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.
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.
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