Yield potential in cotton can be significantly affected by insufficient amounts of potassium. Contrary to other crops, such as corn, where a good portion of the potassium has been absorbed by tassel, in cotton nearly 75% of the potassium is needed during the boll filling period, as bolls are the main absorbers (sinks) of potassium in a cotton plant. Starting with the second-third week of bloom, a cotton plant typically needs 4-5 lb K/acre per day so any shortfalls in the fertility program may be difficult to correct.
Also, when the symptoms of a potassium deficiency are obvious, it probably has already affected yield potential. The picture below shows the onset of a potassium deficiency in a field in Lee County, during the second week of bloom. The symptoms first appear on older leaves, with yellowing in between the veins increasing. They will later develop brown spots, with the tips and margins showing a scorched appearance. Leaves will eventually turn redish-brown in color and will fall from the plant.
Petiole testing will show a deficiency but is not the best way to confirm the problem. The objective of the program is to monitor nutrient status during the season. The levels of nutrients in petioles during a sampling event can be affected significantly by environmental factors. Leaf samples are best suited for diagnostic purposes as they provide information on the nutritional status of the past few weeks, rather than the past few days, as with petioles. The drop in K levels in a K-deficient leaf is also greater and easier to detect than the drop in petiole-K levels. During the early bloom period, levels below 2% are considered deficient, with severely deficient cotton showing levels below 1%.
What can be done to correct or alleviate the problem?
If a cotton plant is using 4-5 lb K/acre per day, it should be obvious that a foliar application of a few pounds of potassium is not going to solve the problem. Research by C. Snyder and others in Arkansas showed that in order to significantly affect the nutritional status of a plant, 4 weekly applications of 4-5 lb K/acre are needed. A recent study by L. Espinoza and others, looked at the effect of dry K applications during the early bloom stage, compared to applications after emergence. The K levels in soil were in the “medium” to “low” range. When yields were compared, the highest one was obtained when potassium was soil-applied at emergence, with soil-applications at early bloom being effective at protecting yield potential. In a related study, a single foliar application (4lb K/acre as potassium nitrate) was not as effective in reducing yield loss as the soil-applied treatments. The purpose of foliar feeding is to supplement a soil-applied program, not to replace it. Under deficient conditions, a single foliar application of K perhaps could be of use for quick plant uptake, to give roots enough time to take up a dry soil fertilizer application. A further consideration regarding foliar feeding is the fact that the efficiency of uptake by a leaf goes down after the third week of bloom due to the increased formation of wax on the leaf surface. This increased wax then becomes a barrier for nutrient uptake by plant leaves. Research by Chang and Oosterhuis in Arkansas and D. Howard and others in Tennessee showed increased yields when the spraying solution was buffered to pH of 4 to 6.
Positive yield responses to foliar feeding under non-deficient conditions are difficult to predict, and tend to be associated with low soil moisture conditions, shallow root systems and perhaps high yield potential. A study funded by Cotton Incorporated across the cotton belt in the mid 90s showed a positive yield response in only 1 out of 3 cases.