(1976) and used for subdivisions of each soil subgroup. Soil
type classification was used as an effective indicator for site
evaluation for forest management aimed at timber production (Hashimoto 1969). Additionally, soil chemical properties showed differences depending on the soil types on a
slope (Ohmasa 1951; Mashimo 1957; Kawada and Takami
1957). This idea of soil type classification is adopted as the
forest soil phase in the Soil Classification System of Japan
(2017). Brown Forest soils can use the following six forest
soil phases (Fig. 4.48):
Dry phase (loose granular structure type): The O horizon
is not thick. Oe or Oe/Oa horizons always develop, but the
development of the Oa horizon is not significant. A black A
horizon is generally thin, and the boundary with the B
horizon is very apparent. A loose granular structure develops
to a considerable depth in the A horizon and B horizon. This
soil has rich mycelial strands and, in extreme cases, forms a
mycelial net horizon. The color of the B horizon is generally
light.
Dry phase (granular structure type): In this soil, a thick
Oe horizon and Oa horizon develop, and a thin black A
horizon or OA horizon is formed. A granular structure
develops in the A horizon, and the boundary between the A
horizon and B horizon is apparent. The color of the B
horizon is normally bright. A granular or nutty structure
develops above the B horizon, and a loose granular or fine
nutty structure is often seen in the lower portion of the B
horizon. A mycelial net horizon rich in mycelial strands is
rarely formed.
Weakly dried phase: An Oe horizon and an Oa horizon
are not well-developed. Humus penetrates relatively deeply;
the humus is light in color and its profile is relatively hard
and dense. A nutty structure frequently develops in the lower
portion of the A horizon and in the upper portion of the B
horizon. Mycelial strands are often recognized in the B
horizon.
Moderately moist phase: This is the typical soil type. Oe
and Oa horizons do not develop well. The A horizon is
relatively thick and is rich in humus of a dark brown color.
An aggregate structure develops in the upper portion of the
A horizon, and a blocky structure is often seen in the lower
portion. The B horizon is brown with a weak blocky structure but no other special structures are recognized. The
transition from the A horizon to the B horizon is generally
gradual.
Moderately moist phase (drier type): The soil profile
shape is almost the same as that of the moderately moist
phase, but these soils show somewhat drier characteristics,
such as the occurrence of a granular structure in the upper
portion of the A horizon or a nutty structure in the lower
portion of the A horizon.
Slightly wetted phase: No O horizon develops. The A
horizon is very rich in humus and very thick, with aggregate
structures developed. The transition to the B horizon, which
is brown to slightly dark gray in color, is gradual. No special
structure is recognized in the B horizon.
Wet phase: A coarse granular or aggregate Oa horizon
develops. The A horizon contains moderate levels of humus.
The permeation of humus to the B horizon is low. The B
horizon has massive soil materials, and is bluish-gray-brown in
color. Rusty mottle is often recognized but a gley horizon is not
recognized in the soil stratum within 1 m of the soil surface.
Fig. 4.48 Morphological characteristics of soil profiles of each soil phase. (Figure supplied by Akihiro Imaya)
4 Major Soil Types
119
type classification was used as an effective indicator for site
evaluation for forest management aimed at timber production (Hashimoto 1969). Additionally, soil chemical properties showed differences depending on the soil types on a
slope (Ohmasa 1951; Mashimo 1957; Kawada and Takami
1957). This idea of soil type classification is adopted as the
forest soil phase in the Soil Classification System of Japan
(2017). Brown Forest soils can use the following six forest
soil phases (Fig. 4.48):
Dry phase (loose granular structure type): The O horizon
is not thick. Oe or Oe/Oa horizons always develop, but the
development of the Oa horizon is not significant. A black A
horizon is generally thin, and the boundary with the B
horizon is very apparent. A loose granular structure develops
to a considerable depth in the A horizon and B horizon. This
soil has rich mycelial strands and, in extreme cases, forms a
mycelial net horizon. The color of the B horizon is generally
light.
Dry phase (granular structure type): In this soil, a thick
Oe horizon and Oa horizon develop, and a thin black A
horizon or OA horizon is formed. A granular structure
develops in the A horizon, and the boundary between the A
horizon and B horizon is apparent. The color of the B
horizon is normally bright. A granular or nutty structure
develops above the B horizon, and a loose granular or fine
nutty structure is often seen in the lower portion of the B
horizon. A mycelial net horizon rich in mycelial strands is
rarely formed.
Weakly dried phase: An Oe horizon and an Oa horizon
are not well-developed. Humus penetrates relatively deeply;
the humus is light in color and its profile is relatively hard
and dense. A nutty structure frequently develops in the lower
portion of the A horizon and in the upper portion of the B
horizon. Mycelial strands are often recognized in the B
horizon.
Moderately moist phase: This is the typical soil type. Oe
and Oa horizons do not develop well. The A horizon is
relatively thick and is rich in humus of a dark brown color.
An aggregate structure develops in the upper portion of the
A horizon, and a blocky structure is often seen in the lower
portion. The B horizon is brown with a weak blocky structure but no other special structures are recognized. The
transition from the A horizon to the B horizon is generally
gradual.
Moderately moist phase (drier type): The soil profile
shape is almost the same as that of the moderately moist
phase, but these soils show somewhat drier characteristics,
such as the occurrence of a granular structure in the upper
portion of the A horizon or a nutty structure in the lower
portion of the A horizon.
Slightly wetted phase: No O horizon develops. The A
horizon is very rich in humus and very thick, with aggregate
structures developed. The transition to the B horizon, which
is brown to slightly dark gray in color, is gradual. No special
structure is recognized in the B horizon.
Wet phase: A coarse granular or aggregate Oa horizon
develops. The A horizon contains moderate levels of humus.
The permeation of humus to the B horizon is low. The B
horizon has massive soil materials, and is bluish-gray-brown in
color. Rusty mottle is often recognized but a gley horizon is not
recognized in the soil stratum within 1 m of the soil surface.
Fig. 4.48 Morphological characteristics of soil profiles of each soil phase. (Figure supplied by Akihiro Imaya)
4 Major Soil Types
119