Sand-sized aggregates in Andosols and their role in P retention

Abstract

P was added to 10 Andosol samples and the allocation of P in 3 size-fractions of the fine earth (< 2 mm) was measured. The silt and clay fraction (< 0.02 mm) in general retained more P than either the fine- or coarse-sand fractions, indicating that the P-active sites were more predominant in the finer fraction. In some samples, however, more P was retained in the sand fractions which contained aggregates of secondary minerals resistant to sonication, dithionite-citrate-bicarbonate (DCB) and oxalate treatments. The amount of added P extracted from these sand fractions with Bray's No. 2 solution decreased following dry grinding, but this effect diminished gradually after successive physical and chemical cleanings. TG and DTA revealed that components which had much adsorption water were removed by DCB and oxalate treatments. Electron-probe micro analysis (EPMA) confirmed that the aggregates were composed mainly of Si and Al compounds. From the energy values of Al-Kα, the Al was deduced to be a mixture of fourfold and sixfold coordination. P was distributed through the interior of the aggregates. With cleaning, sixfold coordinated Al decreased and fourfold coordinated Al became dominant, and the ability to retain P decreased. This indicated that the active sites for P were coupled with sixfold coordinated Al. Fine-sand fractions of older tephras contained some aggregates which persisted after cleaning and these retained P adsorption ability even after oxalate treatment.