Potassium deficiency symptoms of corn and soybean include yellowing/browning of lower (older) leaves with edges exhibiting symptoms first. Potassium deficiency symptoms can indicate low soil test potassium (K), but may also be related to recent dry soils, compaction, and poor root growth.
In 2013-2015, the Lindsey Lab collected 593 soil samples from close to 200 soybean fields in Ohio. Twenty-three percent of the soil samples were in the build-up range, indicating K was likely limiting soybean yield. If you see plants exhibiting symptoms of K defiency, collect a representative soil sample from the “bad” area of the field and “good” area of the field for comparison. In most Ohio soils, a soil test K value of >120 ppm (240 lb/acre) should be adequate. See Table 1 for the new Mehlich-3 K critical levels. If soil test K is <120 ppm, fertilizer may be added in-season according to Tables 2 (corn) and 3 (soybean). We do not recommend using foliar products to supply K.
If soil test K values are >120 ppm and plants are exhibiting K deficiency symptoms, K uptake by the plant is poor and may be caused by dry soil, compaction, poor root growth, or a combination of these factors. Rainfall and continued root growth will help alleviate these issues. Soybean cyst nematode may also limit root growth and K uptake. If poor areas of the field have not been recently tested for soybean cyst nematode, consider collecting a soil sample for analysis.