This paper presents the results obtained in seven experiments conducted in 1952-53 and 1953-54 at various localities of the State of São Paulo, in which three sources of potassium were tested in the presence of nitrogen and phosphorus. No response to potassium was observed in three of the experiments. Based on the average results from the other four trials the yield with NP was 6.66 tons per hectare and the increases due to additions of 30, 60 and 90 kilograms of K(2)0 per hectare were respectively of 20, 23 and 27% with sulphate of potassium and 12, 17 and 17% with potassium chloride; equal K(2)0 applications in the form of leucite did not modify the yields. While potassium sulphate and chloride incfeased a little the size of the tubers, leucite decreased it considerably. Potassium applications did not modify the incidence of internal brown spots.