This contrasts with the dominance of permafrost-affected
soils (Gelisols) in Siberia (Russia), the Northwest Territories
(Canada), and Interior Alaska (USA) under a continental dry
climate, where the absence of ice sheets promoted the
development of permafrost. After the retreat of the ice
sheets, Podzols formed on glacial sediments (till, loess, and
glaciofluvial sands except for lacustrine clays). The formation of Podzols is favored by sandy parent materials that are
poor in Fe, especially glaciofluvial sands or outwash. Since
the mobility of organic acids can be limited by Fe oxides,
podzolization is restricted by parent materials rich in Fe
(>2% Fe 2 O 3 ) (Duchaufour and Souchier 1978). Podzols can
also be formed on clayey parent materials, including volcanic ash, in environments where the intensity of podzolization exceeds clay accumulation (sialitization).
Vegetation and microorganisms: Most Podzols in Japan
occur in forests with a thick O horizon, which is a potential
source of fulvic acids. However, a thick O horizon is not a
prerequisite for Podzol formation; rather, Podzols tend to
have a thick O horizon as a consequence of limited microbial
and faunal activity (e.g., earthworms) under cool, acidic
conditions (Hayakawa et al. 2014). In boreal regions of
Europe and America, Podzols can also develop under a thin
O horizon in open lichen-spruce woodlands. In Japan, Podzols are typically observed under subalpine coniferous forests
dominated by fir (Abies mariesii), hemlock (Tsuga diversifolia), and cypress (Chamaecyparis obtusa). Some conifers
which host ectomycorrhizal fungi are known to be “podzolizers,” that is, to accelerate podzolization (Nielsen et al.
1999); however, Podzols also develop in forests of various
non-ectomycorrhizal conifers, such as Japanese cedar
(Cryptomeria japonica), and egg-cup Podzol under Kauri
trees (Agathis australis) in New Zealand. Podzolization can
also occur in some broad-leaved forests dominated by, for
example, beech (Fagus crenata) in Japan (Fujii et al. 2008).
4.4.6 Boundary Between Andosolization
and Podzolization
Andosols and Podzols are classified by the same diagnostic
property (Al o + 1/2Fe o ). The vertical distribution of Al o and
Al p in an Andosol suggests that organo-mineral complex
was dominant in surface horizons (Fig. 4.28). The release of
Al from volcanic ash can stabilize organic material. The
presence of organic material and low pH conditions (<5)
limit the formation of imogolite and allophane. In Andosols,
in situ weathering by carbonic acids can result in the
incongruent dissolution of minerals derived from volcanic
materials and the accumulation of SRO minerals in the B
horizon (Ugolini et al. 1988). This contrasts with podzolization characterized by eluviation and illuviation. However, as suggested by the fulvic acid-bicarbonate theory, the
Podzol Bs horizon and Andosols share similarities in their
weathering environments in terms of the dominance of
non-complexing acids (e.g., H 2 CO 3 , HNO 3 ), low DOC
concentrations, a pH range between 5 and 6.5 that promotes
Al polymerization, and adequate Si concentrations that
support the stability of imogolite (Fig. 4.29).
Acidity generated in the O horizon can be the determining
factor of pedogenesis for Andosols and Podzols derived
from volcanic ash (Shoji et al. 1982). The lower contribution
of organic acids, as well as the lower soil acidification rate, is
responsible for andosolization (Fujii et al. 2008). This contrasts with podzolization caused by the intensive acidification of the O horizon.
4.4.7 Boundary Between Podzolization
and Brunification
The formation of Brown Forest soils, known as “brunification,” is characterized by the immobilization of organic
matter and Fe oxides in the surface horizons, as suggested by
the vertical distribution of Fe o and Fe p (Fig. 4.28) and higher
saturation of the negative charge of organic acids by Al in
soil solution in a Brown Forest soil (Fig. 4.29). Brown
Forest soils in warmer broad-leaved forests are relatively
acidic, but acidity derived mainly from plant uptake tends to
distribute throughout the profile and be consumed in situ
(Fujii et al. 2008). The dominance of non-complexing acids
results in the accumulation of Fe oxides through incongruent
mineral dissolution (Ugolini et al. 1990). Podzolization can
be differentiated from soil acidification by the amounts of
illuviated SRO minerals, although incipient podzolization
can be involved in the formation of Brown Forest soils.
Large DOC fluxes from the O horizon can induce Al- and
Fe-eluviation in the Ultisol profile of Southeast Asia
(Fig. 4.24). On the other hand, the accumulation of SRO
minerals in the B horizon is not significant (Fig. 4.28) and
the Al concentrations of soil solution are lower than those
for Brown Forest soil (Fig. 4.29). The clay-poor and acidic E
horizon of Ultisols can cause weak eluviation of Al leaching;
however, the leaching of Al and Fe appears to be within
biological cycles of Al and Fe and the recrystallization of
precipitated Al and Fe, except for in tropical Podzols on
extremely sandy materials (Fujii et al. 2011).
Podzols can also be associated with Alfisols (Albeluvisols
and Luvisols in the WRB classification) in temperate
sub-humid regions. The clay migration and formation of the
E horizons in Alfisols and Ultisols could provide a favorable
condition for podzolization. In Japan, clay migration is not
an active process due to the stability of colloids rich in
organic matter, the low dispersibility of SRO clay minerals,
and the absence of a distinct dry season. These factors also
limit the wide distribution of Podzols in Japan.
4 Major Soil Types
97
soils (Gelisols) in Siberia (Russia), the Northwest Territories
(Canada), and Interior Alaska (USA) under a continental dry
climate, where the absence of ice sheets promoted the
development of permafrost. After the retreat of the ice
sheets, Podzols formed on glacial sediments (till, loess, and
glaciofluvial sands except for lacustrine clays). The formation of Podzols is favored by sandy parent materials that are
poor in Fe, especially glaciofluvial sands or outwash. Since
the mobility of organic acids can be limited by Fe oxides,
podzolization is restricted by parent materials rich in Fe
(>2% Fe 2 O 3 ) (Duchaufour and Souchier 1978). Podzols can
also be formed on clayey parent materials, including volcanic ash, in environments where the intensity of podzolization exceeds clay accumulation (sialitization).
Vegetation and microorganisms: Most Podzols in Japan
occur in forests with a thick O horizon, which is a potential
source of fulvic acids. However, a thick O horizon is not a
prerequisite for Podzol formation; rather, Podzols tend to
have a thick O horizon as a consequence of limited microbial
and faunal activity (e.g., earthworms) under cool, acidic
conditions (Hayakawa et al. 2014). In boreal regions of
Europe and America, Podzols can also develop under a thin
O horizon in open lichen-spruce woodlands. In Japan, Podzols are typically observed under subalpine coniferous forests
dominated by fir (Abies mariesii), hemlock (Tsuga diversifolia), and cypress (Chamaecyparis obtusa). Some conifers
which host ectomycorrhizal fungi are known to be “podzolizers,” that is, to accelerate podzolization (Nielsen et al.
1999); however, Podzols also develop in forests of various
non-ectomycorrhizal conifers, such as Japanese cedar
(Cryptomeria japonica), and egg-cup Podzol under Kauri
trees (Agathis australis) in New Zealand. Podzolization can
also occur in some broad-leaved forests dominated by, for
example, beech (Fagus crenata) in Japan (Fujii et al. 2008).
4.4.6 Boundary Between Andosolization
and Podzolization
Andosols and Podzols are classified by the same diagnostic
property (Al o + 1/2Fe o ). The vertical distribution of Al o and
Al p in an Andosol suggests that organo-mineral complex
was dominant in surface horizons (Fig. 4.28). The release of
Al from volcanic ash can stabilize organic material. The
presence of organic material and low pH conditions (<5)
limit the formation of imogolite and allophane. In Andosols,
in situ weathering by carbonic acids can result in the
incongruent dissolution of minerals derived from volcanic
materials and the accumulation of SRO minerals in the B
horizon (Ugolini et al. 1988). This contrasts with podzolization characterized by eluviation and illuviation. However, as suggested by the fulvic acid-bicarbonate theory, the
Podzol Bs horizon and Andosols share similarities in their
weathering environments in terms of the dominance of
non-complexing acids (e.g., H 2 CO 3 , HNO 3 ), low DOC
concentrations, a pH range between 5 and 6.5 that promotes
Al polymerization, and adequate Si concentrations that
support the stability of imogolite (Fig. 4.29).
Acidity generated in the O horizon can be the determining
factor of pedogenesis for Andosols and Podzols derived
from volcanic ash (Shoji et al. 1982). The lower contribution
of organic acids, as well as the lower soil acidification rate, is
responsible for andosolization (Fujii et al. 2008). This contrasts with podzolization caused by the intensive acidification of the O horizon.
4.4.7 Boundary Between Podzolization
and Brunification
The formation of Brown Forest soils, known as “brunification,” is characterized by the immobilization of organic
matter and Fe oxides in the surface horizons, as suggested by
the vertical distribution of Fe o and Fe p (Fig. 4.28) and higher
saturation of the negative charge of organic acids by Al in
soil solution in a Brown Forest soil (Fig. 4.29). Brown
Forest soils in warmer broad-leaved forests are relatively
acidic, but acidity derived mainly from plant uptake tends to
distribute throughout the profile and be consumed in situ
(Fujii et al. 2008). The dominance of non-complexing acids
results in the accumulation of Fe oxides through incongruent
mineral dissolution (Ugolini et al. 1990). Podzolization can
be differentiated from soil acidification by the amounts of
illuviated SRO minerals, although incipient podzolization
can be involved in the formation of Brown Forest soils.
Large DOC fluxes from the O horizon can induce Al- and
Fe-eluviation in the Ultisol profile of Southeast Asia
(Fig. 4.24). On the other hand, the accumulation of SRO
minerals in the B horizon is not significant (Fig. 4.28) and
the Al concentrations of soil solution are lower than those
for Brown Forest soil (Fig. 4.29). The clay-poor and acidic E
horizon of Ultisols can cause weak eluviation of Al leaching;
however, the leaching of Al and Fe appears to be within
biological cycles of Al and Fe and the recrystallization of
precipitated Al and Fe, except for in tropical Podzols on
extremely sandy materials (Fujii et al. 2011).
Podzols can also be associated with Alfisols (Albeluvisols
and Luvisols in the WRB classification) in temperate
sub-humid regions. The clay migration and formation of the
E horizons in Alfisols and Ultisols could provide a favorable
condition for podzolization. In Japan, clay migration is not
an active process due to the stability of colloids rich in
organic matter, the low dispersibility of SRO clay minerals,
and the absence of a distinct dry season. These factors also
limit the wide distribution of Podzols in Japan.
4 Major Soil Types
97