We investigated the effect of hairy-vetch treatment with changes in fertilizer application on nitrogen absorption and yield of cabbage in a heavy clay soil field converted from a paddy field. Hairy vetch incorporated in soil immediately decomposed, and approximately 50％ of the hairy vetch decomposed at the cabbage harvesting time. The results of ¹⁵N tracer experiments indicated that 30％–40％ of nitrogen from the hairy vetch incorporated into the soil was absorbed by cabbages, and half of that remained after cabbages were harvested. Following hairy-vetch treatment, the quantity of nitrogen absorbed by cabbages increased from the early period of growth, and the rate of decline of the quantity of nitrogen absorbed owing to reduced fertilizer application in the hairy-vetch treatment was smaller than that in the no-planting treatment. The yield of cabbage increased following the hairy-vetch treatment and did not significantly decrease despite the fertilizer application being reduced. Therefore, we concluded that cabbages may be cultivated with reduced fertilizer application following hairy-vetch treatment while maintaining productivity and soil nitrogen fertility in a heavy clay soil field converted from a paddy 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 m⁻². 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 N or 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.