yellow leaves on tomatoes
Epsom salts can be used to increase magnesium levels in soil and can be used if a soil test indicates that magnesium levels are low. Otherwise, you can save your money. There can be other causes for yellowing leaves on tomato plants (nitrogen deficiency, diseases such as early blight, insect damage, etc.). Did you amend your soil with compost or add any fertilizer when you planted your tomatoes? After the first fruits appear, you can side-dress your plants with 10-10-10 fertilizer. https://extension.umd.edu/hgic/learn/tomatoes
You can also send us photos of your plant if it has unusual symptoms and we can help to determine if there are any problems.