The frequency of potassium (K) deficiency symptoms in corn has increased in recent years. Observations in Iowa and neighboring states have shown that the reason for these symptoms was a low soil K level in about two-thirds of the instances reported. In the rest of the instances, however, the K deficiency symptoms resulted from a variety of reasons related to limitations in root growth or water uptake and/or K uptake by plants. It is known that factors that limit root activity and growth greatly inhibit K uptake and yield because K is a relatively immobile nutrient in soils. A strong root system and continuous growth of fine roots are required to supply sufficient K to high yielding crops. However, there has not been research on K use efficiency or K fertilizer need of genetically modified modern hybrids with traits that may directly or indirectly affect the physiological needs for K or the plant capacity for K uptake. Of particular interest is the rootworm resistance trait, because a healthier root system without a need for root insecticides and higher yield could result on different root size or surface area that, in turn, can affect K uptake and the yield response to K fertilization. Therefore, a research project has been investigating since 2006 the K nutrition of corn hybrids with or without the rootworm resistance trait.