Sombric horizon: five decades without evolution

Abstract

The sombric horizon is a diagnostic subsurface horizon defined in the soil classification system of the United States (Soil Taxonomy) and FAO (WRB), used to classify the soil at different categorical levels. The sombric horizon has a soil color darker than the overlying surface(s) horizon(s), and must show illuvial humus accumulation features, though they are not associated with aluminum (Al), as in the spodic horizon, nor associated with sodium (Na), as in the natric horizon. There are also criteria to distinguish it from buried A horizons. However, since the first references and proposed concept of the sombric horizon in African soils made by Sys and co-workers in the 1960s, and adopted by the Soil Taxonomy edition of 1975, few modifications have been made to its definition. Moreover, the pedogenic process involved in illuvial humus accumulation in these horizons remains inadequately clarified, making the distinction between the sombric and spodic or buried A horizon difficult and unclear. This review reports the historical evolution of the sombric horizon concept, its definition and inconsistencies under different soil classification systems, and the current hypothesis, together with its fragilities, proposed to explain the soil illuvial humus accumulation. Although it is recognized that further research is necessary, alternative criteria are proposed for the definition of the sombric horizon in the Brazilian System of Soil Classification.

World Reference Base for Soil Resources—Its fourth edition and its history

“…The horizon was introduced to differentiate this type of Fe oxide accumulation from accumulations by other soil‐forming processes like in spodic, plinthic and tsitelic horizons. d)Panpaic horizon: This is a buried A horizon. It is different from the sombric horizon (de Almeida et al., 2015), which is not understood as buried. e)Tsitelic horizon: Accumulation of Fe by subsurface flow, usually from Planosols and Stagnosols further up the landscape. As the limonic horizon, it was introduced to differentiate the various soil‐forming processes leading to accumulations of Fe oxides. f)Protogypsic properties: Accumulation of secondary gypsum, not sufficient for a gypsic or petrogypsic horizon, analogous to the protocalcic properties. g)Aeolic material: Deposited by wind. h)Mulmic material: Mineral material with a high content of soil organic carbon, derived from organic material.…”

Sombric-like horizon and xanthization in polychrome subtropical soils from Southern Brazil: implications for soil classification

“…Sombric and sombric-like horizons are found in well-drained landscape positions with high soil moisture, on high plateaus and/or mountains in tropical and subtropical regions, and several theories exist as to their formation (Almeida et al, 2009;Bockheim, 2012;Almeida et al, 2015). Except for Faivre (1990), sombric horizons described in the literature lack evidence of OM illuviation, and are therefore frequently referred to as sombric-like horizons (De Craene and Laruelle, 1955;Sys et al, 1961;Gouveia et al, 2002;Caner et al, 2003;Almeida et al, 2009;Velasco-Molina et al, 2010).…”