dark iron-rust-colored Bs horizon with rusty mottles, and the
whole soil contains much humus and has a very dark color.
The Humic subgroup is characterized by the accumulation of soil organic matter in the deep horizon because
organic matter slowly decomposes under cool climatic
conditions. This subgroup includes soils that are classified as
“dark brown forest soils” in the Classification of Forest Soils
in Japan (1976). “Dark brown forest soils” were characterized as follows: “Blackish-brown, glossy Oa horizon or
Oa-A horizon is recognized, the A horizon is
blackish-brown, and the Bw horizon is dark brown (brightness and chroma are close to 3). An aggregate structure
develops in the upper part of the A horizon or Oa-A horizon,
and a massive structure is often seen in the Bw horizon and
the lower part of the A horizon. The phenomenon of podzolization or gleization is cannot be recognized by the naked
eye” (Forest Soil Division 1976).
The Thapto-red-yellow subgroup has thinner or deeper
cambic or argic horizons with red-yellow properties compared to Red-Yellow soils. This soil subgroup corresponds
to a part of “reddish-brown forest soils” and
“yellowish-brown forest soils” in the Classification of Forest
Soils in Japan (1976) and a part of “yellow-brown forest
soils” in the Unified Soil Classification System of Japan
(2003). Compared to the “brown forest soil” subgroup, the
“reddish-brown forest soils” have an A horizon that is normally light-colored and thin, while the color of their Bw and
C horizons are strongly reddish in color (the color of the Bw
horizon is less reddish than almost 5YR 5/6 and more reddish than 7.5YR 5/8). These “brown forest soils” were
formed from strongly reddish parent materials as a result of
red-color weathering, and they are strongly acidic. Compared with the “brown forest soil” subgroup, the
“yellowish-brown forest soils” had an A horizon that was
normally light-colored and thin, and the color of their Bw
and C horizons was strongly yellowish in color (the color of
the Bw horizon is less yellowish than almost 10YR 6/6 and
more yellowish than 7.5YR 6/8). These “brown forest soils”
were formed from strongly yellowish parent materials as a
result of yellow-color weathering, and they are strongly
acidic. The soil group of “yellow-brown forest soils” was
considered to be associated with a part of the yellowish and
typical subgroups of “brown forest soils” in the Classification of Forest Soils in Japan (1976). The “yellow-brown
forest soils” were distributed under a warm temperate zone,
whereas the “yellowish-brown forest soils” were derived
from paleo soil materials with yellow-color weathering and
are also distributed under a cool temperate zone (Yambe and
Yagi 1983). The “yellow-brown forest soils” and
“yellowish-brown forest soils” were compared because the
area with past yellow-color weathering greatly overlapped
with the current warm temperate zone and the soils with
yellow brunification derived from yellow-color weathering
materials having features of both “yellow-brown forest soils”
and “yellowish-brown forest soils” were developed (Imaya
2008).
Soils of the Aquic subgroup are characterized by the
lower portion in the soil stratum being saturated with stagnant groundwater for some time during the year. A Stagnic
horizon with endoaquic properties appears in a deeper portion in the solum than it does in Stagnic soils.
Soils of the Epi-gleyed subgroup are saturated with surface water for some time during the year. The surfaces of
these soils contain rusty mottles or reduction spots caused
by the presence of temporary stagnant water. These soils
develop on upland slopes, having massive soil materials.
These soils correspond to “surface gleyed brown
forest soils” in the Classification of Forest Soils in Japan
(1976).
Soils of the Eutric subgroup are derived from base-rich
parent materials, but are not so much strongly affected by the
parent materials as Eutrosols. These soils have a larger
cation exchange capacity, higher pH, and higher base saturation than typical Brown Forest soils that are acidic or
weakly acidic.
Since soil color has been a significant indicator in the
previous soil classification systems in Japan, the dark
red-colored acidic soils such as volcanic dark red soils
derived from parent materials with thermal metamorphism
were classified into the great group of “dark red soils”;
however, most of these soils are now classified as Eutrosols
having dark red color and derived from basic rocks such as
limestone and serpentine. In the Soil Classification System
of Japan, volcanic “dark red soils” are classified as Brown
Forest soils or Red-Yellow soils, because the classification is
based on soil physical and chemical properties rather than
soil color and parent materials.
Brown Forest soils include soils having an argic horizon,
although the majority of Brown Forest soils have a cambic
horizon and may correspond to Cambisols. In order to
classify soils having an argic horizon as an independent soil
subgroup from ones having a cambic horizon, the information of these soils’ characteristics is insufficient.
(4) Further classification for practical use
Brown Forest soils show various morphological features,
such as degree of development of soil horizons and kinds of
soil structure, resulting from various hydrological environments according to mountain slope topography (Ohmasa
1951). Hydrological conditions continuously change due to
topographic position on the mountain slopes, where most of
the forest lands in Japan are located. Additionally, their
vegetation and tree growth performance also change relatively. Based on these facts, six soil types, from dry to wet,
were established in the Classification of Forest Soils in Japan
118
Y. Takata et al.
whole soil contains much humus and has a very dark color.
The Humic subgroup is characterized by the accumulation of soil organic matter in the deep horizon because
organic matter slowly decomposes under cool climatic
conditions. This subgroup includes soils that are classified as
“dark brown forest soils” in the Classification of Forest Soils
in Japan (1976). “Dark brown forest soils” were characterized as follows: “Blackish-brown, glossy Oa horizon or
Oa-A horizon is recognized, the A horizon is
blackish-brown, and the Bw horizon is dark brown (brightness and chroma are close to 3). An aggregate structure
develops in the upper part of the A horizon or Oa-A horizon,
and a massive structure is often seen in the Bw horizon and
the lower part of the A horizon. The phenomenon of podzolization or gleization is cannot be recognized by the naked
eye” (Forest Soil Division 1976).
The Thapto-red-yellow subgroup has thinner or deeper
cambic or argic horizons with red-yellow properties compared to Red-Yellow soils. This soil subgroup corresponds
to a part of “reddish-brown forest soils” and
“yellowish-brown forest soils” in the Classification of Forest
Soils in Japan (1976) and a part of “yellow-brown forest
soils” in the Unified Soil Classification System of Japan
(2003). Compared to the “brown forest soil” subgroup, the
“reddish-brown forest soils” have an A horizon that is normally light-colored and thin, while the color of their Bw and
C horizons are strongly reddish in color (the color of the Bw
horizon is less reddish than almost 5YR 5/6 and more reddish than 7.5YR 5/8). These “brown forest soils” were
formed from strongly reddish parent materials as a result of
red-color weathering, and they are strongly acidic. Compared with the “brown forest soil” subgroup, the
“yellowish-brown forest soils” had an A horizon that was
normally light-colored and thin, and the color of their Bw
and C horizons was strongly yellowish in color (the color of
the Bw horizon is less yellowish than almost 10YR 6/6 and
more yellowish than 7.5YR 6/8). These “brown forest soils”
were formed from strongly yellowish parent materials as a
result of yellow-color weathering, and they are strongly
acidic. The soil group of “yellow-brown forest soils” was
considered to be associated with a part of the yellowish and
typical subgroups of “brown forest soils” in the Classification of Forest Soils in Japan (1976). The “yellow-brown
forest soils” were distributed under a warm temperate zone,
whereas the “yellowish-brown forest soils” were derived
from paleo soil materials with yellow-color weathering and
are also distributed under a cool temperate zone (Yambe and
Yagi 1983). The “yellow-brown forest soils” and
“yellowish-brown forest soils” were compared because the
area with past yellow-color weathering greatly overlapped
with the current warm temperate zone and the soils with
yellow brunification derived from yellow-color weathering
materials having features of both “yellow-brown forest soils”
and “yellowish-brown forest soils” were developed (Imaya
2008).
Soils of the Aquic subgroup are characterized by the
lower portion in the soil stratum being saturated with stagnant groundwater for some time during the year. A Stagnic
horizon with endoaquic properties appears in a deeper portion in the solum than it does in Stagnic soils.
Soils of the Epi-gleyed subgroup are saturated with surface water for some time during the year. The surfaces of
these soils contain rusty mottles or reduction spots caused
by the presence of temporary stagnant water. These soils
develop on upland slopes, having massive soil materials.
These soils correspond to “surface gleyed brown
forest soils” in the Classification of Forest Soils in Japan
(1976).
Soils of the Eutric subgroup are derived from base-rich
parent materials, but are not so much strongly affected by the
parent materials as Eutrosols. These soils have a larger
cation exchange capacity, higher pH, and higher base saturation than typical Brown Forest soils that are acidic or
weakly acidic.
Since soil color has been a significant indicator in the
previous soil classification systems in Japan, the dark
red-colored acidic soils such as volcanic dark red soils
derived from parent materials with thermal metamorphism
were classified into the great group of “dark red soils”;
however, most of these soils are now classified as Eutrosols
having dark red color and derived from basic rocks such as
limestone and serpentine. In the Soil Classification System
of Japan, volcanic “dark red soils” are classified as Brown
Forest soils or Red-Yellow soils, because the classification is
based on soil physical and chemical properties rather than
soil color and parent materials.
Brown Forest soils include soils having an argic horizon,
although the majority of Brown Forest soils have a cambic
horizon and may correspond to Cambisols. In order to
classify soils having an argic horizon as an independent soil
subgroup from ones having a cambic horizon, the information of these soils’ characteristics is insufficient.
(4) Further classification for practical use
Brown Forest soils show various morphological features,
such as degree of development of soil horizons and kinds of
soil structure, resulting from various hydrological environments according to mountain slope topography (Ohmasa
1951). Hydrological conditions continuously change due to
topographic position on the mountain slopes, where most of
the forest lands in Japan are located. Additionally, their
vegetation and tree growth performance also change relatively. Based on these facts, six soil types, from dry to wet,
were established in the Classification of Forest Soils in Japan
118
Y. Takata et al.