Conventional nitrogen (N) and potassium (K) based fertilizers on agricultural soils face efficiency constraints due to high leaching rates and rapid nutrient release. This study aims to synthesize and characterize slow-release fertilizers (SRF-N and SRF-K) based on nano biochar and impregnation using alginate as a binder. Nano biochar was synthesized through ball milling, while impregnation was carried out through a cross-linking process, each method using 0% and 2% alginate. Characterization of the treatment was carried out using Fourier Transform Infrared Spectroscopy (FTIR) for functional group analysis, Scanning Electron Microscopy (SEM) and Image-J for morphology and particle size, and X-ray diffraction (XRD) for mineral crystal structures. The results showed that SRF-Nn and SRF-Kn based on nano biochar had more diverse functional groups, including hydroxyl (-OH), carbonyl (C=O), and carboxyl (-COOH), which could increase cation exchange capacity and nutrient adsorption capacity. Ball milling produced nano biochar with an average size of 40.42 nm, where 92.37% of particles were 1–100 nm. The morphology of nano biochar showed a smoother surface, more even pore distribution, and a larger surface area than conventional biochar and impregnated biochar. SRF-Nn and SRF-Kn with 2% alginate formed a more stable and homogeneous structure. XRD analysis showed that nano biochar had a higher crystallinity level with a dominance of quartz (SiO₂) and calcite (CaCO₃). This content increased the adsorption capacity and stability of the nanostructure, thus potentially increasing the efficiency of nutrient release gradually.