Environmental Engineering Reference
In-Depth Information
or move with the soil water to other parts of the soil horizon or may be leached from
the soil. When Fe and Mn are in their reduced form, they have much less coloring
effect on soil than when they occur in their oxided forms. Of the two, evidence of Fe
reduction is more commonly observed in soils.
4.9.1.4 Sulfate Reduction
Sulfur is one of the last elements to be reduced by microbes in an anaerobic
environment. The microbes convert sulfate (SO
4
2
) to hydrogen sulfide (H
2
S)
gas. This results in a very pronounced “rotten egg” odor in some soils that are
inundated or saturated for very long periods. In unsaturated or non-inundated soils,
SO
4
2
is not reduced and there is no rotten egg odor. The presence of H
2
Sisa
strong indicator of a hydric soil, but this indicator is found only in the wettest sites
in soils that contain S-bearing compounds. It can sometimes be sensed by simply
walking across these areas. This is indicator A4 Hydrogen Sulfide. Caution should
be used when using this as an indicator so that other smells such as those associated
with the decomposition of organic matter are not mistaken for a sulfidic odor.
4.9.1.5 Organic Accumulation
Soil microbes use C compounds found in organic material as an energy source.
However, the rate at which organic C is utilized by soil microbes is considerably
lower in a saturated and anaerobic environment than under aerobic conditions.
Therefore, in saturated soils, partially decomposed organic material may accumu-
late. The result in wetlands is often the development of organic surfaces of varying
thicknesses, such as peat or muck, or dark organic-rich mineral surface layers.
These soils are typically saturated for very long periods of time.
4.9.2 Development of Redoximorphic Features
Redoximorphic features are those formed by the reduction and oxidation of Fe and
Mn compounds in seasonally saturated soils. Fe oxide minerals give the soil red,
brown, yellow, or orange colors depending on which iron minerals are present.
Manganese oxides produce black colors. These oxides tend to coat the surfaces of
the soil particles. Without the oxide coatings, the particles are gray. Areas in the soil
where Fe is reduced often develop characteristic bluish-gray or greenish-gray
colors known as
gley
. Ferric Fe is insoluble but Fe
2+
easily enters the soil solution
and may be moved or translocated to other areas of the soil. Areas that have lost Fe
typically develop characteristic gray or reddish-gray colors and are known as
redox
depletions
. If a soil reverts to an aerobic state, Fe that is in solution will oxidize and
