Magnesium is necessary for the production of chlorophyll. In a sense it promotes blooming in plants that are in the flowering growth cycle because the more the Brug "Y"s, the more flower buds it produces. However, if you use a complete fertilizer complete with micro-nutrients, the plant will get all the magnesium it needs. If the leaves are pale or yellowish and a dose of fertilize doesn't green the leaves, before you add anything else including epsom salts, test the soil pH first. It is possible that nutrients are not being taken up because the pH is too high or too low — either way on the pH scale inhibits the roots from taking up those essential nutrients. It is possible to give plants too much magnesium which might show up as symptoms of calcium or potassium deficiency. Epsom salt is a salt and too much of any salt can damage roots.
While magnesium is needed by plants, it is not needed in large quantities. If the plants are in the ground, there is usually enough magnesium in the soil to provide what plants need, that is, unless there is a pH problem. If the plants are growing in pots, then a complete fertilizer should be able to provide the plant with all the magnesium it needs. Again, unless there is a pH problem.
http://www.ncagr.gov/cyber/kid...
This is an article on hydrangeas, but the principles apply to all plants:
http://en.allexperts.com/q/Fer...
http://www.eplantscience.com/b...
