Aim
This study aimed to investigate the effect of different potassium fertilizer sources and various zeolite modification methods on potassium release from natural zeolite. The research specifically examined how zeolite particle size (powder vs. granular), potassium salt type (KOH, KCl, and K₂SO₄), and application method (spraying vs. immersion) influence the water-soluble and exchangeable potassium concentrations in modified zeolites. The study seeks to develop an efficient slow-release potassium fertilizer from natural zeolite to improve agricultural nutrient management and reduce environmental pollution caused by excessive fertilizer use. Given the increasing global population and rising food demand, agriculture has become heavily dependent on chemical fertilizers. However, uncontrolled fertilizer consumption leads to nutrient leaching due to high solubility, reducing fertilizer effectiveness and causing harmful environmental and health consequences. Zeolites, as aluminosilicate minerals with unique cation exchange capacity, porous structure, and ability to regulate soil pH, have been historically used to enhance agricultural efficiency. They can retain nutrients in the root zone and release them gradually according to plant needs. Potassium, as an essential macronutrient, plays a critical role in plant growth and productivity, but its mobility in soil is moderate, making it susceptible to leaching. Natural zeolites are abundant and easily accessible, making them promising candidates for developing potassium-enriched fertilizers through ion exchange processes.
Materials and Methods
Natural zeolite was obtained from the Navid Sadaf Garmasar mine in Iran, with initial pH of 8.61 and electrical conductivity of 4820 µS/cm. XRF analysis revealed the zeolite composition: SiO₂ (60.91%), Al₂O₃ (10.98%), CaO (6.14%), K₂O (4.24%), Fe₂O₃ (1.53%), Na₂O (2.10%), MgO (0.96%), and other trace elements. The zeolite was sieved into two particle sizes: granular (passing through 2.36 mm sieve, mesh 8) and powder (passing through 600 µm sieve, mesh 28). Three potassium salt solutions (1% K concentration) were prepared using KOH (1.43 g per 100 mL), KCl (1.89 g per 100 mL), and K₂SO₄ (2.23 g per 100 mL). The modification process involved applying these solutions to both zeolite particle sizes using two methods: immersion and spraying, with a 1:1 solution-to-zeolite ratio. For the immersion method, the zeolite samples were completely submerged in the potassium salt solution, while for the spraying method, the solution was uniformly sprayed onto the zeolite surface. The samples were maintained for one week in the laboratory with multiple wetting-drying cycles to ensure proper equilibration and distribution of potassium ions within the zeolite structure. For water-soluble potassium measurement, 5 g of modified zeolite was mixed with 25 mL distilled water in Falcon tubes, shaken, and centrifuged at 3500 rpm for 5 minutes. The supernatant was collected, and the remaining pellet was used for exchangeable potassium extraction using 1 M ammonium acetate solution. The pellet was washed with ammonium acetate, transferred to 250 mL volumetric flasks, and brought to volume. An additional 100 mL of ammonium acetate was added, and the samples were shaken for 30 minutes on a rotary shaker. The filtered samples were then transferred to sample containers. Both water-soluble and ammonium acetate-extractable potassium fractions were analyzed using a flame photometer. The experiment was conducted as a completely randomized factorial design with three replications, resulting in a total of 18 experimental units. Statistical analysis was performed using SAS software, and graphs were prepared using Excel. Analysis of variance was conducted to determine the significance of main effects and interactions, and Duncan's multiple range test was used for mean comparisons at P<0.05 and P<0.01 significance levels.
Results
The physical and chemical characterization of raw zeolites showed neutral to slightly alkaline pH values: 7.07 for powder and 7.46 for granular zeolites, with low electrical conductivity values of 1.01 and 1.13 mS/cm, respectively. These results indicate that both zeolite types are relatively stable materials with moderate alkalinity and low salinity. Water-soluble potassium concentrations in powder zeolites modified with KOH, KCl, and K₂SO₄ were 244.53, 697.7, and 487.1 mg/L, respectively, representing dramatic increases of 1749.7%, 5177.6%, and 3584.8% compared to unmodified powder zeolite. Granular zeolites modified with the same salts showed soluble potassium of 69.45, 190.8, and 129.1 mg/L, with increases of 514.6%, 1588.3%, and 1042.2% over the unmodified granular control. Powder zeolites consistently released significantly more water-soluble potassium than granular zeolites across all salt treatments (P<0.01). The application method showed no significant effect on water-soluble potassium concentration, suggesting that physical factors such as particle size and chemical factors such as salt type are more dominant in determining immediate potassium availability. The effect of salt type was also significant, with KCl yielding the highest water-soluble potassium concentrations, followed by K₂SO₄ and then KOH.
For ammonium acetate-extractable potassium, powder zeolites modified with KOH, KCl, and K₂SO₄ released 9182.3, 9303.4, and 9302.7 mg/L, respectively, showing 240.9%, 245.44%, and 245.41% increases over the unmodified powder zeolite. Granular zeolites released 5973.7, 5436.5, and 5884.6 mg/L under the same treatments, with increases of 166.1%, 142.1%, and 162.1% compared to unmodified granular zeolite. Although no statistically significant differences were observed among the three salt types within each particle size group, powder zeolites consistently released more exchangeable potassium than granular zeolites (P<0.01). In contrast to the water-soluble fraction, the application method significantly affected ammonium acetate-extractable potassium, with spraying (8424.80 mg/L) showing higher efficiency than immersion (5680.60 mg/L) (P<0.01). Analysis of variance revealed significant effects of zeolite particle size and salt type on both potassium fractions (P<0.01), while the application method only significantly affected exchangeable potassium (P<0.01). The interaction between particle size and salt type was also significant for both fractions (P<0.01), indicating that the combined effect of these factors determines the overall potassium release behavior. The coefficient of variation was 33.15% for water-soluble potassium and 16.22% for exchangeable potassium measurements.
Conclusion
This research successfully demonstrated that zeolite modification with potassium salts significantly enhances both water-soluble and exchangeable potassium release from natural zeolite, offering a promising approach for developing slow-release potassium fertilizers. Powder zeolites modified with potassium salts showed superior performance in potassium release compared to granular forms, which can be attributed to the greater specific surface area and increased contact surface of finer particles that facilitate more efficient ion exchange processes. The choice of potassium salt type proved to be a critical factor, with KCl and K₂SO₄ being more effective for water-soluble potassium, while KOH and KCl showed better performance for exchangeable potassium. This differential response indicates that salt type selection can serve as an effective management tool to regulate the rate and sustainability of potassium supply to plants. The spraying method proved more efficient than immersion for increasing exchangeable potassium, likely due to more uniform distribution of potassium solution on zeolite surfaces. However, the application method showed no effect on water-soluble potassium, emphasizing the greater importance of physical and chemical factors compared to the application technique. The zeolite's neutral to slightly alkaline pH and low electrical conductivity make it suitable for soil amendment applications, particularly in acidic soils where it can help buffer pH while supplying nutrients. These findings suggest that potassium-enriched zeolites, particularly in powder form modified with appropriate salts and applied via spraying, can serve as effective slow-release potassium fertilizers. This approach offers a sustainable solution to reduce potassium leaching losses, improve fertilizer use efficiency, minimize environmental pollution, and reduce the frequency of fertilizer applications. Future research is recommended to evaluate the performance of these modified zeolites under field conditions, assess their long-term effects on soil properties and crop yields, and investigate the economic feasibility of large-scale production.