抄録
Natural Farming (NF), characterized by no external input such as chemical fertilizers and biocides, is a farming method aiming to produce safe food by an environmentally sound method. Examination of the long-term change in the productivity under NF will provide us with important information for the low input and sustainable rice production. Yield surveys were conducted in 17 prefectures of three climatically different districts (the Tohoku, Kinki, and Chugoku) in Japan in 1990 and 542 sets of data were collected from NF paddies, differing in years (l-50) afterconvertion to NF (YAC). The average brown rice yields of NF ranged from 352gm-2 in Okayama to 504gm-2 in Akita. The variation in NF yield generally reflected the average regional yield by conventional farming (CF) (p<0.05), but the yield gap between NF and CF was larger, the higher the CF yield. The average yield gap for the overalldata was about -66gm-2, 13% of the regional average in 1990. Although no significant increase in yield with YAC was observed in the data for each district, the yield gap was narrowed particularly after around 15 YAC onward in the Tohoku district. The yield in the Chugoku district did not show any notable increase with the extension of YAC. The yield gain with extension of YAC was modest, but the level of productivity was maintained at about 79-98% of the regional average.
