Significance of the presence of allophane and imogolite in podzol bs horizons for podzolization mechanisms: A review

The widely held view that Al and Fe are transported from A2 to B horizons in podzols in the form of fulvate complexes cannot account satisfactorily for the formation of Bs horizons rich in allophane, although this can be the dominant illuvial horizon in Iron Podzols, and a major feature of the B horizon of Iron Humus Podzols and Peaty Podzols. It is argued that the presence of imogolite and of allophanes with imogolite-like structures in such horizons can be explained only by their deposition from hydroxyaluminium silicate (proto-imogolite) sols, which are known to have the necessary chemical and colloidal stability to act as the agent of transport of Al and, in part, Fe. Evidence is reviewed that indicates that Bh horizons form by the migration of organic colloids through the A2 horizon, and their subsequent precipitation on previously deposited allophanes and hydrous iron oxides at the top of the B horizon. Further processes that could lead to the formation of Peaty Podzols with thin iron pan and other podzol types are briefly sketched.