We have studied the interaction of K atoms with the surface of polycrystalline alkaline-earth metal oxides (MgO, CaC, SrO) by means of CW- and Pulsed-EPR, UV-Vis-NIR spectroscopies and DFT cluster model calculations. The K adsorption site is proposed to be an anionic reverse corner formed at the intersection of two steps, where K binds by more than 1 eV, resulting in thermally stable species up to about 100 K. The bonding has small covalent and large polarization contributions, and the K atom remains neutral, With one unpaired electron in the valence shell. The interaction results in strong modifications of the K electronic wave function which are directly reflected by the hyperfine coupling constant, (K)a(iso). This is found to be a very efficient "probe" to measure the degree of metal-oxide interaction which directly depends on the Substrate basicity. These results provide an original and general model of the early stages of the metal-support interaction in the case of ionic oxides.