a little clip from Wikipedia about magnesium chloride in the garden:
"Gardening and Horticulture
Because magnesium is a mobile nutrient, magnesium chloride can be effectively used as a substitute for magnesium sulfate (Epsom salt) to help correct magnesium deficiency in plants via foliar feeding. It is advisable to use a smaller dose when using magnesium chloride compared to the recommended dose of magnesium sulfate (20 g/L). This is due primarily to the chlorine present in magnesium chloride, which can easily reach toxic levels if over-applied and/or applied too often.
It has been found that higher concentrations of magnesium in tomato and some pepper plants can make them more susceptible to disease caused by infection of the bacteria Xanthomonas campestris, since Mg is essential for bacterial growth."