The distribution area of mineral soil in Indonesia reaches 148 million ha. The acid mineral soil in South Kalimantan for Ultisol reaches 886.186 ha. Ultisols have low fertility and P availability. The low availability of P in the soil is caused by P being absorbed. Isothermal absorption is very useful for measuring P adsorption in the soil due to strong P fixation by Fe and Al in Ultisol soils. Many researchers have suggested determining the amount of P required to reach an optimum level for maximum crop yield to quantitatively describe isothermal adsorption, the most popular of which are Langmuir, Freundlich, and the Brunauer-Emmett-Teller (BET) equations. The purpose of this study was to determine the isothermal adsorption limit of P and describe quantitatively the isothermal adsorption in Ultisols in Banjarbaru City using the Langmuir, Freundlich, and BET equations. From the research results, it can be concluded that P adsorption on Ultisols in Gunung Kupang Village, Cempaka District, Banjarbaru City follows the BET Equation. The high and low P adsorption indicated the availability of P. In other words, the fixation activity of acid mineral soils could result in a smaller amount of available P for plants.