The wet spring has created soil conditions that are not conducive to good corn root growth and development. Potassium (K) deficiency in corn is one result of these conditions.
The peak period of K uptake occurs between four to eight weeks after seedling emergence. During this period, the crop takes up as much as 5 pounds of K per acre per day. If early root growth and the development of nodal roots is restricted or impeded by excessive wet conditions, the plant will simply not have an extensive enough root system to meet the nutrient demands of corn.
Symptoms can appear even if soil K is adequate for corn production. If these wet soil conditions are followed by a dry spell, this can exacerbate the situation. Anything that exerts additional stress or limits growth — such as compacted soils, root pruning and sidewall smearing — will further reduce K uptake.
Also, if K levels are low in the subsoil zone — which has the most active roots — K uptake will not be adequate for the demands of the corn plant.
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Most K deficiency symptoms appear while the corn plant is between 15 inches tall and the tasseling stage. Symptoms appear first on the lower leaves. Potassium is a mobile nutrient, so it can move from older to younger leaves when the K needs of the younger leaves are not met by K uptake by the roots.
The older leaves turn yellow, and the leaf tissue along the margins (leaf edges) starts to dry up and die back (Figures 1 and 2). Potassium deficiency symptoms should not be confused with nitrogen deficiency, which first appears as yellowing on the lower (older) leaves and is located from the leaf tip down the midrib in a “V” shape (Figure 3).
If there is a true soil K deficiency, more severe symptoms develop. Older leaves die back, and yellowing symptoms and marginal necrosis appear on the younger leaves (Figure 4). Plant height may be reduced, silking delayed and mature ear size reduced.
Stalk diameter may remain unchanged, but K-deficient plants may lodge late in the season because stalk strength is lower and plants are more susceptible to stalk rots (Figure 5).
Plant analysis can be used to diagnose K problems during the growing season. Whole plant samples taken 30 to 45 days after emergence should contain 3 to 5% K. Ear leaf samples collected at early silking should contain between 2.5and 3.5% K.
The ear leaf taken at early silking is the best indicator of plant K status, but if the plant is found to be K deficient, very little can be done during the growing season to correct the problem. In general, K applications made later than six weeks after emergence are not economical because the peak period of plant K demands (four to eight weeks after emergence) may have been missed entirely.
For next year’s crop, soil tests from good and affected field areas can help discern true soil deficiency effects from climatic effects.