Potassium (K) leaf deficiency symptoms have recently appeared in some cornfields. Early-season symptoms typically show when corn begins rapid growth, calf to knee-high (V6-V8 growth stages), and the K uptake rate accelerates as plant demand responds to this rapid growth. Because K is maintained in the plant in the K+ ion form, it is readily moved from older tissue to the growth regions, hence deficiency symptoms appear on the older leaves. On corn, symptoms show as yellowing to necrosis of the leaf margins on older leaves.
Iowa State University
Sawyer, John E. and Mallarino, Antonio P., "Corn leaf potassium deficiency symptoms" (2002). Integrated Crop Management News. 1807.
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