|Soil Position||Time Saturated||Time
||% of Year||%||%|
|Foot of Slope||35 ||30||80|
3. Soils were saturated for similar lengths of times
a. rainfall caused water tables to rise, and the rain fell on
both soils at the same time.
b. evapotranspiration caused the water tables to fall, and the
same kends of trees were on both sides.
c. the water in the soil at the foot of the slope was reduced
because the water was stagnant (not moving). water on the slping
soil was moving constantly downslope.
1. Gray colors usually mean "no iron" on particle surfaces.
2. Do "gray colors" always mean the soil has been reduced?
No. Some soil parent materials never had Fe2O3, and have always
3. Wet soils usually have both gray colors and reddish colors.
the gray shows where Fe has been removed, while the red shos
where the Fe has accumulated.
4. Gray color produced by reduction frequently has Fe
concentrations in the same horizon or in lower horizons.
5. Soil color chromas of 2 or less:
a. No Fe is on particle surfaces,
b. soils may be reduced for "significant" periods.
6. Soil color chroma of 3:
a. Soils may be reduced for "short" periods.
b. Soils may be saturated for "significant" periods,
particularly if soil organic matter contents are <1%, and soils
are on slopes.
II. BASIC HYDROLOGY
A. Water Tables
1. Definitions: The surface of the water that fills a well or
open auger hole, or
the upper level of "free water" that can flow out of soil into a
well or hole.
2. The water table may not be the top of the saturated zone if a
capillary fringe is present.
B. Saturation: There are two ways to define this, one
definition is based on water content, the other is based on water
1. Saturation: Water Content definition: Saturation occurs
when all pores are filled with water except for pores containing
a. Problems with definition include: Must measure water content
to know when soils is saturated, and this is not easy to due in
b. This definition includes the capillary fringe.
c. The difinition is difficult for workers to apply during field
2. Saturation: Water Pressure definition. Saturation occurs
where "free water" is present in the soil. The water has a
pressure that is greater than atmospheric pressure.
a. Comments: Free water fills wells and auger holes.
b. The definition can be applied in the field.
c. It does not include the capillary fringe.
Does Reduction occur near septic drainlines?
a. Is the soil waterlogged? Yes
b. Is organic matter present? Yes
c. Are bacteria present? Yes
Then reduction occurs.
Major points for Parts I and II:
a. Oxidation-reduciton reactions causes gray colors to form in
b. These reactions need organic matter, bacteria, and no air to
be present in order to work.
c. The amount of gray color in the soil is usually related to
how long the soils have been reduce, not how long they have been
d. To know how long a soil is saturated, you need to measure
water tables with a well.
III. REDOXIMORPHIC REATURES FOR IDENTIFYING AQUIC
1. In Soil Taxonomy, poorly drained soils had a "Aquic Moisture
Regime". The moisture regime was poorly defined. It was defined
on the basis of saturation and reduction, but was identified on
the basis of soil morphology. This approach meant that it was
never clear if all soils identified as having an aquic moisture
regime were actually saturated and reduced.
2. In international committee was assembled to make improvements
to the regime.
3. the committee recommended the moisture regime be modified,
and the new regime was to be called "Aquic Conditions".
4. Aquic conditions is currently being used for soil
5. Objectives: The purpose of this lecture was to:
a. Describe the requirements for aquic conditions,
b. Discuss the morphology of redoximorphic features,
c. Discuss how redoximorphic features can be interpreted,
d. Review the advantages of redoximorphic features.
B. Aquic Conditions
1. Definition: Aquic conditions requires that each of the
following be documented septarately for ebery soil
a. Saturation depth,
b. Occurrence of Fe reduction, and
c. Redoximorphic features must be present.
2. Basic idea: The intent is to document that each soil series
is saturated and reduced at one or two "local" sites, and then to
exptrapolate this information to similar soils using the
It is now realized that we must calibrate a soils redoximorphic
features to specific periods of saturation and reduction. This
information cannot be obtained from the features
3. Saturation: A horizon is saturated when the soil water
pressure is zero or positive.
a. Water will run into an unlined auger hole or a piezometer
when the horizon is saturated.
b. Saturation may occur at any time of the year, not simply
during the growing season as is required for jurisdictional
c. Saturation is not required if the soils are artificially
d. Three kinds of saturation have been defined:
- i) Endosaturation-soil is saturated in all horizons that lie
between the upper boundary of saturation and a depth of 2 m.
- ii) Episaturation-soil is saturated in a horizon that
overlies an unsaturated horizon, wherer the unsaturated horizon
lies within a depth of 2 m from the surface.
- iii) Anthric saturation-similar to episaturation but is used
for sites having controlled flooding.
4. Reduciton: Iron must be present in a reduced form (Fe(II))
at some time.
a. No minimum time period is required for reduciton.
b. Any method that shows iron reduction is occurring can be
used. this includes platinum microelctrodes to measure redox
potential. it also includes the use of a-a'dipyridyl dye which
is sprayed onto field-moist soil and turns red when reduced Fe is
C. Redoximorphic Freatures-Introduction
1. Definition: Features formed by the reduciton, translocation,
and oxidation of Fe and Mn oxides. Both iron and manganese are
used because the two are virtually inseparable in soils.
2. Redoximorphic features replaces the terms "mottles and low
3. Redoximorphic features are large enough to be seen with the
4. New terms were defined to make the features easy to
D. Kinds of Redoximorphic Features
1. There are three basic kinds of features: Redox
Concentrations, Redox Depletions, and Reduced Matrices.
2. Redox concentrations: Zones of apparent concentration of
Fe/Mn oxides. there are three types:
a. Nodules and concretions (hard bodies),
b. Masses (soft bodies),
c. Pore linings (coatings of Fe or Mn along cracks and root
3. Nodules and concretions: firm to extremely firm irregularly
shaped bodies with diffuse boundaries. these two terms are used
interchangeably. diffuse boundaries are thought to identify
features that are currently forming. Most nodules and
concretions with sharp bnoundaries have stopped forming.
4. Masses: Soft bodies, frequently within the matrix, whose
shape is variable. this term replaces "mottles".
5. Pore linings: Zones of accumulation that may be either
coatings on a proe surface or impregnations of the matrix
adjacent to the pore.
6. Redox Depletions: Zones of low (<2) where Fe-Mn oxides alone
have been removed, or where both Fe-Mn oxides and clay have been
removed. There are two types:
a. Fe depletions
b. Clay depletions
7. Iron Depletions: low chroma bodies with clay contents
similar to that of the adjacent matrix. These occur both along
pores and in the matrix.
8. Clay Depletions: low chroma bodies containing less Fe, Mn,
and clay than an adjacent soil matrix. Clay depletions form in
place. they do not form by deposition of silt onto particle
9. Reduced matrix: A soil matrix that has a low chroma in situ
because of the presence of Fe(II), but whose color changes in hue
of chroma when exposed to air as the Fe(II) is oxidized to
Fe(III) or Fe2O3. The color change should occur within 30
a. Nodules and concretions should be considered relict features
if they are the only kind of redoximophic featyures present.
b. Organic stains (color value <4) are not considered to be
E. Interpretations of Redoximorphic Features
1. Originally, the goal was to use sets of features to identify
and separate endosaturation from episaturation.
2. Field testing showed that this was not possible.
3. Soils with perched and permanent water tables can have the
same kinds of redoximophic features.
4. Features should be interpreted to be indicators of where Fe
reduction and oxidation occur in horizons. Redox depletions show
where reduction has occurred, while redox concentrations show
where oxidation occurs.
5. The arrangement of redox depletions and redox concentrations
in a horizon might be used to interpret how water and air move
through a horizon.
6. To do this effectively, you must specify which features lie
along macropores and which lie in the matrix.
7. Interpretations of redoximophic features should be based
around a concept as to how specific features form.
F. Formation of Fe Depletions and Clay Depletions
1. Both features form in similar ways.
2. The simplest cases occur in horizons having stable
3. See figures for descriptions of formation.
G. Interpreting water movement
1. Case 1: Redox depletions were observed to occur around
macropores, and redox concentrations were seen to occur within
2. The pattern of redoximorphic features suggests:
a. The macropores are stabel and control the movement of
b. Saturation and reduction occur in the macropores which are
root channels or cracks between soil peds.
c. Oxidation occurs in the matrix; it is not clear whether the
matrix gets saturated with water.
d. Water flows from the macropores toward the matrix.
e. The horixon may have a low saturated hydraulic
3. When redox depletions and concentrations occur within the
same horizon, then at some point water must have moved:
From redox depletions toward redox concentrations.
H. Potential advantages of Redoximorphic Features
1. Recognixzing redoximorphic features will correct some of the
problems we've had with low chroma colors.
2. These problems include:
a. Using chromas of 3 or more as indicators of saturation
b. Identifying relict or fossil colors.
c. Estimating rates of formation of colors.
3. How can relict redoximorphic features be identified?
Look for the following characterisitics:
a. Feature boundary characteristics: relict features (e.g.,
nodules) have sharp boundaries, whicl features still forming have
more diffuse boundaries.
b. Relation to macropores: see figures.
c. Feature color: bright red colors may be relict while yellow
and brown colors are more likely to be forming.
4. How long does it take to form redoximorphic features?
a. Reduced matrices can form within 7 days when water is poned
on the surface, temperatures are warm, and fresh organic matter
b. Redox concentrations (pore linings) can form around rice
roots in 30 days or less.
c. Redox depletions can form in 7 days in A horizons where water
is ponded on the surface and organic matters contents are
CONCLUSIONS FOR PART III.
1. Redoximorphic features have been adopted for immediate use as
part of aquic conditions.
2. the features need to be calibrated to specific periods of
saturation for each soil.
3. Redoximorphic features show where oxidation and reduction of
Fe occur in horizons.
4. The features offer several potential advantages over low
chroma colors including more flexibility on color requirements
for aquic conditions, and the ability to distinquish relict
A. How can you identify redoximorphic features the occur in
horizons that are not saturated?
1. this problem occurs with:
a. Capillary fringes: zones above the water table where most
pores are filled with water.
b. Stratified layers: soil layers such as silts over sands
where material with small proes overlies material with large
2. The capillary fringe:
a. Is thick in clays with massive soil structure.
b. Has a depth from the surface which changes as the water table
c. Is important where the water table is static for long periods
below the root zone.
d. Estimated thicknesses of the capillary fringe:
|Soil||Range in Depth||Avg. Depth
||0 - 3 inches||2 inches|
||0.5 - 5 feet||<12
||5 - 100 feet||<60
e. Redoximorphic features of soils within the capillary fringe
and in stratified layers is similar, and consists of redox
depletions within the matrix, with pores linings along macropores
such as roo channels and ped surfaces.
3. Restrictive layers:
a. These are layers of low saturated hydraulic conductivity
which cause perched water tables to form.
b. Their saturated hydraulic conductivity is about 1/10 that of
an overlying horizon.
c. Such restrictive layers are prevalent in the Lower Coastal
Plain where they consist of clays, silts, or organic layers.
d. They are less common in Middle and Upper Coastal Plain
4. Soils with low amounts of Iron: these are soils in which
redoximorphic features cannot form.
a. Both well-drained and poorly-drained soils in this category
will appear gray.
b. Generally such soils are sands or loamy sands.
c. The parent materials of these soils did not contain iron.
d. To identify reduction in these soils we must use features
made of organic matter. These features include:
i) layers of muck at the soil surface,
ii) "splotchy" color patterns,
iii) stratified layers containing organic material,
iv) a dark mineral surface
a. Organic soil material
b. Plant parts can't be recognized
6. "Splotchy" Color Pattern
a. Mineral grains have been stripped of both iron and organic
b. The materials that have moved have "clumped" into masses of
two or more colors.
c. The stripped areas occupy at least 10% of the horizon, and
are at least 1 to 3 cm in diameter.
7. Dark Mineral Surfaces
a. A layer or horizon that is more than 4 inches thick.
b. It has a Munsell value of 3 or less, and chroma of 1 or less
c. The soil matrix in a layer below the surface layer has a
chroma of 2 or less.
8. These organic features are not well-defined, nor is there
formation understood. However, they are probably good indicators
to use if they are properly identified.
Research Technician, I
on November 15, 1996.
Last Updated on April 15, 1998.
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