Epsom salt is composed of magnesium and sulfate, can be beneficial for garden plants. Magnesium is an essential nutrient for plants and is necessary for the synthesis of chlorophyll, which is crucial for photosynthesis. Sulfate can also help improve the absorption of nutrients by the plant roots. If a plant’s leaves turn yellow, it can be a sign they need more sulfate. If lower leaves turn yellow they may need more magnesium.