Dietary potassium in the form of fruits and vegetables or supplements has been shown to lower blood pressure.
In a subset of 300 women in the Nurses' Health Study, potassium emerged as more effective than magnesium or calcium in lowering blood pressure. Women in the study had an average blood pressure of 116/73 mm Hg. Each received a supplement of potassium, magnesium, calcium, a combination of the 3, or a placebo. The study authors reported that a daily potassium supplement of about 1600 mg lowered systolic blood pressure by 2 mm Hg and diastolic blood pressure by 1.7 mm Hg. Calcium and magnesium also lowered blood pressure, but the drop was not statistically significant.
Voelker R. Potassium and Blood Pressure. JAMA. 1998;279(8):574. doi:10.1001/jama.279.8.574-JQU81031-5-1