The soybean aphid (Aphis glycines) (SBA) is a recently introduced soybean pest in North America. In 2000, 2001, and 2003, SBA has reached severe outbreak levels in the Midwest, causing 40-50% yield losses in unsprayed areas. In 2000 and 2001, higher aphid populations as well an unusual top-down deficiency symptom were observed in patches of soybean fields with low potassium levels. During 2002, when aphid population levels were low, these yellowing patches did not occur. In 2003, a field survey was conducted in fields displaying these symptoms. Yellow patches were paired with nearby green areas and aphid populations and plant characteristics were counted. Soil and phloem samples were also collected from the paired areas. Plants in the yellowed patches had fewer leaves, pods, and nodes than paired green plants a short distance away. The yellowed plants also had more SBA per leaf than their green neighbors. The soil in the yellowed patches had significantly lower potassium levels than the green areas. One possible explanation of the increased aphid populations we observed is an increase in aphid reproduction rate due to improved nutrition. In plants potassium plays a role in protein formation from free amino acids. Thus, decreased potassium levels may lead to an increase in phloem amino acids, improved nitrogen nutrition for SBA, and a higher rate of SBA population growth. To test this hypothesis, high-pressure liquid chromatography was performed on phloem samples taken from yellowed and green areas of the sampled fields.