Salt removal from salt-damaged agricultural land using the scraping method combined with natural rainfall in the Tohoku district, Japan

The tsunami that followed the Great East Japan Earthquake in 2011 caused catastrophic salt damage to agricultural lands in Tohoku district on the Pacific coast of Japan. Large amounts of water would be required to leach away the salt from the salt-damaged farmlands. In addition, the salt cannot be washed away by using natural water sources such as rivers and ponds because the irrigation infrastructure, such as water canals, was destroyed and has not yet been repaired. Therefore, it is important to develop new salt-removal techniques to restore agricultural lands without using large amounts of water. We tested a new technique for salt removal: scraping the soil, followed by exposure of the soil to natural rainfall. In the test plots, which were located in a paddy field, soluble salt in the soil accumulated on the soil surface through evaporation. Next, the top 2 cm of the soil surface was scraped away, and the scraped soil was placed in open-topped plastic boxes that were exposed to natural rainfall. Soil electrical conductivity, exchangeable sodium percentage, and ion content decreased exponentially as cumulative rainfall increased. In addition, soil electrical conductivity decreased below the upper threshold limit required for crop cultivation after cumulative rainfall surpassed 217 mm, about 50 days after the start of the experiment. Therefore, our technique may be used to remove salt from agricultural soils in a relatively short time.