This investigation was undertaken as the fundamental studies on the scientific improvement of the soybean plant applied as green manure. The experiment was carried out with the tops and roots at the different stages of grow of soybean, harvested from both the dry-farm and the paddy field ; and the following results were obtained. 1) The nitrogen content was maximum in the plants harvested on the 3 rd or 13th in June. 2) The amounts of water soluble nitrogen and other substance were found in inverse proportion with the age of soybean plant and the pH value was larger at the young stage of growth, and decreased toward the maturity, while the reduction power was contrary to the change of pH value. 3) The carbon and lignin contents were in parallel with the age of soybean plant and other soluble substance decreased at the early stage of growth, and increased afterward. 4) The ash content in the top of soybean increased at the early stage of growth, and decreased afterward, while the change of ash content of the root of soybean was contrary. 5) The decomposition of the top of soybean took place more easily in the harvested plant on the 13 th and 23 rd of June than that of the young stage and was more difficult toward the maturity. The decomposition of root was difficult with the age of soybean plants through the growth. 6) The top of soybean plant decomposed more rapidly than its root. 7) No definite relation was found between the ash content and the rate of decomposition of soybean plants. 8) The carbon-nitrogen ratio seemed to be the most important factor which influence the rate of decomposition of soybean plants as green manure and the importance of carbon-nitrogen ratio in this regard could be understood more clearly if the property of water soluble substance, the amounts of other extract and lignin were considered together.