What is Epsom Salt?
Epsom salt is a very simple chemical consisting of magnesium, sulfate, and some water. The water is tied up in the crystalline structure of the chemical, and we can ignore it.

Magnesium is one of the nutrients plants need to grow. It is however, a minor nutrient which means plants don't need very much of it.

Sulfate consists of sulfur and oxygen. Plants can absorb sulfate directly from the soil and use the sulfur molecule. It too is a minor nutrient for plants.
Epsom Salt Fertilizer
Epsom salt does work as a specific fertilizer. If your soil is deficient of magnesium or sulfur, it will add these nutrients to the soil. As far as garden soil goes neither of these nutrients is usually deficient.

Does Epsom Salt Control Pests
Current research has found no evidence that it controls pests. It does not kill insects or grubs, nor does it repel slugs and rabbits. It is completely useless for pest control.

Does Epsom Salt Make Plants Grow Better?
Epsom salt is not a miracle product. Provided that your soil has enough magnesium it will not make plants grow better, nor will it make more flowers, or make tomatoes grow bigger.

Should You Use Epsom Salts?
This is real simple – only if your soil test shows that you have a magnesium deficiency.