For the added soil, soils from mountains and mountainous
areas in the vicinity of the countermeasure area were selected
based on the results of analyses of soil physicochemical
properties and the amount of soil available. Selection criteria
for the replacement soil include: (1) a low content of heavy
metals such as Cd, and no other harmful substances; (2) the
conventional agricultural production volume should be
secured after the restoration of the soil; (3) a large amount of
the soil can be collected and transported without difficulty;
(4) the area where the additional soil is collected must not be
a designated area of buried cultural assets. From the viewpoint of avoiding recontamination and maintaining the
agricultural productivity of the arable land, it was desirable
that the soil contained as much clay as possible (15% or
more) and had a high cation exchange capacity (CEC).
The replacement soil is the lower soil of mountain forest,
which is usually low in fertility. Therefore, the soil, as it is, is
not suitable for use in the plow layer for cultivating paddy
rice. For this reason, it is necessary to improve the soil by
applying calcareous materials, phosphoric acid materials,
organic materials, and so on, in order to implement stable
rice cultivation after the soil dressing. In paddy fields of the
Jinzu river basin, 6000 kg ha
−1 of silicate lime, 6000 kg
ha
−1 of fused phosphate, and 2000 kg ha
−1 of poultry
manure were sprayed after the soil dressing.
(2) Effects of soil dressing
Before the restoration work, the average Cd concentration in
the plow layer soil of fields in the Jinzu river basin that were
subject to the restoration measures was 1.12 mg kg
−1 , and
the average Cd concentration in brown rice of the area was
0.99 mg kg
−1 . After the restoration work, the average Cd
concentration in the plow layer soil and that in brown rice of
the area had decreased to 0.13 mg kg
−1 and 0.08 mg kg
−1 ,
respectively. Thus, the effect of the restoration work in
reducing cadmium concentrations was clear (Fig. 8.16)
(Toyama Prefecture 2012a).
In 1979, we started conducting soil dressing as part of the
Pollution Control Special Land Improvement Project
(“Kōgai bōjo tokubetsu tochi kairyō jigyō” in Japanese). In
2009, we checked Cd concentrations of the plow layer soil
and brown rice at six sites where the restoration work was
carried out in 1979 and 1980 (three sites for each year). The
average Cd concentration in the plow layer soil and in the
brown rice was 0.12 mg kg
−1 and 0.07 mg kg
−1 , respectively, thus confirming the persistence of the effect of the
restoration work.
Please note that the concentration of Cd in the Jinzu River
has now declined to the natural environmental level due to
the efforts of victims’ organizations and companies. It is
important to remember that those efforts contributed greatly
to the prevention of recontamination.
(3) Crop cultivation after restoration
The conditions of the soil of the paddy fields reconstructed
by soil dressing are quite different from those of the previous
paddy fields at the sites. The features of the soil of the new
plow layer of the restored paddy fields are as follows:
(1) while the capacity to hold fertilizers is large, the content
of humic substances is small; (2) water permeability is poor
as it is clayey soil; and (3) currently, the nutrient supply is
small, including available phosphate, available silicate, and
available nitrogen. As the result, the growth of rice plants
has the tendencies of fewer stalks, light color, and the varied
degree of growth in one field.
Therefore, the Toyama Agriculture and Forestry Promotion Center prepared “rice cultivation guidelines in paddy
fields with additional soil.” The center focused mainly on
teaching so that a conventional yield level could be secured
by conducting proper management for irrigation, fertilization, and promotion of soil maturation (Toyama Prefecture
2012a).
According to a report, the more time that has passed since
restoration work, the more the content of available soil
nitrogen has increased (Toyama Prefecture 2012b). The
report showed that the amount of nitrogen fertilizer required
to ensure a target yield of 5400 kg ha
−1 is logarithmically
reduced in accordance with the number of elapsed years
Fig. 8.16 Effect of the restoration work on the decrease of cadmium
concentration. The values before restoration show the mean values of
the samples of the soil or the brown rice taken from 544 places during
the years 1971 and 1976. The values after restoration show the mean
values of the soil samples of 1010 places or the brown rice samples of
896 places during the years 1981 and 2010. Created by Jun Koike
based on Toyama Prefecture (2012a)
288
N. Ogawa et al.
areas in the vicinity of the countermeasure area were selected
based on the results of analyses of soil physicochemical
properties and the amount of soil available. Selection criteria
for the replacement soil include: (1) a low content of heavy
metals such as Cd, and no other harmful substances; (2) the
conventional agricultural production volume should be
secured after the restoration of the soil; (3) a large amount of
the soil can be collected and transported without difficulty;
(4) the area where the additional soil is collected must not be
a designated area of buried cultural assets. From the viewpoint of avoiding recontamination and maintaining the
agricultural productivity of the arable land, it was desirable
that the soil contained as much clay as possible (15% or
more) and had a high cation exchange capacity (CEC).
The replacement soil is the lower soil of mountain forest,
which is usually low in fertility. Therefore, the soil, as it is, is
not suitable for use in the plow layer for cultivating paddy
rice. For this reason, it is necessary to improve the soil by
applying calcareous materials, phosphoric acid materials,
organic materials, and so on, in order to implement stable
rice cultivation after the soil dressing. In paddy fields of the
Jinzu river basin, 6000 kg ha
−1 of silicate lime, 6000 kg
ha
−1 of fused phosphate, and 2000 kg ha
−1 of poultry
manure were sprayed after the soil dressing.
(2) Effects of soil dressing
Before the restoration work, the average Cd concentration in
the plow layer soil of fields in the Jinzu river basin that were
subject to the restoration measures was 1.12 mg kg
−1 , and
the average Cd concentration in brown rice of the area was
0.99 mg kg
−1 . After the restoration work, the average Cd
concentration in the plow layer soil and that in brown rice of
the area had decreased to 0.13 mg kg
−1 and 0.08 mg kg
−1 ,
respectively. Thus, the effect of the restoration work in
reducing cadmium concentrations was clear (Fig. 8.16)
(Toyama Prefecture 2012a).
In 1979, we started conducting soil dressing as part of the
Pollution Control Special Land Improvement Project
(“Kōgai bōjo tokubetsu tochi kairyō jigyō” in Japanese). In
2009, we checked Cd concentrations of the plow layer soil
and brown rice at six sites where the restoration work was
carried out in 1979 and 1980 (three sites for each year). The
average Cd concentration in the plow layer soil and in the
brown rice was 0.12 mg kg
−1 and 0.07 mg kg
−1 , respectively, thus confirming the persistence of the effect of the
restoration work.
Please note that the concentration of Cd in the Jinzu River
has now declined to the natural environmental level due to
the efforts of victims’ organizations and companies. It is
important to remember that those efforts contributed greatly
to the prevention of recontamination.
(3) Crop cultivation after restoration
The conditions of the soil of the paddy fields reconstructed
by soil dressing are quite different from those of the previous
paddy fields at the sites. The features of the soil of the new
plow layer of the restored paddy fields are as follows:
(1) while the capacity to hold fertilizers is large, the content
of humic substances is small; (2) water permeability is poor
as it is clayey soil; and (3) currently, the nutrient supply is
small, including available phosphate, available silicate, and
available nitrogen. As the result, the growth of rice plants
has the tendencies of fewer stalks, light color, and the varied
degree of growth in one field.
Therefore, the Toyama Agriculture and Forestry Promotion Center prepared “rice cultivation guidelines in paddy
fields with additional soil.” The center focused mainly on
teaching so that a conventional yield level could be secured
by conducting proper management for irrigation, fertilization, and promotion of soil maturation (Toyama Prefecture
2012a).
According to a report, the more time that has passed since
restoration work, the more the content of available soil
nitrogen has increased (Toyama Prefecture 2012b). The
report showed that the amount of nitrogen fertilizer required
to ensure a target yield of 5400 kg ha
−1 is logarithmically
reduced in accordance with the number of elapsed years
Fig. 8.16 Effect of the restoration work on the decrease of cadmium
concentration. The values before restoration show the mean values of
the samples of the soil or the brown rice taken from 544 places during
the years 1971 and 1976. The values after restoration show the mean
values of the soil samples of 1010 places or the brown rice samples of
896 places during the years 1981 and 2010. Created by Jun Koike
based on Toyama Prefecture (2012a)
288
N. Ogawa et al.