A micromorphological study of silt concentrations in some Irish Podzols

Abstract

Concentrations of silt-sized materials occurring in some of the lower solum and C horizons of seven Podzols (Spodosols) in Ireland were studied in thin section by light microscopy. They are most common and distinct in granite-derived soils, least so in dolerite-derived soils and of intermediate occurrence in schist- and shale-derived Podzols. The silt concentrations occur as bands/zones/layers of predominantly silt-sized grains adhering to, surrounding or connecting sand grains and particles of grit. The silt may be sorted or unsorted; if sorted, the finest silt is nearly always contiguous to the grain surface with progressive or step-wise increases in grain size into the bulk matrix. Additionally, individual silty zones or individual layers within zones contrast in one or more of the following properties: colour, mineralogy, packing density, grain orientation and degree of weathering. In general, silt zones are brownish in colour, measure 200–600 μm in thickness, are densely packed, and the long-axes grains are usually oriented more or less parallel to the supporting surfaces. Silt concentrations are best developed in B3x and Cx horizons, show progressive incorporation into micropeds in diffuse B horizons, are fossilized by iron oxides in placic horizons and are bleached and indistinct in eluvial horizons. They are thought to be responsible for the fragipan characteristics of the granite-derived soils and to owe their origin to cryogenic processes which included alternating freeze/thaw conditions, icelens information, frost-heaving, etc., during the Pleistocene era. They are assumed to have undergone progressive destruction and disturbance with advancing pedogenesis.