Most gardeners love a reliable home remedy and are eager to embrace a magic bullet. This is the case with Epsom salts. They are supposed to make flowering plants bushier and blossom more and make peppers and tomatoes more productive. But are they really a miracle cure? Can Epsom salts be harmful? Let’s separate myth from reality.
What Are Epsom Salts?
Epsom salts are the chemical magnesium sulfate (MgSO4)—so they offer both magnesium (about 10%) and sulfur (about 13%).
Magnesium is needed by plants to generate the chlorophyll needed for photosynthesis. On a plant lacking magnesium, the older leaves will turn yellow between the veins and along the edges and growth will be stunted. Sandy, light, low pH soils are prone to magnesium deficiency. If your soil is quickly leached by rainfall and has a pH of 5.5 or less, it probably lacks sufficient magnesium. The addition of compost to raise the level of organic matter in the soil will help.
Sulfur is needed to form some vitamins and plant proteins. If a plant lacks sulfur the young leaves will turn yellow. Sulfur also gives vegetables such as broccoli, onions, and mustard greens their flavors. Sulfur is seldom deficient in garden soils in North America so deficiencies are rare. Plus, if sulfur is overdone, it will build up in the soil and make it hard for plants to make use of the other nutrients they need. If you are lacking sulfur, manure is a good organic source.
The Truth About Epsom Salt
One reason people use Epsom salts is to prevent blossom end rot. This isn’t true. In fact, Epsom salts can be harmful! Why? Blossom end rot is caused by a calcium deficiency, not magnesium or sulfur deficiency. And adding too much magnesium to your soil can actually prevent calcium uptake!
While many vegetables (leafy greens, beans, peas) can grow in soil with low magnesium levels, other plants including tomatoes, peppers, and roses need high levels of magnesium. So what if Epsom salt is added to the soil to correct large soil magnesium deficiencies? Unfortunately, horticultural studies (Auburn University, Delaware Valley College) prove that adding Epsom salts to the soil (directly or through watering) does not link to higher yield or healthier growth from peppers and tomatoes to roses.
When it comes to your soil, do not rely on Epsom salts to adjust deficiencies in magnesium or sulfur! The only real way to know if your soil is lacking magnesium or sulfur is to have it tested. (There are soil tests provided by your cooperative extension.) If you add Epsom salts to soil that already has sufficient magnesium, this can harm plants and contaminate soil.
In addition, spraying Epsom salt solutions on plant leaves can also cause leaf scorch. Excess magnesium can also increase mineral contamination in water that percolates through soil. Remember that Epsom salt is a highly soluble chemical. When you use Epsom salts on plants, it can it will leach out of your soil fast, running into and polluting ponds and streams. The best practice is to avoid adding any extra chemicals to your soil - even things that seem ‘safe’ - because you can easily do more harm than good.
Alternatives to Epsom Salt
Before heading to the drug store for some Epsom salts, get an OMRI certified agricultural grade magnesium sulfate or look for a slow-release form of magnesium that will have longer lasting effects such as magnesium-enriched biochar or kieserite. If your soil is acidic, dolomitic limestone will raise the pH and add magnesium.
Bottom-line: Save the Epsom salts for soothing your tired aching body after a day in the garden!