The right rate of nitrogen varies according to the variety or rice hybrid being grown, a soil’s ability to supply N over the growing season and environmental conditions.

Researchers have identified a range of rates that nitrogen rates that work across many different soils for individual rice varieties. (For a listing of the recommendations, go to the Rice Varieties and Management Tips publication, which can be found at http://www.lsuagcenter.com.)

The right timing and placement of fertilizer N to drill-seeded, delayed flood rice is critical to minimize N losses. Currently, the LSU AgCenter recommends N fertilizer be applied in two seasonal applications:

  • The first should be made just before permanent flood establishment when the rice has four to five leaves or is just beginning to tiller. Two-thirds of the recommended N rate should be applied at this time.
  • The second should be at mid-season, usually at green ring.

“It is critical the first nitrogen fertilizer application be placed on a dry soil,” says Harrell. “When placed on a dry soil, urea or ammonium N fertilizer will be incorporated deeper into the soil with the irrigation water making sure it stays in the anaerobic zone and is thus stable and available for rice uptake throughout the growing season.”

Placing ammonium-based fertilizer on a moist or wet soil will result in gaseous N losses through ammonia volatilization. “The longer the fertilizer stays on the soil before the flood is established, the more N will be lost from ammonia volatilization,” Harrell notes.

Urease inhibitor

“If the flood takes longer than three to five days to flood a particular field, which is the case for most of our commercial rice fields, the use of a urease inhibitor will temporarily delay volatilization losses.”

At midseason, or at the green ring stage, urea or another ammonium N fertilizer can be flown onto a flooded field. “Rice has developed an extensive root system by midseason and the enhanced root uptake can mitigate volatilization losses at this stage of development. So placing N fertilizer into the water at midseason is not a problem.”