Potassium Deficiency in Corn

By Nathan Mueller

In Kansas and Nebraska, I remember seeing several corn fields with severe potassium deficiency symptoms in 2012. Unfortunately, this June I have confirmed one field in Gage County and observed several others in the area with potassium deficiency. Along with many other agronomic challenges during a drought, potassium deficiency in corn is one of them.

Potassium, represented as ‘K’ on the periodic table of elements, is one of six essential plant macronutrients, and is taken up in larger quantities than any other macronutrient by corn. A 200 bushel per acre corn crop takes up an estimated 280 pounds of potassium oxide (K₂O) in the above-ground biomass (stover plus grain). Potassium oxide is the form expressed in fertilizers of USA, i.e. 0-0-62 is 0% N, 0% P₂0₅, and 62% K₂0. Potassium is absorbed by plant roots as a potassium ion via diffusion and mass flow, both of which are negatively affected by dry soil conditions. Typically, deficiency symptoms do not appear until after corn reaches the V6 growth stage when it is more rapidly growing and K demand increases. Low soil potassium, drought/dry soil, and other factors that reduce root growth can limit K uptake and cause visible potassium deficiency symptoms.

What do K deficiency symptoms in corn look like? Potassium is mobile within the plant and can be reallocated from lower older leaves to younger upper leaves. A dark green whorl and upper leaves with middle and lower leaves exhibiting yellowing along the leaf.

It is important to take diagnostic samples now to verify K deficiency, not just drought/soil compacted-induced K deficiency. Take paired soil and plant samples of poor areas (suspected potassium deficiency) and good areas (healthy looking areas) within the field (Figure 3 shows variability from poor to good). Soil samples should be taken to 6-inch or 8-inch depths depending on the calibration/correlation research and recommendations you use. A soil probe can be borrowed from the Gage, Jefferson, or Saline County Extension offices. Corn plant samples should be taken of the uppermost fully developed leaf with a visible collar prior to tasseling or of the corn ear leaf during pollination (R1 growth stage) from 15 plants stored in paper bags. If you need help on these sampling protocols or with lab result interpretations, please feel free to contact me.

If the field is irrigated, sufficient irrigation can help reduce some of these deficiency symptoms and yield impacts. Potassium fertilizer applications are not recommended this far into the growing season. The best plan of action is to document areas of the field where deficiency symptoms are present. This can be done by good scouting notes, GPS locations, or via aerial imagery. This fall, use a soil sampling approach that captures the variability within the field. Be aware that management zone and center-point grid soil sampling can still miss small areas of low soil K within a field, so in-season scouting notes/imagery can be very helpful. Variable-rate potassium fertilizer applications can be very cost-effective with good soil test data.