Sunday, July 25, 2021

Time to start thinking about sidederess fertilizer

• By Audrey Gamble •

albamaFertilizer being transferred to a spreader truck — photo by Josh Thompson, UFL-IFASWith cotton planting dates varying widely across the state, time for sidedress fertilizer may or may not be fast-approaching. Regardless of planting date, demand for nitrogen (N) increases significantly after first square, so sidedress N applications should be made between first square and first bloom to ensure the plant has adequate N when it is needed most

For cotton, our base recommendation for nitrogen is 90 pounds N per acre—split one-third at plant and two thirds at sidedress—regardless of soil type. However, this recommendation should only be used as a guide. There are a couple of scenarios in which we may want to back off the 90 pounds N per acre recommendation.

For example, if excessive vegetative growth has caused problems with late-season insects or boll rot, N rates should be reduced by 20-30 pounds N per acre. For late-planted cotton, reducing N may also help prevent issues with late maturity.

On the other hand, if vegetative growth has been inadequate in the past and planting was timely, N rates may need to be increased slightly.

Remember that 10 to 15 pounds of sulfur (S) per acre is recommended for optimum cotton yields. Sulfur can be applied with at-plant and/or sidedress fertilizer applications. Many N sources already contain S.

It is recommended that S be applied in the sulfate form — not as elemental sulfur — since it can take several months for elemental sulfur to be converted into plant-available forms.

For urea-based fertilizers, significant volatilization losses can occur if fertilizers are not applied before a rain or irrigation event. If rain is not predicted within two to three days after urea application, addition of a urease inhibitor with the active ingredient NBPT can reduce potential N loss.

Dr. Audrey Gamble is an assistant professor and Alabama Extension soil scientist. She may be reached at Audrey Gamble.

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