SUBMICROMORPHOLOGY OF UNDULATING LUSTROUS-FACED PEDS FROM SOME CLAYEYSOILS

Peds bounded by undulating surfaces having a luster or shininess const itute the subsoil of many major soil types world-wide. Characteristics of ped surfaces, such as presence or absence of luster, color, textur e, and porosity, are often used as diagnostic criteria for clay coatin gs at a high category level in soil classification. There is, however, little agreement on either the formation or the physical nature of un dulating lustrous ped surfaces. In particular, soil micromorphological studies have demonstrated that clay coatings apparent on ped faces du ring field examination are often not detectable when using soil thin s ections and optical microscopy. Scanning electron microscopy of fractu res 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 c lay coatings. In 12 of the thirty soil layers, these clay coatings wer e <5 mum thick, and in 11 of the other soil layers these coatings were between 5 and 10 mum thick; these clay coatings would be too thin to be routinely detectable using thin sections and optical microscopy. Cl ay coatings on ped surfaces were clearly evident in the field (using c olor and texture as the discriminating characteristics) on only five o f the soil layers examined: the thickest clay coatings (up to 50 mum) were found around these peds. Some of the thin depositional clay coati ngs around the peds in two of the soil layers examined were deformed b y stress.