Time to start watching for potassium deficiencies in soybeans

Soybeans require a whole lot of potassium (K) and remove a whopping 1.4 pounds of K2O per bushel. The application rate for a 60-bushel per acre soybean crop is 84 pounds of actual K2O or 140 pounds of potash (0-0-60) per acre. As we strive for 70- and 80-bushel beans that amount increases.

The first place to look for a potential potassium deficiency is a soil test. According to the Illinois Agronomy Handbook:
• No K additions are suggested if test levels are above 360 and 400 pounds per acre for the low and high CEC regions, unless crops that remove large amounts of K (such as alfalfa or corn silage) are being grown.
• When soil test levels are between the minimum and 100 pounds above the minimum (260 to 360 pounds and 300 to 400 pounds per acre for the low and high capacity, respectively), apply enough to replace what the crop to be grown is expected to remove.
• When soil test levels are below the desired values (260 and 300 pounds per acre for the low and high capacity, respectively), enough fertilizer should be added to build the test to the desired goal and to replace what the crop will remove.

The second step is to do tissue testing to make sure that K isn’t limiting. Remember that plants can appear normal and still be short on nutrients. We to this as “hidden hunger.”

The critical K tissue concentration ranges are 1.89 to 2.26 percent for young plants and 1.56 to 1.99 percent for leaves later in the season. Keeping K levels near 1.75 to 2 percent means your plants won’t be deficient. An overview of soybean tissue testing can be found here.

As the crop growth is finally underway, K deficiency will show up in some fields. Deficiency symptoms occur on the edges of the lower leaves on most plants. In soybeans, symptoms start as yellowing on the edges of the lower leaves. Eventually the edges of the leaves turn brown and the whole leaf dies. As the deficiency worsens, symptoms appear higher up the plant. If symptoms begin to appear in areas of a field, pull a soil and tissue sample to confirm.

Typical K deficiency symptoms in soybeans

For soybeans, if the soil test is low and/or the crop is showing deficiency symptoms, topdressing with potash can help. Since K isn’t very mobile in the soil, roots must find it. Foliar applications of K will have a more immediate response. Foliar application may be a potential management tool to mitigate reduced yields caused by K deficiency. However, optimal soybean grain yields are best attained using pre-plant K fertilizer application.

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.