Environmental Engineering Reference
In-Depth Information
become concentrated in patches and along root channels and other pores where
oxygen enters or remains in the soil. These areas of oxidized Fe are called
redox
concentrations
. Since water movement in these saturated or inundated soils can be
multi-directional, redox depletions and concentrations can occur anywhere in the
soil and have irregular shapes and sizes. Soils that are saturated and contain Fe
2+
at
the time of sampling may change color upon exposure to the air, as Fe
2+
is rapidly
converted to Fe
3+
in the presence of O
2
. Such soils are said to have a
reduced matrix
(Vepraskas
1994
).
While indicators related to Fe or Mn depletion or concentration are the most
common in hydric soils, they cannot form in soils whose parent materials are low in
Fe or Mn. Soils formed in such materials may have low-chroma colors that are not
related to saturation and reduction. For such soils, features formed through accu-
mulation of organic C may be present.
4.9.3 Types of Redoximorphic Features
4.9.3.1
Iron and Manganese Depletions
Formation is similar for Fe and Mn depletions, and both may occur within the same
or adjacent horizons. It is easiest to visualize these features forming around roots
that grow along stable macropores. These are required so that features continue to
enlarge as succeeding roots grow and die along the same macropore. Roots growing
along a structural crack or channel provide an energy source, organic material, that
is needed by the microbes for Fe reduction. When the root dies and the macropore is
filled with water, the bacteria will consume the root tissue and utilize (reduce) O
2
in
the water if soil temperatures are high enough for the bacteria to be active. The
newly formed bleached layer where Fe and Mn have been removed along the
channels is a redox depletion, specifically an Fe depletion due to its lower content
of Fe and Mn.
4.9.3.2 Masses, Nodules and Concretions
When a horizon has been repeatedly saturated, reduced, and drained, Fe masses will
form where air penetrates into the horizon slowly to oxidize reduced Mn and Fe
ions. Nodules and concretions are believed to form when air penetrates quickly,
perhaps at a point into the wet matrix containing Fe
2+
and Mn
+2
.
4.9.3.3 Reduced Matrices
A reduced matrix forms simply by the reduction of Fe in the soil. This requires that
the soil horizon be saturated to exclude air for a long enough period of time such
that Fe reduction occurs. Reduced matrices can only occur where soluble organic
matter is present and microorganisms are active.
