River until the Akagawa River. The specific properties and
the lower and upper ranges were as follows: exchangeable
potassium (K), lower range 0.01–0.09 g kg
−1 , upper range
0.22–0.43 g kg
−1 ; and exchangeable sodium (Na), lower
range 0.03–0.06 g kg
−1 , upper range 0.14–0.39 g kg
−1 .
Paddy fields in Tsuruoka City receive irrigation water from
the Akagawa, Ohoyama, or Ohoto River. Each river distributes irrigation water to the fields located on the left bank
of the river (Yazawa River land improvement district 1986,
1988; Shoryuji River land improvement district 1998)—that
is, the paddy fields on the right bank of the Yazawa–
Ohoyama River until the Akagawa River receive irrigation
water from the Akagawa River. Granite contains the highest
amounts of K and Na among igneous rocks (Naganori 1984).
Thus, the irrigation water in the area from the right bank of
the Yazawa–Ohoyama River until the Akagawa River may
contain higher amounts of K and Na due to the granitic
geological features that are present in the upper reaches of
the Akagawa River, resulting in higher amounts of
exchangeable K and Na in soils.
The third feature was not clear. The spatial distributions
of pH (4.6–6.3), available P (0.03–0.45 g P 2 O 5 kg
−1 ), and
base saturation percentage (19–97%) had no definite trend.
These chemical properties may not be affected by geological
features in the upper reaches of many rivers, but rather by
cultivation practices such as fertilizer and organic matter
management in each paddy field.
The following is a summary of the above discussion. The
clay mineral composition of alluvial paddy soils in Tsuruoka
City, Yamagata Prefecture, has a spatial characteristic that
can be affected by geological features, namely, green tuff
that is distributed in the upper reaches of all the Akagawa
River and Ohoyama River systems, and granite that is distributed in the upper reaches of the Akagawa River and the
Yazawa River, flow into the plain of Tsuruoka City. The
spatial characteristics of the chemical properties of the paddy
soils in Tsuruoka City can be divided based on three features. One is the spatial distribution of clay mineral composition and organic matter content of the soil that is derived
from different geological features in the upper reaches
between the Akagawa River and the Yazawa and Ohoto
Rivers, such as total N, PAC, CEC, and exchangeable Ca
and Mg. The second is affected by geological features in the
upper reaches of rivers that provide irrigation water, such as
exchangeable K and Na. The third is not affected by geological features in any upper reaches, such as pH, available
P, and base saturation percentage.
6.1.2 Soils in the Hachirogata Land Reclamation
Area
(1) Soil properties of the Hachirogata land reclamation
area
The Hachirogata land reclamation area is located to the east
of the Oga Peninsula, which is situated in the central part of
Akita Prefecture. A crossing point between latitude 40° N
and longitude 140° E is located within this area. Hachirogata
was once the second-largest lake in Japan, with a total area
of 22,024 ha. However, after a severe food shortage after
World War II, the lake underwent extensive reclamation in
1966 under the technical supervision of Dutch experts.
The central land reclamation area occupies 15,640 ha
(71% of the lagoon area), and the entire area is situated
below sea level (average elevation, −4 m above sea level [a.
s.l.]). An extremely moist and soft sludge-like heavy clay
soil covers approximately 80% of the land reclamation area.
This soil is characterized by the following unique properties
that are not commonly present in general farming soils:
(1) It contains an average clay content of 51% (72% at
maximum), half of which comprises minute particles
(diameter < 1 lm). The primary clay mineral is an
expansible 2:1 smectite.
(2) It exhibits relatively favorable chemical properties,
specifically a large base-exchange capacity. It naturally
contains large quantities of nitrogen, potassium, phosphate, silicate, and other compounds. The most abundant phosphate form is the A1 type, followed by the Fe
and Ca types.
(3) Soil pH mainly ranges from neutral to slightly alkaline,
which is uncommon among Japanese soils. This contributes to the loss of fertilizer nitrogen through phenomena such as ammonia volatilization. Accordingly,
the soil requires the application of large quantities of
nitrogenous fertilizer despite its high nitrogen content.
(4) In surface layers, the presence of calcium derived from
seashells significantly affects the soil alkalinity. At
deeper layers, however, marine compounds, such as
sodium, magnesium, chlorine, and sulfur, exert a
greater influence on soil properties, such as soil pH, as
well as on the development of soil structures and the
elongation of crop roots.
188
H. Fujii et al.
the lower and upper ranges were as follows: exchangeable
potassium (K), lower range 0.01–0.09 g kg
−1 , upper range
0.22–0.43 g kg
−1 ; and exchangeable sodium (Na), lower
range 0.03–0.06 g kg
−1 , upper range 0.14–0.39 g kg
−1 .
Paddy fields in Tsuruoka City receive irrigation water from
the Akagawa, Ohoyama, or Ohoto River. Each river distributes irrigation water to the fields located on the left bank
of the river (Yazawa River land improvement district 1986,
1988; Shoryuji River land improvement district 1998)—that
is, the paddy fields on the right bank of the Yazawa–
Ohoyama River until the Akagawa River receive irrigation
water from the Akagawa River. Granite contains the highest
amounts of K and Na among igneous rocks (Naganori 1984).
Thus, the irrigation water in the area from the right bank of
the Yazawa–Ohoyama River until the Akagawa River may
contain higher amounts of K and Na due to the granitic
geological features that are present in the upper reaches of
the Akagawa River, resulting in higher amounts of
exchangeable K and Na in soils.
The third feature was not clear. The spatial distributions
of pH (4.6–6.3), available P (0.03–0.45 g P 2 O 5 kg
−1 ), and
base saturation percentage (19–97%) had no definite trend.
These chemical properties may not be affected by geological
features in the upper reaches of many rivers, but rather by
cultivation practices such as fertilizer and organic matter
management in each paddy field.
The following is a summary of the above discussion. The
clay mineral composition of alluvial paddy soils in Tsuruoka
City, Yamagata Prefecture, has a spatial characteristic that
can be affected by geological features, namely, green tuff
that is distributed in the upper reaches of all the Akagawa
River and Ohoyama River systems, and granite that is distributed in the upper reaches of the Akagawa River and the
Yazawa River, flow into the plain of Tsuruoka City. The
spatial characteristics of the chemical properties of the paddy
soils in Tsuruoka City can be divided based on three features. One is the spatial distribution of clay mineral composition and organic matter content of the soil that is derived
from different geological features in the upper reaches
between the Akagawa River and the Yazawa and Ohoto
Rivers, such as total N, PAC, CEC, and exchangeable Ca
and Mg. The second is affected by geological features in the
upper reaches of rivers that provide irrigation water, such as
exchangeable K and Na. The third is not affected by geological features in any upper reaches, such as pH, available
P, and base saturation percentage.
6.1.2 Soils in the Hachirogata Land Reclamation
Area
(1) Soil properties of the Hachirogata land reclamation
area
The Hachirogata land reclamation area is located to the east
of the Oga Peninsula, which is situated in the central part of
Akita Prefecture. A crossing point between latitude 40° N
and longitude 140° E is located within this area. Hachirogata
was once the second-largest lake in Japan, with a total area
of 22,024 ha. However, after a severe food shortage after
World War II, the lake underwent extensive reclamation in
1966 under the technical supervision of Dutch experts.
The central land reclamation area occupies 15,640 ha
(71% of the lagoon area), and the entire area is situated
below sea level (average elevation, −4 m above sea level [a.
s.l.]). An extremely moist and soft sludge-like heavy clay
soil covers approximately 80% of the land reclamation area.
This soil is characterized by the following unique properties
that are not commonly present in general farming soils:
(1) It contains an average clay content of 51% (72% at
maximum), half of which comprises minute particles
(diameter < 1 lm). The primary clay mineral is an
expansible 2:1 smectite.
(2) It exhibits relatively favorable chemical properties,
specifically a large base-exchange capacity. It naturally
contains large quantities of nitrogen, potassium, phosphate, silicate, and other compounds. The most abundant phosphate form is the A1 type, followed by the Fe
and Ca types.
(3) Soil pH mainly ranges from neutral to slightly alkaline,
which is uncommon among Japanese soils. This contributes to the loss of fertilizer nitrogen through phenomena such as ammonia volatilization. Accordingly,
the soil requires the application of large quantities of
nitrogenous fertilizer despite its high nitrogen content.
(4) In surface layers, the presence of calcium derived from
seashells significantly affects the soil alkalinity. At
deeper layers, however, marine compounds, such as
sodium, magnesium, chlorine, and sulfur, exert a
greater influence on soil properties, such as soil pH, as
well as on the development of soil structures and the
elongation of crop roots.
188
H. Fujii et al.