Phosphorus Adsorption at Different Altitudes and Soil Depths in Mt. Anak Krakatau before the December 2018 Eruption: Analysis using the Langmuir Isothermic Model

Abstract Volcanic soils are generally fertile; however, their high phosphorus (P) adsorption, primarily due to amorphous clay minerals like allophane, poses a significant challenge. This study aimed to examine the effects of altitude and soil depth on the maximum phosphorus adsorption capacity (X max ) and the relative bond energy of phosphorus (K L ) in the soils of Mt. Anak Krakatau (Mt. AK) prior to the December 2018 eruption. Soil samples were collected from four profiles at different elevations (Profile 1: ±5 m above sea level (asl), Profile 2: ±50 m asl, Profile 3: ±90 m asl, and Profile 4: ±105 m asl) and two soil depths (topsoil: 0–20 cm and subsoil: 20–40 cm). Phosphorus adsorption was analyzed using the Langmuir Isothermic Model in the Soil Science Laboratory, Faculty of Agriculture, University of Lampung. Data were evaluated using t-tests and correlation analysis. The results indicated that the highest X max was observed in the subsoil at 105 m asl, while the lowest X max was found in the topsoil at 5 m asl. These findings suggest that phosphorus adsorption increases with both elevation and soil depth. Furthermore, Xmax in both topsoil and subsoil showed a negative correlation with P-potential. In the topsoil, K L was negatively correlated with pH, whereas in the subsoil, K L was negatively correlated with available phosphorus.