Phosphate sorption and desorption by Thai upland soils

Phosphate (P) sorption and desorption characteristics have been investigated for sixty-two representative samples of the major Thai upland agricultural soils which are Oxisols and Ultisols formed on diverse parent materials and under various climatic conditions. P sorption characteristics are well described by Langmuir and Freundlich equations. Values of Langmuir P sorption maximum (Xm) and b coefficient range from 47 to 1250 µg g− 1 soil and 0.5 to 30 mL µg− 1, respectively and the Freundlich k varies between 12 and 1694 µg g− 1 and B coefficient 0.09 and 0.67. Soils derived from basalt have larger values of P sorption maximum than soils on other parent materials. Both P sorption maximum and Freundlich k are closely related to pH (NaF), SSA, total titanium, aluminum and iron, dithionite and oxalate extractable Fe and Al, and other soil properties that reflect specific surface area. Soil pH in NaF is highly effective in predicting the P sorption capacity of Thai upland soils explaining 79% of the variation in P sorption by these soils. P desorption curves for adsorbed P are well described by the Freundlich equation. The Freundlich kd coefficient for P desorption is highly, linearly related to the Freundlich k coefficient of P sorption indicating that the above soil factors that contribute to P sorption similarly affect P desorption.