A study on the present status of nutrients and chemical properties of paddy soil in Fukushima Prefecture

Because no prefecture-wide survey of farmland soil has been conducted in Fukushima Prefecture since the 1990s, the current status of the prefecture's soil nutrients and chemical properties is unclear. Information on local soil fertility and changes over time in soil attributes is important to prefectural agricultural administration. Moreover, after the nuclear power plant accident of March 2011 caused contamination of the soil with radioactive substances, potash was applied as a supplement in Fukushima Prefecture to suppress the uptake of radiocesium by paddy rice. Soil potassium content is consequently thought to have increased, but the variability of this increase in different locations is unknown. Any debate about further measures to suppress the uptake of radiocesium requires the collection and analysis of relevant information. This study was conducted to investigate the nutrient content and chemical properties of paddy soil in Fukushima Prefecture and to elucidate the increase in its exchangeable potassium content since 2011. (1)The Ministry of Agriculture, Forestry and Fisheries collected samples of farmland soil in Fukushima Prefecture in the fall and winter of 2011 to ascertain concentration of radioactive substances. Using these samples, we elucidated the status of nutrients and chemical properties of paddy soil in Fukushima Prefecture as of 2011 and how these have changed since the 1990s. We also examined the relationship between the results of this investigation and application of rice straw and soil amendments during the same period. Our results showed that, compared to levels in the 1990s, nutrients and chemical properties of paddy soil in Fukushima Prefecture as of 2011 had not deteriorated to a level that called for urgent action, but nevertheless indicated remediation of pH and available silicate content. (2)Following the 2011 accident at the Tokyo Electric Power Fukushima Daiichi Nuclear Power Station, large amounts of potassium fertilizer were applied to paddy soils to reduce the uptake of radiocesium by rice. The Fukushima Prefectural Government recommended that extra potassium fertilizer be applied before planting with the goal of increasing the soil exchangeable K content (ExK) to above 250 mg K2O /kg soil. After collecting ExK data for paddy fields in Fukushima Prefecture in 2011 (n = 863), 2014 (n = 730), and 2017 (n = 577), we divided Fukushima Prefecture into 13 regions and investigated the changes in ExK from 2011 onward in these different regions. In addition, we investigated soil chemical properties related to the increase in ExK. In regions where rice with high concentrations of radiocesium was harvested in 2011, supplemental potassium fertilizer (250 kg /ha/year) was applied, resulting in an increase in ExK by 313 mg K2O/kg soil during the period from 2011 to 2014. In 2017, 81% of all sampling sites had ExK greater than 250 mg K2O /kg soil. Simple linear regression analysis identified effective CEC (ECEC) and Esub(K) (K equivalent/ECEC) as soil chemical properties correlated with the magnitude of ExK increase from 2011 to 2014 in all 13 regions. CEC, however, was not correlated with the magnitude of ExK increase. The coefficient of determination (R**2) for the linear model of the relationship between magnitude of ExK increase from 2011 to 2017 and the quantity of additional K fertilizer applied was 0.41. The goodness of fit of the model was improved (R**2 = 0.56) by multiplying the quantity of additional potassium fertilizer by the reciprocal of Esub(K), suggesting that Esub(K) (or ECEC, which is used to calculate Esub(K)) may serve as an indicator of ExK maintenance.