Epsom Salt for Plants: Don’t Do It!


Who has seen the recommendation to add epsom salt to your roses, tomatoes, or just indiscriminately all over the garden!? I certainly have. Epsom salt is a mineral often promoted as an “organic” plant fertilizer for tomatoes, peppers, and gardens in general. As a new gardener, I saw this advice so often that I decided to look into this supposedly-organic soil amendment. Here’s what I found.

Should you use epsom salt for plants? The general consensus from the horticultural community is epsom salt should NOT be used for plants in home gardens. The benefits of epsom salt in the garden have not been proven and the risks to the environment are considerable.

It seems much of the advice to use epsom salts on plants is anecdotal and is not supported by scientific research or advised by prominent horticultural organizations. The risks detailed by the scientific community, however, are enough to persuade me not to start using epsom salts on my plants! Household chemicals are not a cheap substitute for proper organic fertilizer.

Epsom Salt Fertilizer for Rose Plants

Why is Epsom Salt Used for Plants?

Epsom salt IS used effectively for plants in certain, targeted situations. Specifically, a form of magnesium sulphate is used to counteract soil magnesium deficiency in intensively-managed industrial crops. Responsible use of epsom salt in agriculture involves proving that the soil is indeed deficient in magnesium and also that the risk involved with the application is acceptable.

“The science behind the use of Epsom salts is only applicable to intensive crop production in situations where magnesium is known to be deficient in the soil or in the plants. It is irresponsible to advise gardeners and other plant enthusiasts to apply Epsom salts, or any chemical, without regard to soil conditions, plant needs, and environmental health.”

Epsom Salts: Miracle, Myth, or Marketing, by Dr. Linda Chalker Scott, University of Washington

Epsom salts should only be used for plants if a laboratory soil analysis shows a soil magnesium deficiency and a risk analysis deems the application worthwhile. Soil tests for commercial agriculture include specific application instructions for slow-release horticultural magnesium which are custom-prescribed to the plot of land. Industrial agriculture companies also may have their own staff horticulturalists examine soil test results and give recommendations.

“A form of Epsom salts is used as a supplement in commercial agriculture where magnesium is deficient. While magnesium deficiency is an occasional problem for tomatoes in intensive agriculture situations, it would be highly unusual for a casual gardener to have this very specific type of deficiency. Why supply extra magnesium if it is not needed, especially if one runs the risk of creating other issues in the process?”

Epsom Salts are Not Recommended: Unnecessary, Potentially Damaging, University of Saskatchewan

As discussed above, epsom salt is one way to provide a source of magnesium to nutrient-hungry crops growing in magnesium-deficient agricultural soils. That does not, however, make it ok to go out and sprinkle crystals from your bathroom all over your yard.

Organic Epsom Salt for Plants

Is epsom salt safe for organic gardening? While the crystals beside your bathtub are probably not certified as safe for use in organic growing, there are specialized products that are indeed safe for organic use on plants. An example of an industrial epsom salt product is Magriculture, by Giles.

Magriculture plant fertilizer is an organic epsom salt product that is OMRI-Listed for use in organic agriculture – with the restriction that it can only be used if a soil magnesium deficiency has been documented by testing (see OMRI certificate). This agricultural epsom salt is guaranteed to contain a minimum of 9.8% magnesium (all of which is water soluble). It also contains a guaranteed minimum of 12.9% sulfur (see Magriculture Fact Sheet). This stuff is for the pros.

Epsom Salt Jug - Magnesium Sulfate Salt - Half Full of White Crystals

Folk Benefits of Epsom Salt in the Garden

So we have seen that epsom salt CAN be beneficial to some specific plant crops in very specific industrial conditions. This is related solely to high-production soil that is deficient in magnesium. But are there other benefits beyond soil magnesium deficiency? Pinterest certainly indicates that there must be!

“Epsom salt – actually magnesium sulfate – helps seeds germinate, makes plants grow bushier, produces more flowers, increases chlorophyll production and deters pests, such as slugs and voles. It also provides vital nutrients to supplement your regular fertilizer”

Six Ways to Use Epsom Salt in the Garden, The Epsom Salt Council

The quote above is from The Epsom Salt Council, and it sums up many gardeners’ perspective of the usefulness of magnesium sulphate in the garden. There are likely countless more perceived benefits of epsom salt in the garden circulating around out there.

Beyond adding magnesium to the soil, none of these advertised benefits are substantiated by scientific research. The article Miracle, Myth, or Marketing: Epsom Salts,” by Dr. Linda Chalker Scott (Washington State University) is particularly insightful on why each of these perceived benefits are mythical. Trust me, it’s a worthwhile read!

If you still reeeeaaallly want to use epsom salt in your garden, I suggest referring to the article “Fertilize with Epsom Salt” on Garden.org. Written by horticulturalist Charlie Nardozzi, this article describes how home gardeners could use epsom salts, particularly as a foliar spray. But as the article states, “Before you try Epsom salts, test the soil to determine its magnesium content.”

Any fertilizer should only be added to the garden if it is in fact required and the risk of applying it is acceptable. When considering risk, remember that the ratio of magnesium to calcium is very important for nutrient uptake in plants. This includes plants in your yard and plants in the wider ecosystem. Healthy soil generally has 10x more calcium than magnesium. The actions of humans can upset the natural balance of the soil ecosystem. It’s true that plants need magnesium, but there really can be too much of a good thing.

Also know that the epsom salt you buy in a big jug at the pharmacy is not necessarily a “natural” product. Epsom salts that you buy in a big jug probably came out of a chemical plant in Illinois rather than from some kind of artisanal local rock quarry. But we’ll get into that later on!

Using Epsom Salt as an Organic Plant Fertilizer

Using Epsom Salt in the Garden

Wouldn’t it be great if cheap stuff we already had around the house could be repurposed into magic tomato fertilizer? I still have some epsom salts from Costco that my midwife recommended. I’ll be dreaming of a future with relaxing baths instead of using it on my new roses :)

Risks of Using Epsom Salt in the Garden

If you’ve taken a bath in epsom salts, you’ll know that epsom salt is highly soluble. Soluble chemicals are easily transported in solution with groundwater. You can be therefore be pretty confident that much of the epsom salt you place on your soil will wash into local groundwater. It’s not just going to disappear down into the earth, never to be seen again. It’ll be in the soil, throwing off that calcium-to-magnesium ratio that plants depend on.

So, save the epsom salt for the bath. If you do have a legit laboratory soil test indicating a magnesium deficiency, the laboratory can (and generally will) provide site-specific fertilizer instructions. Your local Master Gardeners or State Extension Office may also be helpful in interpreting the soil analysis with you as they’ll be familiar with local soils. Use the data to get your soil in balance!

Epsom Salt for Roses

Roses are no exception. Do not use epsom salt to fertilizer roses in your garden. I feed my roses with a nice top-dressing of homemade compost! If I were looking for a specific rose fertilizer after a soil test, this is one I would consider. The magnesium in that rose food is derived from Sulfate of Potash Magnesia rather than epsom salts and more than half of the magnesium isn’t water soluble. And it’s not sold next to the band-aids…so that’s a good sign.

Here is a good little tip for feeding roses from David Austin Roses:

Do not be tempted to over-feed – this will, in fact, do more harm than good.

How to Feed a Rose, David Austin Roses

Epsom Salt for Tomatoes

Even heavy-feeding plants like tomatoes and peppers only need a tiny bit of magnesium. As was noted in a previous quote, it is also extremely unlikely that your garden soil is deficient in magnesium. It is, however, completely plausible that a percentage of the epsom salt you put on your garden will wash away into the groundwater and pollute the local ecosystem. Not good!

Epsom salts are also not a fix for blossom end rot on tomato plants. Tomato blossom-end rot is caused by a calcium deficiency. Epsom salt does not contain calcium (source: North Dakota State University).

Fertilizing with magnesium salt can actually increase the chances of blossom end rot. Clemson University provides this excellent explanation for this phenomenon:

Avoid excessive potassium or magnesium fertilization as these nutrients will compete with calcium for uptake by the plants. Epsom salts is an example of a magnesium source, so do not apply to garden soil unless a recent soil report indicates a magnesium deficiency.”

Tomato Diseases & Disorders, Clemson University

Adding epsom salts without adding calcium upsets the natural mineral balance of the soil. This can create “magnesium toxicity” in the soil. Epsom salts does not prevent tomato blossom-end rot… it can actually cause it.

Epsom Salt for Houseplants

Epsom salt poses a risk to houseplants. Houseplants in containers are susceptible to magnesium toxicity as the mineral is not able to wash out into surrounding soil. While the magnesium may wash to the bottom, it can still be trapped in the container. This can make it difficult for the houseplant to absorb calcium.

Epsom Salts for Plants - Magnesium Deficiency in Soil

So, epsom salt is not some “miracle cure” for your garden. It’s not even something you should be sprinkling around your plants for “a little extra luck”. Step away from the epsom salts. Here is a nice podcast from the Joe Gardener Show to listen to while you enjoy your epsom salts in the bath instead of the garden:

Joe Gardener Show – Episode 034 – Gardening Myths BUSTED, Pt. 1 with Linda Chalker-Scott

What is Epsom Salt?

What exactly is epsom salt? Epsom salt is a naturally-occurring mineral. It tends to form underground when mineral-rich water evaporates. Epsom salt takes the form of a transparent-white, crystalline, odourless powder. It can be left behind on hard surfaces when seawater or other brine dries up.

Rock deposits of epsom salt are referred to as Epsomite. Deposits selected for extraction via mining are generally large sedimentary deposits. Chemically, this mineral is a hydrated magnesium sulphate (MgSO4·7H2O), or magnesium sulfate heptahydrate crystals.

Further Reading: “Epsomite”, Entry in the “Handbook of Mineralogy, by John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, and Monte C. Nichols. Copyright – Mineralogical Society of America.

Magnesium sulfate is water soluble – meaning that it dissolves in water. An open pit mine of Epsomite therefore would not make sense, as rain would wash away the ore body. Epsomite extraction with solution mining would be more reasonable. These days, however, the epsom salt we buy is made in chemical plants instead of mined as a useable finished product (more on that in the next section).

The Handbook of Mineralogy notes that Epsomite deposits are “widespread and common”. Here are some locations in North America where epsom salts occur naturally:

  • Mammoth Cave, Mammoth Cave National Park, Kentucky;
  • Mt. Princeton Hot Spring, Chalk Creek, Colorado;
  • Leona Heights, Oakland, California;
  • Kruger Mountain, Oroville, Washington;
  • Buckhorn Mine & Claytonia Mine, Idaho;
  • Basque Claims, Ashcroft, British Columbia;
  • Dundas Quarry, Dundas, Ontario.

MinDat.org also has a handy map on their website showing locations of epsom salt deposits around the world. MinDat also offers the following fun-fact about epsom salts.

Epsomite is likely to occur in the Martian soils and on the satellites of Jupiter.

MinDat.org

Neat! But I’m still not pouring epsom salts on my pepper plants.

Where Does Packaged Epsom Salt Come From?

Packaged epsom salts are magnesium sulfate crystals that have been synthesized in a chemical plant. The industrial plant uses common, easily-mined raw minerals to create synthetic epsom salt with a chemical reaction. Synthetic epsom salt is free of impurities. It perhaps doesn’t have the “character” of raw mined “epsomite” epsom salt, but it does have a very nice uniform look about it!

Very few chemical plants produce epsom salts. Here are major epsom salt manufacturers:

Manufacturing of Epsom Salt

The feedstock ore for epsom salt is mined in a similar manner to some other organic plant fertilizers. Non-metallic minerals such as perlite, greensand, diatomaceous earth, and phosphate rock are all extracted from industrial quarries for use in the garden. Extraction is commonly in the form of open pit mining using truck and shovel mining operations.

Major epsom salt manufacturer Giles, uses a magnesium carbonate (MgCO3) feedstock known as Magnesite. This raw magnesium ore is mined by their parent company Premier Magnesia at a large open pit mine in Nevada. Here is an article about the Nevada mine that provides raw magnesium mineral ore for epsom salt production at their Illinois manufacturing plant.

Epsom salt is precipitated and filtered out from feedstock minerals in a chemical plant. It is then generally shipped in bulk either as diluted liquid in hopper trucks or in crystal form in large 2000-pound bulk bags. Bulk buyers package and sell the epsom salts to consumers. Smaller 50-lb bags are also available for sale directly from the manufacturer. Dry storage of epsom salt in crystal form tends to result in caking if the salt is allowed to absorb moisture from the air.

Link to MSDS for Epsom Salts

Other Uses for Epsom Salt

Here are some other industrial processes that use epsom salt:

  • Paper Manufacturing
  • Artificial Snow
  • Cattle Feed 
  • Bath Salts
  • Hair Products
  • Ore Processing
  • Leather Tanning
  • Laxative
  • Adhesive
  • Soap & Sudsing Detergents

Sources: Giles Chemical, PQ Corporation

Just Say No to Epsom Salt in the Garden

Mary Jane

Mary Jane is a home gardener who loves creating healthy, welcoming spaces (indoors and out!) - About Mary Jane (https://www.homefortheharvest.com/authors/about-mary-jane-duford/)

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