There is no scientific basis behind the application of epsom salts to enhance plant growth.

Epsom salts is magnesium sulfate - it doesn't contain any "magic" ingredients for making plants grow better. It is true that both sulfur and magnesium are plant nutrients - but minor ones. Sulfur is nearly always present in sufficient quantity in the soil and with the application of nearly any type of synthetic fertilizer (and via many other common garden products). Magnesium deficiencies are usually present only in heavily cropped soils and over-application of magnesium can be harmful.

As with any other type of fertilizer, routine applications just because it's habit or because the advertisements of the manufacturers (like MiracleGro) would like you to believe your garden will not grow without it should be avoided. Fertilize only if soil tests indicate a need or if your plants are evidencing obvious signs of nutrient deficiency. In most cases, unnecessary fertilization is worse for the plants - and the planet - than not fertilizing at all.

If you mulch well with compost each season you will be replenishing pretty much any nutrient your plants require....no need to fertilize at all unless growing in containers.

Here is a link that might be useful: the myth of epsom salts

I am not a expert. I just want to make that clear. LOL I use a tablespoon of ES per gallon of water once a month. I have crappy soil that I have added compost.