Before specific symptoms occur, potassium deficiency causes growth and yield depressions of the plants. Younger leaves remain smaller, and later, growth of the plants is stopped. Specific deficiency symptoms occur on medium/older leaves first. They show chlorotic zones on the leaf margins which become necrotic later. The younger leaves get a blue-green colour.
Potassium deficiency
Mg deficiency also shows chlorosis and necrosis on older leaves, but these start from the middle part of the leaf whereas K deficiency symptoms occur on the leaf margins first.