The low frequency (acoustic) oscillations of an ionic crystal under pulse loading are considered. Considering the large contribution of the Coulomb energy to the binding energy for ionic crystals, the dynamic equations of the oscillations are written using the Maxwell stress tensor determined by the second moment «EiEk» of the electromagnetic stress field. The analysis of Maxwell's and Euler's equations allows us to establish the dependence of the elastic moduli on the RMS value of the electromagnetic stress field in ionic crystals. At the initial moment of stress, this value is equal to zero and we obtain the value of the volume modulus of the liquid, which is confirmed by experiment.