Why You Should Reconsider Epsom Salt for Plants

It's easy to find recommendations on the internet promoting the use of Epsom salt in the garden, including, unsurprisingly, tips from the Epsom Salt Council. As with coffee grounds, commonly held wisdom isn't always right, and there is little scientific evidence to back it up.

As the respected horticulturalist Linda Chalker-Scott, Ph.D. of Washington State University concluded in MasterGardener Magazine in 2007: “It is irresponsible to advise gardeners and other plant enthusiasts to apply Epsom salts, or any chemical, without regard to soil conditions, plant needs, and environmental health.” Instead of Epsom salts, there is a simpler, more effective, and more sustainable alternative.

What Is Epsom Salt?

Epsom salt (magnesium sulfate) is made of 10% magnesium and 13% sulfur. Magnesium and sulfur are key ingredients in allowing plants to absorb the big three elements essential for plant growth: nitrogen, phosphorus, and potassium. Magnesium and sulfur also are vital in the process of photosynthesis, contributing to the production of chlorophyll, and in enhancing the flavor of many fruits, nuts, and vegetables. Magnesium is key to seed germination and in strengthening cell walls, while sulfur aids in the production of vitamins, enzymes, and amino acids (the precursors to protein).

Epsom salt contains magnesium and sulfur in highly soluble form, which is one of the reasons people recommend it over mineral compounds like Sul-Po-Mag (sulfur, potassium, magnesium) or dolomitic lime (calcium carbonate and magnesium carbonate), which break down much more slowly.

Before You Add Epsom Salt to Your Plants

Before you add anything to your soil, however, test it to see what it needs. You can test your soil's pH to determine how acidic or alkaline is. Contact your state Cooperative Extension Service or garden center about a test that can determine the balance of key nutrients (as well as the potential presence of contaminants) in your soil.

A soil high in calcium and potassium, for example, may be deficient in magnesium. Since plants do not absorb magnesium readily in acidic soil, extra magnesium can rectify the situation. Animal manure and many synthetic fertilizers are high in sulfates, so if you're already adding fertilizers that have a mildly rotten-egg smell, you may not need to add Epsom salt. It is recommended that you test your soil every three years, especially if you are growing edibles, as they eagerly deplete your soil of minerals.

Depending on your test results, your garden may not need Epsom salt at all, and adding it will do more harm than good. Too much magnesium can interfere with the uptake of calcium, which plants also need. In some plants, such as tomatoes or other vining plants, a lack of calcium can result in end rot. If one of the presumed benefits of Epsom salt is its solubility, that can also be one of its detriments, as the salts can easily wash through the soil and into the groundwater. Like other forms of over-fertilization, applying Epsom salt may just be contaminating waterways. In the long run, slow-release fertilizers are more effective than more soluble ones.

Moreover, not every plant needs to be supplemented with Epsom salt. Leafy green vegetables like lettuce and spinach, as well as legumes like peas and beans, do well in soils with low levels of magnesium. If your plants are suffering from yellowing of their leaves, one problem could be that they have a magnesium deficiency. Another problem, however, may simply be that you're watering them too much and leaching all the nutrients out of the soil.

While some plants, such as roses, peppers, and flowering shrubs and trees (such as magnolias, azaleas, rhododendrons), require more magnesium and sulfur than others, there is little scientific research demonstrating the ability of Epsom salt to deliver those key ingredients. Despite recommending Epsom salts, an undated article in The National Gardening Association's Learning Library, “Fertilize with Epsom Salts,” provides only anecdotal evidence from test gardeners, admitting that “little research has been done on the use of Epsom salts as a supplemental fertilizer.” Indeed, the only research cited gives inconclusive results, with one stating: “It's hard to find a direct link between a specific nutrient such as magnesium sulfate and increased yield or plant growth.”

A Simple, Safe Alternative to Epsom Salt

Epsom salt is allowed for agricultural use by the Organic Materials Review Institute (OMRI), but that doesn't mean it's your best choice in the garden. Epsom salt no longer comes from a spring in Epsom, England, where it was first discovered in the 1600s. It is manufactured in the United States primarily by two corporations, Giles Chemical and the PQ Corporation. While both companies produce USDA- and USP-approved products, the manufacturing and transportation of Epsom salt requires energy, and that energy is more likely than not to be produced by fossil fuels. Like any other manufactured product, Epsom salt has a carbon footprint, and it's larger than the alternative.

Most healthy soils already contain ample amounts of magnesium and sulfur, so the best way to make sure your plants are healthy is to have healthy soil. And as with human health, a balanced, well-rounded diet is better than any single supplement. A simpler method of enriching your soil with a lower carbon footprint than Epsom salt is to add locally produced compost to your garden. Regularly top-dressing your garden with organic compost will add a wider array of nutrients, including the same ones provided by Epsom salt.

If your soil test comes back saying your soil lacks magnesium and sulfur, adding Epsom salt to your compost is a safer way to amend your soil than by direct application. Water in your compost by adding 2 tablespoons of Epsom salt per gallon of water to make the nutrients in Epsom salt available to plants.