When to Use Epsom Salts for Tomato Plants—and When Not To

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

Epsom salt, or magnesium sulfate, is a naturally occurring mineral compound consisting of magnesium, sulfur, and oxygen. You might already have it lying around the house, as it’s commonly used as a bath or soak for muscle soreness and pain. But the same micronutrients can also feed plants.

Does that mean Epsom salt will help your tomato plants thrive? Not always. We spoke to expert gardeners to explain best practices when it comes to using this chemical compound on your precious heirloom, cherry, Roma, and beefsteak tomatoes.

Why Epsom Salts Are Used for Plants 

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

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

Magnesium and sulfur are the star players in Epsom salt, according to Carrie Spoonemore, co-creator of Park Seed’s From Seed to Spoon app. They fuel plant protein photosynthesis and enzyme function, and without enough of them, plant leaves may yellow and grow poorly. 

Plants thought to benefit from Epsom salt include tomatoes, under the right circumstance; roses, which gain healthy blooms and greener leaves; peppers, which enjoy stronger roots and improved fruit development; citrus trees, which get greater fruit production and less yellowing of leaves; some houseplants, and some lawns when used as a fertilizer supplement.

When to Use Epsom Salts for Your Tomatoes 

There’s one situation gardening experts agree can call for supplementing with Epsom salt—that’s when your tomato plants have a magnesium deficiency. "Magnesium deficiency usually appears as leaves with bright green ribs and veins and otherwise discolored areas of yellow, red, or brown," Koehn says. Plant growth may also be stunted, and you may see less fruit production. 

To confirm a magnesium deficiency, Spoonemore says, you can conduct a soil test to check magnesium levels or use a foliar analysis (a chemical check of the leaves) to determine nutrient deficiencies in plant tissues.

How to Use Epsom Salts to Treat Magnesium Deficiency in Tomatoes

If you diagnose a deficiency, try these best practices for applying Epsom salt:

• Dissolve one tablespoon of Epsom salt in a gallon of water and apply to the base of the plant.
• Foliar Spray: Mix one tablespoon of Epsom salt into a gallon of water and spray on the leaves.

For optimum results, apply once a month during the growing season, avoid over-application, and then monitor. "Keep an eye on your plants' response," Spoonemore says. "If you notice no improvement or other issues arise, reassess 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 in the absence of a magnesium deficiency. "Applying Epsom salt or additional magnesium when your plants don’t need it, may cause plant growth to stunt and turn too green, which is overall not good for plant health," Koehn says. 

What’s more, Spoonemore adds, "Excess magnesium can interfere with the uptake of other essential nutrients like calcium and potassium, and can lead to salt build-up in the soil, harming plant roots and affecting overall plant health." Overuse may also pollute groundwater, but so can excessive use of any fertilizer. 

To avoid using Epsom salt when it’s not needed and could be harmful, see our list of plant problems you might be tempted to fix with Epsom salt—and what you should do instead to remedy the issue.

Yellowing Leaves

If tests don’t show a magnesium deficiency, tomato leaves can turn yellow because of other issues such as nitrogen deficiency, overwatering, or pest problems, says Spoonemore.

How to Fix: Do soil testing to check nutrient levels, consider cutting back on watering, and inspect the dirt and leaves for pests. Make sure to prune and stake your tomato plants, which will support healthy growth and improve air circulation around plants, helping to prevent diseases.

Not Enough Fruit

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

How to Fix: First, boost 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 such as marigolds, borage, and lavender that attract bees and other pollinators to your garden. Lastly, if you’re growing tomatoes in a greenhouse or indoors, use a fan to create air circulation that mimics natural wind and helps with pollination.

Second, use proper pruning techniques, which means regularly removing small shoots that grow between the main stem and branches—to direct more energy into fruit production, and avoid removing too many leaves, which aid in photosynthesis and fruit development. Prune during the growing season, Spoonemore advises, but avoid heavy pruning during flowering and fruiting stages. Third, use a fertilizer specifically formulated for tomatoes, like Espoma Tomato-tone, which provides essential nutrients without overloading any one element.