Oxidationreduction redox processes in soils

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Oxidation-reduction (redox) processes in soils
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Oxidized Fe (Fe)---reddish colors (ferric
iron) Reduced Fe (Fe)---no color, or dull gray
at best (ferrous iron)
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Fe in rocks is usually 2 (reduced) After
weathering it is released to the soil solution,
where it usually becomes oxidized (3) under
oxidizing conditions, Fe is quickly
precipitated out as insoluble Fe(OH)3 a
gel that never moves very far, and may form
concretions That is why many soils are
brown/red in color!
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Fe in rocks is usually 2 (reduced) After
weathering it is released to the soil solution,
where it usually becomes oxidized (3) under
oxidizing conditions, Fe is quickly
precipitated as insoluble Fe(OH)3 a gel
that never moves very far, and may form
concretions That is why many soils are
brown/red in color!
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Fe in rocks is usually 2 (reduced) After
weathering it is released to the soil solution,
where it usually becomes oxidized (3) under
oxidizing conditions, Fe is quickly
precipitated as insoluble Fe(OH)3 a gel
that never moves very far, and may form
concretions That is why many soils are
brown/red in color!
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Reduction in soils--essentially a biological
process microbes use soil O2 in respiration,
possibly forming reducing conditions
CO2
O2
Food
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LEO GER
GER
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Whether a soil is oxidized or reduced depends on
SUPPLY vs DEMAND of
oxygen Supply Demand
Water table Porosity
--texture --structure
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Whether a soil is oxidized or reduced depends on
SUPPLY vs DEMAND of
oxygen Supply Demand
Water table
Roots Porosity Microbial activity
--texture --organic matter --structure
--temperature (little or no microbial activity
below 5o C) --texture (more in finer soils)
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Microbes take oxygen from the following sources
(in order) 1. entrapped soil air 2.
dissolved oxygen in soil water when O2 supply is
gone, anaerobic conditions occur, and
microbes dehydrogenate OM to get oxygen and in so
doing, reduce certain elements OM----gt
loss of H (H is essentially a proton w/o an
electron) and e- (OM is thus oxidized
something must then be reduced)
H
e-
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Microbes take oxygen from the following sources
(in order) 1. entrapped soil air 2.
dissolved oxygen in soil water when O2 supply is
gone, anaerobic conditions occur, and
microbes dehydrogenate OM to get oxygen and in so
doing, reduce certain elements OM----gt
loss of H (H is essentially a proton w/o an
electron) and e- (OM is thus oxidized
something must then be reduced)
H
e-
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Microbes take oxygen from the following sources
(in order) 1. entrapped soil air 2.
dissolved oxygen in soil water when O2 supply is
gone, anaerobic conditions occur, and
microbes dehydrogenate OM to get oxygen and in so
doing, reduce certain elements OM----gt
loss of H (H is essentially a proton w/o an
electron) and e- (OM is thus oxidized
something must then be reduced)
H
e-
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Microbes take oxygen from the following sources
(in order) 1. entrapped soil air 2.
dissolved oxygen in soil water when O2 supply is
gone, anaerobic conditions occur, and
microbes dehydrogenate OM to get oxygen and in so
doing, reduce certain elements/ions OM---
-gt loss of H (H is essentially a proton w/o an
electron) and e- (OM is thus oxidized
something must then be reduced)
H
e-
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Electrons taken from the dehydrogenated OM are
then donated to (NO3-), Mn and/or
Fe They are reduced, and become NH4, Mn and
Fe (ferrous) 3. nitrate (NO3-) 4. Mn
oxides 5. Fe oxides 6. sulfates
Reduced Fe is soluble and moves along
concentration gradients in the soil solution
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Reduced iron dominant
GLEYED conditions (color chroma lt 2)
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Oxidized iron dominant
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Oxidized iron dominant
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Mottled oxidation-reduction patterns
Redox (redoximorphic) features
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Reduction near root traces they use up the O2
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Redox features/nomenclature
1. Fe depletions (formerly gray mottles)
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Redox features/nomenclature
1. Fe depletions (formerly gray mottles)
2. Fe concentrations (formerly red mottles)
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Redox features/nomenclature
Gleyed dominant soil color has chroma lt2
(wetness is viewed as cause)
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Gleyed colors, gleyed conditions
Add a g to each horizon that applies
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Colors can tell a lot about the long-term water
regime of a soil
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Natural soil drainage classes - a function of
topography and water table depth
WHERE is the water table?
WT usually above the surface Very Poorly
Drained (organic soils) WT usually at or just
below the surface Poorly Drained WT usually in
the upper B horizon Somewhat Poorly Drained WT
usually in the lower B horizon Moderately Well
Drained WT usually below the B horizon Well
Drained WT usually below the B horizon, AND the
soil is sandy Somewhat Excessively
Drained Water table usually below the B horizon,
AND the soil is very sandy or coarse-textured
Excessively Drained
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Natural soil drainage classes - a function of
topography and water table depth
WHERE is the water table?
WT usually above the surface Very Poorly
Drained (organic soils) WT usually at or just
below the surface Poorly Drained WT usually in
the upper B horizon Somewhat Poorly Drained WT
usually in the lower B horizon Moderately Well
Drained WT usually below the B horizon Well
Drained WT usually below the B horizon, AND the
soil is sandy Somewhat Excessively
Drained Water table usually below the B horizon,
AND the soil is very sandy or coarse-textured
Excessively Drained
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Natural soil drainage classes - a function of
topography and water table depth
WHERE is the water table?
WT usually above the surface Very Poorly
Drained (organic soils) WT usually at or just
below the surface Poorly Drained WT usually in
the upper B horizon Somewhat Poorly Drained WT
usually in the lower B horizon Moderately Well
Drained WT usually below the B horizon Well
Drained WT usually below the B horizon, AND the
soil is sandy Somewhat Excessively
Drained Water table usually below the B horizon,
AND the soil is very sandy or coarse-textured
Excessively Drained
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Natural soil drainage classes - a function of
topography and water table depth
WHERE is the water table?
WT usually above the surface Very Poorly
Drained (organic soils) WT usually at or just
below the surface Poorly Drained WT usually in
the upper B horizon Somewhat Poorly Drained WT
usually in the lower B horizon Moderately Well
Drained WT usually below the B horizon Well
Drained WT usually below the B horizon, AND the
soil is sandy Somewhat Excessively
Drained Water table usually below the B horizon,
AND the soil is very sandy or coarse-textured
Excessively Drained
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Natural soil drainage classes - a function of
topography and water table depth
WHERE is the water table?
WT usually above the surface Very Poorly
Drained (organic soils) WT usually at or just
below the surface Poorly Drained WT usually in
the upper B horizon Somewhat Poorly Drained WT
usually in the lower B horizon Moderately Well
Drained WT usually below the B horizon Well
Drained WT usually below the B horizon, AND the
soil is sandy Somewhat Excessively
Drained Water table usually below the B horizon,
AND the soil is very sandy or coarse-textured
Excessively Drained
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Natural soil drainage classes - a function of
topography and water table depth
WHERE is the water table?
WT usually above the surface Very Poorly
Drained (organic soils) WT usually at or just
below the surface Poorly Drained WT usually in
the upper B horizon Somewhat Poorly Drained WT
usually in the lower B horizon Moderately Well
Drained WT usually below the B horizon Well
Drained WT usually below the B horizon, AND the
soil is sandy Somewhat Excessively
Drained Water table usually below the B horizon,
AND the soil is very sandy or coarse-textured
Excessively Drained
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Natural soil drainage classes - a function of
topography and water table depth
WHERE is the water table?
WT usually above the surface Very Poorly
Drained (organic soils) WT usually at or just
below the surface Poorly Drained WT usually in
the upper B horizon Somewhat Poorly Drained WT
usually in the lower B horizon Moderately Well
Drained WT usually below the B horizon Well
Drained WT usually below the B horizon, AND the
soil is sandy Somewhat Excessively
Drained Water table usually below the B horizon,
AND the soil is very sandy or coarse-textured
Excessively Drained
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Natural soil drainage classes
Location of gleying and mottles
WT usually above the surface VPD (organic
soils) WT usually at or just below the surface
PD WT usually in the upper B horizon SPD WT
usually in the lower B horizon MWD WT usually
below the B horizon WD WT usually below the B
horizon, AND the soil is sandy SED WT
usually below the B horizon, AND the soil is
very sandy or coarse-textured ED
Gleyed and mottled throughout Gleyed upper B
and deeper mottled throughout Gleyed lower B
and deeper Mottled upper B Mottled mid-B and
deeper Mottled BC and deeper None None
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BUT, a soils wetness and water conditions are
due to more than just topography CLIMATE also
plays a role!
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Soil moisture regimes a function of water table
AND climate Aquic water table is up in the
profile - can occur in any climate, any area
- a function of topography - SPD and
wetter All others an function of climate Udic
humid climates Ustic semi-arid
climates Aridic/Torric dry climates Xeric
Mediterranean climates
(summer dry, winter wet)
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Udic humid climates Ustic semi-arid
climates Aridic/Torric dry climates Xeric
Mediterranean climates
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Roughly ustic, xeric and aridic
Roughly udic
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Name that soil moisture regime!
Sandy, mixed, frigid Entic Haplorthods
Clayey, smectitic, thermic, shallow Typic
Durixeralfs
Fine-loamy, mixed, superactive, nonacid, mesic
Fluvaquentic Humaquepts
Coarse-loamy, mixed, superactive, mesic Udic
Haplustalfs
Fine-silty, mixed, semiactive, mesic Aquic
Fragiudalfs
Sandy, mixed, mesic Aeric Fluvaquents
Fine, mixed, superactive, mesic Pachic
Argiustolls
Euic, mesic Typic Haplosaprists
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DAILY DOUBLE
Loamy-skeletal, carbonatic, frigid Lithic Xeric
Torriorthents
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Patterns of mottling can tell us a lot about
water/oxidation regime of the soil
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Ground water model uniform parent
materials gleyed in zone of permanent
saturation mottled in zone of fluctuating water
A Bt Cg
Oxidized
Water table highest point
Mottled
Water table lowest point
Gleyed
ENDOAQUIC SMR
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A Bt Cg
Mottled
Water table lowest point
Gleyed
9 mos. saturated, 3 mos. dry.....
interiors of peds gray (gley), exteriors may
be reddish and oxidized ped interiors never
oxidized b/c groundwater is seldom oxidizing, and
it is present most of yr ped exteriors and
cracked areas may be red and oxidized because
roots withdraw water from inter-ped regions
first, O2 moves into these regions and Fe is
oxidized then, reduced Fe from inside peds may
move to ped exteriors along concentration
gradients, and Fe may be concentrated on ped
faces as bright red mottles
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A Bt Cg
Mottled
Water table lowest point
Gleyed
9 mos. saturated, 3 mos. dry.....
interiors of peds gray (gley), exteriors may
be reddish and oxidized ped interiors never
oxidized b/c groundwater is seldom oxidizing, and
it is present most of yr ped exteriors and
cracked areas may be red and oxidized because
roots withdraw water from inter-ped regions
first, O2 moves into these regions and Fe is
oxidized then, reduced Fe from inside peds may
move to ped exteriors along concentration
gradients, and Fe may be concentrated on ped
faces as bright red mottles
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A Bt Cg
Mottled
Water table lowest point
Gleyed
9 mos. saturated, 3 mos. dry.....
interiors of peds gray (gley), exteriors may
be reddish and oxidized ped interiors never
oxidized b/c groundwater is seldom oxidizing, and
it is present most of yr ped exteriors and
cracked areas may be red and oxidized because
roots withdraw water from inter-ped regions
first, O2 moves into these regions and Fe is
oxidized then, reduced Fe from inside peds may
move to ped exteriors along concentration
gradients, and Fe may be concentrated on ped
faces as bright red mottles
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A Bt Cg
Mottled
Water table lowest point
Gleyed
9 mos. saturated, 3 mos. dry.....
interiors of peds gray (gley), exteriors may
be reddish and oxidized ped interiors never
oxidized b/c groundwater is seldom oxidizing, and
it is present most of yr ped exteriors and
cracked areas may be red and oxidized because
roots withdraw water from inter-ped regions
first, O2 moves into these regions and Fe is
oxidized then, reduced Fe from inside peds may
move to ped exteriors along concentration
gradients, and Fe may be concentrated on ped
faces as bright red mottles
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Mottled, Fe concentrations along root traces
Fully gleyed
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A Bt Cg
Mottled
Water table lowest point
Gleyed
Zone of 3 mos. saturated, 9 mos. dry OR
perched water.....
ped exteriors bleached (pseudogley), interiors
brownish red roots and microbes seek out
inter-ped spaces, use up O2, reduce Fe along
ped margins whole ped not gray because water
only perched for short time
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A Bt Cg
Mottled
Water table lowest point
Gleyed
Zone of 3 mos. saturated, 9 mos. dry OR
perched water.....
ped exteriors bleached (pseudogley), interiors
brownish red roots and microbes seek out
inter-ped spaces, use up O2, reduce Fe along
ped margins whole ped not gray because water
only perched for short time
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A Bt Cg
Mottled
Water table lowest point
Gleyed
Zone of 3 mos. saturated, 9 mos. dry OR
perched water.....
ped exteriors bleached (pseudogley), interiors
brownish red roots and microbes seek out
inter-ped spaces, use up O2, reduce Fe along
ped margins whole ped not gray because water
only perched for short time
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Pseudogley locally reduced conditions in an
otherwise oxidizing environment
Common in perched water table settings
EPIAQUIC SMR
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Ground water model coarse-textured lenses within
a finer matrix reddish "linings" of
oxidized Fe around sandy lenses
Fe is reduced in fine-textured layers, into
which O2 cannot move diffuse well which have
higher microbial O2 demand (more humus than
the sandy layers) Fe stays oxidized in
coarse-textured (sandy) strata Fe in the fine
layers diffuses to the edge (contact with the
coarse strata) and oxidizes there, producing
reddish contact zones
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Ground water model coarse-textured lenses within
a finer matrix reddish "linings" of
oxidized Fe around sandy lenses
Fe is reduced in fine-textured layers, into
which O2 cannot move diffuse well which have
higher microbial O2 demand (more humus than
the sandy layers) Fe stays oxidized in
coarse-textured (sandy) strata Fe in the fine
layers diffuses to the edge (contact with the
coarse strata) and oxidizes there, producing
reddish contact zones
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Ground water model coarse-textured lenses within
a finer matrix reddish "linings" of
oxidized Fe around sandy lenses
Fe is reduced in fine-textured layers, into
which O2 cannot move diffuse well which have
higher microbial O2 demand (more humus than
the sandy layers) Fe stays oxidized in
coarse-textured (sandy) strata Fe in the fine
layers diffuses to the edge (contact with the
coarse strata) and oxidizes there, producing
reddish contact zones
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Fine-textured till
Sandy lens
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Whether a soil is oxidized or reduced depends on
SUPPLY vs DEMAND of
oxygen
Remember!
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Browns and reds
Brown ped interiors, gleyed exteriors
Gleyed ped interiors, brown exteriors
Fully gleyed
Light browns
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Browns and reds
Brown ped interiors, gleyed exteriors
O2 supply
Gleyed ped interiors, brown exteriors
Microbial demand for O2
Fully gleyed
Light browns
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Formation of Fragipans Bx or Ex (also Btx)
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FRAGIPAN (definition, and must-haves) dense,
subsurface horizon brittle when dry, firm when
moist virtually no roots air-dried fragments
slake in water
May have bleached vertical streaks coarse
polygonal structure
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Characteristics non-connective porosity most
have evidence of lessivage and redox perch water
(aquitards) low in OM loamy in texture form in
PMs that are low in carbonates or
have been leached many form at a LD in MI, the
Bx is in the lower sequum of a bisequal
soil form only under forest vegetation
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Characteristics non-connective porosity most
have evidence of lessivage and redox perch water
(aquitards) low in OM loamy in texture form in
PMs that are low in carbonates or
have been leached many form at a LD in MI, the
Bx is in the lower sequum of a bisequal
soil form only under forest vegetation
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Characteristics non-connective porosity most
have evidence of lessivage and redox perch water
(aquitards) low in OM loamy in texture form in
PMs that are low in carbonates or
have been leached many form at a LD in MI, the
Bx is in the lower sequum of a bisequal
soil form only under forest vegetation
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Characteristics non-connective porosity most
have evidence of lessivage and redox perch water
(aquitards) low in OM loamy in texture form in
PMs that are low in carbonates or
have been leached many form at a LD in MI, the
Bx is in the lower sequum of a bisequal
soil form only under forest vegetation
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Characteristics non-connective porosity most
have evidence of lessivage and redox perch water
(aquitards) low in OM loamy in texture form in
PMs that are low in carbonates or
have been leached many form at a LD in MI, the
Bx is in the lower sequum of a bisequal
soil form only under forest vegetation
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Characteristics non-connective porosity most
have evidence of lessivage and redox perch water
(aquitards) low in OM loamy in texture form in
PMs that are low in carbonates or
have been leached many form at a LD in MI, the
Bx is in the lower sequum of a bisequal
soil form only under forest vegetation
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Characteristics non-connective porosity most
have evidence of lessivage and redox perch water
(aquitards) low in OM loamy in texture form in
PMs that are low in carbonates or
have been leached many form at or near a LD in
MI, the Bx is in the lower sequum of a bisequal
soil form only under forest vegetation
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Characteristics non-connective porosity most
have evidence of lessivage and redox perch water
(aquitards) low in OM loamy in texture form in
PMs that are low in carbonates or
have been leached many form at or near a LD in
MI, the Bx is in the lower sequum of a bisequal
soil form only under forest vegetation
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Looking down on prism cracks
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Prism crack side view
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Why perched water here?
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TWO issues about fragipan genesis that must be
explained
density attributed to dense packing of grains
and inter-grain bridging some attribute to paleo
processes such as glacial presssures (does not
work in loess paleosols)
Now Self-weight collapse of saturated materials,
followed by desiccation ripening
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TWO issues about fragipan genesis that must be
explained
brittleness attributed to cementation by a
number of different agents aluminosilicates a
morphous compounds (gels) of Si and Al hydrous
oxides of Si and Al clay sesquioxides
Quiz why no fragipans beneath grass vegetation?
Answer grasses biocycle silica
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Most likely cementing agent silica
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Many fragipans occur at a LD, where soluble
agents can be deposited Roots at LD accentuate
deposition of soluble materials
Ava series (Oxyaquic Fragiudalfs) Ap E BE Bt Bt/E
Bt 2Btx1 2Btx2 2C
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Michigan Model of Fragipan Formation
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Fragipan degradation
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Involves FERROLYSIS clay destruction under
redox conditions
- loss of CEC
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Outcomes of ferrolysis -clays destroyed -soil
texture gets coarser -pH lowered, soil gets
more acidic -silica gels available for fragipan
genesis, etc