It could be called a tale of two seasons: Rice farmers have two distinctive periods for fertilizing their crops – pre-flood and post-flood.

Managing nitrogen applications for those two seasons and following the 4R Nutrient Stewardship Concept are key ingredients in improving rice yields, according to researchers at the LSU AgCenter Rice Research Station.

LSU scientists have been studying rice fertility at the station for most of its 102-year history. Their recommendations for applying nitrogen and other crop nutrients have played a major role in nearly continuous improvement in rice yields over the decades since.

The current crop of researchers, led by Dr. Dustin Harrell, assistant professor of agronomy with the AgCenter, says fertilizing rice in environments that are alternately dry and wet is not the easiest task.

“The complex chemistry associated with the alternating dry and flooded conditions in rice can create an environment for enhanced fertilizer nutrient use when correct management procedures are used,” says Harrell. “On the other hand, a profound inefficiency could result from improper fertilizer practices.”

Production aid

Harrell, a speaker at this year’s Rice Research Station Field Day, says the 4R Nutrient Stewardship program, a concept developed by the International Plant Nutrition Institute to help improve nitrogen application efficiency, could be beneficial to Louisiana producers.

“The right source, right rate, right time and the right place are the components which make up the 4R Nutrient Stewardship concept,” says Harrell. “The 4Rs can be used to help make informed and practical site-specific, sustainable fertility decisions in rice production.”

Because the right nitrogen fertilizer source is critical in rice production, Harrell says fertilizers that contain a portion of N in the nitrate form, such as ammonium nitrate, should be avoided.

“Rice fields remain flooded for extended periods of time during the season, creating an anaerobic soil condition,” he notes. “Under anaerobic conditions, nitrate-N can be transformed to gaseous forms and be lost to the atmosphere through denitrification.

“However, ammonium-N will remain in stable in the ammonium form under these same flooded, anaerobic conditions if applied properly. Therefore, only ammonium and ammonium-forming fertilizers should be applied.”

Variety recommendations

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.”