High concentrations of potassium fertilizers in soil tend to reduce the absorption of radioactive cesium in rice plants, according to a study that will be used in discussions on rice-planting standards for next fiscal year.
The results of the study, compiled in an interim report Dec. 25 by Fukushima Prefecture and the farm ministry, showed that paddies yielding rice with cesium levels exceeding the government safety standard of 500 becquerels per kilogram had relatively low potassium concentrations.
In addition, the higher levels of cesium in soil in these paddies were found in layers close to the surface.
Fukushima Prefecture, which asked rice farmers about their techniques and the amount of potassium fertilizers used, plans to continue the study on the assumption that multiple factors, such as soil properties, cultivation methods, amount of water and surrounding environments, can affect the rate of cesium absorption by the plants.
The prefecture collected soil samples from 22 rice paddies that produced rice with cesium levels above the safety standard and nine neighboring paddies where the rice was deemed safe.
The radioactive cesium is believed to have come from the crippled Fukushima No. 1 nuclear power plant.
In fields producing rice with high cesium levels, the concentration of potassium was 6.7 milligrams per 100 grams of soil, or about one-third the average of rice fields in Fukushima city, according to the study.
In those paddies, the cesium level in the soil layer from the surface to 5 centimeters deep was 3.6 times higher on average than for soil 5 to 15 centimeters from the surface, according to the study.
Many of these paddies were in narrow parts of mountain areas that are inaccessible to tractors and other large farming equipment. This landscape may have prevented the rice plants from taking root deep in the soil, allowing them to more easily absorb cesium from the upper layers, the report said.
In paddies carrying the normal concentrations of potassium, the cesium levels of the rice fell within the government's safety standard, the study showed.
(This article was written by Ryo Inoue and Shunsuke Kimura.)
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