If you search the Internet, you will see many sites that advocate the use of Epsom salts in gardening. Some claim that it stimulates extraordinary growth, others that it makes tomatoes bigger, encourages roses to bloom more heavily, makes foliage greener, strengthens plant roots and even keeps slugs at bay! Some of these sites are actually sponsored by Epsom salts producers (it’s soooo easy to hide one’s identity these days!), but others come from sincere gardeners who really think they have found a miracle product and want to share it with the world.
Epsom salts got its name from the town of Epsom in Surrey, England, because it was originally produced by boiling down mineral water taken from the town’s springs.
I have to confess: I myself spread the false information about Epsom salts for many years. I’d learned about it from my father, who used them regularly on his tomatoes and roses, and just never thought to check into it. Then one day a reader questioned me about whether Epsom salts were really as useful for tomatoes as claimed and I started to check into things. Wow! I really was way off base. Mea culpa! I’ll now try to save my reputation by sharing the correct information.
Certainly Not a Miracle Product
If you want to understand what Epsom salts can and cannot do, you have to understand what they are. They are actually just a simple chemical: magnesium sulfate (MgSO4·7H2O). They contain magnesium and sulfur as well as oxygen and water. Yes, you could use them as a fertilizer … but they’ll only be useful if the soil is lacking in magnesium or sulfur. And few soils are.
Their utility is therefore limited to those few cases where either the soil is deficient in either magnesium or sulfur or where those minerals are in some way locked up and unavailable to plants. Those are the cases where plants would benefit from treatment with Epsom salts.
Personally, though, if I suspected a plant was suffering from a mineral deficiency (if its foliage was abnormally yellow, red or stunted, etc.), I certainly wouldn’t choose Epsom salts as my treatment of choice, but rather a complete biological fertilizer, like seaweed fertilizer or fish emulsion, one that contain all the major and trace elements. After all, it may not be magnesium or sulfur the plant is lacking, but another trace element: zinc, iron, boron, molybdenum or whatever. When not cover all the bases rather and give the full range of minerals rather than only two? You can even treat mineral deficiencies with compost, another product that almost always contains the whole range of trace elements, although compost takes longer to give results.
If you apply sulfur or magnesium is applied to a soil that doesn’t need it, especially repeatedly, both products will simply work their way into the water table or lakes and rivers: yet another pollutant! How unfortunate!
Blossom End Rot
A shot of Epsom salts is often recommended to prevent or cure blossom end rot in tomatoes. In this deficiency disease, the end of the fruit opposite the stem blackens and starts to rot.
So, Epsom salts to the rescue! Dilute them in water, water the plant and the fruits that follow will be cured of the disease. It seems to work!
But it didn’t.
The blossom end rot is caused by a calcium deficiency, a not magnesium or sulfur one. Usually it is not so much that calcium is lacking in the soil (it’s very common in most soils), but that the soil was too dry and the roots of the plant therefore couldn’t absorb it properly. So the basic treatment is simply … to water the plant regularly. Avoid irregular watering that leaves the plant drought stressed half the time and soaking in water the other half. Instead, learn to keep the soil evenly moist and blossom end rot will disappear as if by magic. You’d have had the same result whether you added Epsom salts to the water or not, because it’s the water that makes the difference, not the minerals that are dissolved in it.
To grow the biggest possible tomatoes, fertilize your tomatoes correctly with compost or a complete fertilizer (one that contains the full range of minerals, including all the trace elements), water regularly to keep the soil evenly moist, offer good growing conditions (full sun, protection from drying or cold wind, etc.) and you will have beautiful tomatoes as big as that variety ever gets. Applying Epsom salts won’t make the fruits any bigger. If you want really giant tomatoes, grow a variety known for its huge fruits, such as ‘Big Zac’, and care for it well.
Some sites recommend Epsom salts as a sort of all-purpose fertilizer for any plant you grow, claiming that it will give extraordinary results, especially keeping leaves green and stimulating root growth. And it will help give nice green leaves and good root growth… but only if the soil is lacking in magnesium or sulfur. And most soils already contain both elements in sufficient quantities to maintain healthy plant growth.
If, for some reason, you suspect your soil is an exception to the rule (very sandy, acid soils do quite often lack magnesium), have it properly analyzed. Send in a soil sample to a specialized laboratory and if, when the results come back, they indicate a lack of magnesium or sulfur, yes, you could apply Epsom salts.
It’s far more likely that the analysis of naturally poor soil will show other deficiencies as well. If so, the most logical treatment would be to apply a complete fertilizer (one that offers the full range of trace elements) or compost (if you’re not in a hurry) rather than Epsom salts.
From the 1930s through the 1960s, several well-known rosarians recommended Epsom salts applications to stimulate beautiful growth and abundant flowering in roses … but that’s not so today. Although each expert still has their preferred fertilizer regime—some prefer complete fertilizers, others mixtures of products, etc. —. few today still recommend Epsom salts, widely considered outdated.
There’s no use looking for studies showing the efficacy of Epsom salts on roses: they simply don’t exist.
No reliable study shows that treating with Epsom salts prevents or cures infectious fungal, viral or bacterial plant diseases. On the other hand, of course, if you consider mineral deficiencies as diseases, Epsom salts can be used to treat magnesium or sulfur deficiencies .
Insect and Slug Repellent
There is no evidence that Epsom salts can repel harmful insects or mollusks. That one seems to really be only a garden myth.
In conclusion, Epsom salts are only really useful as a garden product in the rare situation of plants or soil suffering from magnesium or sulfur deficiency. Even then, an application of complete fertilizer or even compost would have given the same result without requiring the purchase of a special product. In the gardening world, Epsom salts are pretty much useless.