This paper presents the solubility analysis of potash fertilizer at thermodynamic equilibrium. The Pitzer equations applicable to a KCl-NaCl-MgCl2-H2O system are developed to describe the interaction between the aqueous ions. The equilibrium constant method is used to analyze the dissolution reaction equilibrium. A critical relative humidity around 52% is obtained which characterizes the moisture-potash interaction mechanism as adsorption or condensation. Three distinct stages of dissolution exist for typical potash products that depend on moisture and temperature, with surface brine saturated with three salts (sylvite, halite and carnallite), two salts (sylvite and halite), and single salt (sylvite), respectively. At high relative humidities (i.e., RH > 75%), dissolution depends on the temperature, and moisture content may exceed 3% and dissolution of sylvite can increase without a limit.