MICROMORPHOLOGICAL EVIDENCE OF ILLUVIAL CLAY IN HORIZONS DESIGNATED Bt IN THE FIELD

Abstract

Semi-quantitative counts of areas of oriented, apparently illuvial clay in thin sections of B horizons showed that less than half of the 44 horizons designated Bt in the field had at least 1% apparently illuvial clay, and the counts for more than 1/3 was 0.2% or less. Many of the horizons that did not meet the micromorphological criteria of a Bt (at least 1% illuvial clay) contained much more clay than the associated Ae horizon and a higher proportion of fine clay than the associated C. Most of the soils having Bt horizons with more than 1% apparently illuvial clay were soils of humid to sub-humid regions, Luvisolic soils and Black or Dark Gray subgroups of Chernozemic or Solonetzic soils; none were Brown or Dark Brown Chernozemic or Solonetzic soils. Comparisons of counts of apparently illuvial clay in thin sections by four operators showed that compatibility of results varied among samples and that prior consultation resulted in an increased degree of agreement. Problems of distinguishing illuvial clay in thin sections are discussed and illustrated.