Epsom salt in soil helps tomato growth by warding off the growth-sapping magnesium deficiency often found in older tomato plants. Epsom salt adds necessary magnesium to the soil so that the plant doesn't develop problems such as yellow leaves and poor productivity.
Epsom salt includes about 10 percent magnesium and 13 percent sulfer. When using Epsom salt for tomatoes, add two tablespoons per planting hole before installing the tomato plant. As the plant matures, an additional tablespoon per foot of plant height can be added to the soil by working it in around the base. This should be done every two weeks.