‘French’ prune trees (Prunus domestica syn. ‘Petite d’Agen’) grown on a fine-textured Wyman loam soil were sprayed with KNO3 in Gridley, California (USA). Spray applications (20-22 litres/tree, 43-48 kg/ha) of KNO3 were compared with single annual soil applications of potassium chloride (1,4-2,3 kg/tree) or sprays of urea + KNO3 with respect to leaf potassium and nitrogen concentrations, fruit size, drying ratio and dry yield. KNO3 sprays were as effective or better than soil-applied potassium chloride at maintaining adequate levels of potassium throughout the season. Lowest leaf potassium values, below the adequate level of 1,3% potassium, were found in the trees where no potassium was applied. These trees developed potassium deficiency symptoms. Trees showing below optimum leaf-potassium levels showed a clear yield benefit following spraying. Trees deprived of potassium were the lowest yielding. It was concluded that foliar KNO3 sprays applied four times throughout the growing season can correct relative potassium deficiency in ‘French’ prune an can obtain dry yields equivalent to those obtained with soil applications of KCl.