Effect of Drainage on the Yield of Rice and Occurrence of “Hagare” Symptom in the Paddy Fields in Nagasaki Prefecture

Abstract

In a specific area in the northern mountainous region of Sasebo city, Nagasaki, a strange disorder called ‘Rice Hagare Symptom’ has been observed for over 40 years. In the paddy fields where the Rice Hagare Symptom was observed, we examined the effects of water management on the occurrence of the Rice Hagare Symptom and yield for 3 years. Under the ordinary water management, the redox electric potential of the paddy soil was low showing a strong reduced state and the upper leaves on the rice plant started to wither from 40 days after transplanting. Such a rice community showed many yellowish brown spots in the field, and had reduced yield. We drained and dried the paddy fields 2 or 3 times each for 3–7 days after the midseason drainage period. By this treatment the redox electric potential was elevated, and the Rice Hagare Symptom diminished. The ripening rate and yield were also increased especially in the hot year when yield is reduced. The effect of the drainage was low in the cold year. It was reported that the growth disorder due to soil reduction was caused by excess or lack of nourishment. From the inorganic component content of rice plant and physicochemical property of soil we considered that the Rice Hagare Symptom is probably caused by excess iron.