Genesis of placic horizons in Andisols from Terceira Island Azores—Portugal

The placic horizon is a common feature of Andisols developed under perudic conditions in Terceira Island (Azores). This paper reports morphological, physical and chemical characteristics of three pedons. Placic horizons developed above weakly cemented Bs/BC horizons are approximately 10 mm thick with abrupt and wavy boundaries in two of the pedons (Carvمo and Lavaçal) and up to 20 mm thick in the third (S. Barbara), which shows a gradual lower boundary. Active Al and Fe contents are small in the upper horizons of all three pedons and are mostly complexed with humus, but the placic horizons are Fe rich. In thin sections, a typical component is egg-yellow or red limpid isotropic material, sometimes with a globular aspect. This is apparently superposed on the original micromass, whose amorphous character and limpidity without any identifiable cracked dark coatings suggests that the iron is in the form of an (hydr)oxide rather than complexed with organic matter. Ferrihydrite is indicated by chemical analysis. Formation of the three placic horizons is attributed mainly to reduction in organic-rich horizons under wet conditions, followed by reoxidation in the drier soil horizon below.