Epsom salt (MgSO4•7H2O) is a common household product containing magnesium, sulfur and oxygen. It is used in gardens to ward off pests and to amend the soil. The magnesium level in soil can be adjusted with Epsom salt. Soil that is lacking sulfur for a particular crop, such as onions, may benefit from Epsom salt application. Healthy soils and plants are less prone to insect damage. Slugs are one pest that are deterred by Epsom salt.
Garden pests are both harmful and helpful to the gardener. Advice passed on through gardening cultures in different regions of the country sometimes differ from the advice of scientific, peer-reviewed journals. One example of this is the benefit of spraying plant foliage with an Epsom salt mixture. Many home gardeners swear by this for fuller, beetle-free rose bushes, but there is no scientific evidence of this. Gardeners do report success using an Epsom salt mixture to kill slugs picked from the plants by hand.
Some gardeners claim that a circle of Epsom salts around the base of plants will keep pests away. The Amish have used Epsom salts to cure tomato blight. Their method is to spray the plants with a mixture of one cup Epsom salts to five gallons of water. Other gardeners have used Epsom salts to protect peppers and to make onions more pungent.
When planting from seed in rows, you can sprinkle Epsom salt in with the seed. This is helpful for plants with leaves low to the ground, such as lettuce and spinach. Gardeners use this method to deter pests, such as voles and potato bugs. When intensive fruit growing is practiced, the addition of Epsom salt is helpful to restore magnesium to the soil. This is true for some grains, nuts, cotton and edible plants. The key is the replenishing of magnesium in soils that produce crops that deplete magnesium.
Epsom salt may deter some plant blights and pests, but it is wise to relocate an affected plant type the next year. This will decrease the likelihood of plants being reinfected through the soil. Soils that have the correct pH for your plants and are low in magnesium or sulfur will benefit from an application of Epsom salts. The nutrients from soil are the best support against pest damage.
Scientific literature does not support the claims made about the benefits of Epsom salts as a pest deterrent. However, there is one claim of Epsom salt enthusiasts that is borne out in science. That is the benefit of adding Epsom salt to pasture land as magnesium deficiency is a serious problem for grazing animals. Gardeners have long known that cleaning tools with Epsom salt will help remove oils from plants, such as poison ivy.