75	80.75 ± 1.19	81.33 ± 1.14	81.50 ± 0.91
C2	15.00 ± 1.08	27.78 ± 0.63	40.50 ± 0.41	49.38 ± 1.11	60.10 ± 0.27	61.70 ± 0.36	67.20 ± 1.29	74.43 ± 3.02
C3	14.25 ± 1.19	21.25 ± 1.19	43.30 ± 1.34	49.10 ± 1.34	50.98 ± 1.42	53.63 ± 0.95	67.58 ± 1.15	70.43 ± 0.57
ERP	15.38 ± 0.48	21.75 ± 0.87	39.40 ± 0.78	50.43 ± 0.43	50.63 ± 0.63	52.08 ± 0.30	54.78 ± 1.27	68.13 ± 1.31
Ox-ERP	14.60 ± 2.10	20.58 ± 0.43	35.93 ± 0.98	50.63 ± 0.75	51.25 ± 0.87	51.88 ± 0.25	68.85 ± 2.58	71.33 ± 0.94
TSP	15.08 ± 0.33	24.53 ± 1.02	30.75 ± 0.87	36.63 ± 0.95	50.38 ± 0.26	61.88 ± 0.68	63.00 ± 0.00	69.60 ± 1.10

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Branching patterns showed limited variation among treatments (SI Table 04). All treatments developed 1–4 primary branches over the 10 weeks period. Treatment B5 achieved the maximum of 4 branches (weeks 8–10), matching B7, B8, B9, and C2. By week 10, most treatments had 3–4 branches, while Control, C3, and TSP showed only 2–3 branches. Early branching differences were minimal through week 6, with most treatments showing 1–2 branches.

The exceptional performance of treatments B5 and B8 validates the synergistic benefits of combining Ox-ERP with HA in hydrogel beads. These formulations substantially enhanced plant growth parameters compared to control and conventional fertilizers. The sustained nutrient availability from the CMC-SA hydrogel matrix likely ensured continuous phosphorus and calcium supply throughout the growing season, supporting vigorous plant development.

The improved P and Ca uptake demonstrated in the nutrient uptake study directly translated to superior agronomic performance in this trial. Phosphorus is essential for energy metabolism and reproductive organ development, explaining the exceptionally high pod production in B8 treatment. Calcium's role in cell wall integrity and stress tolerance likely contributed to the robust growth observed in B5 and B8 treatments.¹¹⁰

B5 and B8 exhibited contrasting growth patterns. B5 focused on vegetative growth, reaching 98 cm in height with maximum branching, while B8, at 81 cm, enhanced reproductive development with moderate vegetative growth. B5's 40% ox-ERP content supported vegetative vigor, whereas B8's higher 80% nano Ox-ERP and improved P availability led to earlier flowering and more pod development.¹¹⁰ This represents an important agronomic consideration: B5 is suitable for biomass or fodder production, while B8 is optimal for yield-focused cropping. In nano Ox-ERP treatments, the 2% HA concentration (B5, B8) outperformed both 1% (B7) and 3% (B9) formulations. This indicates an optimal humic acid level for enhancing nutrient bioavailability in the hydrogel system. Higher concentrations (3% in B9) may have increased viscosity and altered nutrient release, leading to reduced uptake efficiency. These findings agree with earlier studies on optimal HA application rates in soil systems.¹⁰⁹

The poor performance of ERP and TSP treatments is significant. While TSP contains readily available phosphate, its effectiveness was severely limited in alkaline soil, where rapid fixation by calcium and magnesium occurs.¹¹¹ ERP similarly failed to provide adequate phosphate bioavailability. The stand-alone nano-Ox-ERP treatment, while performing better than ERP and TSP, was inferior to hydrogel-formulated versions, indicating that the hydrogel matrix's controlled-release mechanism provided distinct advantages over raw amendments.

The improved performance of the optimized formulation (B8) is likely associated with the combined effects of hydrogel structure, oxalic acid modification, and humic acid incorporation.

3.11. Impact of fertilizer application on soil microbial populations

Table 5 presents the bacterial populations observed in soil after different fertilizer treatments. Among, SA-CMC-Ox-ERP-HA beads, B8 was selected for microbial test due to its best performance in the plant trial. The B8 treatment, which combined nano Ox-ERP with HA, produced significantly higher bacterial populations (7.533 log CFU g⁻¹) compared to all other treatments (p < 0.05). In contrast, C3 treatment with ERP alone yielded only 5.901 log CFU g⁻¹, showing minimal improvement over the control. TSP produced the lowest bacterial count at 5.473 log CFU g⁻¹, actually lower than the control soil.

Table 5. Bacterial population^a.

Sample	Bacterial population (average log CFU g⁻¹)
B8	7.533 ± 0.037^a
C3	5.901 ± 0.138^b
TSP	5.473 ± 0.275^b
Control	5.632 ± 0.080^b

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Different letters indicate statistically significant differences between groups (Tukey's HSD test, p < 0.05). (n = 3).

The superior bacterial proliferation observed in the B8 treatment can be attributed to the complementary roles of oxalic acid and humic acid. Oxalic acid enhances phosphorus solubility by chelating calcium and magnesium ions that otherwise immobilize phosphate under alkaline soil conditions, thereby increasing phosphorus bioavailability to soil microorganisms.¹¹² However, microbial growth is frequently limited not only by nutrient availability but also by the availability of organic carbon.¹⁰⁹ Humic acid serves as a readily utilizable carbon source, improves soil moisture retention, and facilitates micronutrient availability, collectively creating a favorable microenvironment for microbial metabolism and proliferation.¹¹³ The combined presence of bioavailable phosphorus and organic carbon in the B8 treatment therefore supported significantly higher bacterial populations than treatments lacking humic acid. The lower bacterial population observed in the TSP treatment, which was even lower than that of the control soil, may be attributed to the rapid dissolution of soluble phosphate salts, leading to localized increases in ionic strength and osmotic stress. Such conditions can negatively affect microbial survival and activity, particularly in alkaline soils with limited buffering capacity.⁹⁹

Four main types of fungi were observed on the PDA plates Supplementary Fig. S5). Dark brown, velvety colonies presumptively identified as Aspergillus species appeared in B8, C3, and control samples. These are common soil decomposers that break down organic matter. Blue-green colored colonies were identified as Penicillium species, which were present in all treatment groups. White to cream-colored fluffy colonies appeared to be Trichoderma species, which were particularly abundant in the TSP treatment. Small numbers of pale, slow-growing colonies, likely representing Fusarium species were occasionally observed in control plates.

The B8 and C3 treatments showed diverse fungal colonies with good growth, reflecting the improved nutrient conditions. However, TSP treatment showed a distinct shift toward Trichoderma dominance with fewer Aspergillus colonies, suggesting that the high salt content of TSP creates osmotic stress that favors salt-tolerant fungi. The control plates had the lowest fungal diversity and colony density, suggesting that nutrient inputs are necessary to support active microbial communities in this alkaline soil.

These results demonstrate that integrated organic-inorganic fertilizer approaches are more effective than conventional soluble phosphate fertilizers for promoting soil microbial communities in alkaline soils.¹⁸ The B8 treatment not only increased bacterial populations substantially but also maintained a balanced and diverse fungal community, indicating good overall soil health.

It should be noted that microbial populations in this study were assessed using culture-dependent methods, which estimate only the fraction of microorganisms capable of growing on PDA under laboratory conditions. A substantial proportion of soil microbial communities remain non-culturable or require specific growth conditions not captured by standard media. Therefore, the observed microbial counts represent relative trends rather than absolute microbial abundance. Despite this limitation, culturable methods remain suitable for comparative evaluation of treatment effects on dominant microbial groups. The results obtained in this study demonstrate enhanced phosphorus release behavior, improved bioavailability, and superior plant performance compared to conventional systems. To further contextualize these findings within the existing body of literature, a component-wise comparison of previously reported fertilizer systems is presented in Table 6.

Table 6. Component-wise comparison of reported studies and the proposed integrated fertilizer system.

Study	System/Study type	Use of humic acid	Use of Oxalic acid modified rock phosphate	Use of alginate/CMC hydrogel	pH responsive behavior	Reported Outcomes	Key limitation	Advancement in the present study
Khatun et al., 2025 (ref. 72)	Humic acid–alginate hydrogel fertilizer	Humic acid incorporated	Not used	Alginate-based hydrogel used	Limited	Water absorption reached about 945 percent; urea release about 80–90 percent over 30 days; improved wheat yield (∼4199 kg ha⁻¹)	Focused on soluble fertilizer; no phosphate activation	Integrates humic acid with phosphate activation and responsive release system
Ma et al., 2025 (ref. 16)	Starch-based hydrogel humic acid fertilizer	Humic acid incorporated	Not used	Hydrogel system used	Not considered	Improved slow-release behavior and soil fertility enhancement	No control of mineral phosphorus; no responsiveness	Combines humic acid with nano-modified rock phosphate and pH responsiveness
Liu et al., 2025 (ref. 77)	Humic acid slow-release hydrogel fertilizer	Humic acid incorporated	Not used	Hydrogel system used	Not considered	Germination increased from 9.4 percent to 62.5 percent; water retention increased about sevenfold	No targeted nutrient release; no mineral activation	Introduces controlled and triggered nutrient delivery system
Hidayat et al., 2024 (ref. 71)	Alginate–polymer hydrogel with humic acid and struvite	Humic acid incorporated	Not used	Alginate-based hydrogel used	Limited	High nutrient retention; phosphorus removal efficiency up to 99 percent	No activation of low-solubility phosphate sources	Applies hydrogel system to modified rock phosphate with enhanced dissolution
Xiong et al., 2023 (ref. 114)	Humic acid–phosphate fertilizer system	Humic acid combined with phosphate	Not used	Not used	Not considered	Improved phosphorus transformation and plant uptake	Bulk system; no release control	Integrates controlled release + nano-scale activation + responsiveness
Present study (this work)	pH-responsive nano-hydrogel fertilizer system using OA-modified ERP and HA	Humic acid incorporated within hydrogel	Oxalic acid modified Eppawala rock phosphate used in nano form	Alginate–CMC hydrogel system used	Strong and targeted pH-responsive behavior	Controlled and pH-triggered phosphorus release; enhanced bioavailability; improved plant performance	Not applicable	First integrated system combining chemical activation, biological stimulation, nano-scale enhancement, and pH-responsive controlled release in a single platform

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As shown in Table 6, previous studies have primarily focused on individual strategies such as humic acid-mediated biostimulation, organic acid-induced phosphate solubilization, or hydrogel-based controlled release systems. While these approaches have demonstrated improvements in nutrient efficiency, they remain largely isolated and lack functional integration. In contrast, the present study combines oxalic acid-modified nano rock phosphate, humic acid incorporation, and an alginateCMC hydrogel matrix within a pH-responsive framework. This integrated system enables the simultaneous coupling of chemical activation, biological enhancement, and controlled nutrient release, thereby providing a more efficient and environmentally adaptive approach for phosphorus delivery. Therefore, the present work represents a significant advancement beyond conventional single-mechanism systems by introducing a multi-functional and responsive fertilizer platform that aligns with the requirements of sustainable and precision agriculture.

4. Conclusions

This study successfully integrates mechanochemically activated nano Ox-ERP with SA-CMC hydrogel and humic acid to create a sustainable phosphorus fertilizer. Nano-scale particle refinement (to 100–120 nm) combined with the formation of calcium oxalate and monetite phases dramatically enhanced phosphorus solubility across pH conditions. The optimal formulation B8 (80% nano Ox-ERP + 2% HA) achieved 91.5% phosphate encapsulation, pH-responsive swelling, and sustained diffusion-controlled release, with biodegradation suggested within 20 days.

Agronomic trials demonstrated clear superiority over conventional fertilizers: B8 achieved 2.9-fold higher pod production, 5-fold greater phosphorus uptake, and significantly stimulated soil microbial populations compared to controls and commercial TSP. The system effectively addresses key limitations of conventional phosphate fertilizers: enhanced bioavailability in alkaline soils, reduced leaching losses through controlled release, elimination of phytotoxicity risks, and improved soil health through microbial stimulation.

The mechanochemical approach using oxalic acid modification offers distinct sustainability advantages over conventional acid treatment, avoiding hazardous byproducts while achieving equivalent or superior phosphate activation. This integrated platform represents a viable pathway toward efficient, environmentally responsible phosphorus management suitable for implementation in diverse agroecological systems.¹¹⁵ Further studies focusing on isolating individual components and quantifying their respective contributions are required to establish the mechanistic interactions more conclusively.

Author contributions

Tharushi Perera – conduct experiment, formal analysis of data, and write the manuscript. Sanduni Dabare – Conduct experiment, formal analysis of data, and write the manuscript. Imalka Munaweera – conceptualization, funding acquisition, methodology, supervision, writing, review, and editing the original draft. Kasun M. Thambugala – supervision for microbiology testing, writing, review, and editing the original draft. Saranga Diyabalanage – supervision for characterization of materials, writing, review, and editing the original draft. All authors have approved the final version of the manuscript.

Conflicts of interest

The authors declare that there is no conflict of interest.

Supplementary Material

RA-016-D6RA03574D-s001

RA-016-D6RA03574D-s001.pdf^{(1.7MB, pdf)}

Acknowledgments

Financial support for this study is acknowledged by the University of Sri Jayewardenepura, Sri Lanka, under the research grant number RC/URG/SCI/2024/12. The Instrument Centre in the Faculty of Applied Sciences at the University of Sri Jayewardenepura for their support with the relevant instrumental operation and analyses. The Genetics and Molecular Biology Unit in the Faculty of Applied Sciences in the University of Sri Jayewardenepura and the Department of Botany in the Faculty of Applied Sciences in the University of Sri Jayewardenepura for their support with the microbiology testing.

Data availability

All the data are included in the manuscript.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra03574d.

References

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-016-D6RA03574D-s001

RA-016-D6RA03574D-s001.pdf^{(1.7MB, pdf)}

Data Availability Statement

All the data are included in the manuscript.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra03574d.

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PERMALINK

A sustainable pH responsive hydrogel-based phosphorus fertilizer using nano oxalic acid-modified rock phosphate and humic acid

Tharushi Perera

Sanduni Dabare

Imalka Munaweera

Kasun M Thambugala

Saranga Diyabalanage

Abstract

1. Introduction

Fig. 1. Research design.

2. Methodology

2.1. Materials

2.2. Preparation of nano oxalic acid-modified rock phosphate (nano-Ox-ERP)

2.3. Characterization of ERP, nano-ERP, and nano-Ox-ERP

2.4. Determination of total phosphate

2.5. Preparation of SA-CMC-based hydrogel beads loaded with nano-Ox-ERP and humic acid

Table 1. Nano-Ox-ERP and HA compositional variations of the synthesized SA-CMC hydrogel beads loaded with Ox-ERP and humic acid.

2.6. Characterization of the prepared SA-CMC-Ox-ERP-HA hydrogel beads

2.7. Encapsulation efficiency of phosphate in SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel bead fertilizer

2.8. Encapsulation efficiency of humic acid in SA-CMC-Ox-ERP-HA hydrogel bead fertilizer

2.9. Swelling capacity of the SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

2.10. Release behavior of phosphate from SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads in solutions of different pH

2.11. Biodegradability test of SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

2.12. Germination study and plant bioassay

2.13. Plant trail

2.14. Determination of soil microbial population

3. Results and discussion

3.1. Characterization of ERP, Nano-ERP and OX-ERP

3.1.1. SEM morphology characteristic of ERP, nano-ERP and nano-Ox-ERP

Fig. 2. (A) SEM image of nano-ERP (C) nano-Ox-ERP at a scale bar of 200 nm and a magnification of 25 000×. Particle diameter distribution of (B) nano-ERP (D) nano-Ox-ERP.

3.1.2. PXRD analysis of raw ERP, nano ERP and nano-Ox-ERP

Fig. 3. (A) PXRD spectrum of (a) nano-Ox-ERP (b) raw ERP (c) nano-ERP (d) Fluoroapatite (JCPDS card No- 01-073-1727) (e) hydroxyapatite (JCPDS card No- 01-072-1243) (f) Chloroapatite (JCPDS card No- 01-073-1728) (g) monetite (JCPDS card No- 01-071-1759) (h) whewellite (JCPDS card No- 20-0231).

3.1.3. FTIR analysis of ERP, nano-ERP and nano-Ox-ERP

Fig. 4. FTIR spectra of (a) nano-Ox-ERP (b) nano-ERP (c) ERP (d) Oxalic acid in the range 4000–500 cm−1.

3.2. pH-dependent phosphorus release behavior of nano-Ox-ERP, nano-ERP, and TSP in aqueous systems

Fig. 5. Cumulative phosphorus release of (A) nano-Ox-ERP (B) TSP (C) nano-ERP in acidic, neutral, and alkaline pH solutions. (Nano Ox-ERP and ERP fitted graphs according to Hill function with R2 = 0.9500–0.9963, TSP fitted graphs according to bidose response model with R2 = 0.9500).

3.3. Characterization of SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

3.3.1. FTIR analysis

3.3.2. SEM analysis

3.4. Phosphate and humic acid encapsulation in SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel bead fertilizer

Fig. 7. Encapsulation efficiency of (A) phosphate (B) HA in SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads.

3.5. pH-dependent swelling behavior of the SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

3.6. pH-dependent phosphorus release in aqueous solutions and kinetic analysis of SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

Fig. 9. Cumulative phosphorus (P) release profiles from SA-CMC-Ox-ERP-HA at acidic pH: A, D, and G, at neutral pH: B, E, and H, at alkaline pH: C, F, and I. (curves fitted using Bi dose response model and R2 = 0.9800–0.9990).

Fig. 10. Cumulative phosphorus release of SA-CMC-Ox-ERP (control beads) (A) C1 (B) C2 (C) C3 in neutral, acidic and alkaline pH solutions (curves fitted using hill function and R2 = 0.9800–0.9990).

Table 2. Korsmeyer–Peppas parameters – release exponent (n) and release rate constant (k).

3.7. pH-dependent release behavior of humic acid from SA-CMC Ox–ERP-HA hydrogel beads in aqueous solutions

Fig. 12. Humic acid (HA) release profiles from SA–CMC–Ox–ERP–HA hydrogel beads B1–B9 at (A) acidic, (B) neutral, and (C) alkaline conditions (n = 3).

3.8. Biodegradability of SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel beads

3.9. Germination study and plant bioassay for SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel fertilizers

Fig. 15. Effect of different SA-CMC-Ox-ERP-HA and SA-CMC-Ox-ERP hydrogel bead fertilizer applications in alkaline soil on early-stage banana pepper plant nutrient content. (A) Phosphorus uptake per plant, (B) calcium uptake per plant. (n = 6).

3.10. Plant growth assessment of banana pepper treated with nano-ERP, SA-CMC-Ox-ERP-HA, SA-CMC-Ox-ERP, and TSP

Fig. 16. Plant trial for banana pepper (a) Control, (b) B5, (c) B7, (d) B8, (e) B9, (f) C2, (g) C3, (h) ERP, (i) nano-Ox-ERP, and (j) TSP. (Photographs show plants at week 10 (onset of fruiting) and circled pods at week 12 (maturity stage).).

Table 3. Effect of treatments on pod number at reproductive stages (Mean ± SD, n = 6)a.

Table 4. Effect of treatments on plant height (cm) at different growth stages (Mean ± SD, n = 6).

3.11. Impact of fertilizer application on soil microbial populations

Table 5. Bacterial populationa.

Table 6. Component-wise comparison of reported studies and the proposed integrated fertilizer system.

4. Conclusions

Author contributions

Conflicts of interest

Supplementary Material

Acknowledgments

Data availability

References

Associated Data

Supplementary Materials

Data Availability Statement

ACTIONS

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RESOURCES

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Fig. 4. FTIR spectra of (a) nano-Ox-ERP (b) nano-ERP (c) ERP (d) Oxalic acid in the range 4000–500 cm⁻¹.

Fig. 5. Cumulative phosphorus release of (A) nano-Ox-ERP (B) TSP (C) nano-ERP in acidic, neutral, and alkaline pH solutions. (Nano Ox-ERP and ERP fitted graphs according to Hill function with R² = 0.9500–0.9963, TSP fitted graphs according to bidose response model with R² = 0.9500).

Fig. 9. Cumulative phosphorus (P) release profiles from SA-CMC-Ox-ERP-HA at acidic pH: A, D, and G, at neutral pH: B, E, and H, at alkaline pH: C, F, and I. (curves fitted using Bi dose response model and R² = 0.9800–0.9990).

Fig. 10. Cumulative phosphorus release of SA-CMC-Ox-ERP (control beads) (A) C1 (B) C2 (C) C3 in neutral, acidic and alkaline pH solutions (curves fitted using hill function and R² = 0.9800–0.9990).

Table 3. Effect of treatments on pod number at reproductive stages (Mean ± SD, n = 6)^a.

Table 5. Bacterial population^a.