Sustainable slow-release fertilizers have been reported as environmentally friendly alternatives to highly soluble commercial products. Their main advantages are that they dissolve and release nutrients into soils in a way that assures bioavailability of nutrients to plants over a long period of growth. In addition, novel formulations can reduce or eliminate environmental problems caused by excess use of conventional fertilizers, such as eutrophication and atmospheric pollution. In this study, the solid-state mechanochemical activation method was used to prepare potential fertilizers by milling montmorillonite (MMT) or talc with K2HPO4. Characterizations by several instrumental techniques evidenced phase transformations, while kinetic studies and modeling indicated promising release performance. Even though the potassium release behavior was similar for both systems, the kinetic studies showed that phosphorus release profiles were different. Since potassium struvite (K-struvite—MgKPO4·6H2O) was formed during the release experiments, talc based potential slow-release fertilizers displayed slower release behavior compared to MMT.