Potassium ions are very important to organisms. That is why fertilizers have potassium compounds in them. The ions send messages from cells to other cells. It helps biological membranes "depolarize". This means go from a negative to a positive electrical charge. This is needed for muscles to "contract" (get shorter and move things.) It is needed for the heart to "beat" (push blood through blood vessels.) If the potassium level in the blood is too high or too low it can cause death because the heart stops. A few good sources of potassium are bananas, apricots and raisins.