Effects of lime on phosphate sorption characteristics, and exchangeable and soluble phosphate in fifteen acid soils

Abstract

Fifteen acid soils (pH <5.2 in 0.02 M KCl) from eight Great Soil Groups were used to examine the effects of lime on exchangeable and soluble phosphate in relation to changes in soil phosphate sorptivity, affinity and sorption capacity. Soils varied widely in their reactions to liming, which raised the pH to as high as 7.0 in a sandy solodized soil but only to 4.85 in a yellow earth. Liming to a pH of about 4.8 decreased sorptivity and sorption capacity in four soils whose initial pH was <4.5. Higher sorptivity in these unlimed soils was attributed to precipitation by soluble aluminium. Liming to pH values of about 4.8 to 5.8 minimized soluble aluminium, but increased sorption and greatly increased sorptivity and affinity in most soils. As the pH rose above 5.8 sorptivity and affinity decreased, while sorption capacity remained constant or increased. The results suggested that in most soils sorption was dominated by adsorption, but in a sandy solodized soil liming appeared to change the dominant sorption process from precipitation to adsorption. Soluble phosphate was increased by liming in most soils, but these increases were often inconsistent, with small or negative increases in exchangeable and extractable phosphate and large increases in sorptivity. These results suggest that phosphate solubility in some acid soils is controlled more by the dissolution of iron and aluminium phosphates than by the desorption of exchangeable phosphate.