If fields are showing boll loss late season, petiole testing is needed to determine whether there is potassium or other nutrient deficiency.
Petiole sampling should typically begin at least one week before first bloom and continue weekly until the first open boll.
It is best to take at least 25 to 35 samples from representative areas in the field.
Petiole samples should be taken from the first full-sized mainstem leaf from the terminal, pulling the leaf off at the mainstem. This is usually the fourth vegetative leaf from the top of the mainstem. Separate the leaf blade and leaf stem (petiole). Discard the leaf blade and submit the petioles to your local county or state office for analysis and recommendations.
Since local sampling techniques may vary, check with your county extension office or consultant for sampling instructions.
Urea is the most common foliar nitrogen material applied to cotton because it is inexpensive, easily absorbed by the leaf, and has a low potential for leaf injury.
The rate of uptake by the leaf is dependent on the rate of urea applied, temperature and the condition of the leaf surface. Healthy, actively growing cotton and warm temperatures favor uptake.
Research has shown that approximately 30% of the urea is absorbed during the first hour and 70% within 24 hours. It is important to use “Feed Grade65533;? or low biuret urea and to avoid applications to stressed cotton.
Typical rates of foliar urea range from 10 to 15 lbs. of urea (5 to 7 lbs. of actual nitrogen) applied in 10 to 15 gallons of water. It may be necessary to buffer the urea solution if pH is above 7. Applications should be made within 2 to 3 hours of mixing the urea to avoid possible leaf burn due to ammonium toxicity. Applications made either early in the day or late in the evening are best.