Test shows lower nitrogen rates may be more cost-effective

Frances Gould  |  10/9/2013 7:49:19 PM

LSU AgCenter agronomist Dr. Dustin Harrell is conducting a study of micronutrient requirements of rice. The center rice plot in this fertility trial in Acadia Parish shows the effects of phosphorous deficiency. Harrell is studying optimal rates and application timing for the primary nutrients phosphorus and potassium, as well as the micronutrient zinc at the Acadia location.

Photo By: LSU AgCenter

LSU AgCenter agronomist Dr. Dustin Harrell admits to his surprise and skepticism about a new nitrogen soil test that showed lower nitrogen rates for a rice crop may be more cost-effective than the rates currently recommended.

"I had some serious doubts about some of the N-ST*R (nitrogen soil test for rice) recommended rates that we evaluated this year at off-station locations," Harrell said. "I was very surprised at the outcome."

While he’s still not ready to make any recommendations based on the first year’s results, Harrell said the test could be beneficial if it continues to show positive results the next couple of years.
 
The work is in collaboration with the University of Arkansas, Texas A&M University and Mississippi State University.

Harrell said University of Arkansas soil scientists refined the nitrogen soil test for rice, also known as N-ST*R , extraction methodology and he collaborated on the calibration of the soil test to determine the correlation between the extraction results and the crop response. The extraction method calls for sampling deeper in the soil profile to get a truer picture of the amount of nitrogen that will become available to a rice plant over the growing season. 

"Now we’re in the validation phase with trials on commercial rice fields," Harrell said. "After three years of positive results, we may have the confidence to begin using the soil test for nitrogen recommendations, but we are not there yet." Harrell said making individual field recommendations for nitrogen has been difficult because many variables can affect how much of the element can be released from the soil and eventually absorbed by the plant during the growing season.

A test of the N-ST*R recommendations on a commercial rice field this year saw a field with no nitrogen applied yielded 37 barrels, while a field that received 30 pounds of nitrogen per acre yielded 43.5 barrels. The farmer’s usual rate of 140 pounds yielded 45 barrels. "Maybe we don’t have to use such high rates of nitrogen that we have traditionally used," Harrell said. An excessive plant uptake of nitrogen actually can cause crop losses from lodging, shattering and higher disease pressure, he said.

The N-ST*R soil test doesn’t always recommend low nitrogen amounts, he said, and the testing has recommended a 150-pound rate in some cases, but those are not the fields he deems important to test at this time.

"For the next two years, I hope to get to evaluate the N-ST*R soil test recommendations on new fields where we don’t know any of the field history, and re-test this," Harrell said.

The LSU AgCenter agronomist also said it’s not known if yields will be consistent if low nitrogen rates are used in successive years. He also said the new soil testing method is expensive.

Harrell said researchers in Arkansas are working on clay soil calibration, while the calibration he is validating is for silt loam soils.

Checkoff funds for this
project in 2011: $131,250

(This article was published in the 2012 Louisiana Rice Research Board Annual Report.)



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Please click on the links above to go to the Rice Research Board Reports home page, to go to the 2013 report, and to go to the PDF version of the 2012 report.

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