SIGNIFICANCE OF TIMING AND METHOD OF N FERTILIZER APPLICATION FOR THEN-USE EFFICIENCY IN FLOODED TROPICAL RICE

N-use efficiency in flooded tropical rice is usually low. Fertilizer N losses result mainly from losses of volatile NH3 after broadcast appl ication of urea into floodwater between transplanting and early tiller ing which is a common practice of farmers. Losses appear predominantly during the first week after urea application. With broadcast and inco rporation of N into soil at transplanting losses may be reduced but ar e still substantial. Deep placement of urea supergranules (USG) has no t been adopted by farmers because it is very laborious. A new applicat ion technique, namely injection of dissolved urea into the upper soil layer, was developed by which fertilizer N losses were effectively min imized while at the same time allowing flexible timing of application independent of crop stage and water management. It provides N-use effi ciency equal to that achieved by USG point placement but is less labor -intensive.