Aluminium species and pH in soil solution under different crops

Abstract

The concentration of monomeric Al in soil solution was estimated using both chrome-azurol-S (CAS) and pyrocat-echol violet (60 sec) (PCV). In some samples the concentrations measured with PCV were greater than with CAS, suggesting PCV was less discriminating against other Al species, such as some less labile AI-organic complexes. Monomeric (CAS) Al ranged from 0 to 1000 pM, and soil solution pH values were as low as 3.6 for an Ultisol in an apple orchard. For Andisols and Inceptisols under pasture and forest, the concentration of monomeric Al was 0 to 20 pM in the soil solution pH range 5.3 to 6.8. In this range the concentration of monomeric (CAS) Al showed no clear trend with increase in pH. The apparent activity of the AI3+ species, however, did decrease to <0.2 pM above pH 5.5. CAS was probably measuring labile AI-organic complexes together with AIOH2+ and Al(OH)t in solution but it is not known ifthese complexes, which were detected above pH 5.5, are phytotoxic.