Soils with fragipans in the USA

Fragipans are dense subsurface soil layers that severely restrict water flow and root penetration. They are considered a pedogenic horizon, but their exact genesis is not well understood. In Soil Taxonomy (2010) a fragipan is defined as a layer that is at least 15cm thick, contains a coarse structure, and is reversibly cemented. We examined 362 pedons with fragipans in the Natural Resources Conservation Service (NRCS) SSURGO database and 36 case studies in the USA from the literature. Soils with fragipans are classified primarily at the great-group but also at the subgroup level. Fragipans occur in four orders (Alfisols, Ultisols, Inceptisols, Spodosols), 10 suborders, 23 great groups, and 57 subgroups. From the NRCS database, fragipan soils have silty or loamy particle-size classes (95% of total soil series), an udic or aquic soil-moisture regime (98%), a mesic soil-temperature regime (60%), and a mixed mineralogy class (75% of total soil series). Although they occur in 28 states in the USA, the most extensive areas of soils with fragipans occur in the southern Mississippi River valley and the central and southern Appalachian Mountains. Soils with fragipans are most common east of the 97°W meridian and in forested areas where precipitation exceeds evaporation. These soils are derived from silt-rich materials, especially loess and low-lime tills (61% of total soil series) and occur primarily in lower, moderately well-drained topographic positions. The genesis of fragipans involves wetting and drying, possibly influenced by the presence of lithologic discontinuities, which lead to hydro-consolidation that causes close-packing of grains and reduces porosity. Although silica may bind clays and Fe oxides and bridge skeletal grains, this property is not recognized in the definition of the fragipan. In the conterminous USA soil mapping units containing soils with fragipans occupy 970,000km2 (12% of the land area).