Submicromorphology of undulating lustrous-faced peds from some clayey soils

Peds bounded by undulating surfaces having a luster or shininess constitute the subsoil of many major soil types worldwide. Characteristics of ped surfaces, such as presence or absence of luster, color, texture, and porosity, are often used as diagnostic criteria for clay coatings at a high category level in soil classification. There is, however, little agreement on either the formation or the physical nature of undulating lustrous ped surfaces. In particular, soil micromorphological studies have demonstrated that clay coatings apparent on ped faces during field examination are often not detectable when using soil thin sections and optical microscopy. Scanning electron microscopy of fractures across the undulating lustrous surfaces of peds from 30 layers of a wide variety of soil types (Alfisols, Mollisols, Vertisols, Ultisols and Oxisols) indicates that these peds were covered by depositional clay coatings. In 12 of the thirty soil layers, these clay coatings were <5 μm thick, and in 11 of the other soil layers these coatings were between 5 and 10 μm thick; these clay coatings would be too thin to be routinely detectable using thin sections and optical microscopy. Clay coatings on ped surfaces were clearly evident in the field (using color and texture as the discriminating characteristics) on only five of the soil layers examined: The thickest clay coatings (up to 50 μm) were found around these peds. Some of the thin depositional clay coatings around the peds in two of the soil layers examined were deformed by stress.