Deficiencies will show in older leaves first. Leaves turn deep green on a uniformly smaller, stunted plant. Leaves show brown or purple spots. Phosphorus flocculates when concentrated and combined with calcium. Potassium increases chlorophyll in foliage and helps regulate stomata openings so plants make better use of light and air.
Potassium encourages strong root growth, water uptake and triggers enzymes that fight disease.
Potassium is necessary during all stages of growth. It is especially important in the development of fruit. Older leaves mottle and yellow between veins, followed by whole leaves that turn dark yellow and die. Flower and fruit drop are common problems associated with potassium deficiency. Potassium is usually locked out by high salinity.