What does Epsom salt do for tomatoes?

The chemical formula for Epsom salt is magnesium sulfate. Most tomatoes don't lack sulfur, but many suffer from magnesium deficiency. This can be caused by soil depletion or by an imbalance of nutrients preventing absorbtion (such as excess calcium). Adding Epsom salt to irrigation water delivers magnesium to the plant alleviates the deficiency. Spraying the compound directly on the leaves is reputed to work within 48 hours, but the soil does also need to be amended as a long-term fix.

NB: tomato blossom end rot is caused by calcium deficiency, not a magnesium deficiency, so Epsom salt is not a recommended treatment.