Ammonium adsorption tests were performed to investigate comprehensively the adsorption of ammonium by soil during the land-treatment process of nutrient-enriched wastewater. Accordingly, the conceptual pattern of ammonium adsorption isotherms, as affected by the associated phosphate concentration, and the ratio of ammonium to phosphate and nutrient cations represented by potassium, is clearly illustrated. The findings contribute to a better understanding of the treatment effect and are a useful guide for the design of wastewater land-treatment systems.