There are trace minerals your plants cannot live without.
Magnesium and iron are both important to the plant's production of chlorophyll. Magnesium is an activator for many enzymes in plant growth processes, and assists iron in the formation of chlorophyll during photosynthesis. It, 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 with liquid seaweed.
Calcium (Ca) and boron (B) are essential to any plant's plumbing system. Calcium is essential for proper water uptake, and both are important for proper cell formation. Calcium is present in gypsum, lime, and oyster shells. Boron is available in borax and a chelated boron spray.
Sulphur (S), zinc (Zn), and manganese (Mn) are the "catalysts" that help other nutrients such as nitrogen become usable by your plants. Gypsum and flowers of sulfur are good sources of sulphur. The others are available in chelated form, usually as a spray.
All the above additives are to be found in plant feed sprays from your local garden centre.