Significance of timing and method of N fertilizer application for the N-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 application of urea into floodwater between transplanting and early tillering which is a common practice of farmers. Losses appear predominantly during the first week after urea application. With broadcast and incorporation of N into soil at transplanting losses may be reduced but are still substantial. Deep placement of urea supergranules (USG) has not been adopted by farmers because it is very laborious. A new application technique, namely injection of dissolved urea into the upper soil layer, was developed by which fertilizer N losses were effectively minimized while at the same time allowing flexible timing of application independent of crop stage and water management. It provides N-use efficiency equal to that achieved by USG point placement but is less labor-intensive.