Naturally occurring polymers such as organic matter have been known to inhibit aggregation and promote mobility of suspensions in soil environments by imparting steric stability. This increase in mobility can significantly reduce the water filtering capacity of soils, thus jeopardizing a primary function of the vadose zone. Improvements to classic filtration theory have been made to account for the known decrease in attachment efficiency of electrostatically stabilized particles, and more recently, of sterically stabilized particles traveling through simple and saturated porous media. In the absence of an established unsaturated transport expression, and in the absence of applicable theoretical approaches for suspensions with asymmetric and nonindifferent electrolytes, this study presents an empirical correlation to predict attachment efficiency (α) for electrosterically stabilized suspensions in unsaturated systems in the presence of nonideal electrolytes. We show that existing models fall short in estimating polymer-coated colloid deposition in unsaturated media. This deficiency is expected given that the models were developed for saturated conditions where the mechanisms controlling colloid deposition are significantly different. A new correlation is derived from unsaturated transport data and direct characterization of microspheres coated with natural organic matter over a range of pH and CaCl2 concentrations. The improvements to existing transport models include the following: adjustment for a restricted liquid-phase in the medium, development of a quantitative term to account for unsaturated transport phenomena, and adjustments in the relative contribution of steric stability parameters based on direct measurements of the adsorbed polymer layer characteristics. Differences in model formulation for correlations designed for saturated systems and the newly proposed correlation for unsaturated systems are discussed, and the performance of the new model against a comprehensive set of experimental observations is evaluated.