In light of skyrocketing fertilizer costs, the last couple of years provided an excellent opportunity for LSU AgCenter researchers to reevaluate optimal fertilizer timing and rates. Experiments were conducted during 2006 and 2007 at the LSU AgCenter’s Macon Ridge Research Station near Winnsboro and the Northeast Re-search Station near St. Joseph.
Don Boquet, professor of agronomy at the LSU AgCenter, and Gary Breitenbeck, professor of agronomy and soil microbiology also at LSU in Baton Rouge, worked with John Kruse, Georgia-Pacific Chemicals, LLC, Decatur, Ga., on the research project, and a presentation on their research was given at the Beltwide Cotton Conferences earlier this year.
“The objective was to evaluate soil- and foliar-applied nitrogen (N) formulations for cotton lint yield responses,” said Boquet in the presentation.
Currently, Boquet says, the recommended nitrogen rates range from 75 pounds per acre on silt-loam soils up to 100 pounds per acre on clay soils with consideration given to crop sequences and cover crop effects that either reduce or increase nitrogen needs of the following cotton crop.
Formulation Not A Yield Factor
The research was conducted on two soil types: a Gigger silt loam and Commerce silt loam. Four nitrogen formulations – ammonium nitrate, urea, Nitamin 43G and 30L were soil-applied at rates of 0, 25, 50, 75, 100 and 125 pounds per acre. The application timings were: 1) 100 percent at-planting and 2) split applications with 50 percent or 75 percent at-planting and the remainder at flower initiation.
As for foliar-applied products, the three formulations used were urea, CoRon and Nitamin 30L.
Application rates of urea and Nitamin 30L were 10 pounds of nitrogen applied three times at flower initiation and then at 10-day intervals. The application rate of CoRon was two gallons per acre applied at the same times and intervals.
The researchers found that the different forms of soil-applied nitrogen had little to no effect on the lint yield, nor were there any differences in leaf response to nitrogen rate based on the different formulations.
“The different formulations were very close,” says Boquet. “There was no difference in performances until you get out to the high end, and that was a negative response. We found that ammonium nitrate tends to drop off in yield more than others. The optimal rate was right around 75 pounds on the silt-loam soil, which confirms our years of data.”
The optimal rate for soil-applied N was 90 pounds per acre on the Commerce silty-clay loam.
Research Supports At-Planting Only
In Louisiana, Boquet says, they do not recommend split applications, and their research bore out that recommendation as well.
“Split applications of soil-applied nitrogen did not increase yield above that obtained with a 100 percent at-planting application,” he says. “Why we aren’t getting response to split application like other folks are, I’m not sure. We do know there is a lot of residual nitrogen in the soil, which is fortunate for growers in the state because of the cost of fertilizer.”
In conclusion, Boquet says their results validate the current nitrogen recommendations of applying 100 percent of nitrogen fertilizer at-planting at 75 pounds per acre for cotton grown on silt loam and 90 pounds per acre for cotton grown on silt-clay loam soils.
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