Encapsulation of NPK in snake fruit peel biochar (Salacca edulis) and activated natural zeolite as a slow-release fertilizer for water spinach (Ipomoea reptans Poir)

The declining of efficiency of chemical fertilizer and rapid nutrient leaching in tropical soils shows the need for affordable slow-release systems based on agricultural waste and natural minerals. Therefore, this study aims to investigate the use of biochar from snake fruit peel as a slow-release fertilizer (SRF) and evaluate the effectiveness of the combination with activated natural zeolite. The process was carried out by drying the snake fruit peel at 70°C, activating it with HNO₃ for 4 hours, and pyrolyzing it at 200°C for 2 hours. Activated natural zeolite was prepared using NaOH concentrations of 4, 6, and 8 M for 1.5 hours, then characterization was conducted through atomic absorption spectroscopy (AAS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), gas sorption analysis (GSA), and scanning electron microscopy coupled with energy-dispersive X-ray analysis (SEM-EDX). The results showed that AAS had a potassium content of 7.175 mg g^-1 in 100 mL and 358.785 mg g^-1 in 500 mL of snake fruit peel. FTIR analysis showed the presence of O–H and Si–O functional groups, while sharp XRD peaks were consistent with the JCPDS data. GSA characterization identified the optimal zeolite concentration as 6 M, and SEM-EDX confirmed the presence of essential NPK elements. The NPK-Biochar-Zeolite SRF fertilizer was found to contain nitrogen (1786.40 ppm), phosphorus (3151.67 ppm), and potassium (15.06 me%), showing superior potassium retention compared to other nutrients. In terms of plant growth, the NPK-Biochar-Zeolite SRF significantly increased water spinach, locally known as “kangkung†in Indonesian, plant height by 2.2% and leaf width by 0.3% compared to other treatments. These results suggest that biochar from snake fruit peel, in combination with activated zeolite, can enhance fertilizer efficiency and support optimal plant growth for sustainable agricultural practices.

1. INTRODUCTION

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

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