When things are going wrong in the garden, it can be tempting to turn to household solutions as a magic potion to solve them. One common remedy that has been passed down through generations of gardeners is Epsom salt, which is said to jumpstart slow-growing plants.
But does it actually work?
Epsom salt is the chemical magnesium sulfate. In its marketing, Epsom salt is touted as increasing seed germination, improving uptake of other nutrients and enhancing growth and overall health of plants because it adds the essential element magnesium back into the soil.
According to Bruce Hoskins, a scientist at the University of Maine Soil Testing Lab, Epsom salts can be useful for gardens with magnesium deficiencies, which over time, can impact leaf growth and the plant’s chlorophyll’s ability to capture energy from the sun. The symptoms of magnesium deficiency vary from crop to crop, but generally manifests as lightened leaves.
“Magnesium deficiency typically shows up in lower leaves,” said Caleb Goossen, Organic Crop and Conservation Specialist at the Maine Organic Farmers and Gardeners Association. “The veins themselves typically stay green because the plant is mobilizing the magnesium from lower leaves and bringing it to newer leaves for new growth.”
However, magnesium deficiencies are unlikely to cause plants to grow slowly, so the idea that it can jump start germination and growth is less sound. Hoskins said that a more likely culprit is nitrogen deficiency or an imbalance in pH for slow growing plants.
“Compost will always give you organic matter, but almost no detectable nitrogen,” Hoskins said. “It’s very common for midseason poor growth.”
Hoskins emphasized that the only way to know what is ailing your garden — and how to solve it — is by getting soil tests, which are conducted throughout the year at the University of Maine’s soil testing laboratory. Properly diagnosing the mineral imbalance in your garden is especially important because magnesium deficiencies can be difficult to accurately identify.
“The classic [hallmark of a] magnesium deficiency is yellowing of the leaves between the veins but sometimes other things show up like that as well,” Hoskins said. “A lot of people think they have magnesium deficiency, [but] it winds up being potassium or something like that.”
By incorrectly addressing the issue in your garden — say, by sprinkling the soil with Epsom salt when magnesium deficiencies aren’t your problem — you may also further exacerbate the issues because of the chemistry of the soil.
“There are three nutrients — calcium, potassium, magnesium — [that] all have plus-charge ions, all potentially mutual competitors held in soil and taken into plants,” Hoskins explained. “We don’t want to have people get their soil get way out of whack by over-applying any of those three. We don’t just say, ‘Well, go ahead and throw some on.’ I’m not going to tell someone to do that unless I have something to look at.”
Besides, Hoskins said that Epsom salt is a rather expensive way to address the issue of magnesium deficiency in your soils. Popular alternatives like magnesium lyme and K-Mag, a common potassium magnesium sulfate fertilizer, are less expensive to buy in bulk and tend to have higher concentrations of magnesium than Epsom salt.
“Epsom salt is kind of the fallback,” Hoskins said. “If you don’t need potassium and you don’t need lyme but you do need magnesium, we recommend Epsom salt.”
Hoskins said that this is most common with acid-loving plants.
“Blueberries is probably where it’s most commonly recommended because it’s a neutral salt,” he said.
The only way to know if your soil needs magnesium, though, is through a soil test. The best time to get a soil test is before the growing season so you can address these problems before the plants go in the ground, but the good news is that you can get a soil test midseason if you think you might have a magnesium deficiency or any other problem that you need to address.
“You can test anytime,” Hoskins said. “We have a lot of people sending in now because their gardens are not doing well. You can make midseason adjustments for things like nitrogen, and in the case of magnesium, Epsom salt is highly water soluble so it can be put on and watered in. We do recommend it occasionally but only where it’s needed.”