Sweet, juicy tomatoes are the highlight of summer—in a fresh green salad, topped with basil and mozzarella and drizzled with balsamic, or on a simple slice of sourdough bread. So if your tomato plants aren’t producing the bumper crop you expected, you might be tempted to fertilize them with Epsom salts, a gardener’s secret weapon.

Epsom salt, or magnesium sulfate, is a naturally occurring mineral compound made up of magnesium, sulfur, and oxygen. You may already have it at home, as it is often used as a bath or bath additive for sore muscles and aches. But these same micronutrients can also nourish plants.

Does this mean that Epsom salts will help your tomato plants thrive? Not always. We spoke to experienced gardeners to explain the best practices for using this chemical compound on your prized heirloom, cherry, Roma and beefsteak tomatoes.

Why Epsom salt is used for plants

Under the right circumstances, Epsom salt promotes plant growth and overall health. “Magnesium is the center of the chlorophyll molecule, so it’s crucial for the beautiful green color of the leaves and therefore good for overall plant health,” says Andrew Koehn, a gardener at Scribner’s Catskill Lodge in Hunter, New York.

When used correctly, Epsom salt can help plants absorb nutrients so they grow stronger and can help fight mildew and other fungal diseases. Epsom salt is known to increase flower size, promote bushier plants, improve seed germination and increase fruit production.

According to Carrie Spoonemore, co-developer of Park Seed’s From Seed to Spoon app, magnesium and sulfur are the main ingredients in Epsom salt. They promote protein photosynthesis and enzyme function in plants, and without sufficient amounts, plant leaves can turn yellow and grow poorly.

Plants said to benefit from Epsom salts under the right circumstances include tomatoes, roses which produce healthy flowers and greener leaves, peppers which have stronger roots and better fruit development, citrus trees which produce more fruit and have less yellowing leaves, as well as some houseplants and some lawns when used as a fertilizer supplement.

When to use Epsom salt for your tomatoes

There is one situation where gardening experts agree that supplementing with Epsom salts is necessary — namely, when your tomato plants are magnesium deficient. “Magnesium deficiency usually manifests itself in leaves with light green ribs and veins and otherwise discolored areas of yellow, red or brown,” says Koehn. Plant growth may also be stunted and you may produce less fruit.

To confirm a magnesium deficiency, Spoonemore says you can do a soil test to check magnesium levels or do a leaf analysis (a chemical examination of the leaves) to identify nutrient deficiencies in plant tissue.

How to use Epsom salt to treat magnesium deficiency in tomatoes

If you diagnose a deficiency, try these best practices for using Epsom salts:

• Dissolve one tablespoon of Epsom salt in 1 gallon of water and sprinkle it on the stem of the plant.
• Foliar spray: Mix one tablespoon of Epsom salt with 1 gallon of water and spray on the leaves.

For best results, apply once a month during the growing season, avoiding over-application, and then monitor afterward. “Keep an eye on your plants’ response,” says Spoonemore. “If you don’t see improvement or other problems arise, review your soil and nutrient management practices.”

When not to use Epsom salt on tomatoes – and what to do instead

Avoid using Epsom salt on tomatoes unless there is a magnesium deficiency. “Applying Epsom salt or extra magnesium when your plants do not need it can cause stunted growth and to Green, which is overall not good for plant health,” says Koehn.

Additionally, says Spoonemore, “excess magnesium can interfere with the absorption of other important nutrients such as calcium and potassium and can lead to salt deposits in the soil, which can damage plant roots and affect overall plant health.” Excessive use can also pollute groundwater, but the same is true of excessive use of fertilizers.

To avoid using Epsom salt when it’s not needed and could be harmful, check out our list of plant problems you may want to fix with Epsom salt – and what to do instead to fix the problem.

Yellowing leaves

If tests don’t show a magnesium deficiency, tomato leaves can turn yellow due to other causes, such as nitrogen deficiency, too much water or pest infestation, Spoonemore says.

How to fix: Do soil tests to check nutrient levels, consider watering less, and inspect soil and leaves for pests. Be sure to prune and support your tomato plants to encourage healthy growth and improve air circulation around the plants, which helps prevent disease.

Not enough fruit

According to Spoonemore, low fruit yields could be due to poor pollination, improper pruning or nutrient imbalances.

How to fix: First, increase the chances of pollination by gently shaking the plants or using a small brush to transfer pollen from flower to flower, Spoonemore says. Also consider planting flowers like marigolds, borage and lavender that attract bees and other pollinators to your garden. And finally, if you’re growing tomatoes in a greenhouse or indoors, use a fan to create air circulation, which mimics natural wind and helps with pollination.

Second, use proper pruning techniques, meaning regularly removing small shoots that grow between the main stem and branches – to channel more energy into fruit production, and avoid removing too many leaves that help with photosynthesis and fruit development. Prune during the growing season, advises Spoonemore, but avoid heavy pruning during the flowering and fruiting phases. Third, use a fertilizer specifically designed for tomatoes, such as Espoma Tomato-tone, which provides essential nutrients without overloading any single element.