Use of sorption isotherms to evaluate P requirements of some savannah soils of Western Nigeria

Olufemi J. Ayodele, Raufu A. Sobulo, Akinola A. Agboola


The P sorbed by nine savannah surface soils (0-15 cm) and one subsoil was determined after equilibration in 0.01 M CaCl2 containing various P concentrations for 15 min, 30 min, 1 h, 4 h, 6 h, 12 h, 17 h and 24 h. The capacity of soils to decrease solution P concentrations increased with equilibrium time. Amounts of P sorbed from a solution containing 100 mg P/100 g initially increased rapidly and appeared to attain an equilibrium with the surface soil samples in ? 6 h. The soils had very low P sorption levels when compared with many tropical soils. P sorption at 6 h equilibration time varied between 0.8 to 23 and 3.4 to 42 mg kg-1 soil at 0.2 and 2.0 mg c1m-3 equilibrium concentrations, respectively, and correlated with sorption determined at other shaking periods. Adsorption maximum and P sorption capacity were significantly correlated negatively with pH and with exchangeable acidity and exchangeable and citrate-dithionite oxides of Al and Fe, while clay, organic matter and Bray's P1 available P were not significantly correlated.


Soils; Phosphorus; Sorption levels

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