INSIGHTS

• Drought and dry soil conditions influence multiple processes in crops, specifically potassium uptake from the soil.
• Potassium deficiency may impact corn or soybean yield potential depending on the growth stage when nutrient uptake is decreased.

What is Potassium Deficiency in Corn and Soybeans?

Yellowing of leaves could be occurring in fields for many reasons such as disease presence, low fertility or management factors. In growing seasons with droughty conditions, potassium (K) deficiency in corn and soybeans may be causing these symptoms.

Even though symptoms of deficiency are present, actual soil tests may be in the normal range for potassium. This deficiency is because the plant is unable to access and uptake K from the soil due to limiting factors such as drought or compaction. Potassium moves to the roots by diffusion in the soil solution.

Potassium deficiency impacts are more commonly seen at early, rapid growth stages, and less often in late reproductive stages. Potassium is very mobile within the plant and much is remobilized from roots and stems right before seed fill is initiated. It is an important nutrient for photosynthesis, enzymatic reactions, starch synthesis, nitrogen fixation and energy metabolism in plants. Plants take up large quantities of K during their life cycle and K deficiency may limit plant growth, ultimately impacting yield potential.

Symptoms

Potassium deficiency in Corn:

• Yellowing or browning starting at leaf tip, then along leaf margins, followed by necrosis and dieback.
• Usually appears in older leaves first. Generally, from a distance, leaves appear light green.
• Common during rapid growth periods when plant demand goes up, V6-V8 growth stages.¹

Potassium Deficiency in Soybeans:

• Yellowing along leaf margins is visible in middle and upper leaves later in season and on lower leaves early in season, and the impacted leaves may fall off.
• Leaf margins may become brown or necrotic with prolonged deficiency.
• K deficiency may advance soybean maturation, along with other nutrient deficiencies and excessively wet or dry soil.²

Factors Impacting K Deficiency

• Drought: Potassium is made available in the soil solution. In drought conditions, the diffusion of K to the roots is slowed, so soils with marginal K levels will likely show even more symptoms with low soil moisture.
• Inadequate K levels: Soils can become depleted of K.
• Stunted root system: An active root system is required to take up K, so factors like temperature, compaction, seed furrow side-wall compaction, hard, dry soils, shallow planting depth, or pathogen/insect pests may stunt a root system.
• Growth stage: Soybeans demand a high amount of K during the R1-R5 growth stages where 75% of the total K uptake occurs.³

Management

• Test soil and leaf samples for K in normal and affected areas to help determine K levels.
• If soil K levels are adequate, precipitation will likely increase availability to the plants.
• Prevention is the best treatment. Apply K fertilizer as recommended before the next crop. Fertilizer programs will vary because the amount of K supplied by the soil varies from large differences in soil parent materials.⁴
• Prevent soil compaction or limitations to root development and activity.
• There are no certain economically effective rescue treatments. In-season rescue fertilizers are only recommended if not enough K was applied in early fertilizer applications. Generally, precipitation will improve potassium availability.

References:

¹ Sawyer, J. and A. Mallarino. 2002. Corn leaf potassium deficiency symptoms. ICM News, Iowa State University Extension and Outreach. https://crops.extension.iastate.edu/encyclopedia/corn-leaf-potassium-deficiency-symptoms

² Mallarino, A. 2018. Soybean potassium deficiency symptoms during early and late growing stages. ICM News, Iowa State University Extension and Outreach. https://crops.extension.iastate.edu/cropnews/2018/08/soybean-potassium-deficiency-symptoms-during-early-and-late-growing-stages

³ Camberato, J., S. Casteel, G. Ruhl, and D Telenko. 2018. Potassium deficiency on upper soybean leaves Is not uncommon during seed fill. Pest and Crop Newsletter, Purdue University Extension. https://extension.entm.purdue.edu/newsletters/pestandcrop/article/potassium-deficiency-on-upper-soybean-leaves-is-not-uncommon-during-seed-fill/  

⁴ University of Minnesota Extension. 2018. Potassium for crop production. Nutrient Management Extension, University of Minnesota. https://extension.umn.edu/phosphorus-and-potassium/potassium-crop-production#soil-moisture-598160