Classification and development of shallow soils (< 50 cm) in the USA

Abstract

In Soil Taxonomy, soils in lithic subgroups have hard rock within 50cm of the surface. An examination of the digital databases of the USDA Natural Resources Conservation Service revealed that soils in lithic subgroups occur in 10 soil orders (excludes Oxisols and Gelisols, for which there were insufficient data) and can be ranked by area: Mollisols>Aridisols, Entisols, Inceptisols>Spodosols, Alfisols>Histosols, Ultisols, Andisols≫Vertisols. This ranking is comparable to the overall ranking of these 10 orders in the USA, except that Aridisols and Mollisols are over-represented while Alfisols and Ultisols are under-represented in lithic subgroups. Seven soil great groups account for nearly two-thirds (65%) of the soils: Calciustolls, Torriorthents, Dystrudepts, Haplargids, Haplocalcids, Haplustolls, and Argixerolls. For mineral soils, the minimum and maximum average depths to bedrock were 23 and 47cm, respectively. Although 47% of the soil series had either a cambic horizon or no diagnostic subsurface horizon, 26% had a calcic horizon, 21% an argillic horizon, and 5% a spodic horizon. Nearly two-thirds (63%) of the soils on an area basis are skeletal, i.e., contain >35% coarse fragments. More than one-quarter (26%) of the soil series have “no competing series,” meaning that they can be considered endemic to the region. Pedogenic processes in lithic soils appear to be analogous to those in soils derived from deeper unconsolidated materials, with the dominant processes being humification, calcification, argilluviation, cambisolization, and podzolization. There is abundant evidence that lithic soils are as developed—if not more developed—than nearby soils derived from deeper unconsolidated sediments. Lithic soils may be important reservoirs of SOC, clays, carbonates, and dust. In that lithic soils are often in upslope or at higher elevations, they play an integral role in landscape evolution.