Annual changes in soil fertility after ceasing fertilization in an unfertilized paddy field and factors limiting rice growth in the field

To estimate the environmental conditions that enable soil fertility to be maintained without fertilization, fertilization in a paddy field at Kyoto University Farm was ceased in 2010. We then evaluated annual changes in soil fertility in the field until 2015. Planted and unplanted plots were established, and surface soil samples were collected from both plots before transplanting and after harvesting of rice plants. In 2012, surface soil samples and rice shoots were collected five times during the cropping period; the physicochemical properties of the soils, yield of the rice plants, and uptake of nitrogen (N) and potassium (K) by rice plants were analyzed. During the experimental period, the yield of unhulled rice was relatively stable, ranging from 551 to 639 g /square m. However, the levels of mineralizable N, total N, nonexchangeable K, and particulate organic matter in the surface soil of both planted and unplanted plots began to significantly decrease after three cropping seasons. During the cropping period, soil ammonium N and exchangeable K content also decreased with increasing rice growth. Moreover, in 2013, fertilizer trials with Nor K application were conducted under both field and pot conditions to identify which element limited rice growth. Distilled water was used for irrigation in the pot experiment, whereas river or underground water was used for irrigation in the field experiment; total N and K concentration was measured at each irrigation event. The fertilizer trials demonstrated that the element limiting rice growth was K or N under pot or field conditions, respectively. The amount of K supplied by irrigation was considered sufficient to overcome the low K status of the unfertilized soil and meet plant demands. We compared these results with previous reports and discussed the soil and geographical conditions necessary to maintain soil fertility without fertilization.