Some farmers have reported corn and soybeans showing symptoms of potassium deficiency this year. During a prolonged drought, it is not uncommon to see potassium deficiency symptoms (Photos 1 and 2). This was a common occurrence during previous drought years.

Potassium is highly mobile in the plant. Consequently, potassium deficiency symptoms occur first on the lower leaves and progress toward the top as the severity of the deficiency increases. One of the most common signs of potassium deficiency is the yellowing along leaf margins followed by scorching and dieback. In most severe cases, the necrotic leaves may be shed. The potassium deficiencies will most likely to appear on low soil test potassium soils or under prolonged dry topsoil conditions. Potassium moves to the roots by diffusion in the soil solution. Therefore, prolonged lack of soil moisture can drastically reduce potassium uptake.

Photo 2. Potassium deficiency in soybean. Photo credit: University of Missouri Extension

Although potassium (unlike nitrogen and phosphorus) does not become a part of the chemical structure of plants, many plant physiologists consider potassium as second only to nitrogen in importance for plant growth. This is because potassium “activates” as many as 60 enzymatic and plant hormonal reactions, regulating many physiological and biochemical processes.

Plants depend on potassium to regulate the opening and closing of leaf stomates, the pores that allow gases and water vapor to pass through. When there is adequate potassium in the plant, potassium moves into the guard cells surrounding the stomate, which takes up water (Photo 3). These guard cells become turgid and swell. As the inner walls of the guard cells are thicker than the outer walls, the pores open. This allows free exchange of gases and water. When the water supply becomes short, potassium is pumped out of guard cells and the pores are closed tightly, conserving water. This minimizes drought stress.

Photo 3. Open stomate with expanded guard cells. Photo credit: Pearson Education, Inc.

However, if potassium supplies are inadequate, the stomate becomes slow to respond and may take hours, rather than minutes, to close. As a result, plants become much more susceptible to drought.

For crops showing deficiency symptoms this year, there are no certain economically effective rescue treatments. Crops take up most of their potassium requirements during early vegetative stages. The best preventive treatment is to soil test and apply potassium as recommended before or at planting of the next crop. Also, try and avoid induced deficiency situations where root growth may be restricted, such as soil compaction and pest damage. If only certain areas of the field are affected, those areas should be recorded, post-harvest soil tested and treated. Fertilizer applicators with GPS and variable rate technologies would find it feasible to spot treat these areas for the benefit of the next crop.

For Michigan State University corn and soybean fertilizer recommendations, see “Nutrient Recommendations for Field Crops in Michigan,” MSU Extension bulletin E2904. To see video footage of potassium deficiency, watch this Potassium Deficiency in Soybean YouTube video from the University of Wisconsin Extension.