Despite the fact that the growth and dissolution of sparingly soluble minerals in aquatic and biological systems usually occur under nonstoichiometric conditions, little attention has been paid to the effect of solution ionic ratio on the rates. Equations for kink density along an atomic step of an AB crystal with NaCl lattice in super- and undersaturated solutions have been derived using a kinetic approach previously developed for a Kossel crystal. The equations reduce to that obtained thermodynamically at solubility equilibrium. The rate of step movement can subsequently be calculated. For growth and dissolution controlled by integration and detachment, respectively, it is shown that the rate cannot be defined solely in terms of the ionic activity product; it also depends upon the activity ratio and the relative integration frequencies of the lattice ions. At a given thermodynamic driving force, a maximum rate occurs at a certain ratio of lattice ion activities. The present theory is expected to be applicable to sparingly soluble salts, which are characterized by a low kink density. The effect of solution stoichiometry is also discussed for diffusion-controlled crystal growth and dissolution.