Epsom Salts

Epsom salts are wonderful in the bath, but they do not belong in the garden.

Popular culture touts epsom salts as a garden miracle worker, but the truth is, adding Epsom salts to soil nearly always ends badly for plants.

Epsom salts (Public Domain)

False claims about Epsom salts

The sheer volume of claims made about Epsom salts should be the first clue that there is a problem. Epsom salts are said to improve flower blooming, germination, photosynthesis, the uptake of other nutrients, fruit production and flavor, and to act as a valuable nutrient for overall health. Epsom salts are said to “enhance a plant’s green color” and “help plants grow bushier” while deterring pests, reducing blossom end rot, transplant shock, leaf curling, and chlorosis (yellowing). Epsom salts are also said to be a safe, reliable weed killer. 

With all these miracles provided by Epsom salts, why isn’t everyone using it all the time? Many advertisements and articles also state that Epsom salts are not persistent in the soil, so you cannot overuse it. Wouldn’t it be wonderful if all those claims were true? Unfortunately, they are not.

The truth about magnesium sulfate

Epsom salts, also known as magnesium sulfate (MgSO4), is made up of magnesium, sulfur, and water. Magnesium and sulfur are plant micronutrients. This means plants use them, but only in very small amounts. If a magnesium or sulfur deficiency has been identified via a lab-based soil test, Epsom salts can be safely used to counteract those deficiencies (assuming your soil is equally deficient in both nutrients at the same time). That’s also only after you are certain that the deficiency is not being caused by too much or too little of another nutrient. For example, too much phosphorus in the soil can make it difficult for plants to absorb magnesium, regardless of how much is present. [Are you beginning to see how important that soil test is?]

Adding too much magnesium can cause just as many problems as having too little. More often than not, your soil probably has more magnesium than plants need. When I bought my house in 2012 and sent a sample out for a soil test, my results came back with 798 parts per million (ppm) for magnesium. The optimal range is 50 - 120 ppm. Whoever lived here before me kept adding more fertilizer (or Epsom salts), long after it was needed.

Crops that commonly need magnesium include apples, beets, citrus, peppers, potatoes, tomatoes, and watermelon. Usually, deficiencies only occur when these and other crops are being grown intensively, and rarely in the home garden. 

There is no scientific evidence of Epsom salts repelling slugs, insects, grubs, voles, rabbits, or anything else. As for the blossom end rot claims, adding Epsom salts may actually interfere with your plants’ ability to take up calcium, making it more likely that your tomatoes will develop blossom end rot, rather than preventing it.

Epsom salts may sound like a garden cure-all, but those claims should be filed under “too good to be true”. A healthy, productive garden does not come with quick fixes. Before jumping on a bandwagon, be sure you take the time to read the science behind those claims. 

And get a soil test.

Your plants will be healthier and more productive, and you can save Epsom salts for the bath, where they belong.