Potassium deficiency symptoms often appear on drought stressed corn, grain sorghum and soybeans. These symptoms may occur even if the soil tests high for potassium (K). Dry soil conditions limit crop root growth. Potassium moves slowly within soil, so roots must continually exploit additional soil volume for potassium. If root growth is inhibited by dry soil or compaction, potassium uptake is depressed.
Perhaps more important is the effect of dry soil on potassium diffusion to root surfaces. Roots actively take up potassium so a potassium concentration gradient (low concentrations near the root, higher concentrations in soil solution about 1/2 inch from the root) develops. Potassium ion diffusion toward the root surface is stimulated by this gradient. In dry soils, the water film around the soil particles becomes much thinner than in soil at field capacity. The route of the K ions becomes much longer and circuitous as water films shrink. So, not only is root growth slowed, but potassium diffusion is inhibited by dry soil. Under these situations, potassium deficiency symptoms soon appear on older leaves.
The vast majority of the potassium present in Missouri soils is unavailable for plant growth. Some of the potassium is "fixed" within clay mineral layers and becomes available slowly with time. Dry weather can interfere with the availability of fixed potassium because some types of clay minerals collapse (shrink) when water is removed. The collapsed layers trap the K ions. When the soil is wetted, the clay layers move apart and some of the fixed potassium is freed.
Dry weather and its effects on soil properties have contributed to poor potassium uptake by plants. Soybean is more likely to exhibit potassium deficiency than corn, because most corn plants are mature enough that potassium uptake is nearly complete. Soybean seeds contain much more potassium than corn grain, so many soybean plants in Missouri are only now entering the time period of rapid potassium uptake. Deficiency symptoms first appear as pale to lemon-yellow margins on older leaves. Browning followed by necrosis of leaf margins denotes an increase in severity of the deficiency.
Rain, if timely, can stimulate new root growth and help alleviate some of the potassium deficiency symptoms. A rescue treatment with knifed liquid or broadcast KCl will probably not improve yield. Research data are limited, so definitive recommendations are not possible. However, there is a possibility of harming the crop by root pruning or leaf burning.
Updated 5/4/06
