Epsom salt is magnesium sulfate. It consists of magnesium and sulfate, which is, simply put, sulfur. On average, Epsom salt consists of 16% magnesium and 13% sulfur. However, these levels can vary from provider to provider or depending on the product used. You can always find the exact content of your Epsom salts on the packaging. Magnesium sulfate is essential for tomato plants, which is why fertilization with Epsom salt is often recommended.