Revisiting the definitions of the sombric horizon in soil taxonomy and world reference base for soil resources

A review of the literature suggests that the sombric horizon (from French sombre, dark) was established in Soil Taxonomy (ST) and the World Reference Base for Soil Resources (WRB) from limited data and without a clear understanding of how this horizon forms. This paper reviews data on sombric horizons, evaluates four hypotheses regarding their origin, and offers suggestions for improving the identification of sombric horizons. Of the 30 pedons recognized in the literature as having sombric or sombric-like horizons, 12 fully satisfied the existing criteria in ST and the WRB. Soils with a true sombric horizon may be restricted to the highlands of central Africa (Burundi, Rwanda, Congo) on relatively cool (mean annual air temperature 16–20 °C), moist (mean annual precipitation 1450–2000 mm) plateaus and mountains at elevations ranging from 1450 to 2000 m. Soils with a sombric horizon occur primarily on highly weathered materials from a variety of crystalline rocks. The surface of the sombric horizon occurs at depths of 40 to 110 cm from the surface (average = 76 cm) and ranges from 27 to 100 cm in thickness (average = 63 cm). The sombric horizon commonly is dark reddish brown (5YR 3/3), acidic (average pH = 4.7), low in exchangeable bases (average base saturation = 4%), high in organic C (average = 1.3%), and despite abundant clay (average = 56%) has a low cation-exchange capacity (average = 12 cmol(+)/kg soil). Based on existing data, the sombric horizon contains humus that has migrated downward in the soil, possibly in response to climate and vegetation change. Sombric horizons are not to be confused with sombric-like horizons which may contain andic soil properties or spodic materials. In Soil Taxonomy, soils with sombric horizons are classified primarily as Sombriudoxes (8 pedons) and Sombrihumults (4 pedons). In the World Reference Base for Soil Resources, sombric horizons occur primarily in Umbric Ferralsols (Sombric).