Soil phosphorus adsorption characteristics and kinetics under different land use types: The case of Dire Inchine District, West Showa Zone, Oromia, Ethiopia

Abstract

Soil phosphorus is an essential plant nutrient, and its insufficiency severely limits crop yields. However, its adsorption study plays a vital role in devising appropriate soil phosphorus, “P”, management practices for its use proficiency. This study was initiated to evaluate phosphorous adsorption characteristics and kinetics of soil of three different land uses.  A representative topsoil samples (0-20 cm) depth from the forest, grazing, and cultivated lands were collected and weighed 1g of composited soil samples were equilibrated in 20 ml of 0.01 M CaCl₂ solution containing KH₂PO₄ at rates of( 0, 5, 25, 100, 150 and 250) mg P L^-1 for assessment of Langmuir adsorption isotherm using batch type experiment. Then, the equilibration time was deduced from the kinetic experiment and fixed at 5 hours. The textural class of soils of forest land was clayey loam, whereas that of grazing and cultivated lands was clayey. Soil pH values under the three land uses were found to be strongly acidic. The experimental soils differed considerably in sorption characteristics under the three land uses. The highest mean value of the product of biding energy and sorption maximum (Kq_max) was recorded in the soil of cultivated land (1.08 Lmg^-1) followed by forest land soil (0.7313 Lmg^-1), while the lowest was recorded for the soil of grazing land (0.672 Lmg^-1), indicating considerable variation soil phosphorus adsorption characteristics under three land-uses. The value of the regression equation (R²) indicated that higher values (R² > 0.9864) have better soil binding strength than the others. Thus, the good fitness of the model was ascertained by witnessing the R² values. The kinetic data showed that the values of concentration of soil “P” adsorbed (q_e) under the three land uses were closer and the R² values were comparably lower and almost analogous for the pseudo 1^st order kinetic model than the pseudo 2^nd order model. Soil “P” sorption kinetics was fast in the first 120 minutes and then showed no change with the increase of the contact time.