Chelated minerals have already gone through the chemical changes that make the minerals usable to your plants. They're both important to the plant's production of chlorophyll. Magnesium, in fact, makes up the core of the chlorophyll molecule. Dolomitic lime and epsom salts are good sources of magnesium. To supply your plants with iron, try spraying liquid seaweed or chelated iron .
Calcium is present in gypsum, lime, and oyster shells.
Boron is available in borax and a chelated boron spray. Gypsum and flowers of sulfur are good sources of sulfur. The others are available in chelated form, usually as a spray. Older leaves turn a pale green and the veins are usually a reddish color. New growth will be stunted.