Sombric Horizon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 231

in Soil Taxonomy (Soil Survey Staff 2010 )andtheWRBforSoilResources
(IUSS Working Group WRB 2007 ) as to whether or not sombric horizons were
indeed present.

21.2 Properties of Sombric Horizons

In ST and WRB, the sombric horizon is a dark-colored, strongly leached, subsurface,
mineral horizon that is enriched in illuvial organic carbon (Table 21.1 ). Originally
observed by Ruhe ( 1954 , 1956 ), soils with sombric or sombric-like horizons have
been reported in well-drained landscape positions on cool, moist high plateaus and
mountains of tropical and subtropical regions (Beinroth et al. 1996 ), including central
Africa (Sys et al. 1961 ;Sys 1972 ; Frankart 1983 ;Mutwewingabo 1989 ;van
Wambeke 1961 , 1992 ; Nizeyimana and Bicki 1992 ), South America (Volkoff and
Cerri 1987 ; Faivre 1990 ; Schaefer et al. 2002 ; Almeida et al. 2009 ), and southern
India (Caner et al. 2003 , 2007 ). The sombric horizon must be in a position so that it
can be traced laterally and it must more or less follow the topography of the ground
surface (Soil Survey Staff 2010 ; IUSS Working Group WRB 2007 ); therefore, buried
A horizons do not qualify as sombric horizons. In the literature the sombric horizon is
variously designated as a subsurface, altered A horizon (Caner et al. 2003 ), an AB
and/or BA horizon (Almeida et al. 2009 ), a Bh horizon (Nizeyimana and Bicki 1992 ),
an Hsom horizon (Frankart 1983 ), a Bso horizon (Sys 1972 ), or an S (for sombrique)
horizon (Faivre 1990 ). The sombric horizon is continuously traceable in this roadcut
showing a Sombriudox in Rwanda (Fig. 21.1 ). A more detailed image of a sombric
horizon from Rwanda is given in Fig. 21.2 .
Although the central concept of a sombric horizon is a somewhat continuous
subsurface horizon, some investigators have recognized “spots” and horizons with
“dark polyhedra” in early stages of sombric horizon development (Frankart 1983 ).
The “spots” sombric horizon is a subsurface horizon with diffuse, discontinuous,
and unequal impregnation of humic substances in the subsoil and/or some peds. The
sombric horizon with dark polyhedra is characterized by the presence of variable
quantities of dark blocky structural units. Eswaran ( 1988 ) described “sombrovites”
as tongues of sombric material occupying more than half of the horizon that extends
into the underlying saprolite.
As originally defined by Sys et al. ( 1961 ) and Frankart ( 1983 ), amplified by
Eswaran ( 1988 ), and eventually adopted by ST and the WRB, the sombric horizon
is a subsurface horizon that is dark colored and has a lower chroma and/or value than
the overlying horizon. Many of the horizons that qualified as sombric in the present
study have a dark reddish brown (5YR 3/3) color (Table 21.1 ). Soil Taxonomy does
not have a thickness requirement for the sombric horizon. The WRB requires that the
sombric horizon be 15 cm in thickness. All of the designated sombric horizons in

Table 21.2 meet this requirement. Horizons qualifying as sombric ranged from 27 to
100 cm in thickness and averaged 63 cm. The depth to the surface of the sombric
horizon ranged from 40 to 110 cm and averaged 76 cm.

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