Take care of cotton nutrient problems now

As cotton begins to bloom, North Carolina growers face a critical window of opportunity. To optimize yield, they must identify and resolve crop nutrient problems within a few short weeks. The best way to do this is by submitting samples for plant tissue analysis as soon as possible, according to the N.C. Department of Agriculture and Consumer Services.

Plant tissue analysis is a quick and inexpensive agronomic test that measures nutrient levels within a crop, said Agriculture Commissioner Steve Troxler. For a fee of $7 per sample, growers can find out in two business days whether their crop needs an extra application of fertilizer, and receive precise recommendations for meeting that need and heading off possible yield loss.

Growers are urged to scout their fields and submit samples now, because after the fifth week of bloom, adding fertilizer will not provide any economic benefit, Troxler said.

When checking fields for problems, look for areas of off-color or stunted growth. Pay particular attention to sandy areas that may have lost nutrients due to heavy rains. For more effective troubleshooting, always collect matching sets of soil and tissue samples from areas of good and poor growth. Since cotton is very susceptible to damage by plant-parasitic nematodes, it makes sense to submit separate soil samples for nematode assay ($3 per sample).

Side-by-side comparisons of test results from  good and poor areas are very helpful in problem diagnosis. Results of plant tissue analysis and nematode examination can provide answers to questions such as:

  • Is there enough boron in the plant and should I make an additional application?
  • Do I need to apply additional potassium?
  • Are my plants pale or yellow due to a lack of nitrogen or sulfur?
  • Did I apply enough sulfur this year? Is the nitrogen-to-sulfur ratio sufficient?
  • Are nematodes a contributing factor to poor crop growth?
  • Based on the physiology of nutrient uptake in cotton, the best time to collect tissue samples is between 9 a.m. and 1 p.m. If the weather has been especially dry, wait for rain, if possible, and collect tissue samples 48 to 72 hours afterward. During drought, plants take up fewer nutrients, so tissue test results are often abnormally low.

For problem diagnosis, select the most recent mature leaf (MRML), which is usually about four to five nodes down from the terminal bud. As you collect the MRMLs in the field, immediately separate leaf blades from leaf stalks, or petioles, to stop the transfer of nutrients between the two plant parts. There should be 25 to 30 leaf blades and petioles in each sample.

Cotton tissue samples must contain both leaf blades and petioles because each provides different information. Chemical testing of petioles measures nitrate nitrogen accumulated during the previous 24 hours; leaf analysis measures nutrients accumulated over the previous week. With these two measurements, agronomists can determine how much fertilizer a crop needs to reach its full yield potential.

When filling out the Plant Sample Information form for problem cotton samples, provide as many details as possible, including weather extremes. It is especially important to indicate week of bloom at the time the samples are collected, because the crop's maturity affects its need for nitrogen and potassium. As soon as there are five open flowers per 25 row feet, cotton is in its first week of bloom, or B1.

NCDA&CS regional agronomists are always available to give advice on sampling technique, sample submission, interpreting reports and following recommendations. To identify the agronomist for your area, visit www.ncagr.gov/agronomi/rahome.htm, or call Kent Messick at 919-733-2655. Additional information on collecting and submitting cotton tissue samples is available online at www.ncagr.gov/agronomi/uyrplant.htm.

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