the influence of irrigation water show horizon differentiation
as a result of leaching and accumulation of iron and manganese (Fig. 4.33) and/or have a thick gray subsurface
horizon. In the Japanese soil classification system (2017),
Fluvic Paddy soil is defined as “soils that have (1) an iron
accumulation horizon (subsurface horizon 2 cm or more in
thickness with plenty of thread-like and cloud-like iron
mottles and two times or more the amount of iron (DCB
extracted) contained in the plow layer); and/or (2) a grayed
subsurface horizon with abundant cloud-like iron mottles
and well-developed structures having shiny ped faces with
its lower boundary being 50 cm below the mineral soil
surface.” These soils are generated by strong influence of
artificial irrigation water and are typically found in natural
levee areas and alluvial fans.
The Japan Soil Classification System (2017) defines the
following subgroups: soils with surface soil strongly depleted of iron (Albic; generally called degraded paddy soils),
soils having an inverted gley horizon beneath the plow layer
(Epi-gleyed), soils having a yellowish-brown oxidizing
subsoil (Endoaeric), soils having a groundwater aquic mottled subsoil (Aquic), and other soils (Haplic)
Fluvic Paddy soils are used for paddy rice and upland
crops. These soils are easily used for upland crops due to
their rather good drainage capacity.
(2) Gley Fluvic soils
Gley Fluvic soils were saturated almost all year round. It has
gley horizon
1 and its upper boundary is within 50 cm of the
inorganic soil surface. These soils are widely found in flood
plains, deltas, tidal flats, and reclaimed land. The groundwater
level is high, and the soil is generally poorly drained (Fig. 4.34).
The Japan Soil Classification System defines the following subgroups: soils with actual and potential acidity due
to the presence of sulfuric acid (Thionic); soils with intervening organic layers (Peaty); soils with a humic or highly
humic epipedone (Humic); soils with mottled surface gray
layers (Epi—Gray); soils with iron mottles being confined to
the upper 25 cm (Strong); and soils with thick gley horizons
with iron mottles (Mottled).
Gley Fluvic soils have mostly been used for paddy fields.
However, multipurpose uses of paddy fields are extending
under the “gentan” policy (a governmental policy aimed at
keeping rice prices high by reducing the supply of rice), and
Fig. 4.34 Profiles of Fluvic soil groups. Photograph Obara et al. (2015)
1
A gley horizons is defined as “a horizon that gives a positive reaction
instantly to a,a’-dipyridyl, or is physically unripened and
bluish-gray-colored.” Gley horizons are separated into groundwater gley
horizons, formed under the influence of groundwater, and stagnant-water
gley horizons, formed under the influence of stagnant water.
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Y. Takata et al.
as a result of leaching and accumulation of iron and manganese (Fig. 4.33) and/or have a thick gray subsurface
horizon. In the Japanese soil classification system (2017),
Fluvic Paddy soil is defined as “soils that have (1) an iron
accumulation horizon (subsurface horizon 2 cm or more in
thickness with plenty of thread-like and cloud-like iron
mottles and two times or more the amount of iron (DCB
extracted) contained in the plow layer); and/or (2) a grayed
subsurface horizon with abundant cloud-like iron mottles
and well-developed structures having shiny ped faces with
its lower boundary being 50 cm below the mineral soil
surface.” These soils are generated by strong influence of
artificial irrigation water and are typically found in natural
levee areas and alluvial fans.
The Japan Soil Classification System (2017) defines the
following subgroups: soils with surface soil strongly depleted of iron (Albic; generally called degraded paddy soils),
soils having an inverted gley horizon beneath the plow layer
(Epi-gleyed), soils having a yellowish-brown oxidizing
subsoil (Endoaeric), soils having a groundwater aquic mottled subsoil (Aquic), and other soils (Haplic)
Fluvic Paddy soils are used for paddy rice and upland
crops. These soils are easily used for upland crops due to
their rather good drainage capacity.
(2) Gley Fluvic soils
Gley Fluvic soils were saturated almost all year round. It has
gley horizon
1 and its upper boundary is within 50 cm of the
inorganic soil surface. These soils are widely found in flood
plains, deltas, tidal flats, and reclaimed land. The groundwater
level is high, and the soil is generally poorly drained (Fig. 4.34).
The Japan Soil Classification System defines the following subgroups: soils with actual and potential acidity due
to the presence of sulfuric acid (Thionic); soils with intervening organic layers (Peaty); soils with a humic or highly
humic epipedone (Humic); soils with mottled surface gray
layers (Epi—Gray); soils with iron mottles being confined to
the upper 25 cm (Strong); and soils with thick gley horizons
with iron mottles (Mottled).
Gley Fluvic soils have mostly been used for paddy fields.
However, multipurpose uses of paddy fields are extending
under the “gentan” policy (a governmental policy aimed at
keeping rice prices high by reducing the supply of rice), and
Fig. 4.34 Profiles of Fluvic soil groups. Photograph Obara et al. (2015)
1
A gley horizons is defined as “a horizon that gives a positive reaction
instantly to a,a’-dipyridyl, or is physically unripened and
bluish-gray-colored.” Gley horizons are separated into groundwater gley
horizons, formed under the influence of groundwater, and stagnant-water
gley horizons, formed under the influence of stagnant water.
102
Y. Takata et al.