Epsom salts are often offered as a sort of cure-all for the garden. If only it were true!
By Larry Hodgson
If you do an Internet search, you’ll see many sites that advocate the use of Epsom salts in gardening. When I entered the words Epsom salts garden use in Google, there were 258,000 hits … and I’m assuming that most were recommending its use. Old gardening books often make similar claims, although I would hope the authors of modern ones did their research!
So, what are Epsom salts supposed to do? Claims include that they stimulate extraordinary growth, make tomatoes bigger, prevent blossom end rot, encourage roses to bloom more heavily, make foliage greener, strengthen plant roots and even keep slugs at bay! Even that they kill weeds! (Odd that a product supposed to stimulate growth of the plants we like would also be deadly to unwanted plants, don’t you think?) 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.
Far From 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). 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 only 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 were abnormally yellow, red or stunted, etc.), I certainly wouldn’t choose Epsom salts as my treatment of choice, but rather a complete fertilizer, like seaweed fertilizer or fish emulsion, one that contains 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: the symptoms of mineral deficiencies are so hard to interpret! Why not cover all the bases 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 may take a bit longer to give results.
If you apply sulfur or magnesium to a soil that doesn’t need it, especially repeatedly, both products will either build up in the soil, making it possibly toxic for plant growth (magnesium) or seriously acidifying it (sulfur) or they’ll work their way into the water table or into nearby 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 actually doesn’t.
The blossom end rot is caused by a calcium deficiency, a not magnesium or sulfur one. And usually, it’s 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.
You can learn more about blossom end rot in the article Preventing Blossom End Rot on Tomatoes.
To grow the biggest tomatoes possible, 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’ll 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.—, rosarians today consider the use of Epsom salts seriously 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 strictly a garden myth.
There are all sorts of DIY recipes for weedkiller (herbicide) floating around on the Internet and some involve Epsom salts. They almost always use Epsom salts not on their own, but mixed into some sort of recipe: boiling water, strong vinegar, dishwashing detergent, etc. The thing is, Epsom salts simply aren’t a great product for killing weeds. You could take any of these recipes, but replace the Epsom salts with sodium chloride (table salt) and that would give you a more effective weedkiller at much less expense.
Of course, if you apply a strong enough solution of Epsom salts (or of any mineral!), sure you can kill weeds, because you’ve rendered the soil toxic to plant growth. You’ll probably never be able to garden there again. Is that what you really want? And if that’s fine with you, again, table salt will give the same results more cheaply … and both chemicals will likely end up polluting the water table.
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.