Soybean fields are beginning to mature very rapidly. For the past several weeks “yellow areas” have been showing up in many fields throughout the Knox County. One possible explination could be potassium (K) deficiency.
Late season K deficiency is usually found on leaves near the top of the plant. These symptoms typically occur in areas of the field where K fertility values are low however, this symptomology can sometimes be found in fields with adequate K levels during dry periods in a growing season. Additionally these symptoms can appear later in the growing season after a significant rain following an extended dry period.
Sometimes K deficiency is confused with Sudden Death Syndrome (SDS) symptomology. Picture 2 shows SDS symptoms. The presence of the green veins is the key identifying characteristic for SDS. The veins on the leaves will remain green while the leaf tissue between the veins will turn yellow and then brown.
Picture 3 shows soybean with K deficiency. The leaf discoloration starts on the outer edge of the leaf and moves inward, including leaf veins.