Abstract | Reddish brown horizons of silicate clay accumulation occur in various desert soils of a study area stride the Rio Grande Valley of southern New Mexico (fig. 1). These horizons of clay accumulation are considered to be argillic horizons, although clay skins on ped surfaces and on pore walls are usually absent. Presence of clay skins on soil features that formed in place, such as peds, indicates clay movement subsequent to parent material deposition. Thus, clay skins demonstrate illuviation and are an important marker of an argillic horizon (15). In this desert area, however, clay skins must be waived as a requirement for argillic horizons. Instead, the features characteristic of the argillic horizon are coatings of oriented clay on sand grains and pebbles, or bodies of oriented clay within peds, or both. Illustrative soils have been selected to demonstrate the range in morphology of the argillic horizon as influenced by texture, authigenic carbonate, and soil age. Reasons for the absence of clay skins are discussed. |