In this research, the ability of calcareous soil to absorb ammonium was investigated and the effect of compost amendment as a natural organic source on ammonium adsorption was also studied. pH, dissolved ammonium concentrations, ammonium adsorption capacity, and cation exchangeable capacity for soil samples were analyzed in relation to an untreated control and treatment with compost under ambient temperature and different initial ammonium concentrations (10, 20, 30, 40, 50, 70, and 100 mg/L). The amounts of ammonium adsorbed increased with increasing initial concentrations of dissolved ammonium for control and treatment with compost. Models evaluated in this study including Langmuir and Freundlich isotherms. Ammonium adsorption was found to conform to Freundlich equation isotherm more than that with Langmuir. The highest adsorption capacity was exhibited by 100 mg/L added initial ammonium concentration. Adding compost as a source of organic matter enhanced the adsorption capacity of calcareous soil with an increase in the adsorption of 36%, which indicated that the organic matter is a key limiting parameter in the mechanism of ammonium adsorption. Ammonium adsorption by calcareous soils is an important process in identifying the ammonium nitrogen route in soil-water systems.