Extractable iron and aluminium predict the P sorption capacity of Thai soils

The objective of this study was to determine if dithionite- and oxalate-extractable Fe and Al can be used to predict the P sorption capacity of Thai soils. Forty-five samples from diverse soil types were taken from surface and subsurface horizons of soils on sandstone, shale/limestone, granite, and basalt. The samples were analysed for P sorption, dithionite- and oxalate-extractable Fe and Al (Fe-d, Fe-o, Al-d, Al-o), specific surface area (SSA), and other soil properties. Generally P sorption data for these soils were slightly better fitted by the Langmuir equation than the Freundlich equation. The Langmuir P sorption maximum ranged from 35 to 1111 mu g/g with a median value of 370 mu g/g soil. Soils developed on basalt had higher values of P sorption maximum (x(m)) (range 400-1111 mu g/g, median 597 mu g/g) than soils on other parent materials. Fed concentrations in soils (4-74 g/kg) were much higher than Feo concentrations (0.2-13.8 g/kg) with values of Fe-o/ Fe-d ranging from 0.01 to 0.28 ( median 0.09), indicating that most of the free iron oxides were crystalline. Amounts of Al-d and Al-o were about equal with median values of 1.6 and 1.0 g/kg, respectively. About 80% of the samples had SSA values <40 m(2)/g. Both the P sorption maximum and Freundlich k were linearly related to SSA (R-2 = 0.77, 0.74), Al-d (R-2 = 0.78, 0.79), Al-o (R-2 = 0.64, 0.74), Fe-d (R-2 = 0.48, 0.41), Fe-o (R-2 = 0.43, 0.72), and clay content (R-2 = 0.48, 0.36). Stepwise regression indicated that 81% of the variability in P sorption by these soils could be explained by a combination of dithionite and oxalate Fe and Al, however, Al-d alone is almost as effective in predicting the P sorption capacity of Thai soils.