Epsom salts add the essential nutrient, magnesium. If your soil is lacking in this nutrient, a small amount of Epsom salts added to the soil could improve the fertility. When you add a little bit of Epsom salts to your plants—as long as they need it—it can increase the colors of your blooms. Be sure not to add too much, and don’t add Epsom salts if your soil isn’t deficient in magnesium as too much can cause toxic soil buildup and inhibit calcium absorption.