What does Epsom salt do for tomatoes?

The chemical formulation of Epsom salt is magnesium sulfate. Most tomatoes don't lack sulfur, but many suffer from magnesium deficiency (usually due to soil depletion). Applying Epsom salts alleviates the deficiency. Spraying on the compound is reputed to work within 48 hours, but the soil does also need to be amended as a long-term fix.