Epsom salts for vegetation: good or dangerous?

People have long used Epsom salts on plants. All over the internet it is recommended in ardent words that really speak to the emotions.

People claim that spraying their plants with Epsom salts dissolved in water kills pests, makes their tomatoes huge, kills fungal diseases on the leaves of their roses, and it is an effective weed killer. Epsom salt is said to cure flower end rot and "make plants stronger".

But the strange thing is that when you combine all of these miraculous claims, they don't make any sense. How can a product be an effective weed killer without damaging your desired plants and at the same time making them somehow "stronger"? Can a product really kill all garden pests?

Let's start with what Epsom salt is, and then let's go over some of the claims about what it supposedly does.

What is Epsom Salt?

Is Epsom Salt For Plants A Good Or Bad Idea?

In the 17th century a small town in Surrey, England called Epsom was targeted.

A villager named Henry Wicker had looked after the animals there, but a lack of water had made it difficult for him and his animals. He saw water in a cow's hoof print in the mud and dug a shallow hole there, hoping it would fill with groundwater overnight. When he returned that morning, he was filled with groundwater, but the cattle would not touch him.

Wicker tasted the water and found it bitter. He also discovered that the water had a laxative effect. Having water that he believed was a miracle, he began promoting his discovery as a cure, and as time went on, Epsom became known for its water. It became a health resort that attracted people from near and far who came for the "healing" elements of its water.

But what was in that water? It was thought to contain magnesium sulfate, a compound made up of magnesium and sulfur. Later tests during the 20th century found that most of the material dissolved in the water was calcium sulfate. Unfortunately, by this time it was long believed to be magnesium, and an entire Epsom salt industry had been created.

Nowadays the dry salt crystals that we can buy as Epsom salts are magnesium sulfate, also called MgSO₄. They get their name from the town of Epsom in Surrey, UK, but they may or may not actually be part of what collects in the cow's hoofprint. Still, Epsom prides itself on the fact that their town became popular with the discovery of Epsom salts over 400 years ago, though they more often attribute the discovery to the cow than the villager!

When was Epsom salt used for gardening purposes? The answer to that is less clear. Many Epsom salt companies started printing the salt on the box for more sales. It is likely that the use of Epsom salts in the garden was inspired by one of these boxes. Various health benefits have also been printed on the boxes, some of which have been scientifically refuted over the years.

Epsom salt in the garden: pros and cons

There are tons of ways to use Epsom salts when you browse the internet. Let's examine some of the ways people use Epsom salts and whether or not they are actually effective!

Flowering end red

By and large, the most common recommendation for using Epsom salt is treating tomatoes and peppers for flower end rot. It is recommended to add a tablespoon of Epsom salt to the soil before planting or to dissolve Epsom salt in a gallon of water and use it to spray plants as a foliar spray.

Blossom end rot is caused by a lack of calcium in your plants. Most often it is directly related to irregular watering. When the soil is dry, plants cannot readily absorb nutrients, which can lead to a number of deficiencies. Timed drip irrigation can prevent flower end rot.

Epsom salt does not contain calcium. It contains magnesium and sulfur, but applying this to your tomatoes to treat flower end rot can make the condition worse. Another common cause of calcium deficiency is an excess of other nutrients in the soil; too much of a single nutrient can lead to imbalances. An overdose of magnesium can negate the effects of normal calcium in the soil, making it a dangerous additive in the treatment of flower end rot.

Pepper plants and tomato plants require nitrogen for their initial plant growth and development, but once the plants are old enough to bear fruit, phosphorus becomes more important. When caring for your tomatoes and peppers, fertilize regularly at the beginning of the year with a balanced fertilizer according to the manufacturer's instructions, but switch to a lower-nitrogen, higher-phosphorus fertilizer for fruit development. It's also advisable to do a soil test annually to see if you need to fertilize at all, as many home gardeners have rich soil to begin with.

Avoid Epsom salts to treat flower end rot. While it is a common gardening myth, it is imprecise.

Pest and disease control

Epsom salt boxThis Epsom salt box describes it as "natural" to make buyers feel completely safe. Source: Kazuhiro Keino

Until the 1950s and 1960s, there were sporadic claims that using Epsom salts in the garden would reduce the pest population. There are also claims that Epsom salt is used as a foliar spray to treat plant diseases and improve plant health.

Tests on various pests have shown no evidence that Epsom salt helps keep plants pest or disease free. In fact, spraying Epsom salts on many plants causes leaf burns.

While sulfur alone is effective against some types of pests, higher levels of magnesium have little benefit. Plants require other garden pest or disease control methods that target the pests or diseases identified, rather than a home-brewed solution.

But what about the sulfur in Epsom salts? Sulfur is widely used to treat diseases, especially rust or powdery mildew. Unfortunately, the amount of sulfur in magnesium sulfate is not enough to be effective. If you want to use sulfur, opt for soluble sulfur instead and leave out the magnesium in Epsom salts.

Epsom salt as fertilizer

Some people claim that Epsom salt is a useful fertilizer to help prevent magnesium deficiency problems.

It is important to note that the main nutrients most plants need are nitrogen, potassium, and phosphorus. Nitrogen promotes leaf growth. Phosphorus promotes healthy flowering and fruiting. Potassium is used for general plant health and root and tip development.

While magnesium is an essential micronutrient that plays a huge role in photosynthesis, many plants only require a tiny amount in their soil. Adding Epsom salts to the garden only helps if your soil is really magnesium deficient. In fact, using Epsom in the garden can create an imbalance that can reduce the intake of other, more important nutrients.

An excess of phosphorus can often lead to soils showing a magnesium deficiency. Since home gardeners often do not conduct annual soil tests, it is very easy to find an excess of NPK nutrients, especially if the gardeners fertilize heavily. This can simulate a magnesium deficiency that actually doesn't exist.

Magnesium deficiency can occur in commercial arable land, especially those with acidic sandy soils. This is due to the intensive planting of these areas, and a magnesium deficiency is usually not the only deficiency. Commercial arable land is often deficient in NPK, magnesium, occasionally iron or calcium, and sometimes other micronutrients. It is for this reason that the overall nutrient content is increased through the use of highly effective fertilizers in commercial agriculture. Before changing magnesium or other nutrients, a soil test is always done to make sure the correct mix of nutritional supplements are being used.

Germination aid

Some claim that the magnesium and sulfur in Epsom salts act as a seed germination aid or speed up the germination of seeds. Unfortunately, this is also a common gardening myth.

A fully developed seed is a complete package. Inside it contains the plant embryo and a supply of food, all neatly surrounded by the seed coat. Eating is not a problem, which is why you can germinate seeds on a paper towel or grow edible sprouts in a jar. What is required from the outside world for seed germination is light (sometimes), moisture, and the right temperature for germination. The semen also needs oxygen, otherwise the embryo in the semen will die.

In most cases, the seeds germinate without help. The use of magnesium and sulfur does not bring any particular germination benefits, so it is best not to use Epsom salts for this either.

Larger fruits and flowers or more foliage

Claims that using Epsom salt will improve the size of tomatoes or make plants "bushier" are somewhat misleading. As mentioned in the discussion of deficiencies above, what tends to improve the size of tomatoes, peppers, or other fruits is usually phosphorus. To get plants to produce more foliage it is usually good nitrogen uptake and a healthy pruning program, not the addition of magnesium sulfate.

Another common myth is that it is an excellent fertilizer for roses. The claims for this came from members of the Rose Society from 1930-1960, all of whom swore their roses would bloom more and that the plants would fill nicely with foliage. Unfortunately, studies have found no evidence of this, and it was dismissed as an option by most rose growers at this point. Epsom salt in the garden does not improve anything related to flowering, fruiting or foliage of plants, whether it is roses, tomatoes or something else.

Improves nutrient absorption

One of the claims for adding Epsom salts to the garden is that it improves the nutrient uptake of the standard NPK nutrients. As described in the fertilization section above, this is a bit inaccurate. It can actually reduce the absorption of nutrients, especially when used in large quantities.

If anything, one can best hope that adding more magnesium would potentially aid photosynthesis. It is an integral part of the chlorophyll molecules that are found in the cell walls of your leaves. However, if your plant is fertilized regularly, it will already be rich in magnesium as companies include magnesium as part of their micronutrients in feed. Magnesium levels don't have to be sky high as they would if you added Epsom yourself. This is just not going to help plants photosynthesize, regardless of what people would hope for.

Plant and Epsom Salt Safety

Enlarged MgSO4 crystalsMgSO4 crystals seen under a microscope. Source: ptc24

Finally, there are two related claims that suggest that Epsom salts do not linger in the soil and are harmless to plants.

Unfortunately, these are actually wrong. Although it is very soluble in water, large amounts of undiluted Epsom salt remain in the soil. There are recommendations for sprinkling a cup of Epsom salt in the garden, and that can do serious harm to your plants. Epsom salt has been linked to certain forms of root diseases as well as fruit flavors. Even a tablespoon of Epsom salts per gallon of water as a spray carries risks; it can cause leaf burn, especially if used in hot weather.

In addition, dissolved Epsom salts can pollute the groundwater. This can pose risks to the ecosystem, not just your own garden. There is evidence of rising magnesium levels in areas where this practice is regularly performed, in addition to other pollutants such as excess NPK fertilizer. Too much of a good is bad, especially when it comes to your soil.

weed killer

There is literally no science to suggest that Epsom salt is a weed killer on its own. Yes, it can damage plants, but only in large doses in the soil or with foliar application.

The only recipe I could find for this practice recommends mixing 1 cup of Epsom salt with two gallons of vinegar and then spraying that mixture on weeds.

Regular household vinegar does have some weed effect, but is limited at best. Agricultural vinegars used for weed control usually contain 15-20% acid, while household vinegars usually contain between 3-5% acid. In fact, agricultural vinegar is such a strong acid that it can burn your skin. If it can burn you, it is very likely that it will be effective against weeds.

In contrast, household vinegar is so mild that you can put your whole hand in a bowl without harming it. Soaking your hand with it for a while may leave you feeling slightly irritated, but it's safe enough for us to eat, so it's likely not very effective against stubborn weeds. Boiling water can actually be more effective and less acidic in your soil afterwards.

Needless to say, Epsom salt really isn't that useful as a weed killer. Even the vinegar treatment itself is pretty questionable.

So is it a good idea?

Unfortunately, the only viable horticultural use for magnesium sulfate is when your property is on farmland that has been worked to death and only when you have a soil test done to see if your soil is magnesium deficient. It doesn't help promote good plant growth in peppers or tomatoes, it won't make larger blooms on your roses, and it won't help other plants grow.

The allure of using Garden Epsom is that it is cheap. You can buy a large box from your local pharmacy. But how often do you buy your garden items from the pharmacy?

We recommend that you use a good quality organic fertilizer or high quality compost to provide the vital nutrients for your plants. Skip the sprinkling of Epsom salts around the base of the plants, don't apply it on your trees or in the planting hole for your newest seedling, or mix it in a gallon of water to spray on your plants.

There are still plenty of gardeners out there who swear it will work for them, but if it can't be replicated in your garden and there are known risks associated with using Epsom salts, there is simply no reason to take the risk. It may have been a recommended method before, but after a century of study it turns out to be unnecessary for the garden.

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