The application of chemical fertilizer combined with farm manure is popularly practised in Chinese farming.In order to clarify the characteristics of nitrogen supply in such a fertilization system,pot experiments were conducted using ¹⁵N labelled crotalaria and ammonium sulphate as organic and inorganic nitrogen sources respecLively.Some preliminary results are summarized as follows:1.Rice plant utilized much more soil nitrogen than fertilizer nitrogen.treatments only about one third of the nitrogen assimilated by the rice plant came from the fertilizers.The ratio of soil nitrogen to fertilizer nitrogen taken up by rice palnt was closely related to the kinds of soil.In general,the more fertile the more soil nitrogen was assimilated by rice plant.