Potassium is an activator of many enzymes that are essential for photosynthesis and respiration, and it also activates enzymes needed to form starch and proteins. It is required in the opening and closing of stomata by guard cells. Potassium encourages root growth and increases crop resistance to disease.
Deficiency symptoms of potassium first appear on older leaves (mobile element). These leaves initially become slightly chlorotic, especially close to dark necrotic lesions (dead or dying spots) that soon develop. In many cereal crops, cells at the tips and margins of leaves die first, and the necrosis spreads along the margins toward the younger, lower parts of the base of the leaves.
Potassium helps move photosynthate out of the leaves. The photosynthesis process has a negative feedback system. If the photosynthate builds up above a specific level photosynthesis is reduced. By keeping the photosynthate moving away from the point of production, photosynthesis can carry on at the maximum rate. Potassium is also very important in getting the sugars and starches translocated to the fruit, grain, tubers, and roots.
In humans, potassium deficiency causes irregular heart action, poor muscular control and/or nervousness.