Epsom Salt Myths – learn the truth about using it in the garden

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Robert Pavlis

Almost daily, I see a post in social media about using Epsom salt to cure all manner of plant problem. Planting a new plant; add Epsom salt to the planting hole. Are bugs your problem? Epsom salt will get rid of them. It also gets rid of diseases, and blemished on leaves. It makes tomatoes grow bigger, and produces a higher yield, with no Blossom End Rot. Roses are absolutely dependent on the stuff – you must put it in the planting hole every time.

If Epsom salt is such a miracle cure for plants, why is it that the scientific community does not know about it? Time to debunk this myth once and for all.

Epsom salt for plants
Epsom salt for plants – or is it best for a bath?

What is Epsom Salt?

Epsom salt is a very simple chemical consisting of magnesium, sulfate, and some water. The water is tied up in the crystalline structure of the chemical, and we can ignore it.

Magnesium is one of the nutrients plants need to grow. It is however, a minor nutrient which means plants don’t need very much of it.

Soil Science for Gardeners book by Robert Pavlis

Sulfate consists of sulfur and oxygen. Plants can absorb sulfate directly from the soil and use the sulfur molecule. It too is a minor nutrient for plants.

Epsom Salt Fertilizer

Epsom salt does work as a specific fertilizer. If your soil is deficient of magnesium or sulfur, it will add these nutrients to the soil. As far as garden soil goes neither of these nutrients is usually deficient. If you are adding any kind of organic material or organic mulch to the soil, your soil will likely have enough of both magnesium and sulfur.

Sandy soil and acidic soil may have a deficiency of magnesium (ref 1).

It should not be added to soil unless a soil test shows you that you need to add more. If you need only sulfur and not magnesium, then horticultural sulfur is a much better product to use.

Just to be clear – the NPK numbers for Epsom salts is 0-0-0.

Video Version

This is the video version of this post.

YouTube video

Magnesium Deficiency in Plants

What about a plant that shows a magnesium deficiency? First off, it is hard to identify a nutrient deficiency by looking at plants – that is another myth. But lets say you are sure your plant has a magnesium deficiency. It seems to make sense to add Epsom salt to the soil – right? Not necessarily.

High levels of phosphorus in the soil prevents plants from absorbing magnesium even though there might be lots in the soil. The solution in this case is to either reduce the phosphorus level, which is hard to do quickly. In this case ading Epsom salt will not work. Soil chemistry is complicated – don’t mess with it unless you know what you are doing.

Does Epsom Salt Control Pests

Current research has found no evidence that it controls pests. It does not kill insects or grubs, nor does it repel slugs and rabbits. It is completely useless for pest control

Does Epsom Salt Control Diseases

There is no clear evidence that any disease is controlled by Epsom salt.

Does Epsom Salt Make Plants Grow Better?

Epsom salt is not a miracle product. Provided that your soil has enough magnesium it will not make plants grow better, nor will it make more flowers, or make tomatoes grow bigger.

Magnesium is part of the chlorophyll molecule and vital for plants to grow. If it is missing in the soil, plants won’t grow as well, and adding magnesium to the soil will help. But that is only true if you have a deficiency.

What About Roses?

Epsom salt is recommended most frequently for roses. You put some in the planting hole and you feed with it yearly. Is this advice scientifically sound?

A review of the literature found no scientific evidence that roses need more magnesium than other plants. The Rose Society of America (ref 2) does not recommend Epsom salts for the ‘casual rose grower’, but does recommend it if you are a ‘rose specialist’. Why would the depth of your interest in roses affect which fertilizer is required?? That makes no sense.

The marriage of roses and Epsom salt has been with us a long time, and bad habits are hard to break.

I grow some roses. I’ve never added Epsom salts to any plant in the garden and my roses grow just fine.

Preventing Blossom End Rot in Tomatoes

Epsom salt is regularly recommend for tomatoes to prevent blossom end rot. I have discussed this in Blossom End Rot. Blossom End Rot is a calcium deficiency in the fruit – not a magnesium deficiency. With respect to Blossom End Rot, reference 3 says “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 soil unless a recent soil report indicates a magnesium deficiency.”

Adding magnesium can cause Blossom End Rot – it is not fixing the problem.

Should You Use Epsom Salts?

This is real simple – only if your soil test shows that you have a magnesium deficiency.

References:

1) Miracle, myth…or marketing, Epsom salts; https://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/epsom-salts.pdf

2) American Rose Society: http://www.rose.org/rose-care-articles/fertilizers-when-and-how/

3) Blossom End Rot of Tomato – an Update: http://www.clemson.edu/extension/hgic/hot_topics/2009/08blossom_end_rot.html

4) Photo Source: Stacie Biehler

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

92 thoughts on “Epsom Salt Myths – learn the truth about using it in the garden”

  1. Hi Robert, Thank you for your article. I recently purchased 22 kilos of magnesium sulphate from my local farm store thinking whata great deal I got on Epsom Salts for the bath to alleviate muscular pain and have a better sleep.

    I noticed on the bag that it said “Agricultural Use Only” and that amongst the various minerals with the magnesium sulphate is 6 mg of Lead per kilo. I know that lead is poisonous and was wondering if I soak in a tub with one cup of magnesium sulfate a few times a weak, am I poisoning myself with lead?

    Any thoughts on this please?

    I am assuming that food grade epsom salts have extracted the Lead from it?

    Gaura

    Reply
    • Gaura, like much knowledge we research on the Internet, the knowledge about Epson salts in agriculture is not as black and white as some bloggers would have us to believe. Its use is very specific to a class of chemical deficiencies. It is not a panacea, but, likewise it is not categorically harmful to plants. Keep in mind that commercial agriculture and home gardening are too very different animals. Commercial agriculture can be hard on soils, plus the desire to increase yield changes things for the commercial grower. Never, never, never take one person’s view on the subject. Be especially wary of excessive bias, one way or another. Do your own research. BTW, the USDA does list magnesium sulfate as soil additive for a small class of crops.

      Reply
      • “USDA does list magnesium sulfate as soil additive for a small class of crops” – I find that hard to believe – do you have a link to this?

        Adding magnesium to soil is dependent on soil conditions – not the crop.

        Reply
        • https://www.ams.usda.gov/sites/default/files/media/MGSuTechnical%20Evaluation%20Report%20Crops.pdf is a good starting point, Robert. Mag sulfate has many uses for crops, animals, even humans. And, yes adding nutrients to the soil is based on a soil analysis, but the interpretation and subsequent action is based on the specific crop’s nutritional needs. Now, if I can just stem the recent looper larvae attack on my Bougainville. A jug of BT should arrive tomorrow.

          Reply
          • It says very clearly, “In agriculture, magnesium sulfate is added to soil to correct for magnesium deficiency
            53 (Kawamura and Rao, 2007).”

            I found no reference that says it should be used on specific crops. Nor does it say it does anything for crops other than provide magnesium and sulfur nutrients.

          • Lines 51-54 from the USDA report::
            51 Magnesium sulfate has a wide variety of uses in agriculture, food processing, personal care products, and
            52 medicine. In agriculture, magnesium sulfate is added to soil to correct for magnesium deficiency
            53 (Kawamura and Rao, 2007). Crops that heavily depend on magnesium-rich soil include potatoes, peppers,
            54 tomatoes, and roses. Magnesium sulfate is also commonly added to potted plants

            Again, yes it corrects soil for deficiency in magnesium, presumably because you want to grow plants that require magnesium. Those are listed in lines 53-54 of the USDA report. Otherwise, just soak your feet in it.

            You’re the chemist, what am I missing here?

          • So … the quote does not support your position that magnesium sulfate does anything for the listed crops except add nutrients – which is what I have been saying. They just happen to list crops that use more magnesium than others. That does NOT mean you have to add magnesium to grow these crops.

            You are reaching an illogical conclusion that is not based on the facts.

            My soil has very high magnesium levels – I can grow potatoes without adding any more. You fertilize soil – not the plants.

        • I think we’re going in circles, Robert. I did not get a PhD in engineering for reaching illogical conclusions. And when my Soybeans need a particular nutrient that is insufficient in the native soil, we add it to the soil for the benefit of the beans. If we don’t, I lose income at harvest time.

          “My soil has very high magnesium levels – I can grow potatoes without adding any more. You fertilize soil – not the plants.”

          Congratulations on your potatoes. But, guess what. The soil did not “eat” the magnesium, the potatoes did. The soil itself does not benefit from any nutrients. It is not a consumer. (except, of course for certain chemical reactions that occur.) Just a vehicle. Get your horse in front of the cart and you’ll get somewhere. You can have the last word. 🙂 Thanks for the mental joust.

          Reply
  2. Thank you for all the knowledge I have gained in reading the “discussions” in this column. I daresay you’ve kept me from making some serious mistakes concerning use of Epsom Salts. Is there a way to sign up for your column, Mr. Pavlis? Thank you! Pauletta

    Reply
  3. Wow! So much excitement.

    I agree with what Robert is arguing about Epsom salts and plants: not often needed. But, I myself have horrible arthritis and my rheumatologist tells me Epsom salts are proven to pull fluid from swollen joints. So, I would be reluctant to give them to my plants, but just bought 20 lbs of Epsom for my bath…

    We have hard water in our building and salt buildup in plant soil is a big problem for my plants. I will be cautious fertilizing mine. They seem to like a fish solution that stinks like hell a few times a year, though.

    Reply
  4. Epsom salts are really magnesium sulfate, two chemicals that are required nutrients for plant growth but, it is not a complete fertilizer since it lacks nitrogen, phosphorous and potassium.  Most home gardens are not deficient in magnesium.  So save your Epsom salts for your bath.

    Reply
  5. Thank you so much for this column, and the clear, patient replies to all the comments. I’m not deeply trained in science but I sure know the difference between old wives’ tales or casual observation, and actual scientific testing. I’ll take science any day – every day – over the crazy made-up or mismeasured (or more likely, unmeasured) things people pass along as “wisdom.”

    My own experience with Epsom Salts is that I’m in an Extension-sponsored Master Gardener program and was appalled this spring to hear a fellow Advanced Master Gardener include the old “everything grows better with Epsom Salts” advice in the public presentation he’s been giving all over this year — even to master gardeners. Ugh! We are strictly told not to give out any advice that is not research-backed, i.e., published by or referenced by an Extension publication. Now I have to ask our Extension educator, who has a PhD in plant pathology himself, to please remind our group that we’re supposed to be spreading good gardening knowledge, not myths!

    I came here today because someone swore to me that Epsom Salts would keep the slugs from decimating our dahlias. Having studied soil science in grad school, I didn’t like the idea of dosing my soil with unnecessary amounts of Mg, S, or especially “salts.” (We are already alkaline.) THANK YOU for putting that nonsense idea to rest! I’ll stick with beer traps and leave my healthy soil alone.

    Reply
    • Thanks for the last paragraph. The rest of the comment was not useful. When someone says “xxx is not scientifically proven” I would rather they say “I have researched and not found any peer reviewed papers …” I have alkaline soil so the comment is very useful. Sulphur I use carefully. My hydrangea all seemed to just die then they tested my lawn and found the soil to be rather alkaline and added sulphur. The color changed dramatically. I think I will have the soil in all the beds tested and see what is missing. The new beds were heavily amended by me with peat moss, Milorganite, Holly Tone and Sweat Peat. The rhododendron I planted took off like a shot! So I must have gotten it right on that bed but testing would have been a lot better approach. Thank you for your comment.

      Reply
      • Years ago I overdosed on Sulphur to turn a hydrangea red. It turned brick red to match the brick red window boxes on our blue house. After a while my wife wanted them back to blue. I’m still working on that; they are pink now. “Better things for better living through chemistry.” (I was a chem major in college who went into computer communications; my chemistry is now limited to CaCl2 in the winter and Chemical Warfare against weeds in the summer!) 😉

        Reply
        • Hmmm, this sounds a little backwards… or should I say, ‘upside down’🙃 Doesn’t acid make hydrangea more blue and alkali make it pink/red – not the other way around?🤔 It is contrary to what happens with litmus paper, which turns red with acid and blue with alkali😊

          Reply
  6. Oh to live in a location like Canada, nice fertile soil. Sadly I live in Tucson, Az. and have garden soil that always wants to go to 8.5 pH. This is due to the hard water we have and the constant supply of CaCO3 we keep putting into our gardens everyday as we water. So, depending on your location, which I think really should always be the rule of thumb, you may well indeed always need to add Epsom Salts, Gypsum, Soil Sulfur to your garden just to keep the pH at a useable level for your plants. Location, Location, Location. Of course, more compost fixes everything also 🙂

    Reply
    • We don’t have fertile soil everywhere, I have hard water and my pH is 7.4. Even in your soil, Epsom salts will not help unless you are magnesium deficient. And gypsum should only be used to replace deficiencies.

      Reply
  7. Magnesium sulfate will reduce the heat or increase the heat source?
    Magnesium sulfate can be use for cucumber plant?
    Pls reply fast……

    Reply
    • Magnesium sulfate will not increase or reduce heat. Its only purpose is to add magnesium if your soil is deficient.

      Reply

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