Sombric Horizon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 246

other Fe percentage

(by ammonium oxalate) totaling 0.50 or more and half that amount or less in an
overlying umbric

requirements, spodic materials require an “Al +

½

, ochric epipedon, or albic horizon” (Soil Survey Staff 2010 ,
p. 21). Soil materials with andic soil properties must have, among other properties,
an Al +

...

Fe % (by ammonium oxalate) equal to 2.0 % or more, or 0.4 % or more
when the volcanic glass content is 5 % or more. To eliminate the confusion among
the sombric horizon, spodic materials, and andic soil properties, the sombric
horizon could be required to have an Al +

½

Fe % (by ammonium oxalate) less
than 0.50 % where volcanic glass is present in quantities less than 5 %, and less
than 0.40 % where volcanic glass is 5 % or more. This would be preferable to the
present requirement that the sombric horizon “contains illuvial humus that is [not]
associated with aluminum, as the humus in the spodic horizon . . .” (Soil Survey
Staff 2010 , p. 14; see also IUSS Working Group WRB 2007 , p. 35). Future work
should include a search for soils with sombric horizons in other mountain plateau
regions of the intertropics. Complete pedon characterizations should be made,
including extractable forms of Fe and Al. The genesis of sombric horizons remains
poorly understood.

½

21.7 Summary

According to ST and the WRB, the sombric horizon is a subsurface horizon with a
darker color and greater organic C than the overlying horizon. Of the 30 pedons
reported as containing a sombric-like or sombric horizon in the literature, 12 fit the
criteria established by ST and the WRB. These soils have a base saturation less than
50 %, show evidence of humus accumulation and have a greater organic C concen-
tration than the horizon above, and do not meet the requirements of a spodic materials
or andic soil properties. The 12 pedons with true sombric horizons were classified as
Sombriudoxes (8 pedons) and Sombrihumults (4 pedons). Soils containing sombric
horizons appear to be restricted to cool, moist high plateaus and mountains of central
Africa, including Burundi, Rwanda, and Congo. The sombric horizon originates from
illuviation of humus, as evidenced by organans and cutans on ped faces, spots, dark
polyhedra, and sombrovites reflecting their progressive development, and possibly
narrow humic acid/fulvic acid ratios. Soils with sombric horizons are derived almost
exclusively from highly weathered materials.

References

Ahrens RJ, Rourke RV (2000) The International Committee on Spodosols (ICOMOD): delibera-
tions and rationale. Soil Surv Horiz 41:6-14
Almeida JA, de Cararo DC, Uberti AAA (2009) Genesis of the sombric horizon in Ultisols
(Red Argisols) in southern Santa Catarina, Brazil. Rev Bras Ciˆnc Solo 33:405-416