The world first took notice of Epsom salts when a farmer in the town of Epsom discovered that the water tasted bitter. Although drinking it was unpleasant, ChemistryExplained.com notes that Epsom residents found other uses for the mineral that was causing the bad taste. Gardeners around the world realized that a few sprinkles of the mineral helped some plants to grow. People are still trying to sort out what Epsom salts can and cannot do for plants and soil.
Epsom salt is a compound of magnesium and sulfate. Magnesium forms part of the chlorophyll molecule. Plants that do not receive enough magnesium cannot make enough chlorophyll, and the result is the leaves of the plant turning yellow. Chlorophyll controls how plants convert carbon dioxide and water to carbohydrates. Although magnesium is one of the earth's most abundant minerals, it may not be present in the soil in the form where the plant can access it when it needs it.
Epsom salts will give you better results in the garden only if your soil suffers from magnesium deficiency. Look for symptoms that your soil needs magnesium, including yellowing plant leaves that starts at the lower leaves and moves up the plant. According to the University of Wisconsin Extension service, the leaves may turn reddish purple or rosy colored as the deficiency increases. Analyze the soil and plant to determine actual mineral levels. Plant roots cannot absorb magnesium efficiently when there is excess potassium present.
The Epsom Salt Council, a trade group, promotes Epsom salts as a gardening supplement that will produce more flowers, increase chlorophyll, make plants grow bushier, and increase the likelihood that seeds will germinate. Some gardeners also believe that Epsom salts will deter slugs, voles and other pests. The University of Washington's Dr. Linda Chalker-Scott cautions that scientific tests on Epsom salts only show that it increases magnesium levels and therefore, she concludes, all other effects that people attribute to Epsom salts are possible misconceptions.
Epsom salts are readily available and relatively inexpensive. Applying Epsom salts is not physically demanding because you can find it in small cartons or bags. Plants rarely require more than a small amount of Epsom salts, applied at discrete intervals. Using Epsom salts in the garden is also advantageous because it is organic, and appeals to gardeners who want to reduce their dependence on chemically produced fertilizers.
You'll need to conduct tests on you garden soil for magnesium to determine if it has a deficiency. Many gardeners do not want to go through the effort of sending away samples for testing. Epsom salts are highly soluble and likely pass through the soil, however, if that is true then the excess magnesium will pass to another area that may not need the mineral. Scientists are divided on whether or not excess magnesium leaves the soil, some take the position that the magnesium accumulates in the top soil and create toxicity problems for the plant.