Classification and distribution of soils with lamellae in the USA

a r t i c l e i n f o Lamellae are thin, often discontinuous layers of clay-enriched material that are associated with iron oxides and may occur in Alfisols, Ultisols, Mollisols, Entisols, Inceptisols, or Spodosols. Our analysis of the SSURGO database revealed that there are 118 soil series with lamellae that represent six orders, 14 suborders, and 25 great groups, and 25 subgroups. In Soil Taxonomy, lamellae are identified at the subgroup level, and a horizon is classified as lamellic if the combined thickness of the lamellae is less than 15 cm. Lamellae occur primarily in soils with a mixed mineral class (73%), a sandy or sandy-skeletal textural class (59%), a frigid or cryic soil-temperature regime (59%), and a udic or ustic soil-moisture regime (89%). The lamella (Bt horizon) most commonly has a loamy sand (33% of total pedons), loamy fine sand (33%), or sandy loam (29%) texture and is usually one soil textural class finer than the interlamellae areas. Differences in clay content between the interlamellae areas and Bt average over 5%. Two or more lamellae are common in soils with lamellae, and the thickness of a single lamella commonly ranges between 6 and 22 mm. The depth to the surface of the first lamella averages 72 cm; the maximum depth exceeds 155 cm. The pedogenic origin of lamellae involves clay movement (argilluviation), with clays bridging and coating sand grains and the eventual formation of micro-laminae in response to a varying wetting front. In the USA, soils with lamellae cover 3.6 million ha and occur in 30 states. Lamellae play a role in the flux of water and nutrients in the soil, and have been used as soil stratigraphic markers and relative-age indicators in archeological and soil geomorpho-logical studies. Soils in many parts of the world contain fine-textured pedogenic layers that have been referred to as " fibers " , " clay bands " (Berg, 1984), " texture bands ", where soils containing lamellae are distinguished at the subgroup level. In Soil Taxonomy, a lamella is an illuvial soil horizon less than 7.5 cm thick that " contains an accumulation of oriented silicate clay on or bridging sand and silt grains " (page 18). Lamellae play an important role in the flux and retention of water and nutrients, especially in coarse-textured soils and, therefore, on plant growth (Hannah and Zahner, 1970). Lamellae have been used as soil …