Effect of Irrigation Rate On Urea Incorporation.

Urea is the leading nitrogen (N) fertilizer used in the United States.  Although it possesses a high percent of N (46%), the process of hydrolysis lends it to N loss through volatilization.  Incorporating urea into the soil will limit volatilization, however; mechanical incorporation is not always feasible so incorporation with irrigation water can substitute.  There is limited field research on irrigation rates needed to limit N volatilization after urea is applied.  Ammonia volatilization was monitored at six different irrigation rates; 0.00, 2.54, 6.35, 12.70, 19.05, and 25.40 mm from a center pivot irrigation system.  These trials were on a sandy soil in winter wheat following potatoes.  The N loss as ammonia 24 days after application was 60.06, 53.91, 38.73, 17.31, 5.55, 2.80 % N applied for the 0.00, 2.54, 6.35, 12.70, 19.05, and 25.40 mm of irrigation respectively.  Every irrigation rate experienced immediate N loss with volatilization rates increasing until 7 days following application where volatilization rate began to decrease.  Nine days after application most treatments showed little to no volatilization losses.  Total N volatilization was greatest at the 0.00 through 6.35 mm irrigation rates and losses were significantly decreased at rates greater than 6.35 mm of irrigation.  Plotting the loss by irrigation rate gave an exponential expression; (% N loss) = 62.6558e^(-0.1616*irrigation rate).  This relationship confirms that 12 mm of irrigation is enough water to reduce N loss from volatilization below 10% N applied.  Soil nitrate and ammonium levels, along with wheat total N are currently being analyzed to confirm the loss of N at the site.