It is essential for plant growth, however the exact functions within plants are not well understood. The primary function may be related to plant metabolism. It is known to be vital to photosynthesis, because it declines when potassium is deficient. Respiration increases when potassium is deficient. Combined, increased respiration and decreased photosynthesis reduces the plant carbohydrate supply.
Potassium is known to be essential for protein synthesis and enzyme activation.
The following are guidelines, and thus are not infallible. If doubt exists, plant tissue samples should be analyzed by a reputable laboratory. This is shown first on the oldest leaves and then younger leaves as the deficiency worsens. The first indication of phosphorus deficiency is an over-all stunted plant. Leaves may be unusually dark green at some stages, may have distorted shapes, and may become purple.