(3) reducing red-brown coloured ferric iron minerals (with
Fe
3+ as insoluble FeOOH) to soluble, colourless, ferrous (Fe
2+ ) iron minerals; and
(4) sulphate (SO
2À
4 ) reduction to hydrogen sulphide (H 2 S).
Both Mn and Fe in their soluble (reduced) states are
mobile (dissolve in water, Mn especially so) and so can
easily move with the soil water to other parts of the profile or
possibly to adjacent soils in the landscape. As Mn is oxidised, it may precipitate around pre-existing nuclei forming
concentric patterns within concretions or non-concentric
nodules. Such concretions or nodules may additionally
contain Fe oxides.
Saturation is not mapped directly because of its high
temporal and spatial variability. Instead the presence of pale,
grey, colours (redox depletions or low-chroma mottles) and
bright orange/brown mottles (redox concentrations or segregations), together known as reductimorphic or redoximorphic features, are used as indicators of prolonged soil
saturation. The connection between soil colour and water
saturation is a result of biochemical reduction. The presence
of reducing conditions, which can sometimes be uncertain
where the parent materials are themselves pale-coloured
(such as white rhyolitic pumiceous alluvium), can be easily
shown by using the Childs’ test for ferrous iron in solution
(developed by Cyril Childs). A dye solution a,a′-dipyridyl is
sprayed on to the soil matrix or a ped and a positive response
is indicated by the development of pink to red colours. If the
presence of Mn-oxides (e.g. MnO 2 ) is uncertain, rapid
effervescence following the addition of hydrogen peroxide
(H 2 O 2 ) will confirm their occurrence and abundance.
Alternatively, black MnO 2 concretions give a brown streak
on paper (whereas black charcoal fragments make black
marks).
Where there is a lack of oxygen (anaerobic conditions)
then the reduction process, the gain of electrons (the opposite process to oxidation, the loss of electrons), is triggered
which causes oxygen to be removed from a number of soil
minerals. Soil in an oxygenated state is normally pigmented
in red, yellowish-brown, brown, or other ochreous colours.
The pigments are derived from Fe, Mn, and Al species that
occur as stable oxides or hydroxy-oxide minerals, including,
for Fe
3+ the polymorphs goethite, lepidocrocite, ferrihydrite,
or haematite; and for MnO 2 the polymorphs birnessite or
pyrolusite. Under reducing (anaerobic) conditions the Fe,
Mn, and Al species become soluble and colourless. In areas
where rust brown, insoluble ferric (Fe
3+ ) iron minerals are
reduced to colourless, soluble ferrous (Fe
2+ ) iron minerals
the soil colour changes from a rusty brown colour to grey or
bluish grey. Similarly, black or brown, insoluble Mn
4+ or
Mn
3+ minerals are reduced to soluble pale Mn
2+ minerals.
Once in the soluble form, the iron and manganese may
migrate to zones of higher oxygen content where oxidation
returns the minerals to the insoluble, coloured, form which
thus precipitates out of solution.
Permanently high water tables result in soils that have
continuously reduced forms and pale grey colours (moist
chromas
2, or
3 with value ! 6). The precipitated
manganese or iron minerals accumulate in pores and along
cracks or in root channels where oxygen is available, as
finely dispersed particles, as discontinuous coatings on ped
surfaces, or they may form concentrations that are observed
as red or brown mottles (redox segregations), concretions, or
nodules. In some Gley Soils an iron pan may form if there is
a horizontal layer in which oxygen is present. The reduced
iron may migrate to and precipitate out in a distinct layer that
typically marks a textural change (e.g. from silt loam to
sandy gravel) in the profile. A pan may also form at a point
where a seasonally fluctuating water table repeatedly rises
and falls. If all the solubilised manganese and iron have
migrated out of the soil then the grey colours may remain
even if a soil is drained and oxidised.
Thus in Gley Soils, the redox depletions (pale colours)
together with the redox concentrations (reddish brown Mnand/or Fe-rich spots, concretions, or ped coatings) reflect the
depth of the water table or the persistence of perching, and
the length of time the soil is saturated (Fig. 5.4). Gley Soils
generally have a dark-coloured topsoil that may be quite
high in organic matter. To qualify as a Gley Soil, generally,
the soil must have a soil horizon with at least 50% pale grey
colours (i.e. a reductimorphic horizon which usually extends
to at least 90 cm depth).
5.3 Soil-Landscape Relationships
Gley Soils form in any area of the landscape where water
accumulates in the soil such that saturation occurs reasonably frequently, or for at least a few weeks per year. Thus the
occurrence of Gley Soils is strongly influenced by topography and rainfall, and is, therefore, somewhat predictable.
Gley Soils form in areas with high groundwater tables or in
areas where slowly permeable layers result in perched water
tables (Fig. 5.5). Flat, low-lying areas, and hollows that
prevent surface runoff of excess water are often saturated for
extended periods. Back swamp areas on river flood plains
accumulate water during floods and if a levee (raised
flood-depositional area on the river margin) prevents the
flood water from returning to the river, then water may lie on
the soil surface for several weeks after a flood event, saturating the soil and leading to development of a Gley Soil.
Such ‘back swamp’ areas are invariably low lying and are
likely to have high water tables near the soil surface at times
in winter and spring.
76
5 Gley Soils
Fe
3+ as insoluble FeOOH) to soluble, colourless, ferrous (Fe
2+ ) iron minerals; and
(4) sulphate (SO
2À
4 ) reduction to hydrogen sulphide (H 2 S).
Both Mn and Fe in their soluble (reduced) states are
mobile (dissolve in water, Mn especially so) and so can
easily move with the soil water to other parts of the profile or
possibly to adjacent soils in the landscape. As Mn is oxidised, it may precipitate around pre-existing nuclei forming
concentric patterns within concretions or non-concentric
nodules. Such concretions or nodules may additionally
contain Fe oxides.
Saturation is not mapped directly because of its high
temporal and spatial variability. Instead the presence of pale,
grey, colours (redox depletions or low-chroma mottles) and
bright orange/brown mottles (redox concentrations or segregations), together known as reductimorphic or redoximorphic features, are used as indicators of prolonged soil
saturation. The connection between soil colour and water
saturation is a result of biochemical reduction. The presence
of reducing conditions, which can sometimes be uncertain
where the parent materials are themselves pale-coloured
(such as white rhyolitic pumiceous alluvium), can be easily
shown by using the Childs’ test for ferrous iron in solution
(developed by Cyril Childs). A dye solution a,a′-dipyridyl is
sprayed on to the soil matrix or a ped and a positive response
is indicated by the development of pink to red colours. If the
presence of Mn-oxides (e.g. MnO 2 ) is uncertain, rapid
effervescence following the addition of hydrogen peroxide
(H 2 O 2 ) will confirm their occurrence and abundance.
Alternatively, black MnO 2 concretions give a brown streak
on paper (whereas black charcoal fragments make black
marks).
Where there is a lack of oxygen (anaerobic conditions)
then the reduction process, the gain of electrons (the opposite process to oxidation, the loss of electrons), is triggered
which causes oxygen to be removed from a number of soil
minerals. Soil in an oxygenated state is normally pigmented
in red, yellowish-brown, brown, or other ochreous colours.
The pigments are derived from Fe, Mn, and Al species that
occur as stable oxides or hydroxy-oxide minerals, including,
for Fe
3+ the polymorphs goethite, lepidocrocite, ferrihydrite,
or haematite; and for MnO 2 the polymorphs birnessite or
pyrolusite. Under reducing (anaerobic) conditions the Fe,
Mn, and Al species become soluble and colourless. In areas
where rust brown, insoluble ferric (Fe
3+ ) iron minerals are
reduced to colourless, soluble ferrous (Fe
2+ ) iron minerals
the soil colour changes from a rusty brown colour to grey or
bluish grey. Similarly, black or brown, insoluble Mn
4+ or
Mn
3+ minerals are reduced to soluble pale Mn
2+ minerals.
Once in the soluble form, the iron and manganese may
migrate to zones of higher oxygen content where oxidation
returns the minerals to the insoluble, coloured, form which
thus precipitates out of solution.
Permanently high water tables result in soils that have
continuously reduced forms and pale grey colours (moist
chromas
2, or
3 with value ! 6). The precipitated
manganese or iron minerals accumulate in pores and along
cracks or in root channels where oxygen is available, as
finely dispersed particles, as discontinuous coatings on ped
surfaces, or they may form concentrations that are observed
as red or brown mottles (redox segregations), concretions, or
nodules. In some Gley Soils an iron pan may form if there is
a horizontal layer in which oxygen is present. The reduced
iron may migrate to and precipitate out in a distinct layer that
typically marks a textural change (e.g. from silt loam to
sandy gravel) in the profile. A pan may also form at a point
where a seasonally fluctuating water table repeatedly rises
and falls. If all the solubilised manganese and iron have
migrated out of the soil then the grey colours may remain
even if a soil is drained and oxidised.
Thus in Gley Soils, the redox depletions (pale colours)
together with the redox concentrations (reddish brown Mnand/or Fe-rich spots, concretions, or ped coatings) reflect the
depth of the water table or the persistence of perching, and
the length of time the soil is saturated (Fig. 5.4). Gley Soils
generally have a dark-coloured topsoil that may be quite
high in organic matter. To qualify as a Gley Soil, generally,
the soil must have a soil horizon with at least 50% pale grey
colours (i.e. a reductimorphic horizon which usually extends
to at least 90 cm depth).
5.3 Soil-Landscape Relationships
Gley Soils form in any area of the landscape where water
accumulates in the soil such that saturation occurs reasonably frequently, or for at least a few weeks per year. Thus the
occurrence of Gley Soils is strongly influenced by topography and rainfall, and is, therefore, somewhat predictable.
Gley Soils form in areas with high groundwater tables or in
areas where slowly permeable layers result in perched water
tables (Fig. 5.5). Flat, low-lying areas, and hollows that
prevent surface runoff of excess water are often saturated for
extended periods. Back swamp areas on river flood plains
accumulate water during floods and if a levee (raised
flood-depositional area on the river margin) prevents the
flood water from returning to the river, then water may lie on
the soil surface for several weeks after a flood event, saturating the soil and leading to development of a Gley Soil.
Such ‘back swamp’ areas are invariably low lying and are
likely to have high water tables near the soil surface at times
in winter and spring.
76
5 Gley Soils