Distribution and classification of soils with clay-enriched horizons in the USA

a r t i c l e i n f o In Soil Taxonomy three diagnostic subsurface horizons reflect clay enrichment: the argillic, kandic, and natric horizons. Clay illuviation is recognized in Soil Taxonomy at some level in 10 of the 12 orders, including the percent of the soil series in the USA contain taxonomically defined clay-enriched horizons. However, many other soils contain Bt horizons that do not qualify as an argillic or related horizons. Several soil-forming factors are important in their development, including udic and ustic soil climates, lithological discontinuities, parent materials enriched in carbonate-free clays and coarse fragments, well-drained conditions, backslopes rather than eroding shoulders, and a time interval of N 2000 yr or more. The genesis of argillic, kandic, and natric horizons is also dependent on electrolyte concentration, the amount and distribution of precipitation, clay charge, and microfabric. Nearly all classification systems recognize clay-enriched subsoils at a high hierarchical level. Some of the most productive soils in the World for food and fiber production have clay-enriched horizons. Clay-enriched horizons are important for the nutrient status of soils, water retention, and geomorphic stability (Hopkins and Franzen, 2003). In Soil Taxonomy (ST) (Soil Survey Staff, 2010), Alfisols and Ultisols are defined on the basis of clay-enriched horizons and many Aridisols and Mollisols have clay-enriched subsoils. Argillic and related horizons have been particularly important in soil stratigraphy, relative dating, pedodiversity studies, and climate-change research reviewed the genesis of soils with argillic and related horizons, focusing on field and laboratory data, thin-section and scanning electron microscope (SEM) analysis, and mass-balance studies. In summary, clay-enriched subsoils are the result of translocation, in situ formation, and relative loss of clay from the topsoil. The objectives of this study are to (i) identify the soil taxa in ST featuring clay enrichment; (ii) show the distribution of clay-enriched soils in the USA with clay enrichment; (iii) discuss the relative importance of the soil-forming factors on the development of clay-enriched horizons; and (iv) compare and contrast the pedogenetic processes involved in forming argillic, kandic, and natric horizons. 2. Historical overview of clay-enriched horizons That fine soil particles moved through the soil profile was recognized as early as the late 1800s (King, 1895; Sibirtsev, 1900). The importance of clay was stressed by Hilgard (1906), who reviewed the physico-chemical properties in relation to soil development and plant growth. At that time, no size boundary for these fine particles …