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.
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.
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.
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
If epsom helps things grow, than why are we told to use it to kill weeds and sterilize soil for no regrowth?
Epsom salts is magnesium sulfate. Plants need very small amounts of magnesium and sulfur – but your soil usually has enough of both. In that case it does NOT help things grow.
It does not kill weeds or sterilize soil unless you put huge amounts on the soil. Neither magnesium or sulfur is particularly toxic to plants, but lots of just about anything will kill things.
Why are you told lies? People don’t know any better, and some web sites just plain lie to sell advertising.
Well I hear what you are saying Robert and believe it, I believe in science. But maybe there is something else going on chemically. I can’t argue with facts, but I planted 1 gallon size Palms in my yard at the time my neighbors palms were almost 20 years old. Mine, at the age of 8 were the same size as his. Now they are 11 yo and 39′ feet. Trunk diameter @ 5′ is 20″ These things are huge for their age. They get Mag sulfate couple times a year, and organic mulch at the base, no chemical fertilizers. My banana trees are producing bananas like crazy. Over 300 this year from 20 trees, here in central FL. Either I got the green thumb or it’s the Eps Salts. What do you think? 😉
What is the magnesium level in your soil?
I have used epsom salt in gardening for years and have been very pleased with the results. Maybe my garden doesn’t like science. I don’t have weeds, or pests (except mosquitoes) , or plant diseases and my roses, grass and tomatoes look wonderful.
It is not that your garden does not like science. It is that you have not tested the Epsom salts scientifically and therefore you don’t know if it works. Study the scientific method to understand this better or have a look here: The Real Value of Anecdotal Evidence
Thanks for the article. This is my first year using Epsom Salts so I can’t tell quite yet if they seem to be working for the plants. However, I had serious problems w/ slugs the past few years and Epsom Salts is definitely helping this year. Earlier in the summer, I put a couple on a pile of E.S. and they definitely died. I have almost no slug damage now (partially due to spreading the crystals on the dirt occasionally I believe). Thanks again for the rest of the info!
Ref #1 was rewritten when Dr. Chalker-Scott was pressured to justify her claims:
https://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/epsom-salts.pdf – article
https://puyallup.wsu.edu/lcs/reference-epsom-salts/ – references to above article
“The solution in this case is to either reduce the phosphorus level – hard to do quickly, or increase the nitrogen level.”
Can you please explain how the nitrogen would help correct this. Thank you, all your info is so helpful to this garden novice!
No I can’t – sorry. I believe this advice came from reference #1, and it is no longer a valid link. I’ll have to do some more digging to clarify this and If I get any good answers, I’ll let you know.
“No I can’t – sorry. I believe this advice came from reference #1, and it is no longer a valid link.”
That link is most definitely valid, sir. It’s a downloadable PDF. ardicle. I just clicked on it and read it….
ok,now i have some blueberry plants. i need acid soil to be sure.well…………..will magnesium sulfate increase the acidity of my soil?
Sort of. The sulfate part of Epsom salts will convert to sulfuric acid and neutralize soil. While this is going on the magnesium levels could rise to toxic levels. For this reason it is much better to use agricultural sulfur – same acidifying effect, and no toxic magnesium.
Here is the real problem. Sulfate will neutralize alkaline soils, but that does not mean the soil will become acidic. If your soil contains things like limestone, the acid from sulfate will simply dissolve some of the limestone. the pH of the soil will still be alkaline.
to understand this better have a look at Increasing Soil Acidity
Good
I used Epsom salts to enhance the colour of a ‘New Dawn’ rose. The effect on the rose was negligible, but shortly afterwards the Clematis ‘Elasa Spath’ planted to climb through the rose, developed extreme chlorosis. Several years later, speaking at an ORG&HPS meeting, Peter Keeping told us that magnesium damages the root system, making it susceptible to viruses. Yep – it would appear to have happened in this case.
I did a little digging about the toxicity of magnesium. Excess in soil is not very toxic and as long as their is lots calcium in the soil should not be a problem. I don’t think magnesium would kill the clematis roots but high salt concentration can affect plant roots. I’ll probably do a followup post on magnesium toxicity some time.
How can I flush Epsom salts from my soil? If too much has been added.
You can’t. If you tried to flush the soil you would also flush away all kinds of other nutrients. The first step is to get a soil test done to measure magnesium levels – are they at toxic levels. if not – don’t worry about it.
Add more organics – they absorb excess nutrients.
Are you an organic grower?
Yes – but you might not agree.
https://www.gardenmyths.com/organic-gardening-are-you-an-organic-gardener/
Extremely sound as ever Robert.
Although both sulphur (sorry about English spelling) and magnesium are sometimes called minor elements, other classifications include them as a significant second tier below NPK and calls them major also. They are certainly needed by plants in larger quantities than ‘trace elements’ such as iron.
I tend to think that my extremely sandy soil does require magnesium and the general fertiliser I use on my tomatoes, vegetables etc contains some.
I would never dream of using epsom salts as a general soil additive.
I must say in my younger days our college advised amateurs, Solanum the house plant called winter cherry was very popular and always seemed to be losing its leaves to magnesium deficiency. Standard advice was give them a dose of epsom salts. (The plants not the customers). Epsom salts does have the merit to give magnesium rapidly as a liquid feed.
I had a good chuckle at the NPK numbers for Epsom salts; that’s your result after using them – zero (multiplied by three) 😉
But maybe I shouldn’t say anything, I am really just a ‘casual’ rose grower 😉
I’m genuinely trying to follow this! My mother was a “master gardener” whom I’ve seen bring dry sticks to life, and lover of ES. I’m easily appeased by testable scientific evidence, relying on such as to avoid the unexplained. She’d never been able to give me an explanation for why she would use the stuff, none to my satisfaction. I however have none regarding her ability to proliferate the flora around her. This has definitely peaked my intrest in what’s REALLY going on in my soil, and brought a lovely reminiscence with it. I only wish to have been able to share the delight of research and knowledge of it with the person who was the first to teach me to “sew”. In a nutshell…I’m torn between the reasonable hypothesis supported by the “laws” of Science, and between mom’s rediculous hippie dippy plant psychic malarkey that somehow always seemed to prove right for her.
I’ll take old wives tales over ‘science’ any day of the century!