From a garden web article:
A little over 2 years ago I asked Dr Elaine Ingham the following question, "What is the effect of magnesium sulfate (Epsom salts), used to adjust the salt balance in their soil or simply to add magnesium, on the beneficial microbes in the soil or compost tea?" She answered as follows:
Concentration is the major factor here. Small amounts [of magnesium sulfate] are fungistatic (meaning they inhibit growth of fungi), and with time, fungi recover. Higher amounts clearly kill beneficial fungi. Testing has been limited [in determining] the precise threshold of negative impact.