Environmental Engineering Reference
In-Depth Information
Table 4.2 Element reduction sequence in inundated soils
Eh threshold, mv
Element
Oxidized form
Reduced form(s)
+350
Oxygen
O
2
H
2
O
NO
3
N
2
O, NO
2
+220
Nitrogen
Mn
+4
Mn
+2
+200
Manganese
Fe
+3
Fe
+2
+120
Iron
SO
4
2
150
Sulfur
H
2
S
250
Carbon
CO
2
CH
4
3. Anthric saturation-paddy soil with a created perched water table. Like
episaturation but must occur under controlled flooding, for example, wetland
rice or cranberries.
It should be noted that the term
water table
is not used in the definition of
saturation. Also, horizons within the capillary fringe are technically not considered
saturated since this contains soil water that has pressures less than atmospheric
pressure. Under ideal circumstances, horizons that are saturated by the above
criteria will have all their soil pores filled with water. However, for a horizon to
be considered saturated it is not necessary that all pores be filled with water.
Horizons that have soil water pressures of zero or positive are considered saturated
even if they contain entrapped air in some pores. Saturation can occur at any time
during the year.
4.9.1.2 Anaerobiosis
When aerobic conditions exist, bacteria decompose organic matter and consume O
2
in soil pores containing air. Under anaerobic conditions, bacteria decompose
organic matter by consuming dissolved O
2
until it is gone. At this point, the soil
water is reduced. The bacteria continue to consume organic matter, but at a slower
rate. They produce organic chemicals that reduce nitrates (NO
3
) and minerals,
including Fe and Mn oxides. The sequence is shown in Table
4.2
. While nitrate
reduction is the first indication of anaerobic conditions, it does not leave a visible
indicator that can be used for the easy identification of a hydric soil.
4.9.1.3
Iron and Manganese Reduction, Translocation,
and Accumulation
Both oxidized Fe and Mn can be chemically reduced under certain soil conditions.
Reduction occurs when oxidized forms of Fe (ferric, Fe
3+
) or Mn (manganic, Mn
3+
or Mn
4+
) accepts electrons from another source such as organic matter to produce
ferrous Fe (Fe
2+
) and manganous Mn (Mn
+2
). When these elements are reduced in a
soil, several processes occur: (1) Fe and Mn oxide minerals begin to dissolve in
water; (2) the soil colors change to gray; and (3) Fe
2+
and Mn
+2
ions diffuse through
