Cotton following corn or soybeans likely needs less nitrogen fertilizer than cotton after cotton—possibly a lot less.

Too much nitrogen fertilizer may hurt yield instead of boost it, says LSU Extension agronomist Dan Fromme, who works out of the LSU AgCenter in Alexandria.

Fromme, speaking at the February Louisiana Technology and Management Conference in Marksville, said several factors affect nitrogen fertilization decisions for cotton, including rotation, irrigation, and soil type.

 Fromme says research data show the detrimental effects of too much nitrogen on cotton. Studies have shown that applying more than the needed nitrogen rate could produce excessive growth and fewer productive structures than cotton that receives recommended rates. Other research shows that nitrogen “in excess of that required for optimum crop performance can reduce yield or fiber quality.”

Studies have also shown that excess nitrogen, especially combined with late-season moisture, may delay maturity, reduce harvest and ginning percentages and promote boll shedding, disease and insect damage.

Preceding crop

The preceding crop makes a big difference, Fromme said. Following a soybean crop, producers should cut nitrogen rate by 10 to 20 pounds per acre. Behind a good winter legume crop, nitrogen rate should be reduced by 30 to 50 pounds.

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Those lower rates are especially important on fields with a history of excessive stalk growth. Excessive applications and applications made late also contribute to delayed maturity, increased boll rot and tends to make the cotton more attractive to destructive insects.

Fromme recommends adjusting nitrogen rates to soil types as well. For cotton planted dryland on clay, clay loam, silt clay, and silt clay loam soils, recommended nitrogen rate ranges from 90 to 120 pounds. If those soils are irrigated, nitrogen rate adjusts to 100 to 120 pounds per acre.

On lighter soils — fine sandy loam, loamy sand, silt loam and very fine sandy loam — recommendations call for 60 to 90 pounds of nitrogen per acre in dryland and irrigated production.

Fromme says continuing studies indicate that over-fertilization may produce more plant than fiber.

“Cotton that yielded the most was not the tallest and was not the greenest,” he said. “The soil type and the previous crops have an impact on nitrogen rates, and the 2016 and 2017 results validate the LSU nitrogen rate recommendations.”

Fromme showed data from research plots that show cotton following soybeans on some soil types did better with no additional nitrogen. In some cases, adding more nitrogen to cotton that followed soybeans resulted in rank stalks but a lighter boll load.

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