Adequately high potassium (K) nutrition is essential for high sugar yield and quality of beet crops.: We quantified the effect of potassium deficiency on sugar beet dry matter and sugar yield formation during the growing season. Sugar beets were grown on low, medium, and high soil K concentrations in a long-term K fertilizer field experiment on a silt loam alluvial soil. Plants were harvested at four time points during the growing season, including the final harvest. At each harvest, the dry matter yield and K concentration of leaves and beets (when applicable) were recorded. In autumn, sugar concentration and beet quality were measured. Low soil K concentrations resulted in an approximately 10% lower sugar yield compared to high soil K supply. Reduced growth of low K plants already occurred during the germination phase and in the first weeks thereafter, whereas later growth rates were similar. Higher soil K levels increased K uptake rate and thus a higher final total K uptake. High K plants showed rapid early growth and maximum growth rates were achieved earlier than by low K plants. The initial differentiation between the low and high K treatments persisted due to similar growth rates in subsequent phases at both low and high soil K. It is concluded that under overall low soil K conditions, adequate amounts of K fertilizer should be applied to meet the demand of sugar beet seedlings for maximum growth and final sugar yield. Banded K fertilizer application next to the plant rows is supposed to be efficient for this purpose.