Phosphate absorption of red and/or yellow colored soil materials in relation to the characteristics of free oxides

Abstract

The contribution of various forms of free oxides and exchangeable cations to phosphate absorption was evaluated by simple, partial, and multiple correlation analyses. Among the following components, i.e., Fe and Al soluble in dithionite-citrate-bicarbonate (DCB), Fe and Al soluble in acid ammonium oxalate (oxalate), and exchangeable Ca, Mg, and Al, the content of oxalate soluble Al was highly correlated with the phosphate absorption coefficient (PAC) (r =0.916), followed by that of oxalate soluble Fe (0.749), DCB soluble Al (0.65) and exchangeable Al (0.511). The fact that the content of oxalate soluble Al is highly correlated with pH(NaF) indicates that active hydroxy Al is the main source of phosphate absorption of red and/or yellow colored soil materials. Multiple regression analysis gave the following equation; PAC=0.77(Alo -ex.AI)+0.02(Fed -Feo)+0.20ex.Mg+ 1.02ex.AI+0.98ex.Ca+O.02 R=O.96 (The values of all the variables were expressed in mmol (element)/g soil.) Since phosphate absorption can be expressed as; PAC/(Fed+Ald) can be considered to represent the reactivity of free oxides toward phosphate absorption or considered as an intensity factor of phosphate absorption, and (Fed+Ald), as a capacity factor of phosphate absorption. The high positive correlation between PAC/(Fed+Ald) and (Feo+Alo)/(Fed+Ald) and their negative correlations with the weathering index (WI) indicate that the activity of free oxides and their ability to absorb phosphate decreased in the course of weathering. © Taylor & Francis Group, LLC.