Gley Fluvic soils are now also used for a wide range of crops
with surface and underground drainage systems.
In recently reclaimed land, the drainage system develops
from an early stage of reclamation. The soils are physically
ripened after reclamation. In the first stage of ripening, gley
horizons are dehydrated from the surface layer and are
compacted, and cracks develop in deeper layers. At the same
time, the upper part of the soil develops iron mottles in pores
and crack surfaces. If the dehydration continues until soil
matrices are unsaturated with water, the gley horizons
should change to gray-colored mottled horizons. If a
gray-colored mottled horizon develops at a depth of 50 cm
or more from the surface, Gley Fluvic soils should change to
Gray Fluvic soils. The change of a gley horizon to a
gray-colored mottled horizon occurs rather quickly when the
groundwater level drops. However, under traditional paddy
rice cultivation, the change of a gley horizon to a
gray-colored mottled horizon at greater depth takes place
over a longer time. In the case of reclaimed land in Kojima,
it took more than 70 years after reclamation for a gley
horizon to change into a gray-colored mottled horizon at
50 cm depth.
(3) Gray Fluvic soils
Gray Fluvic soils have a mottled horizon formed under the
seasonal saturation of groundwater and with its upper
boundary being within 50 cm of the mineral soil surface
(Fig. 4.34). Mottled horizons formed under the influence of
groundwater are characterized by iron mottles along pores,
such as tubular and root-like mottles occurring along root
channels. These soils occur widely in nearly flat coastal plains,
flood plains, delta plains, valley bottoms, and alluvial fans.
Seven subgroups are provided: Thionic, Peaty, Humic,
Epi-gleyed (having an inverted gley horizon), Gleyic (having a groundwater gley horizon in the subsoil), Thapto-andic
(having a buried Andosol in the subsoil), and Haplic
(others).
In Gray Fluvic soils, the groundwater level is low relative
to Gley Fluvic soils and soils are somewhat poorly drained
in many cases. These soils are mostly used as paddy fields,
and partly as upland crop fields.
(4) Brown Fluvic soils
These are soils which occur on alluvial plains. The subsurface horizon is yellowish-brown in color with no influence
of groundwater within 50 cm of the surface and with no or
weak influence of irrigation water (Fig. 4.34). Due to the
earlier stage of weathering and the liberation of free iron
oxides, soil materials of alluvial lowlands are normally dull
yellowish-brown in color, with chroma ranging from 3 to 4.
These soils occur mainly in lowlands with a low groundwater table, such as natural levees and alluvial fans.
Brown Fluvic soils are divided into four subgroups: soils
with the upper boundary of a mottled horizon within 75 cm
of the surface (Aquic), soils with a humic to highly humic
epipedon (Humic), soils with a weak influence of irrigation
water (Protoanthraquic), and others (Haplic). Brown Fluvic
soils are used for upland crop fields or for residential areas.
(5) Regosolic Fluvic soils
Regosolic Fluvic soils are composed of unweathered clastic
sediments in lowlands. These soils are normally gray in color
(i.e., the color of the clastic materials is normally gray) and do
not have rusty mottles due to the lack of free iron. Soils are
generally sandy or sandy-skeletal in texture and are found in
the present and past river beds. Part of these soils is seasonally
saturated with underflow water, but the soil is not reduced.
Regosolic Fluvic soils are separated into two subgroups:
seasonally saturated soils (Aquic) and others (Haplic).
Because these soils are generally well-drained, they are
mostly used for upland crop fields and orchards.
4.5.3 Relationships with Other Soil Great
Groups
Fluvic soils are young soils developed from recent alluvial
deposits and have only some diagnostic horizons related
with aquic conditions, for example, gley horizons, mottled
horizons, iron accumulation horizons, and grayed horizons.
The important linkages of Fluvic soils are with other soils
distributed in lowlands, namely Peat soils, Andosols, and
Regosols.
Peat soils are usually distributed with Gley Fluvic soils.
Gley Fluvic soils and Peat soils have similar wet conditions
and only differ in their ratio of organic matter content and
mineral sediment content. In some regions, Peat layers have
been decomposed by drainage, and soils have been changed
to Gley Fluvic soils or Gray Fluvic soils. Where Andosols
are distributed on terraces or hills, materials with andic soil
properties are transported by erosion or other processes, such
as landfill, and are deposited in flood plains. These soils are
classified as Gleyic Andosols when the thickness of layers
with andic soil properties is at least 25 cm within 50 cm of
the soil surface (Fig. 4.35). Gleyic Andosols are usually
found in narrow valleys within terraces or hills that are
covered by Andosols (“Yachida” in Japanese). These soils
have been used for paddy rice cultivation since ancient
times. The soils on slightly elevated lands along the sea coast
such as sand dunes, sand bars, and sand banks are classified
as Sandy Regosols.
4 Major Soil Types
103
with surface and underground drainage systems.
In recently reclaimed land, the drainage system develops
from an early stage of reclamation. The soils are physically
ripened after reclamation. In the first stage of ripening, gley
horizons are dehydrated from the surface layer and are
compacted, and cracks develop in deeper layers. At the same
time, the upper part of the soil develops iron mottles in pores
and crack surfaces. If the dehydration continues until soil
matrices are unsaturated with water, the gley horizons
should change to gray-colored mottled horizons. If a
gray-colored mottled horizon develops at a depth of 50 cm
or more from the surface, Gley Fluvic soils should change to
Gray Fluvic soils. The change of a gley horizon to a
gray-colored mottled horizon occurs rather quickly when the
groundwater level drops. However, under traditional paddy
rice cultivation, the change of a gley horizon to a
gray-colored mottled horizon at greater depth takes place
over a longer time. In the case of reclaimed land in Kojima,
it took more than 70 years after reclamation for a gley
horizon to change into a gray-colored mottled horizon at
50 cm depth.
(3) Gray Fluvic soils
Gray Fluvic soils have a mottled horizon formed under the
seasonal saturation of groundwater and with its upper
boundary being within 50 cm of the mineral soil surface
(Fig. 4.34). Mottled horizons formed under the influence of
groundwater are characterized by iron mottles along pores,
such as tubular and root-like mottles occurring along root
channels. These soils occur widely in nearly flat coastal plains,
flood plains, delta plains, valley bottoms, and alluvial fans.
Seven subgroups are provided: Thionic, Peaty, Humic,
Epi-gleyed (having an inverted gley horizon), Gleyic (having a groundwater gley horizon in the subsoil), Thapto-andic
(having a buried Andosol in the subsoil), and Haplic
(others).
In Gray Fluvic soils, the groundwater level is low relative
to Gley Fluvic soils and soils are somewhat poorly drained
in many cases. These soils are mostly used as paddy fields,
and partly as upland crop fields.
(4) Brown Fluvic soils
These are soils which occur on alluvial plains. The subsurface horizon is yellowish-brown in color with no influence
of groundwater within 50 cm of the surface and with no or
weak influence of irrigation water (Fig. 4.34). Due to the
earlier stage of weathering and the liberation of free iron
oxides, soil materials of alluvial lowlands are normally dull
yellowish-brown in color, with chroma ranging from 3 to 4.
These soils occur mainly in lowlands with a low groundwater table, such as natural levees and alluvial fans.
Brown Fluvic soils are divided into four subgroups: soils
with the upper boundary of a mottled horizon within 75 cm
of the surface (Aquic), soils with a humic to highly humic
epipedon (Humic), soils with a weak influence of irrigation
water (Protoanthraquic), and others (Haplic). Brown Fluvic
soils are used for upland crop fields or for residential areas.
(5) Regosolic Fluvic soils
Regosolic Fluvic soils are composed of unweathered clastic
sediments in lowlands. These soils are normally gray in color
(i.e., the color of the clastic materials is normally gray) and do
not have rusty mottles due to the lack of free iron. Soils are
generally sandy or sandy-skeletal in texture and are found in
the present and past river beds. Part of these soils is seasonally
saturated with underflow water, but the soil is not reduced.
Regosolic Fluvic soils are separated into two subgroups:
seasonally saturated soils (Aquic) and others (Haplic).
Because these soils are generally well-drained, they are
mostly used for upland crop fields and orchards.
4.5.3 Relationships with Other Soil Great
Groups
Fluvic soils are young soils developed from recent alluvial
deposits and have only some diagnostic horizons related
with aquic conditions, for example, gley horizons, mottled
horizons, iron accumulation horizons, and grayed horizons.
The important linkages of Fluvic soils are with other soils
distributed in lowlands, namely Peat soils, Andosols, and
Regosols.
Peat soils are usually distributed with Gley Fluvic soils.
Gley Fluvic soils and Peat soils have similar wet conditions
and only differ in their ratio of organic matter content and
mineral sediment content. In some regions, Peat layers have
been decomposed by drainage, and soils have been changed
to Gley Fluvic soils or Gray Fluvic soils. Where Andosols
are distributed on terraces or hills, materials with andic soil
properties are transported by erosion or other processes, such
as landfill, and are deposited in flood plains. These soils are
classified as Gleyic Andosols when the thickness of layers
with andic soil properties is at least 25 cm within 50 cm of
the soil surface (Fig. 4.35). Gleyic Andosols are usually
found in narrow valleys within terraces or hills that are
covered by Andosols (“Yachida” in Japanese). These soils
have been used for paddy rice cultivation since ancient
times. The soils on slightly elevated lands along the sea coast
such as sand dunes, sand bars, and sand banks are classified
as Sandy Regosols.
4 Major Soil Types
103