Nursery container production involves growing plants in organically-based material that contains humic and fulvic acids. Leachates loaded with nitrate and phosphate from container crops may become contaminants due to the low anionic exchange capacity (AEC) of the growing media. Goethite is a variable charge mineral that produces positive surface charge when the pH of the solution is less than zero point of charge (pH0). In this research study, we investigated the adjustment of the variable charge at the surface of Leonardite Humic Acid (LHA), goethite and the mixture, and the effect of phosphate addition on the change of the charge adjustment. The proton adsorption to LHA, goethite and their mixture was examined using potentiometric titrations under different salt concentrations and free concentrations of phosphate and LHA. The pH0 of the mixture decreased linearly with loadings of LHA and phosphate at rates below saturation. Modeling the change of charge adjustment will provide information on manipulating surface charge of the mixture to increase AEC.