PHOSPHATE SORPTION BY THAI RED OXISOLS AND RED ULTISOLS.

Phosphate sorption characteristics of 28 samples of red Oxisols and red Ultisols from agricultural areas of Thailand were described by using Langmuir and Freundlich equations. The soils were three Typic Kandiudults, a Typic Paleustult, a Typic Kandiustult, a Typic Kandiustox, three Rhodic Kandiustox, two Rhodic Kandiudox, two Typic Kandiudox, and a Kandiudalfic Eutrudox. Values of Langmuir P maximum (Xm) ranged from 18 to 1111 mg kg^-1 soil and the Freundlich k coefficient varied between 9 and 652 mg kg^-1. Many soil properties were closely correlated with these measures of P sorption capacity including total titanium, goethite, total iron, amorphous aluminum, and specific surface area. Goethite, microcrystalline kaolin, and amorphous forms of Fe and Al oxides were major contributors to P sorption by these soils. The Freundlich B coefficient, which is a measure of P bonding energy, was weakly related to clay content but there were no close relationships between the equivalent Langmuir b coefficient and soil properties. Soil pH in 1 M NaF (pH 8.0) was highly predictive of P sorption and may therefore be used as a quick economical field test for evaluating P fertilizer requirement.