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between
14.8, implying that the organic matter originated from C4
(grasses) plants, rather than C3 plants (Caner et al.
2003
). Humic/fulvic acid ratios
were 0.8, suggesting that fulvic acids were predominant. From
14
C dating, the mean
residence time of the organic matter in the sombric horizon was 20-21 ky which
matches with the Last Glacial Maximum as measured in many parts of the world.
However, the studies of Caner et al. (
2003
) and Almeida et al. (
2009
) also report
high levels of extractable Fe and Al in associated with the soil organic C in sombric-
like horizons, which is inconsistent with the currently accepted definitions of the
sombric horizon (Soil Survey Staff
2010
; IUSS Working Group WRB
2007
).
The accepted concept of the sombric horizon is that it is an illuvial horizon in
which humus has migrated into the subsoil (Frankart
1983
; Faivre
1990
). Evidence
for this process as accounting for the development of sombric horizons includes
(1) a darker color in the sombric horizon than in the horizon above due to the
accumulation of organic C (Figs.
21.1
and
21.2
); (2) microscopic evidence of
organans on ped faces in the sombric horizon (Frankart
1983
; Eswaran
1988
);
(3) the occurrence of spots, dark polyhedra, and sombrovites reflecting the progres-
sive development of sombric horizons (Frankart
1983
; Eswaran
1988
); (4) more
organic C in the sombric horizon than in the overlying eluvial horizon; and possibly
(5) the occurrence of narrower humic acid/fulvic acid ratios in the sombric horizon
than in the overlying A horizon, reflecting movement of more active forms of
C (Volkoff and Cerri
1987
; Caner et al.
2003
). Finally, soils with sombric horizons
are derived almost exclusively from highly weathered crystalline rocks.
12.9 and
21.6 Proposal for Improving the Definition
of the Sombric Horizon
The following suggestions are offered to improve definitions of the sombric horizon
in ST and WRB. The definition of the sombric horizon could be strengthened
by including a requirement that it must contain at least 0.6 % organic C and that
the level in the sombric horizon be at least 0.2 % more than the overlying horizon
(Eswaran
1988
). A thickness requirement should be added to ST; the WRB requires
that the sombric horizon is 15 cm or more in thickness. ST and the WRB require that
the color value or chroma, or both, be lower than an overlying horizon. The sombric
horizon could be required to have a color value of less than 4 and a chroma of less
than 4 (moist; less than 5.5 dry) and that the value and chroma be darker by one or
unit than the overlying horizon. To recognize the free draining and strongly leached
nature of the sombric horizon, the pH could be required to be 5.0 or lower.
Limits need to be placed on the amounts of oxalate-extractable Fe and Al so that
the sombric horizon is not merely a special kind of spodic horizon or has andic soil
properties. This is especially critical to avoid the proliferation of “sombric-like”
horizons in the literature. The spodic horizon currently is defined as an illuvial
horizon with 85 % or more spodic material (Soil Survey Staff
2010
). In addition to
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