Potassium (K) is one of "the big 3" elements found within lawn fertilizer. When applied and the correct amount is maintained in the soil it strengthens the outer cell membrane of the grass. An illustration of this can be likened to a warrior going to battle, the thicker the armour he wears, the more it can cope with from external forces. Like the warrior, grass when the membrane is at peak strength from the correct amount of potassium in the soil, the lawn can withstand the elements from things such as drought, extreme heat etc. This in turn assists you keeping a nice green lawn all throughout summer.