**8. Conclusion**

Potential soybean seed yield may over 10 ton ha-1 based on recent world record soybean production in the USA. However, the average soybean seed yield is only 2.56t. Soybean seed yield is proportional to the total assimilation of nitrogen. Soybean seeds contain an extraor‐ dinary high concentration of protein (about 35%), therefore, one t of seeds requires about 70-90 kg of N. To get a high seed yield, soybean plants need to assimilate a sufficient amount of nitrogen not only during vegetative stages but also during reproductive stages. At first, a high and long lasting activity of nodules during the pod filling stage is the most important factor to get high seed yield. To obtain optimum nodulation and nitrogen fixation activity, physical, chemical and biological conditions in the soil are very important in addi‐ tion to appropriate cultivation method (fertilization, planting date, planting density, weed and pest management etc.). Inoculation of effective bradyrhizobium strains may improve soybean growth and seed yield. Although it is well known that a heavy supply of nitrogen fertilizer severely depresses nodulation and nitrogen fixation of soybean, a continuous sup‐ ply of a low level of combined nitrogen from the subsoil (eg. soil organic matter) may sup‐ port the plant vigor and photosynthetic activity and promote nitrogen fixation. The use of coated urea and lime nitrogen for deep placement has been successful in various types of soil in Japan.
