This study describes the behavior of potential slow-release fertilizers (SRF), prepared by the mechanochemical activation of calcined Mg2Al-CO3 or Mg2Fe-CO3 layered double hydroxides (LDH) mixed with dipotassium hydrogen phosphate (K2HPO4). The effects of LDH thermal treatment on P/K release behavior were investigated. Characterizations of the inorganic composites before and after release experiments combined X-Ray diffraction (XRD), Fourier-transform infra-red spectroscopy (FTIR), solid-state nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The best release profile (<75% in 28 days and at least 75% release) was obtained for MgAl/K2HPO4 (9 h milling, 2:1 molar ratio, MR). Compared to readily used K2HPO4, milling orthophosphate into LDH matrices decreases its solubility and slows down its release, with 60% and 5.4% release after 168 h for MgAl/K2HPO4 and MgFe/K2HPO4 composites, respectively. Mechanochemical addition of carboxymethylcellulose to the LDH/K2HPO4 composites leads to a noticeable improvement of P release properties.