The experiment was conducted on a low fertility soil (Typic Acrustox), during two years, at Jaboticabal -SP - Brazil (21º14’S; 48º17’W; 550m asl). The treatments consisted of five K rates: 0-25-50-100, and 200 kg ha^-1 of K₂O. In order to evaluate the nutritional status of the plants, soil and leaves + 4 (fourth leaf from the top) were sampled at flowering stage. At the stage of soft grain were harvested in order to determine the weight of the sorghum stalks and to estimate the ethanol production. The potassium fertilization increased the stalks and ethanol (l ha^-1) yields, but did not significantly alter the total sugar content. The maximum yield of stalks (56 t ha^-1) was obtained with about 140 kg ha^-1 of K₂O. The K adequate contents in leaves were within 15 -17 g kg^-1. The K critical level (soil) was 0.14 cmol_c dm^-3.