We’re seeing signs of potassium deficiency in cotton more and more, but it’s not all bad. The signs are stemphyllium leaf spot. These are the circular spots on the leaves with red borders that may have “shothole” appearance. Where K deficiency is more severe, we see discoloration on the leaf also. UGA Extension Fertility Scientist Dr. Glen Harris says because it is late in the year, we should see more stemphyllium. If the plant is loaded with bolls, it has used up a lot of its potassium. If the plant doesn’t have many bolls, stemphyllium or K deficiency may have occurred earlier and caused more of an issue.
Correcting K Deficiency
UGA recommends putting out K with pre-plant fertilizer. However, even with 90 lb in this field, it may have not been enough. Where a known K deficiency exists, foliar K applications should be considered. Even with foliar K, we may not be putting enough where this stemphyllium is heavy. Two foliar applications of 5 – 10 lbs/K2O in each application during early bloom (1st through 4th week) need to be considered.
Dr. Harris says in most cases, the best way to avoid K deficiency is to:
- Soil test
- Apply recommended K fertilizer at planting
- Consider foliar feeding K during peak bloom
How late is too late to foliar feed?
We’re now pushing 5 – 7 weeks of bloom or more in most fields. Based on research, foliar fertilization is most effective when applied during peak bloom or 4th week of bloom. Foliar feeding during 5th and 6th week of bloom may not be effective depending on cotton variety. Once we hit 7th and 8th week of bloom (and after), foliar feeding is too late – not recommended.