Evaluation of Adsorption Kinetics and Isotherm Models for Phosphorus in Luvisols and Vertisols Under Different Land Uses in Central Ethiopia

Adsorption isotherms and kinetic models are essential for predicting phosphorus (P) adsorption mechanisms, which aid in P fertilizer recommendations. Despite their importance, investigations into effective P fertilizer management and the adsorption kinetics and isotherm models are lacking in the study areas. The study aimed to evaluate adsorption kinetics and isotherm models for P in Luvisols and Vertisols under cultivated, forest, and grazing land. Soil samples were collected from Luvisols and Vertisols under cultivated, forest, and grazing land uses. P adsorption was studied by equilibrating with KH 2 PO 4 at 0, 1, 2, 5, 10, 15, and 20 mg P L −1 . Freundlich and Langmuir models were used to test the P adsorption. For P release kinetics, 10 mL of CaCl 2 (0.01 M) containing 50 mg P L −1 was added. The equilibrium was studied at 0.5, 1, 3, 6, 12, 24, 36, 48, and 72 h. Data were formulated using zero‐order, pseudo‐first‐order, pseudo‐second‐order, power function, Elovich, and intraparticle diffusion kinetic models. The Freundlich model provided the best fit to the data. Freundlich coefficient (Kf) varied 199.2 and 123.1 mg·P·kg −1 (Luvisols), and 202.1 and 120.21 mg·P·kg −1 (Vertisols). External P requirements of Freundlich (EPRf) (kg·P·ha −1 ) for cultivated were 138.4 kg P ha −1 (Luvisol) and 114 kg P ha −1 (Vertisol). Cultivated soils of EPR are higher by 118.4 kg·P·ha −1 (Luvisols) and 94 kg·P·ha −1 (Vertisols) from the current blanket recommendations. The pseudo‐second‐order kinetics model provided the best fit for describing P adsorption. The pseudo‐second‐order ( q e ) cal varied (7.11–22.27 mg·kg −1 ) (Luvisols) and (9.01–31.85 mg·kg −1 ) (Vertisols); ( q e ) exp varied (2.76–8.38 mg·kg −1 ) (Luvisols) and (8.02–16.24 mg·kg −1 ) (Vertisols). In conclusion, this finding further indicates a need to revise the current P fertilizer application rate of 20 kg·P·ha −1 , currently applied across all soil types. However, this revision should only occur after validating the models and the EPR values estimated in this study for each soil in both greenhouse and field environments.