A new fertilization method with deep placement of slow-release N fertilizers, such as coated urea and lime nitrogen (LN) (calcium cyanamide) at 20 cm depth was found to promote soy bean seed yield. In the present study, the effect of deep placement of LN was investigated on different parameters such as growth, N accumulation, NZ fixation activity and yield of soy bean by applying LN at different rates in the rotated paddy field of Niigata, Japan. In addition to the basal fertilizer, ammonium sulphate (16 kg N ha(-1)), deep placement of LN was conducted by applying various amounts such as 50 kg N ha(-1) (A50), 100 kg N ha(-1) (A100) and 200 kg N ha(-1) (A200) at 20 cm depth in separate plots. A N-15-labelled LN fertilizer was also employed for each of the above treatments to calculate N utilization from LN in separate plots. Soya bean plant growth and N-2 fixation activity were periodically analysed. Both plant growth and N accumulation were found to increase with LN treatment compared with control plants. An increase in N-2 fixation activity was found in the A100 plots. The total seed yield was the highest in the deep placement of LN with A100 (73 g per plant) compared with other treatments. The visual quality of harvested seeds also showed that A100 enhanced the quality of seeds compared with other treatments. Thus, it is suggested that N fertilization management with particular reference to optimum amount of fertilizers is important for maximum growth. N-2 fixation and enhancement of seed yield of soy bean.