FERTILIZER NITROGEN UPTAKE BY RICE FROM UREA-AMMONIUM NITRATE SOLUTION VS GRANULAR UREA

Granular urea is the primary N fertilizer source for rice (Oryza sativ a L.) production in the southern USA. New cultivars respond to higher rates of N fertilizer applied preflood. Application of high rates ofte n results in distribution problems when granular urea is used as the N source. Because liquid fertilizer sources can reduce distribution pro blems, this study was initiated to evaluate the efficiency of urea-amm onium nitrate solution (UAN, 28-0-0) as a N source for rice. Each N fe rtilizer source was applied in three split applications at a rate of 8 4 kg N ha(-1) just prior to flooding (preflood), 33.6 kg N ha(-1) at 1 .3-cm internode elongation (IE), and 33.6 kg N ha(-1) 10 d after IE (I E + 10d) onto 'Newbonnet' rice that was seeded into microplots (0.58 m (2)). The fertilizer sources consisted of granular urea of UAN with ei ther the urea, NH4, or NO3 labeled with N-15 at each application time. At all three application times, the highest fertilizer N uptake occur red when granular urea was the N source. Application of granular urea resulted in 72.5% recovery of the fertilizer N in the plant compared w ith 52.6% recovery of the fertilizer N from UAN. Application of granul ar urea also resulted in significantly higher grain yields than UAN. G ranular urea was superior to UAN as a N source for rice production due to the inefficient uptake of NO3-N from the preflood application (9.8 %). However, these data show that rice can efficiently utilize NO3-N f rom applications made at IE or later (73.7% recovery).