AGRI-BRIEFS   AGRONOMIC NEWS ITEMS

From Agronomists of the Potash & Phosphate Institute 655 Engineering Drive, Suite 110 Norcross, Georgia 30092-2837 Phone (770) 447-0335

Spring 1997, No. 6

Much is known about potassium nutrition of cotton...but not enough to give growers and crop advisers the confidence that soil test results and applied fertilizer will always provide the crop with its needs at vital growth stages for optimum yield, quality and profitability.

Potassium nutrition and fertilizer management were prominent topics of discussion by researchers, crop advisers and growers during the 1997 Beltwide Cotton Conference. Many were seeking the answer(s) to the question of why late season potassium deficiency symptoms are developing in today's high yielding cotton. Specialists believe that no single reason covers all fields and regions. Instead, it might be due to any one or more of the following.

• Cotton has a large requirement for potassium as bolls begin to develop and fill. Soil nutrient reserves plus applied fertilizer might not have been enough to provide for peak need. Also, nutrient availability during critical growth stages can be reduced by heavy rains on sandy soils...leaching nitrogen, potassium, sulfur, magnesium and/or boron below the plant root feeding zone...and causing an unexpected nutrient shortage.
• High yielding varieties with a heavy boll set tend to become more determinate in nature. This results when sugars from photosynthesis are translocated to the developing bolls rather than to roots for nutrient absorption. Without this energy supply, nutrient uptake by roots will shut down and the plant must rely upon nutrient reserves already within the plant to maintain a high photosynthetic rate, translocation of sugars and complete boll development.
• Potassium-deficient cotton is more susceptible to root and leaf diseases. With inadequate potassium, diseases such as Verticillium wilt produce mycelium in the xylem vessels of cotton which slows the movement of water and nutrients up the plant to the leaves. Also, leaf cells of potassium-deficient cotton are more susceptible to infection by leaf diseases.
• With high yield management and higher plant populations, each plant tends to have a smaller volume of root and stem tissue for storing nutrient reserves for late season use. When early season nutrient storage is inadequate to provide for late season crop needs, plants tend to show potassium deficiency symptoms on newer leaves, reduce the rate of photosynthesis, slow the transport of sugars to the bolls, build up sugars and starch grains in leaf cells, increase plant respiration, and trigger early plant cut out.