Potassium or nitrogen?

Looks like a magnesium deficiency. "As a fixed nutrient, any drawback, as a rule, is manifested in a new growth of the leaves. Yellow spots appear on the leaves, which over time become brown, causing dying of the leaves. Areas between residents of old leaves yellow (exclusive chlorosis), and stains can also appear. Colors of rust. If not treated, magnesium deficiency can seriously reduce the ability of plants to assemble the harvest of any type. "