Geoscience Reference
In-Depth Information
international system of classii cation for the
whole population of known and unknown soils”
(Soil Classii cation Working Group 1998).
commonly recognized in soils than is manga-
nese. Under anaerobic conditions, soil microbes
reduce iron (Fe
3
+
to Fe
2
+
) and manganese (Mn
4
+
to Mn
2
+
). Ferric iron is highly insoluble, but
ferrous iron is quite mobile and may migrate
easily in solution. Transitions from ferric to
ferrous and back to ferric may take place in
different locations within a soil or at different
times, depending mainly on water level. In this
manner, iron may be relocated such that some
zones become depleted in iron and other zones
are enriched. Ferric iron tends to precipitate as
soft, rusty masses, particularly along root chan-
nels, cracks, and other pores where air is able
to penetrate. Such redoximorphic features are
likely to develop in soils that alternate between
aerobic and anaerobic chemistry.
Microbes likewise reduce sulfate (SO
4
2
−
) to
hydrogen-suli de (H
2
S) gas, which has a strong
“rotten-egg” odor. This olfactory indicator is a
sure sign of a hydric soil (Fig. 5-9). However,
the rotten-egg odor is present only in the wettest
5.3 Hydric soil criteria
Wetlands are areas of predominantly hydric soils
that may support a prevalence of water-loving
plants, known as hydrophytic vegetation, which
are adapted for growing in anaerobic conditions.
Anaerobiosis refers to the combined chemical
and biological processes operating within soils
for which oxygen is depleted. The i eld criteria
for recognizing hydric soils are mostly con-
sequences of anaerobiosis. These criteria are
uniquely associated with wet soils and allow
in-i eld identii cation of hydric soils (Vasilas,
Hurt and Noble 2010). Four key elements are
involved
-
iron, manganese, sulfur, and carbon.
Iron and manganese behave in a similar way
chemically, although iron is more abundant and
Figure 5-9.
Hydrogen-suli de odor is common in salt marshes and other extremely wet environments. View over
Atlantic coast and salt marsh at Moody Division, Rachel Carson National Wildlife Refuge, Maine, United States.
Blimp aerial photo by S.W. Aber, J.S. Aber, and V. Valentine.
