Nitrogen Fertilizer Sources and Timing before Flooding Dry‐Seeded, Delayed‐Flood Rice

Urea is the primary N source used for the large preflood N application in delayed‐flood rice ( Oryza sativa L.) production in the southern United States. Urea is prone to substantial NH 3 volatilization losses, however, if fields are not flooded quickly. Most delayed‐flood rice fields require 5 to 10 d to flood. Consequently, a study was conducted to evaluate the use of less NH 3 –volatile N sources for the preflood N application. The objectives were to evaluate the NH 3 volatilization loss and impact on N uptake and rice yield when urea, urea plus the urease inhibitor N ‐( n ‐butyl)thiophosphoric triamide (NBPT), (NH 4 ) 2 SO 4 , or a urea‐(NH 4 ) 2 SO 4 (UAS) blend were applied preflood and a flood established 1, 5, or 10 d after N application. When flooding was delayed for 5 or 10 d after N application, NH 3 volatilization was the least for urea + NBPT (2–10%) and (NH 4 ) 2 SO 4 (4–5%) and they produced the highest rice N uptake and yield. The UAS blend that had NH 3 volatilization losses (11–15%) at 5 and 10 d after application that were intermediate between urea (17–24%) and (NH 4 ) 2 SO 4 or urea + NBPT also had N uptake and grain yield intermediate between these N sources. Urea should only be used if ∼2 d are required to flood a field. If 3 to 5 d are required to flood a field, then UAS has some merits but it is not as consistent as (NH 4 ) 2 SO 4 or urea + NBPT. When >5 d are required to flood, (NH 4 ) 2 SO 4 or urea + NBPT should be used.