The solution chemistry of podzolic soils from the eastern Canadian shield: a thermodynamic interpretation of the mineral phases controlling soluble Al 3+ andH 4 SiO 4

SUMMARY Thirty‐one soil solutions were extracted by immiscible displacement with CCl 4 under high speed centrifugation from sub‐horizons of three podzolic soils from north‐eastern Ontario, Canada. The solutions were analysed for major cations and anions and a speciation of dissolved Fe and Al was attempted to distinguish ‘free’, ‘organically bound’ and ‘inorganically bound’ species. Results indicated that the Ae (E) horizon solutions were of low pH and contained mainly organically bound Fe and Al. With depth, p Hs increased, ionic strengths decreased and the relative proportion of inorganically bound Fe and Al increased. Although application of phase diagrams permitted only a semi‐quantitative interpretation of the data, all horizon solutions, with the exception of some Ae solutions, appeared supersaturated with respect to likely occurring crystalline and amorphous aluminosilicates [kaolinite, halloysite, allophane (Al:Si=l) and imogolite]. Of the phases considered, reactions involving imogolite‐allophane, gibbsite‐halloysite, gibbsite‐allophane and gibbsite‐imogolite all appeared reasonable in controlling the content of Al 3+ and H 4 SiO 4 in solution, although the presence of gibbsite and imogolite could not be definitely confirmed in these soils.