Genesis of soils with an abrupt textural contrast in the United States

Identified from the USDA Natural Resources Conservation Service databases, soils with an abrupt textural change (ATC)occur in four orders (Alfisols > Mollisols, Ultisols > Aridisols), 14 suborders and 31 great groups and represent nearly 1000 series in the USA. More than three-quarters (79%) of the soils have an argillic horizon and 21% have a natric horizon. An albic horizon is present in 40% of the soils. More than half (58%) of the soils with an ATC occur in Pale- great groups, suggesting older materials. Soil series with an ATC occur primarily in fine (56%) and clayey (14%) particle-size classes; smectitic (49%) and mixed (40%) mineral classes; mesic (39%) and thermic (33%) soil-temperature classes; and ustic (31%), xeric (27%), and aridic (21%) soil-moisture classes. On average, the texture of the eluvial and illuvial horizons varies by two classes on the soil-textural triangle. Of the several theories accounting for the origin of soils with an ATC, clay illuviation from the eluvial to the illuvial horizon must be considered a major process. More than halfof the soils are derived from alluvium and other stratified materials; residual soils may also contain sedimentary layering, which is important in the development of an ATC. Elevenpercent of the soils have a lithologic discontinuity at the eluvial-illuvial junction, which may influence the development of an ATC. The ATC in soils with an aquic soil-moisture regime (13%), such as the Argialbolls, may reflect plugging of the argillic horizon with fine clays which causes lateral subsurface flow that removes clay from the albic horizon.Climate may be important in the development of ATC in the USA, in that many of the soils are subject to extended dry periods, followed by intense thunderstorms. A third (34%) of the soils with an ATC occur in Texas. There are about 271,000 km2 of soils with an ATC in the USA, which accounts for 3.2% of the total land area.