Potassium nutrition requirements may vary between cotton cultivars with differing growth habits and maturity, due to its effect on carbohydrate and biomass partitioning. Our objective was to characterize an indeterminate and more determinate cultivar on yields, earliness and partitioning responses to potassium. A three year field study was conducted in long-term K fertility plots at Jackson, TN. Two cultivars, DP555 BG/RR a relatively indeterminate cultivar, and PM1218 BG/RR, a more determinate cultivar, were compared by their aboveground biomass, carbohydrate partitioning, lint yields, and earliness under two potassium fertility treatments.  The potassium levels were 56 kg K ha^-1 yr^-1 to represent adequate K nutrition and 112 kg K ha^-1 yr^-1 representing excessive fertility, based on Tennessee Extension recommendations for cotton. The experimental design was a RCB split plot with six replications. Plants were sampled on two dates, early bloom and cutout, to examine both carbohydrate and biomass partitioning. Plots were mechanically harvested twice each year for yield, and earliness was measured as the percent of total yield picked at first harvest.  The more determinate cultivar, PM1218, had higher above ground dry weight, higher starch concentration in stem tissue, a greater proportion of biomass in reproductive organs, and higher percent first harvest, representing an earlier maturing plant compared to DP555.  Excessive potassium delayed maturity by decreasing percent first harvest. Lint yields were decreased at 56 kg K in PM1218, but this rate did not affect DP555’s yields. Yields of the two cultivars were equivalent at the 112 kg K rate. This result indicates that PM1218 yields were not maximized at the recommended K rate and suggests that cultivars differing in earliness and growth habit may also differ in K fertilization required for optimal yield response.