Adsorption Isotherms Behaviors of Phosphorus with Langmuir Equation for Some Acid Soils in South Kalimantan, Indonesia

Abstract

Background: Phosphorus availability in acidic mineral soils is often limited due to strong adsorption onto iron and aluminum oxides, reducing its effectiveness for plant uptake. Understanding phosphorus adsorption behavior in different soil types is essential for improving fertilizer efficiency. This study applies the Langmuir isotherm model to compare phosphorus sorption characteristics in Oxisols, Inceptisols, and Ultisols of South Kalimantan. Aims: To examine the natural characteristics of adsorption in newly opened paddy fields on the solubility of macronutrient P by using the adsorption isotherm equation. Study Design: This research was a laboratory incubation study using soil samples from three different soils in South Kalimantan Province, Indonesia. Place and Duration of Study: Soil samples were from three separate locations (Oxisols, Tanah Laut Regency), Inceptisols (Barito Kuala Regency), and Ultisols (Banjarbaru City). The research was conducted from January to May 2025. Methodology: After soil preparation, the adsorption isotherm of P was measured using the established method using six soil samples with three replicates.  Added P (0, 2, 5, 10, 15, 30, and 50 mg P kg-1) from KH2PO4 were applied in 1 g soil in 25 mL 0.01 M CaCl2.  The suspension was shaken in a 50 mL centrifuge tube at 25 ℃ for 17 hours.  The tube was centrifuged at 3,000 rpm for five minutes, and the supernatant was filtered (Whatman #42) for P analysis.  P remained in the solution after equilibration was measured.  The common adsorption isotherm equation (Langmuir) was used to fit the adsorption data and to compare their behaviors among the soils. Results: The comparison of Langmuir equation variables in three soil types (Oxisols, Inceptisols, and Ultisols) demonstrated the importance of soil chemical characteristics influencing P cycle in soils.  This study found that the maximum P sorption capacity (xm) follows the order Inceptisols > Oxisols > Ultisols, indicating significant variability driven by intrinsic soil properties. Phosphorus adsorption was most efficient at low to medium dissolved P concentrations, following the Langmuir isotherm model.  Environmental aspects and plant interactions are necessary to assess the relevance of P in acid mineral soils such as leaching risk and long-term sorption-desorption dynamics. Conclusion: The maximum P sorption capacity (xm) follows the order Inceptisols > Oxisols > Ultisols, indicating significant variability driven by inherent soil properties, especially chemical properties.  Further implication may also be useful to generate recommendation doses of precision fertilization in such soils.