The soybean aphid is an invasive pest in the midwest United States, with frequent population outbreaks. Previous work has shown that aphid population densities are higher on potassium-deficient soybean than on healthy soybean. The experiments reported here test the hypotheses that the potassium nutrition of the host plant affects the forms of phloem nitrogen available to soybean aphids, and subsequently, their abundance. In field surveys and an exclusion cage study when aphid populations were high, soybean plants with potassium deficiency symptoms had a higher density of soybean aphids than plants without deficiency symptoms. In clip cage experiments, this effect was caused by earlier aphid reproduction and higher numbers of aphid nymphs per mother on plants growing in lower-potassium soil. In phloem exudation samples, the percentage of asparagine, an important amino acid for aphid nutrition, increased with decreasing soil potassium, perhaps because of potassium’s role in the nitrogen use of the plant. Taken together, these results show that soybean potassium deficiency can lead to higher populations of soybean aphid through a bottom-up effect. A possible mechanism for this relationship is that soybean potassium deficiency improves the nitrogen nutrition of these N-limited insects. By releasing these herbivores from N limitation, host plant potassium deficiency may allow soybean aphid populations to reach higher levels more rapidly in the field.