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Potassium deficiency symptoms can occur in cotton late in the season. Deficiency symptoms include: damaged leaves, poorly developed bolls, reduced fiber quality, and reduced yield potential. Another source of K deficiency in cotton could be the steadily increasing soybean yields, with some fields producing more than 90 bushels/acre.
Figure 1. By the time K deficiency symptoms appear on cotton leaves, yield potential has already been reduced.
For every bushel of soybean harvested, 1.4 lbs of K/acre is removed from the soil. A 90-bushel/acre soybean crop will remove 120 lbs of K/acre. A 50 to 60 bushel/acre soybean crop will remove 70 to 85 lbs of K/acre. If a grower is not replacing the amount of nutrient a crop removes, the following crop is forced to “mine65533;? the nutrients from the topsoil and subsoil. If potassium is not replaced, soil test levels will eventually fall, and a potassium-sensitive crop, such as cotton, could suffer a significant decline in yield potential.
Cotton plants will demand the most K near the start of flowering, with maximum daily K uptake rates of 2.2 to 3.5 lbs/acre/day during flowering. Nearly 75% of the K is needed during the boll filling period, increasing demand to 4 to 5 lbs/acre/day.1 If K is limited during this period, there is a reduction in the turgor pressure of the fiber, resulting in less cell elongation and shorter fibers. It is very difficult to apply enough K during this period to overcome deficits in soil K levels. With cotton plants demanding 4 to 5 lbs of K/acre/day, foliar application of a few lbs of K/acre will not completely overcome the deficit. By the time K deficiency symptoms are visible, yield potential may have already been reduced.
Petiole and leaf analysis can help to determine if potassium levels are low in the soil. Tissue tests are excellent guides for determining fertilizer recommendations during the growing season. Because the levels of nutrients in petioles during any time period can be affected by environmental factors, leaf samples may be the best choice for detecting K deficiency in cotton. Leaf samples provide nutrient levels over the past few weeks, while petiole samples provide nutrient levels over the past few days. A drop in K levels in a K-deficient leaf may be greater and easier to detect than a drop in petiole-K levels.
Following soil test guidelines, cotton growers should keep K levels in the topsoil and the subsoil high to ensure an adequate supply throughout the growing season. Even with high levels of K in soil tests, drought during boll filling can result in reduced K uptake. When plants are small enough to allow sidedress applications of fertilizer, additional K can be applied to the soil.
Foliar K applications can be used to supplement soil-applied K. When a potential K deficiency is diagnosed by tissue samples, foliar K applications can correct the deficiency within 20 hours.1 Growers should collect tissue sample data and be prepared to begin foliar K applications from square initiation through peak boll development. Three to four foliar K applications of approximately 3 lbs K/acre should be used at each application.1
Sources: 1Abaye, A. Potassium fertilization of cotton. http://pubs.ext.vt.edu. Espinoza, L. Potassium deficiency in cotton. http://www.arkansas-crops.com Reiter, M. Manganese and potassium fertility for soybean production in Virginia. http://www.vasoybean.com . Web sources verified 9/8/15. 131213013720