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

Abstract

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 NH3 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 NH3–volatile N sources for the preflood N application. The objectives were to evaluate the NH3 volatilization loss and impact on N uptake and rice yield when urea, urea plus the urease inhibitor N‐(n‐butyl)thiophosphoric triamide (NBPT), (NH4)2SO4, or a urea‐(NH4)2SO4 (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, NH3 volatilization was the least for urea + NBPT (2–10%) and (NH4)2SO4 (4–5%) and they produced the highest rice N uptake and yield. The UAS blend that had NH3 volatilization losses (11–15%) at 5 and 10 d after application that were intermediate between urea (17–24%) and (NH4)2SO4 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 (NH4)2SO4 or urea + NBPT. When >5 d are required to flood, (NH4)2SO4 or urea + NBPT should be used.