Watch for Potassium (K) Deficiency Symptoms in Soybeans

Richard Taylor, Extension Agronomist;

Although it shouldn’t be a surprise, a few fields in the state have started showing potassium (K) deficiency symptoms. In the past, this condition usually shows up in the eastern sections of New Castle County at least every few years but reports this year indicate that other areas of the state are showing the symptoms. Since potash prices have been quite high the past few years, many growers have been cutting back K application rates or skipping a year or two between applications. These practices have lead to low soil test levels of K and now to at least some fields, usually with very light sandy soil and low organic matter content, showing K deficiency symptoms.

Many dryland corn fields have long had the corn leaves fire up or turn necrotic and even a lot of the irrigated fields I’ve been in during the past week are showing dead or firing lower leaves. The full dent growth stage is evident in even the irrigated fields probably due to the large number of heat units or degree days we’ve had this summer. So in the case of corn, we’re probably too late to notice K deficiency symptoms but for soybeans that’s not the case.

In soybeans the deficiency shows up as yellowing along the trifoliate leaflet edges on the oldest leaves of the plant (Photo 1). The yellowing of the edges intensifies and expands and can move up the plant (Photos 2 and 3). The reason the deficiency begins on the lower leaves is that the element K is mobile in the plant so that when there is an insufficient supply the plant will cannibalize the lower leaves and mobilize the K up to the new growing leaves. If the deficiency persists or worsens as the plant becomes larger and begins the reproductive phase, the yellowing gradually turns brown and the leaf edges die and become ragged (Photo 4).

Photo 1. Note the very beginning of leaf yellowing along the leaflet margins on the very lowest leaves.

Photo 2. Note that leaf yellowing along the leaflet margins has expanded and has moved up the plant closer to the terminal.

Photo 3. Leaf edges are almost fully involved as the deficiency worsens.

Photo 4. At this stage, the leaf edges begin to show necrosis while other leaf edges have turned completely necrotic.

What can be done if you find deficiency symptoms? Various liquid formulations of K are available that could be applied as a foliar spray although the experiences I and other researchers have had suggests that the foliar burn from these products could easily cancel out the benefit of adding K. A problem with foliar sprays is that it is difficult to apply enough to the plant to supply the plant’s need for K. If you increase the concentration or the amount of solution K applied, the burn potential increases and will cancel out the benefit of foliar K. Some work at Rutgers University (unpublished) showed that both potassium chloride and potassium carbonate significantly burned soybean plants and cancelled any yield increase.

Where does this leave the producer with a K deficiency? Essentially, we’re back to broadcasting or flying on muriate of potash (0-0-60) at about 100 lbs/acre and then either irrigating the field if irrigation is available or hoping for a rain event to begin to dissolve the fertilizer and move it into the upper soil layer where many of the soybean roots are present. Again with the element being a plant mobile nutrient, the K will be moved by the plant to the growing points which will be the terminal where new leaves are being formed and to the reproductive sinks (flowers, pods, and seeds). This should be effective in helping to reduce the yield reduction possible with K deficiency.

If you can, look at the soil test to make sure there is adequate phosphorus (P) available for the crop. It’s been shown that the addition of both P and K can have a synergistic effect where the yield increase is greater when both nutrients are applied versus just applying a single nutrient. If the soil test level of P is already high, as it is on many Delaware soils, then there likely will not be a benefit to adding both nutrients so an accurate soil test is essential when considering the combination product.