Symptoms
Symptoms of potassium deficiency are visible on older leaves first. Chlorosis starts from the leaf tips in the intercostal areas, but the chlorotic zones coalesce and the whole leaf tip becomes yellow.
Later, it shows dark brown color and dies off. Basal parts of the leaves remain green for a longer time.
Reasons
Potassium deficiency
Manganese and phosphorus deficiency result in chlorosis followed by necrotic zones, but these start from the leaf margin, not from the leaf tip.Furthermore, Mn deficiency is visible on younger leaves first.
Magnesium deficiency causes interveinal chlorosis with necrotic zones later, but these are spread over the whole leaf area and are not concentrated on the leaf tip.