When you have to delay flooding, it's worth the higher cost of ammonium sulfate to prevent loss of yield, Arkansas researchers say.

Arkansas Agricultural Experiment Station researchers have found a way to provide adequate nitrogen fertilizer for rice under less than ideal conditions.

Preflood nitrogen fertilizer is typically applied aerially as urea at around the four- to five-leaf growth stage onto a dry soil surface immediately prior to flooding, said Rick Norman, a University of Arkansas Division of Agriculture scientist who studies soil fertility for rice.

Once this preflood nitrogen fertilizer has been applied, a permanent flood should be established as quickly as possible, preferably within five days of the nitrogen application.

“The flood incorporates the nitrogen fertilizer into the soil,” Norman said. “If flooding is delayed, urea loses nitrogen to the air because of ammonia volatilization, especially on soils with higher pH levels. If the flood can not be established in a timely manner then ammonia volatilization losses from urea can be substantial.”

Because nitrogen fertilizer is essential for obtaining good rice yields, Norman and Division of Agriculture scientists Nathan Slaton and Chuck Wilson looked for an alternative nitrogen source that would be more stable before flooding.

“Ammonium sulfate is a granular nitrogen fertilizer that is less prone to ammonia volatilization losses than urea,” Norman said. “Unfortunately it's about twice as expensive as urea.”

Norman, Slaton and Wilson conducted a two-year study on Calhoun/Calloway silt loam. These soils have high pH levels that promote ammonia volitization and cause nitrogen loss. When urea or ammonium sulfate were applied seven days prior to flooding, grain yields of rice were not significantly reduced compared to when these nitrogen fertilizers were applied the day of flooding if an extra 30 pounds of nitrogen per acre was applied.

The differences between urea and ammonium sulfate were greater when these nitrogen fertilizers were applied 10 to 14 days prior to flooding.

“Even when urea applications were increased up to 60 pounds of nitrogen per acre, it showed significant reduction in grain yields,” Norman said. “However, grain yields where not significantly reduced when ammonium sulfate was applied 10 to 14 days prior to flooding and the nitrogen rate was increased by 30 pounds per acre.

“Many producers don't have the pumping capacity to flood all their fields quickly after an aerial application of nitrogen,” he said. “Now we know they can maintain their yields if they apply ammonium sulfate in place of urea and increase the preflood nitrogen fertilizer rate by 30 pounds per acre.

“When you have to delay flooding, it's worth the higher cost of ammonium sulfate to prevent loss of yield.”


Fred Miller is a science writer with the Arkansas Agricultural Experiment Station.