Answer: Epsom salt is a good source for the trace nutrients magnesium and sulfur. Late in the season tomatoes and pepper plants may start to show signs of deficiencies, such as leaves yellowing between the veins and a decrease in fruit production. This looks similar to blight, but blight will appear as blotchy brown spots on the leaves and stems. Be sure to confirm which it is.
Before randomly adding nutrients it’s always best to obtain a soil test to evaluate the levels of trace minerals in your growing area. Too high a level of calcium will inhibit the plants’ uptake of magnesium, and depending on your soil type you might require more than just what one nutrient can add.
Best advice is to watch your plants toward the end of the growing season. If they start to show yellowing leaves and you’ve ruled out other diseases, you could try adding a tablespoon of epsom salt around the base of the plant, making sure to water it in well. Some report that peppers are heavy magnesium feeders so this could be the reason for the use of epsom salts. Some also suggest starting the treatment at first bloom to thwart the late season deficiency.