According to FAO projections, the world population is estimated to reach 9.7 billion by 2050. This rapid population growth will significantly increase global food demand. Meanwhile, agriculture remains the main sector supplying food, and a large proportion of production significantly relies on inorganic fertilizer, particularly NPK, to achieve optimal yields (Chen et al., 2025; Wang et al., 2022). However, continuous use of chemical fertilizer may lead to various negative impacts, such as soil acidification, compaction, and structural degradation, which in turn result in nutrient loss and reduced crop yield and quality (Chen et al., 2025). Poor agricultural practices and environmental factors, such as high rainfall, irrigation, erosion, and fertilizer volatilization, also contribute to the leaching of essential nutrients, including nitrogen (N), phosphorus (P), and potassium (K), from the soil, subsequently causing water pollution and increased greenhouse gas emissions (Tahery et al., 2022).

One promising approach is the development of slow-release fertilizer (SRF). In general, SRF is engineered to release nutrients gradually according to crop demand, showing a low nutrient-release rate, which extends soil nutrient availability for plant uptake and reduces leaching or dissipation into the environment, ultimately improving fertilization efficiency (Priyadi & Mangiring, 2019; Rafique et al., 2025; Wang et al., 2022). Several types of SRF have been developed, including polymer-based (El-Aziz et al., 2022), biochar-based (Wang et al., 2022), and zeolite-based (Legese et al., 2024) fertilizers. Polymer-based fertilizers, such as polyurethane, polyolefin, and polyvinylidene chloride, can effectively slow nutrient release but are difficult to degrade, may accumulate in the environment, and have relatively high production costs. On the other hand, biochar and zeolite, which are derived from natural and environmentally friendly sources, have been identified as more sustainable alternatives for SRF. These materials also offer significant advantages, including enhanced functional activity and improved performance in promoting soil and nutrient-release processes (Ramesh & Raghavan, 2024).

Biochar, a carbon-based material, offers numerous advantages when used as a soil amendment, including the ability to enhance nitrogen recycling in soil-plant systems. The high surface area, porous structure, and diverse functional groups allow biochar to adsorb various nutrient ions such as ammonium, nitrate, phosphate, and potassium (Rashid et al., 2025). Biochar can also retain and gradually release nutrients, thereby reducing nutrient leaching, lowering fertilizer application frequency, and increasing nutrient use efficiency (Jia et al., 2021). Previous studies have shown that biochar derived from rice husks, empty fruit bunches of oil palm, or other organic materials effectively slows nitrogen loss due to leaching, enhances nutrient retention, and supports plant growth (Dong et al., 2020; Setyawan et al., 2022). Among various feedstocks, snake fruit peel (Salacca edulis) shows high potential as biochar due to the high carbon content, large surface area, and good sorption properties (Maneesri et al., 2024). However, a major limitation of biochar-based fertilizer is the insufficient slow-release capability, largely driven by the weak interfacial binding forces that affect nutrient retention on the biochar surface (Dong et al., 2025). To overcome this limitation, composite biochar fertilizers have increasingly been engineered to enhance nutrient stabilization, regulate diffusion pathways, and prolong the availability of nutrients in the soil system (Luo et al., 2021; Ramesh & Raghavan, 2025).

Recent advancements have shown that incorporating mineral phases into biochar matrices can substantially reinforce nutrient adsorption and slow surface desorption kinetics. Among these minerals, zeolite, as a crystalline aluminosilicate mineral, is distinct due to its ion-exchange capacity and uniform microporous channels, which are highly effective for trapping fertilizer ions (Ginting et al., 2025; Sharma et al., 2022). Zeolite also improves soil aeration, enhances soil structure, and promotes root growth by reducing soil compaction. Studies have shown that combining zeolite with biochar or natural polymers slows nitrogen release, improves fertilizer efficiency, and maintains nutrient availability in the soil (Dong et al., 2025; Permana et al., 2023; Suci et al., 2025). For example, (Dong et al., 2025) developed a nanozeolite-coupled biochar-based phosphorus fertilizer (ZEO/BC-PSRF) using a simple disc granulation process with kaolinite as a binder. The controlled-release performance of this fertilizer was evaluated through water release testing, indicating that ZEO/BC-PSRF showed the slowest phosphorus release, with only 29.4% released after 24 h (Dong et al., 2025). The use of coconut shell biochar (matrix) and activated natural zeolite (slow-release agent) in fertilizer successfully increased red onion yields (Permana et al., 2023). However, the potential of snake fruit peel biochar combined with activated natural zeolite in NPK-based fertilizer remains underexplored.

Water spinach (Ipomoea reptans Poir) was selected as the test crop due to the high consumption and cultivation popularity in Indonesia, along with a strong nutritional profile, particularly the high vitamin, mineral, and iron content (Peng et al., 2024). The rapid growth and substantial nutrient requirements make it a responsive model for assessing fertilizer efficiency. Due to the short cultivation cycle as a leafy vegetable, brief nutrient deficits quickly influence biomass, chlorophyll levels, and leaf quality, enabling a clear comparison between slow-release and conventional nutrient delivery (Mohd Noor et al., 2022). Additionally, the frequent cultivation near water bodies and role in nutrient-rich environments are directly associated with concerns of nutrient runoff and eutrophication, supporting the relevance for evaluating controlled-release biochar–zeolite fertilizer systems. Previous studies have shown that the incorporation of biochar or zeolite as an SRF matrix enhances plant growth, improves nutrient retention, and increases fertilizer use efficiency (Fistarani & Handayani, 2023; Pohan, 2021). Therefore, this study aims to develop an NPK-based SRF using snake fruit peel biochar and activated natural zeolite, and to evaluate the effectiveness in promoting the growth as well as productivity of water spinach. The results are expected not only to improve fertilization efficiency but also to provide an environmentally friendly, sustainable, and cost-effective alternative for tropical agriculture.

The major discovery of this study is that the combination of activated natural zeolite and snake fruit peel biochar as a matrix for NPK SRF significantly enhances nutrient retention in soil and improves water spinach growth. Among the treatments, NPK-Zeolite–Biochar SRF (A4) showed the highest plant height increase (2.2 % higher than other SRF formulations) and indicated superior performance in retaining potassium, while NPK-Zeolite SRF excelled in nitrogen and phosphorus retention. These results show that the integration of biochar and zeolite not only provides a controlled nutrient-release system but also optimizes soil nutrient availability for plant uptake.

The observed enhancement can be attributed to the physicochemical properties of both materials. Activation of natural zeolite with 6M NaOH resulted in the largest surface area, reaching 34,7306 m^² g^-1. This shows that zeolite treated with 6M NaOH is suitable for use as a matrix in the preparation of SRF fertilizer. A similar study by Permana et al. (2023) reported that activation improved the surface area of zeolite-based SRF fertilizer (Permana et al., 2023). The increase in surface area is mainly attributed to the removal of impurities from the zeolite framework during the NaOH treatment, which exposes more pores and active sites (Hong & Um, 2021). In contrast, activation with 4M and 8M NaOH resulted in lower surface areas. The decline may be associated with agglomeration, where individual zeolite particles cluster into larger masses, thereby blocking access to the internal pores. During surface area analysis, gas adsorption tends to occur on these aggregates rather than on isolated particles, which potentially leads to inaccurate surface area measurements (Hong & Um, 2021). Furthermore, at higher concentrations, such as 8M, NaOH treatment can damage the zeolite framework. Strong alkaline conditions promote the dissolution of silica, and under more extreme conditions, aluminum atoms are also released from the structure. This leaching effect weakens the framework and reduces the material porosity. Previous studies have also shown that excessive NaOH concentrations cause partial removal of Al^³⁺, significantly decreasing the surface area.

Beyond chemical changes, NaOH activation also alters the surface morphology of the zeolite, further influencing the structural and textural characteristics (Permana et al., 2023).

The presence of O–H and C–O bonds in the FTIR spectra shows the successful activation of snake fruit peel biochar, which possesses polar characteristics and is suitable for use as a slow-release agent in fertilizer formulations. The Si–O stretching vibration of siloxane observed in the sample shows the formation of a stable silica framework. The broad absorption band in the wavenumber region of 1100-1000 cm^-1 is attributed to the internal and external asymmetric stretching vibrations of tetrahedral Si–O from siloxane, which is a characteristic feature of silica-based materials (Liu et al., 2019). The FTIR spectra of NPK-Biochar, NPK-Zeolite, and NPK-Biochar-Zeolite SRF showed remarkable differences compared to the spectrum of biochar alone, including the appearance of new peaks and peak shifts. Compared to biochar, the granulated NPK-Biochar, NPK-Zeolite, and NPK-Biochar-Zeolite SRF show higher intensity absorption bands associated with aliphatic –OH and –NH groups from primary amines or amides, observed around 3410 cm⁻^¹ and 3070 cm⁻^¹. These chemical bonds show that both biochar and zeolite have reacted with urea and ammonium (Tahery et al., 2022; Wang et al., 2022). Additionally, peaks observed around 1430 cm⁻^¹ and 1460 cm⁻^¹ correspond to the presence of carbonate and NH bending vibrations from ammonium and or urea. The peak near 1000 cm⁻^¹ may show the stretching vibrations of aromatic C–O, Si–O, and P–O bonds (Shi et al., 2020). Absorption bands in the wavenumber range of 910 cm⁻^¹ to 740 cm⁻^¹ are associated with phenolic –OH, C–H bending, and the presence of various inorganic or mineral components such as Al–O–Si and Fe–O groups (Liu et al., 2019).

The XRD diffractograms of biochar, NPK-Biochar SRF, NPK-Zeolite SRF, and NPK-Biochar-Zeolite SRF showed distinct peaks at 2θ = 28.1°, 29.7°, 30.8°, 34.0°, and 40.5°, which are primarily attributed to the crystalline structures of sylvite (KCl) and calcite (CaCO₃). These minerals are commonly present in biochar, suggesting the potential for soil amendment applications (Sahoo et al., 2021). After loading with NPK fertilizer, the NPK-Biochar sample showed new diffraction peaks at 2θ = 22.1°, 24.0°, 31.8°, 42.9°, and 45.2°, indicating the successful incorporation of NPK into the biochar matrix. This shift in the XRD pattern signifies a modification in the crystalline structure of the biochar due to the NPK loading (Priya et al., 2024). In the case of NPK-zeolite, a prominent peak at 2θ = 26.7° was observed, corresponding to quartz minerals, according to JCPDS 05-0490. Additionally, sharp peaks at 2θ = 10.0°, 12.0°, and 20.8° were identified as clinoptilolite minerals, in accordance with JCPDS 25-1349, while peaks at 2θ = 16.7°, 22.4°, 26.7°, 28.3°, and 29.0° are characteristic of mordenite, based on JCPDS 11-0155 (Betriani et al., 2023). The presence of these distinct mineral phases in the NPK-Zeolite sample reflects the stability of the zeolite structure even after NPK loading. Finally, the XRD pattern of NPK-Biochar-Zeolite showed characteristic peaks from all three materials, confirming the successful combination of NPK, biochar, and zeolite. This synergy suggests the potential for a novel composite material combining the nutrient delivery capabilities of NPK, the adsorptive properties of biochar, and the ion-exchange characteristics of zeolite, which may offer enhanced soil fertility and sustainable agricultural applications (Dong et al., 2025).

SEM analysis in Figure 3b shows that natural zeolite activated with NaOH forms irregular particle aggregates with relatively small sizes stacked and closely packed, indicating the successful activation of the zeolite (Ates & Akgül, 2016). The surface morphology of the fertilizer is relatively rough due to the adherence of micronutrients on the surface, which can influence the gradual release of nutrients. This shows that the incorporation of snake fruit peel biochar and activated natural zeolite into NPK SRF fertilizer produces a more porous surface, enhancing the capacity to absorb and retain nutrients (Shao et al., 2025). On the other hand, the nutrients contained in the fertilizer contribute to a slower release.

The EDX results showed that snake fruit peel biochar consisted of 32.83 % carbon, 12.92 % nitrogen, 33.70 % oxygen, 1.00 % magnesium, 0.76 % silica, and 9.33 % potassium. These values indicate that the biochar not only serves as a carbon source but also contributes macro- and micronutrients essential for plant growth (Omokaro et al., 2025). For the activated natural zeolite, the EDX analysis showed that NaOH treatment significantly altered the elemental composition. The desilication process removed part of the silica from the zeolite framework, increasing mesoporosity while maintaining micropore size and acidity. The reduction of silica content created more space for accommodating particles and enhanced adsorption capacity. Additionally, the desilication treatment resulted in an increased sodium (Na) content within the zeolite structure.

The addition of snake fruit peel biochar had a significant effect compared to zeolite SRF fertilizer. The NPK–Zeolite–Biochar SRF in Table 4 contained 9.42 % carbon, 34.61 % oxygen, and 11.52 % potassium. The use of activated natural zeolite enhanced the retention of essential nutrients, resulting in higher elemental contents than those observed in other synthetic fertilizer (Suci et al., 2025). However, the presence of residual impurities blocking some zeolite pores during activation may have reduced the capacity of the zeolite to capture specific elements, which explains the loss of certain nutrients in the final product.

The plant growth test results in Figure 4 showed that fertilizer application can positively affect plant height. Among the treatments, SRF A4 (NPK zeolite–biochar SPK) was the most effective, increasing water spinach plant height by 2.2 % compared to A1, A2, and A3. The significant increase in plant height observed in the biochar and zeolite treatments can be explained by the ability to enhance nutrient uptake and increase soil organic matter, which helps reduce leaching of nitrogen, potassium, and phosphorus (Kamrunnahar et al., 2024; Suraya et al., 2023). In contrast, water spinach without fertilizer (A0) only reached a height of 6 cm and showed no further growth until harvest. These results show that the application of SRF can enhance leaf expansion in water spinach (Yang et al., 2021).

Soil analysis results indicated that NPK SRF with activated natural zeolite was most effective in retaining nitrogen and phosphorus in the soil compared to other synthetic fertilizer shown in Table 5. On the other hand, NPK SRF Zeolite–Biochar showed superior potassium retention, while NPK SRF Biochar performed better in retaining phosphorus and potassium. Overall, all three synthetic fertilizer showed good ability to maintain soil NPK levels.

Despite these differences in nutrient retention, harvested water spinach showed uniform shape and size across all treatments. With commercial Phonska, the NPK content in the soil after water spinach cultivation was more balanced in retaining nutrients. Conversely, soil without synthetic or commercial Phonska fertilizer had lower NPK levels due to the lack of a substance that could inhibit nutrient release. Water spinach grown in this soil was smaller and less nutritious.

Regarding the NPK-Biochar-Zeolite SRF, the comparison among the three synthesized fertilizer shows that the N and P contents were lower compared to those in the NPK SRF with activated natural zeolite. The biochar included in the NPK-Biochar-Zeolite SRF formulation helped prevent nutrient loss. Additionally, the dosage of snake fruit peel biochar used influences nutrient availability in the soil (Hossain et al., 2020). Further studies are needed to determine the optimal biochar content, because biochar contributes significant amounts of macro- and micronutrients, allowing it to function effectively as an SRF (Gwenzi et al., 2018; Marcińczyk & Oleszczuk, 2022).

Zeolite is a hydrated aluminosilicate mineral with a porous structure that contains alkali cations, which can directly influence the availability of nutrients in the soil, such as nitrogen (N) and phosphorus (P). Therefore, the amount of activated natural zeolite applied affects the levels of N and P available in the soil. The application of activated natural zeolite combined with snake fruit peel biochar has a significant effect on the availability of N and P nutrients in the soil (Legese et al., 2024; Villada et al., 2024).

In conclusion, this study underscores the significant potential of snake fruit peel as a sustainable alternative to KCl in the production of SRF. The high potassium content, with values of 7.175 mg g^-1 in 100 mL and 358.785 mg g^-1 in 500 mL, along with a substantial K₂O content, positions snake fruit peel as a valuable source of potassium for fertilizer production. Energy Dispersive X-ray (EDX) analysis of the snake fruit peel biochar showed a diverse elemental composition, including carbon (32.83 %), nitrogen (12.92 %), oxygen (33.70 %), magnesium (1 %), silica (0.76 %), and potassium (9.33 %). When combined with activated natural zeolite, the biochar significantly improved the morphology of NPK slow SRF, resulting in a more porous surface structure. This feature enhances the fertilizer ability to absorb and temporarily store essential nutrients, enabling a slower and more controlled release, thereby improving overall efficiency. In terms of plant growth, the use of commercial Phonska fertilizer led to a more balanced nutrient content in water spinach plants, with nitrogen at 2390.09 ppm, phosphorus at 2063.63 ppm, and potassium at 10.51 me%, yielding superior results compared to soils without synthetic or commercial fertilizer. Among the treatments, NPK-Biochar-Zeolite SRF (A4) was the most effective, increasing plant height by 2.2 % and leaf width by 0.3 % compared to the other treatments (A1, A2, and A3). More importantly, the NPK SRF Zeolite–Biochar treatment also showed superior potassium retention, while the NPK SRF Biochar had better phosphorus and potassium retention. These results suggest that incorporating biochar from snake fruit peel and zeolite into SRF formulations can significantly improve fertilizer efficiency and support optimal plant growth, making it a promising strategy for sustainable agriculture.