Strawberry (Fragaria x ananassa Duch) cv. "Oso Grande" was grown in sand culture out-of-doors to investigate the effectiveness of supplementary potassium (K) applied to the root zone at high (35 mM) NaCl and high pH (8.5). Treatments were (1) normal nutrient solution alone (C), (2) normal nutrient solution plus 35 mM NaCl (C + S), and (3) plants receiving normal nutrient solution and 35 mM NaCl plus 3 mM K2SO4 supplemented in nutrient solution (C + S + K). Two pH levels (5.5 or 8.5) were combined with the above treatments (C, C + S, and C + S + K). The plants grown at high NaCl and pH produced less dry matter, fruit yield and chlorophyll than those in normal nutrient solution (C). Potassium supplemented into nutrient solution resulted in increases in dry matter, chlorophyll concentrations and fruit yield of strawberry. The deleterious effect of high salinity on plant growth was more striking than that of high pH (8.5). Membrane permeability increased with addition of 35 mM NaCl and increasing pH from 5.5 to 8.5. Supplementary K decreased membrane permeability. Sodium (Na) concentration in plant tissues increased in the elevated NaCl and pH treatments. Concentration of K was much lower in the plants grown at high NaCl and supplementary K application enhanced the K concentration within the plants. These findings suggest that supplementary K can improve plant growth and fruit yield and quality grown under saline conditions.