Emerald Harvest - Cal-MagHow Cal Mag Works Cal Mag delivers a rich source of calcium and magnesium, two secondary macronutrients that are essential for plant health and vitality. If plants don't get an adequate supply of calcium and magnesium, then you'll start to notice some very negative consequences. Growth can slow down to a crawl and you'll see clear signs of deficiencies, like yellowing between the veins on leaves (magnesium) and dark brown spots (calcium), as well as