Environmental Engineering Reference
In-Depth Information
4.9.4 Location of Redoximorphic Features
Pore linings occur along ped surfaces as well as root channels. They are also found
on the roots of living plants that can transport O
2
to their roots in saturated soils
(oxidized rhizospheres). These form by diffusion of Fe
2+
and Mn
+2
ions toward
aerated macropores, where the ions are oxidized adjacent to the macropores and
even on root surfaces. If both Fe and Mn are in solution, the Fe tends to precipitate
first because it will oxidize at a lower Eh value than will Mn. Therefore, pore linings
may appear to consist of clearly separated Mn oxides (in the macropore) and Fe
oxides (in the matrix).
In terms of location, reducing conditions will occur near the root channels if the
soluble carbon source required by the bacteria comes from dead roots. If the organic
compounds are dissolved and dispersed in the soil water, then reduction can occur
at any place in a soil horizon where the pores are filled with water.
4.10 Using Field Indicators of Hydric Soils
4.10.1 Overview
To fully understand this section, we recommend that the reader downloads and
prints out a copy of the most recent version of
Field Indicators of Hydric Soils in the
United States
along with any subsequent errata from the NTCHS website at
http://
soils.usda.gov/use/hydric/
and a copy of Chapter 3 of the Corps Delineation
Manual Regional Supplement for the area of interest at
http://www.usace.army.
ence. The publication
Field Indicators of Hydric Soils in the United States
is a
comprehensive list of all the Field Indicators approved for use by the NTCHS. All
Field Indicators listed in the Corps Regional Supplements are a subset of the
NTCHS national list of indicators. The Regional Supplements also contain the
appropriate data sheet for determination and delineation of wetlands and hydric
soils in that region.
Not all of the Field Indicators are appropriate for each situation. The Field
Indicators are regionalized, and each indicator is only valid in specific Land
Resource Regions (Fig.
4.12
). In addition, some indicators are restricted by soil
texture. There are three categories of Field Indicators which are distinguished by
soil texture: All Soils, Sandy Soils, and Loamy and Clayey Soils.
All soils
refers to
soils with any USDA soil texture. Examples include A1 Histosol, A4 Hydrogen
Sulfide, and A12 Thick Dark Surface, among others. Sandy soils have a USDA
texture of loamy fine sand and coarser (sandier). Examples include S1 Sandy
Mucky Mineral, S6 Stripped Matrix, and S10 Alaska Gleyed. The loamy and clayey
soils category has USDA textures of loamy very fine sand and finer (more clay).
Examples include F1 Loamy mucky mineral, F6 Redox Dark Surface, and F9
Vernal Pools.
