In this paper, the important properties required to model moisture transfer in granular porous potash fertilizer by capillarity, gravity, and diffusion within a particle bed (i.e., porosity, permeability, specific surface area, and irreducible saturation) are investigated experimentally and theoretically for narrow ranges of particle size. Special attention is directed toward minimizing the uncertainty of each measurement and calculation. The irreducible saturation level (or moisture content) was deduced by using experimental data and theoretical/numerical simulations of moisture movement by capillarity, gravity, and diffusion within a granular potash bed. It is shown that, for a mixture with a wide range of particle sizes, the potash bed properties can be predicted from the properties for each narrow range of particle size in the mixture.