Argillic

Argillic describes a subsurface soil horizon in which clay minerals have been translocated downward from upper horizons, resulting in an illuvial accumulation of clay. In many soil classification schemes, this horizon is designated Bt and is recognized by evidence of clay illuviation, such as clay skins or argillans on pore surfaces and peds, and a higher clay content than the overlying horizons.

Formation and characteristics: The argillic horizon forms through downward movement of fine clay minerals with percolating water, typically in humid or subhumid climates where time allows illuviation to occur. It is usually located below an E or A horizon and marks a transition to a more clay-rich B horizon. The horizon often exhibits increased cation exchange capacity and, depending on the soil, may show changes in color associated with mineral accumulations. In Alfisols, the Bt horizon commonly has a relatively high base saturation; in Ultisols, the Bt horizon tends to have a low base saturation and more acidic conditions.

Occurrence and significance: Argillic horizons are a key feature of several soil orders, most notably Alfisols and Ultisols, and can occur in other orders that develop substantial illuviation. The presence of an argillic horizon indicates a soil has experienced clay translocation and is typically more structured with higher clay content, which can influence fertility, drainage, and rooting depth. The diagnostic contrast with horizons formed mainly by eluviation (such as an E horizon) helps distinguish illuvial clay accumulation from surface processes.

Terminology note: The term argillic is derived from argilla, meaning clay, and is related to other horizon designations for illuvial clay (Bt) and related subhorizons used in soil taxonomies around the world.