Phosphate sorption and desorption by Thai upland soils

Abstract

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.