However, more than 2000 ha of agricultural fields remain
untouched in a difficult return zone. For further decontamination and subsequent agricultural usage, it is important to
investigate the behavior of radiocesium in these areas after
7 years.
6.6.5 Potassium Fertilizer Application
for Mitigation of Crop Radiocesium
Uptake
Because of their long half-lives, there is concern that radiocesium isotopes will remain on the surface of agricultural
land and persist for a long time. Therefore, from March 2011
we started monitoring the radiocesium in the soil and agricultural products that have been collected from agricultural
land in Fukushima Prefecture, and we have investigated the
distribution of radiocesium in farmland. Based on these data,
the Nuclear Emergency Response Headquarters has indicated that there are rice planting areas in all the regions of
the prefecture, except in the 20-km exclusion zone and the
deliberate evacuation zone (DEZ). However, the brown rice
produced in some areas of Fukushima Prefecture exceeded
the provisional regulation radioactivity level for agricultural
crops at that time (>500 Bq kg
−1 ). We investigated the radiocesium content of soil and brown rice and the
exchangeable potassium (Ex-K) content of soil. There was a
high correlation between the radiocesium in the brown rice
and the Ex-K in the soil (Fig. 6.40), while no correlation was
observed between the radiocesium in the soil and the radiocesium in the brown rice from these fields (Fig. 6.41).
(1) Effect of potassium fertilizer application timing on
the root uptake of radiocesium in brown rice
We investigated the optimum timing and amount of K
application needed to reduce the
134 Cs and
137 Cs uptake by
brown rice. A field was contaminated with radiocesium; the
brown rice produced in this field in 2011 exceeded the
provisional regulation value for radiocesium and
radiostrontium at that time (500 Bq kg
−1 ), and the planting
of rice in this area was restricted in 2012.
The measured
137 Cs radioactivities in brown rice were
32 Bq kg
−1 without K application, 5.0 Bq kg
−1 with
8.0 g m
−2 of basal K, 15 Bq kg
−1 with K applied at 50 days
after transplanting, and 36 Bq kg
−1 with K applied at
80 days (Fig. 6.42). Therefore, the basal application of K
fertilizer decreased the
137 Cs uptake the most. The
137 Cs
radioactivity of brown rice was 32 Bq kg
−1 without K
application, 4.0 Bq kg
−1 with 8.0 g m
−2 of K, and 2.0 Bq
kg
−1 with 16 g m
−2 of K (Fig. 6.43). The application of K at
a level above 16 g m
−2 had no further effect. Thus,
increasing the K rate to 16 g m
−2 decreased the
137 Cs concentration in brown rice. We focused on exchangeable radiocesium as a proxy for available radiocesium because
there was no correlation between total radiocesium in paddy
soil and that in brown rice (Saito et al. 2012, 2015)
(Table 6.23). Heavy applications of K decreased the mean
radioactivity of exchangeable
137 Cs in the soil from 234 to
25 Bq kg
−1 dry weight. Hence, heavy applications of K
fertilizer during the early growing period could decrease the
137 Cs uptake by rice plants and the concentration of
exchangeable
137 Cs in the soil.
Table 6.21 Aboveground plant
dry weight, concentration and the
plowing amount of various green
manure crops
Experimental
plot
Aboveground part
dry weight
T–C
T –N
137
Cs
(t ha
−1
)
(%)
(t
ha
−1
)
(%)
(kg ha
−1
)
(B q
kg
−1
)
(kBq ha
−1
)
Sesbania
5.2
44.4
229
2.1
107
88
467
Crotarralia
2.2
42.5
95.2
1.4
33
369
841
Buck wheat
1.1
44.3
49.6
0.4
3.4
121
140
Part of the data is based on Farming resumption demonstrated technical information (Fukushima Agricultural
Technology Centre 2019)
Table 6.22 Above-ground part
dry weights and
137
Cs
concentration of Italian ryegrass
Experimental plot (Previous crops)
Above-ground part dry weight (t ha
−1
)
137
Cs (Bq kg
−1
)
Sesbania
4.6
71
Crotarralia
2.4
36
Buckwheat
1.1
29
No cultivation
0.8
24
Part of the data is based on Farming resumption demonstrated technical information (Fukushima Agricultural
Technology Centre 2019)
6 Tohoku Region
233
untouched in a difficult return zone. For further decontamination and subsequent agricultural usage, it is important to
investigate the behavior of radiocesium in these areas after
7 years.
6.6.5 Potassium Fertilizer Application
for Mitigation of Crop Radiocesium
Uptake
Because of their long half-lives, there is concern that radiocesium isotopes will remain on the surface of agricultural
land and persist for a long time. Therefore, from March 2011
we started monitoring the radiocesium in the soil and agricultural products that have been collected from agricultural
land in Fukushima Prefecture, and we have investigated the
distribution of radiocesium in farmland. Based on these data,
the Nuclear Emergency Response Headquarters has indicated that there are rice planting areas in all the regions of
the prefecture, except in the 20-km exclusion zone and the
deliberate evacuation zone (DEZ). However, the brown rice
produced in some areas of Fukushima Prefecture exceeded
the provisional regulation radioactivity level for agricultural
crops at that time (>500 Bq kg
−1 ). We investigated the radiocesium content of soil and brown rice and the
exchangeable potassium (Ex-K) content of soil. There was a
high correlation between the radiocesium in the brown rice
and the Ex-K in the soil (Fig. 6.40), while no correlation was
observed between the radiocesium in the soil and the radiocesium in the brown rice from these fields (Fig. 6.41).
(1) Effect of potassium fertilizer application timing on
the root uptake of radiocesium in brown rice
We investigated the optimum timing and amount of K
application needed to reduce the
134 Cs and
137 Cs uptake by
brown rice. A field was contaminated with radiocesium; the
brown rice produced in this field in 2011 exceeded the
provisional regulation value for radiocesium and
radiostrontium at that time (500 Bq kg
−1 ), and the planting
of rice in this area was restricted in 2012.
The measured
137 Cs radioactivities in brown rice were
32 Bq kg
−1 without K application, 5.0 Bq kg
−1 with
8.0 g m
−2 of basal K, 15 Bq kg
−1 with K applied at 50 days
after transplanting, and 36 Bq kg
−1 with K applied at
80 days (Fig. 6.42). Therefore, the basal application of K
fertilizer decreased the
137 Cs uptake the most. The
137 Cs
radioactivity of brown rice was 32 Bq kg
−1 without K
application, 4.0 Bq kg
−1 with 8.0 g m
−2 of K, and 2.0 Bq
kg
−1 with 16 g m
−2 of K (Fig. 6.43). The application of K at
a level above 16 g m
−2 had no further effect. Thus,
increasing the K rate to 16 g m
−2 decreased the
137 Cs concentration in brown rice. We focused on exchangeable radiocesium as a proxy for available radiocesium because
there was no correlation between total radiocesium in paddy
soil and that in brown rice (Saito et al. 2012, 2015)
(Table 6.23). Heavy applications of K decreased the mean
radioactivity of exchangeable
137 Cs in the soil from 234 to
25 Bq kg
−1 dry weight. Hence, heavy applications of K
fertilizer during the early growing period could decrease the
137 Cs uptake by rice plants and the concentration of
exchangeable
137 Cs in the soil.
Table 6.21 Aboveground plant
dry weight, concentration and the
plowing amount of various green
manure crops
Experimental
plot
Aboveground part
dry weight
T–C
T –N
137
Cs
(t ha
−1
)
(%)
(t
ha
−1
)
(%)
(kg ha
−1
)
(B q
kg
−1
)
(kBq ha
−1
)
Sesbania
5.2
44.4
229
2.1
107
88
467
Crotarralia
2.2
42.5
95.2
1.4
33
369
841
Buck wheat
1.1
44.3
49.6
0.4
3.4
121
140
Part of the data is based on Farming resumption demonstrated technical information (Fukushima Agricultural
Technology Centre 2019)
Table 6.22 Above-ground part
dry weights and
137
Cs
concentration of Italian ryegrass
Experimental plot (Previous crops)
Above-ground part dry weight (t ha
−1
)
137
Cs (Bq kg
−1
)
Sesbania
4.6
71
Crotarralia
2.4
36
Buckwheat
1.1
29
No cultivation
0.8
24
Part of the data is based on Farming resumption demonstrated technical information (Fukushima Agricultural
Technology Centre 2019)
6 Tohoku Region
233