Epsom salt isn't really salt -- it's a combination of magnesium, sulfur and oxygen -- magnesium sulfate -- that occurs naturally in a spring in Epsom, in the county of Surrey in England. Both magnesium and sulfate are good for plants, and Epsom salt has long been the secret ingredient of successful horticulturalists. It is also useful for removing old tree stumps but for a different reason than the one that makes it a nutrient for living plants.
Benefits of Epsom Salt
You don't have to look far to find Epsom salt -- it's available at any drugstore because of its many health benefits, which include relief for muscle pain and elimination of toxins in the body. It's even a treatment for athlete's foot. It benefits plants by supplying magnesium, which plants need to help process the macronutrients nitrogen, phosphorus and potassium. Moreover, both magnesium and sulfate are essential components of chlorophyll. Adding Epsom salt to the soil prevents magnesium deficiency that can turn leaves and grass yellow, and it helps flowers to blossom more vibrantly. It's also good for trees and shrubs that absorb it through their roots.
How Epsom Salt Kills Stumps
With all these benefits for plants, it's difficult to understand how Epsom salt could deteriorate a tree stump, but the explanation is simple: It dehydrates it. Like any naturally occurring salt, Epsom salt absorbs moisture through osmosis. When you infuse a tree stump with a high concentration of the inorganic compound, it absorbs the moisture the roots are still pulling out of the soil, even in the absence of foliage. In the absence of Epsom salt, this moisture would keep the stump alive, and perhaps even produce new growth, but instead, the stump dries out and decays.
Saturate the Stump
The most effective way to kill a stump with Epsom salt is to drill six to eight 1-inch holes into the top of the stump that are about 8 inches deep. You can fill these about halfway with Epsom salt crystals and top off with water, but a faster method is to fill them with a saturated solution of Epsom salt crystals and water -- a saturated solution is one in which no more of the salts will dissolve. Pour more of the solution over the top of the stump, especially in indentations where it can form pools, then cover the stump with black plastic.
Three or Four Months Later
You don't need to do much else to kill the stump, and little monitoring is required. Epsom salt creates a minimal health hazard -- at worst, ingestion can causes a condition known as hypermagnesemia, which is an overdose of magnesium. This is rarely problematic because the kidneys effectively regulate magnesium. Uncover the stump periodically to check its condition and add more Epsom salt. It takes three to four months for a good-sized stump to dry out and deteriorate to the point at which it's easy to chop it up and remove it with an ax and a shovel.