Efficiency of urea fertilizer and its improvement for lowland rice in Thailand

Recent research in rice production in Thailand emphasizes the improvement the efficacy of urea on rice.The emphasis is on minimizing N losses from lowland rice fields after application of urea and maximizing fertilizer N uptake by the rice crop as well as improving grain yield.This paper reviews the current literature dealing with some practical aspects of N fertilizer application on the efficacy of urea applied to lowland rice in Thailand as assessed by grain yield and 15N balance techniques.Data from the field 15N experiments revealed that recoveries at harvest ranged from 26-58% of the applied urea.The inefficient use of fertilizer is largely attributed to N losses from the floodwater-soil plant systems following urea application.It varied widely from 9 to 56% of the applied N, depending on factors such as soil type, climate, season, cultural practices etc.Ammonia volatilization and denitrification are considered the major processes of N loss.Balance studies using 15N labelled fertilizer have shown that N losses from urea are much greater reaching 50% of the applied N with broadcasting of urea directly into the floodwater 10 days after transplanting.Deep placement of urea as urea supergranules (USG) and proper incorporation of broadcast urea into the drained soil before transplanting have been shown frequently to reduce N loss and increase grain yield.N loss was found to be much lower when urea was broadcast onto drained soil for two days prior to flooding than when broadcast into floodwater 10 days after transplanting.The use of urease inhibitors either N-(n-butyl) thiophosphoric trianmide (nBTPT) or phenyl phosphorodiamidate (PPD) with urea was found ineffective in improving fertilizer N use by the rice crop or in reducing N loss or in increasing gram yield.Some possibilities for the improvement of the efficacy of urea fertilizer can be brought about by deep placement of USG or incorporating the fertilizer into soil without stagnant floodwater.Broadcast application of urea or granular urea onto drained soil followed by flooding two days later can offer considerable potential for improving the efficacy of urea in situations where proper placement of urea into the soil is impractical.