Sombric Horizon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 242

21.3 Classification of Soils with a Sombric Horizon
An additional requirement is that the sombric horizon must not underlie an albic
horizon. None of the pedons in Table 21.1 contain an overlying albic horizon.
The definition of a sombric horizon precludes it from being buried; none of the
sombric horizons in Table 21.1 appear to have been buried. A key site requirement
is for soils containing a sombric horizon to be free draining. All of the pedons in
Table 21.1 except pedon 19 appear to be well drained and lack redoximorphic
features or gleying.
According to ST and the WRB, key chemical properties of the sombric horizon
are (1) a base saturation less than 50 %; (2) evidence of humus accumulation
by “commonly” containing “more organic matter than an overlying horizon”
(ST, p. 14) or by having “illuvial humus on ped surfaces or in pores” (WRB,
p. 35); (3) not meeting the requirements of a spodic horizon, i.e., low amounts of
oxalate-extractable Fe and Al; and (4) not meeting the requirements of a natric
horizon, i.e., lacks dispersion by Na. All of the pedons listed in Table 21.1 for which
data were provided have a base saturation less than 50 % except pedons 9 and
possibly 20. The following pedons in Table 21.1 lack a horizon containing more
organic matter than the overlying horizon: pedons 1, 3, 4, and 6, 7, 9-11, 16-17,
23, and 29. Descriptions of these pedons did not indicate the presence of illuvial
humus on ped surfaces or in pores. Therefore, it is unclear as to whether these
pedons contain a true sombric horizon. The mean organic C concentration of
horizons meeting the sombric requirements is 1.3 %, and the mean C concentration
of the overlying horizon is 1.0 %.
Pedons 1 from Mexico and 5 from Brazil meet the requirements of a spodic
horizon and, therefore, cannot have a sombric horizon. Pedons 11 and 12 have andic
soil materials and abundant oxalate-extractable Al; therefore, they are disqualified
from having a sombric horizon. In cases where analyses were provided, none of the
pedons contains high levels of exchangeable Na. These data suggest that 12 of the
30 pedons listed in Table 21.2 meet the requirements of a sombric horizon,
including pedon 13 from Burundi, pedons 14 and 15 from Rwanda, and pedons
18-19, 21-22, and 24-28 from the Congo. Pedons 3, 4, 6, 7, 9, 10, 16, 17, 23,
and 29 contain insufficient evidence for illuvial humus accumulation to warrant
identification of a sombric horizon. Until the definition of the sombric horizon is
improved, these soils should be identified as having a “sombric character” or
“sombric-like” horizon.
Of the 12 pedons with a true sombric horizon, 8 are classified as Sombriudoxes
and 4 as Sombrihumults (Table 21.2 ). In the WRB these soils are classified
primarily as Umbric Ferralsols (Sombric). Although soils containing sombric and
sombric-like horizons have been reported in highlands of Mexico, Amazonia, the
Nilgiri Hills of India, and Rwanda, Burundi, and the Democratic Republic of
Congo, our analysis suggests that only the central African soils qualify as having
a sombric horizon (Table 21.2 ). Based on the literature, soils with sombric horizons
apparently do not occur in mountains of subtropical or tropical Kenya (Muchena
and Sombroek 1983 ), Jamaica (Scholten and Andriesse 1986 ), PR (Beinroth 1982 ),