50 cm from the soil surface. Wet Andosols are defined as
Andosols in which a soil horizon showing endoaquic properties appears within a depth of 50 cm from the soil surface.
These groups of Andosols are referred to as Aquands in Soil
Taxonomy. However, the Gleyed Andosols in the Japanese
classification system are classified as Gleysols in the WRB
system, because Gleysols are keyed out before Andosols
(IUSS Working Group WRB 2015).
In the Japanese classification system, the other Andosols
are keyed out as non-allophanic Andosols (Fig. 4.16) and
thereafter as Allophanic Andosols (Fig. 4.17). These correspond to Aluandic Andosols and Silandic Andosols,
respectively, in the WRB. Both groups of Andosols are
referred to as Udands in the USDA Soil Taxonomy.
The soil names “Andosols” and “Andisols” are derived
from the Japanese characters an meaning dark and do meaning
soil. Therefore, these soil names originally connote the typically dark soils derived from volcanic ejecta, which correspond to Humic Andosols in the Japanese classification
system. Subdivision based on soil color is defined at the
subgroup level because color is less important for andic soil
properties. Among Humic Andosols in the Japanese classification system, surface horizons having a high organic carbon
content (6% or more) are equivalent to “melanic” horizons in
the WRB classification and USDA Soil Taxonomy.
4.3.4 Distribution of Andosols in Japan
Of the approximately 1500 active volcanoes in the world,
111 are located in Japan (Japan Meteorological Agency
2017). In the vicinity of the Japanese Archipelago, two
plates, the Pacific Plate and the Philippine Sea Plate, are
subducting. In parallel the subduction zone, the East Japan
Volcanic Belt and the West Japan Volcanic Belt are distributed. The former includes volcanoes in the Kuril Islands,
Hokkaido, Tohoku, the Chubu and Kanto regions, and the
Izu-Ogasawara Islands, while the latter is composed of
volcanoes in the Chugoku and Kyushu regions and the
Nansei Islands (refer to Sect. 2.2).
Reflecting this fact, the spatial distribution of soils
belonging to the Andosol great group is the second largest
among all soil great groups in Japan, behind that of the
Brown Forest soil great group (Table 2.4 in Sect. 2.5);
Andosols are estimated to occupy about 30% of Japan’s
territory. Previously, the presumed distribution of Andosols
was 17% of Japan’s territory, whereas that of Brown Forest
soils was 53% (Kanno et al. 2008). The change in the estimated distribution is due to a change in the Japanese soil
classification system; now, Brown Forest soils that possess
andic properties are classified as Andosols.
By region, the distribution of Andosols is particularly
high in Hokkaido (42% by area), Kanto (40%), Tohoku
(36%), and Kyushu (34%), while the distribution is also
considerable in Tokai–Hokuriku (22%) and Kinki–Chugoku–Shikoku (15%). The high percentage of Andosol
present in the Hokkaido, Tohoku, Kanto, and Kyushu
regions is due to the high concentration of volcanoes in these
areas, which provide the parent materials of Andosols.
Andosols are distributed nationwide in Japan, including
areas with a near absence of volcanoes. This is partially due
to the influence of widespread tephra. Machida (2002)
identifies 31 types of widespread Quaternary tephra. Among
them, the representative ones are the Kikai-Akahoya (K-Ah)
tephra (Kikai volcano, southern Kyushu), the Aira-Tanzawa
(AT) tephra (Aira volcano, southern Kyushu), the
Shikotsu-1 tephra (Shikotsu caldera, central Hokkaido), the
Aso-4 tephra (Aso caldera, central Kyushu), and the Toya
tephra (Toya caldera, central Hokkaido) (Machida 2002).
These tephras are distributed over areas covering from
between one quarter and three quarters of the Japanese territory (Machida 2002).
As noted already, Andosols are often divided into two
groups based on the major colloidal composition of surface
horizons: Allophanic Andosols, dominated by allophanic
clays (allophane and imogolite), and non-allophanic Andosols, dominated by Al–humus complexes and often containing 2:1 layer silicates. Allophanic Andosols
preferentially form in thick Holocene tephra deposits in
Hokkaido, northern Tohoku, Kanto, and Kyushu regions. On
the other hand, non-allophanic Andosols form in areas with
minimal Holocene tephra deposition (Saigusa and Matsuyama 1998) (Fig. 2.35 in Sect. 2.5).
4.3.5 Environmental Significance and Utility
of Andosols
(1) Organic carbon (OC) accumulation
The accumulation of soil organic matter (SOM) is a characteristic property of Andosols. Andosols cover 0.84% of
the Earth’s land surface, but they contain approximately
1.8% of the global soil organic carbon (SOC). Even though
the contribution of Andosols to the SOC stock is not so large
at a global level, it is relatively important in Japan because
Andosols cover 30% of the country’s territory.
The large accumulation of SOC in Andosols results from
a combination of high detritus input resulting from the
generally high productivity of Andosols and from the
effective stabilization of SOM against decomposition. Stabilization mechanisms of SOM were summarized to be
attributed to: (1) the formation of SOM in organo-mineral or
organo-metallic complexes; (2) the low activity of soil
microorganisms due to low soil pH, aluminum toxicity, low
base cation content, and/or phosphorus deficiency; (3) the
4 Major Soil Types
89
Andosols in which a soil horizon showing endoaquic properties appears within a depth of 50 cm from the soil surface.
These groups of Andosols are referred to as Aquands in Soil
Taxonomy. However, the Gleyed Andosols in the Japanese
classification system are classified as Gleysols in the WRB
system, because Gleysols are keyed out before Andosols
(IUSS Working Group WRB 2015).
In the Japanese classification system, the other Andosols
are keyed out as non-allophanic Andosols (Fig. 4.16) and
thereafter as Allophanic Andosols (Fig. 4.17). These correspond to Aluandic Andosols and Silandic Andosols,
respectively, in the WRB. Both groups of Andosols are
referred to as Udands in the USDA Soil Taxonomy.
The soil names “Andosols” and “Andisols” are derived
from the Japanese characters an meaning dark and do meaning
soil. Therefore, these soil names originally connote the typically dark soils derived from volcanic ejecta, which correspond to Humic Andosols in the Japanese classification
system. Subdivision based on soil color is defined at the
subgroup level because color is less important for andic soil
properties. Among Humic Andosols in the Japanese classification system, surface horizons having a high organic carbon
content (6% or more) are equivalent to “melanic” horizons in
the WRB classification and USDA Soil Taxonomy.
4.3.4 Distribution of Andosols in Japan
Of the approximately 1500 active volcanoes in the world,
111 are located in Japan (Japan Meteorological Agency
2017). In the vicinity of the Japanese Archipelago, two
plates, the Pacific Plate and the Philippine Sea Plate, are
subducting. In parallel the subduction zone, the East Japan
Volcanic Belt and the West Japan Volcanic Belt are distributed. The former includes volcanoes in the Kuril Islands,
Hokkaido, Tohoku, the Chubu and Kanto regions, and the
Izu-Ogasawara Islands, while the latter is composed of
volcanoes in the Chugoku and Kyushu regions and the
Nansei Islands (refer to Sect. 2.2).
Reflecting this fact, the spatial distribution of soils
belonging to the Andosol great group is the second largest
among all soil great groups in Japan, behind that of the
Brown Forest soil great group (Table 2.4 in Sect. 2.5);
Andosols are estimated to occupy about 30% of Japan’s
territory. Previously, the presumed distribution of Andosols
was 17% of Japan’s territory, whereas that of Brown Forest
soils was 53% (Kanno et al. 2008). The change in the estimated distribution is due to a change in the Japanese soil
classification system; now, Brown Forest soils that possess
andic properties are classified as Andosols.
By region, the distribution of Andosols is particularly
high in Hokkaido (42% by area), Kanto (40%), Tohoku
(36%), and Kyushu (34%), while the distribution is also
considerable in Tokai–Hokuriku (22%) and Kinki–Chugoku–Shikoku (15%). The high percentage of Andosol
present in the Hokkaido, Tohoku, Kanto, and Kyushu
regions is due to the high concentration of volcanoes in these
areas, which provide the parent materials of Andosols.
Andosols are distributed nationwide in Japan, including
areas with a near absence of volcanoes. This is partially due
to the influence of widespread tephra. Machida (2002)
identifies 31 types of widespread Quaternary tephra. Among
them, the representative ones are the Kikai-Akahoya (K-Ah)
tephra (Kikai volcano, southern Kyushu), the Aira-Tanzawa
(AT) tephra (Aira volcano, southern Kyushu), the
Shikotsu-1 tephra (Shikotsu caldera, central Hokkaido), the
Aso-4 tephra (Aso caldera, central Kyushu), and the Toya
tephra (Toya caldera, central Hokkaido) (Machida 2002).
These tephras are distributed over areas covering from
between one quarter and three quarters of the Japanese territory (Machida 2002).
As noted already, Andosols are often divided into two
groups based on the major colloidal composition of surface
horizons: Allophanic Andosols, dominated by allophanic
clays (allophane and imogolite), and non-allophanic Andosols, dominated by Al–humus complexes and often containing 2:1 layer silicates. Allophanic Andosols
preferentially form in thick Holocene tephra deposits in
Hokkaido, northern Tohoku, Kanto, and Kyushu regions. On
the other hand, non-allophanic Andosols form in areas with
minimal Holocene tephra deposition (Saigusa and Matsuyama 1998) (Fig. 2.35 in Sect. 2.5).
4.3.5 Environmental Significance and Utility
of Andosols
(1) Organic carbon (OC) accumulation
The accumulation of soil organic matter (SOM) is a characteristic property of Andosols. Andosols cover 0.84% of
the Earth’s land surface, but they contain approximately
1.8% of the global soil organic carbon (SOC). Even though
the contribution of Andosols to the SOC stock is not so large
at a global level, it is relatively important in Japan because
Andosols cover 30% of the country’s territory.
The large accumulation of SOC in Andosols results from
a combination of high detritus input resulting from the
generally high productivity of Andosols and from the
effective stabilization of SOM against decomposition. Stabilization mechanisms of SOM were summarized to be
attributed to: (1) the formation of SOM in organo-mineral or
organo-metallic complexes; (2) the low activity of soil
microorganisms due to low soil pH, aluminum toxicity, low
base cation content, and/or phosphorus deficiency; (3) the
4 Major Soil Types
89