Professional opinions vary about whether or not Epsom Salt is good for plants. It is true that many master gardeners and commercial growers recommend and use Epsom Salts. But there are also many horticulturists who strongly advise home gardeners against using it.
Despite the differences of opinion, there is evidence that using Epsom Salts, which is magnesium sulfate, can provide vital nutrients that you can use to supplement your regular fertilizer. But the caveat is that you should only use it if you are sure your soil is deficient in magnesium – and you need to use it carefully and correctly.
What is Epsom salt?
Epsom Salt is a crystallized form of magnesium sulfate, a mineral salt that occurs naturally in the earth. A compound of sulfate and magnesium, it is found in rock formations.
Epsom salt was discovered in the early 17th century at a place called Epsom, very close to London in the United Kingdom.
The story goes that during a drought in 1618, Henry Wicker, a cow herder, was thirsty and found a pool of water at Epsom Common. He tried drinking it, but it tasted acidic and very bitter.
When the water had evaporated, he noticed there was a white residue on the ground. He also realized that the water had a laxative effect.
Unsurprisingly, this discovery led Epsom Salts to become a popular cure for constipation. Today, it is approved as a laxative by the U.S. Food and Drug Administration.
It is also commonly used for bathing and soaking, and as a fertilizer for plants. While all Epsom Salt is the same, the Pennsylvania-based Epsom Salt Council points out that there are different grades.
The grades that are suitable for human use are labeled USP (United States Pharmacopeia) and usually have “drug facts” on the packaging. This means that it has been tested and certified to meet the strict regulatory standards of the USP and Federal Drug Administration (FDA).
But, you can also buy Epsom Salts that are good for agricultural applications including plant growth.
Barry Gehm, an assistant professor of chemistry and biochemistry, points out that Magnesium Sulfur can also be used as a fertilizer to provide plants with magnesium and sulfur. But, as he says, it is a very different substance to Magnesium Sulfate that already contains a soluble form of sulfur, sulfate iron.
Benefits of Epsom salt in Plants
Dr. Linda Chalker-Scott is a master gardener who has a Ph.D. in horticulture. She is also both an extension urban horticulturist and associate professor at Washington State University.
In an online article, miracle, myth…or marketing Epsom salts, she says the reality is that Epsom salts have been used worldwide to relieve typical magnesium deficiency found during the intensive cropping of many different fruit and vegetable species. But she emphasizes that the science behind using Epsom salts is only applicable to intensive crop production where farmers know that the soil and/or plants have a magnesium deficiency.
According to the Epsom Salt Council, magnesium sulfate is an inexpensive way to improve or start a new garden. This is because it:
- Provides vital nutrients that supplement regular fertilizers
- Increases the production of chlorophyll
- Helps seeds to germinate
- Makes plants grow bushier
- Produces more flowers on plants
- Deters pests including voles and slugs
The Council states that Master Gardeners recommends Epsom Salt. They also say that the National Gardening Association confirms certain benefits.
For example, when roses are fertilized with Epsom Salt plants grow bushier and produce more flowers. They also say that it makes pepper plants grow a lot bigger than those that are only treated with regular commercial fertilizer.
Mattson himself adds Epsom Salt to his own fertilizer and uses it for plants that include roses, pansies, impatiens, and petunias.
How do you use Epsom salt in plants?
The Epsom Salt Council says that Neil Mattson, an assistant professor at Cornell University, is one of many professionals who advocate the use of Epsom Salt. His advice to gardeners is to mix it with fertilizer and add it to their soil every month.
Alternatively, home gardeners can mix one tablespoon with a gallon of water and spray the leaves every couple of weeks.
Their tips for using Epsom Salt in home gardens include recipes for houseplants, roses, shrubs, lawns, and trees. All involve slightly different recommendations.
Tips for Different Plant Applications
For houseplants in general, add 2 tablespoons of Epsom Salt to a gallon of water. Feed once a month.
Shrubs like rhododendrons, azaleas, and other evergreens can be fed with a mix of 1 tablespoon in a gallon of water. This will be enough for 9 square feet and should be applied at the root level every 2-4 weeks.
Trees need double the amount (2 tablespoons) of Epsom Salt in a gallon of water per 9 square feet. Only apply it to the root zone three times a year.
They advise spreading 3 pounds of Epsom salt over every 1,250 square feet of lawn. You can do this with a spreader or dilute it with water and use a sprayer to apply.
The tips for roses are a little more involved. Use 1 tablespoon per foot of plant height for each plant and apply every two weeks.
You can also scratch 1/2 cup of Epsom Salt into the soil at the base of rose plants. This will encourage flowering canes and promote new, healthy basal cane growth.
You can also soak unplanted bushes in 1 cup of Epsom Salt per gallon of water to help their roots recover. Add 1 tablespoon to each hole when you plant the bushes.
They also have a garden startup tip which advocates sprinkling a cup of Epsom Salts over every 100 square feet of soil. Mix into the soil thoroughly before you plant.
When should you put Epsom salt on plants?
Like Dr. Chalker-Scott, Anne Sawyer, extension educator at the University of Minnesota, warns that you should only use Epsom Salt for plants if a soil test shows that there is a magnesium deficiency. In Minnesota, she says, this kind of deficiency is most likely to occur when the soil is sandy and has a low pH.
Quoting Neil Mattson, the Epsom Salt Council says that he doesn’t only focus on magnesium sulfate (Epsom Salts), but also on magnesium sulfur.
Mattson maintains that you’ll know when plants need more magnesium sulfate because all the leaves start turning yellow. When the leaves turn yellow but the veins stay green, this indicates that they need more magnesium sulfur.
Either way, he says that plants need both these “building blocks,” which are essential nutrients. He is, of course, referring to magnesium sulfur and Epsom Salts (magnesium sulfate).
Which plants like Epsom salts?
Dr. Chalker-Scott lists some of the North American crops that benefit from Epsom Salts. She says that plants Epsom Salt will benefit include (in alphabetical order), alfalfa, apples, beets, carrots, citrus, cauliflower, cotton, grains, hops, kale, nuts, okra, peppers, plums, potatoes, snap beans, sugar beets, sweet potatoes, tobacco, tomatoes, watermelon, and wine grapes.
Other more exotic species include bananas, cacao, coffee, rubber, Swedish turnips, and tea. Some conifer species grown for their timber are also treated with Epsom Salts when soil conditions are magnesium-deficient.
The Epsom Salt Council tips above mention other typical garden plants you might grow in your own backyard.
How to Water Plants with Epsom Salts
Generally, when watering plants with Epsom Salts, it’s best to target the roots rather than water over the leaves. After all, you’re aiming to up the magnesium content of the soil.
Also, it will help to avoid leaf scorch.
Can too much Epsom salt hurt plants?
Anne Sawyer warns that adding too much Epsom Salt to the soil can prevent enough calcium from getting to your plants. Too much magnesium sulfate can also increase mineral contamination in the water that filters through the soil.
Sometimes people spray hydrated magnesium sulfate onto their plants. Be aware that this can easily scorch the leaves.
So, is Epsom Salt good for plants? Sometimes, yes. But generally, it’s best for big-scale commercial growers and not home gardens or pot plants.
Many professionals, like Dr. Chalker-Scott, dispute what general articles say. Lots of them reference outdated research that has subsequently been disproven.
This doesn’t mean that they dispute the need to do something about magnesium deficiency. Rather, they believe that it’s too easy to use too many Epsom Salts, and this is not necessarily an environmentally sustainable practice. Dr. Chalker-Scott is particularly critical of the Epsom Salt Council, disputing their recommendations. She believes it is irresponsible to advise gardeners to use any chemicals, including Epsom salts unless they have a thorough knowledge of soil conditions, environmental health, and the specific needs of plants.