Development of sustainable slow-release mineral fertilizer systems from agricultural wastes are of great significance. Struvite-K (MgKPO4·6H2O), a struvite (MgNH4PO4·6H2O) analogue, has received much attention in recent years due to its low water-solubility and fertilizing properties, which offer important environmental and economic benefits in agriculture. Use of slow-release organic fertilizers greatly improves availability of nutrients by agricultural crops and minimize fertilizer losses and the risks associated with contamination of the environment. This study reports development of a slow-release fertilizer system consisting of chitosan, oleic acid and struvite-K crystals. In the study chitosan, a biodegradable polysaccharide polymer, was used to encapsulate oleic acid-coated struvite-K crystals derived from pumpkin (Cucurbita pepo L.) pulp waste. The capsules were characterized using FT-IR, SEM and SEM–EDX analyses. K release profiles of the synthesized capsules were tested in acidic, neutral, and alkaline conditions. The K release rate of oleic acid-treated capsules was 4% at the end of the 10th day. Chitosan-oleic acid-struvite-K capsules can be tested as slow-release mineral fertilizer in sustainable agriculture activities.