Magnesium: 

From Hands-On Agronomy by Neal Kinsey and Charles Walters, published by Acres U.S.A.

Calcium and magnesium go hand in hand. Magnesium is a constituent of chlorophyll. In fact, chlorophyll and photosynthesis rely on its presence and availability. Magnesium aids in phosphate metabolism. And it activates several enzyme systems….

Magnesium, in conjunction with calcium, is the key to air and water in the soil. Magnesium helps hold the soil together and tightens it up. Too much of this wonder element hardens the soil when it is dry and makes a clay soil sticky when it is wet….

Magnesium deficiency causes whitish strips along the veins and often a purplish color on the underside of the lower leaves.