- The magnesium in Epsom salts is an important part of a plant's chlorophyll cell function, which is essential for photosynthesis. Without photosynthesis, the plant can't make enough energy and will wither and die. Leaves usually turn a pale yellow, sometimes with dark spots in the center. Sulfur in Epsom salt helps the plant create useful proteins. Without these proteins, the plant cannot grow properly, leading to stunted shoots, wilting leaves or discoloration.
- Gardeners add Epsom salt to their soil when the ground is deficient in magnesium and/or sulfur. However, because some of the symptoms of a lack of magnesium or sulfur are similar to those of other mineral deficiencies, a soil test is the only sure way to know if you need Epsom salt. In general, the acidic soils found in the northeast U.S. tend to lack magnesium, while more southwestern states such as Utah often contain ample amounts.
- Some plants in particular grow better with Epsom salts. Both non-native palms and citrus plants can benefit from a dose of Epsom salt. General fertilizer may not contain enough magnesium for citrus plants. Tomatoes also grow well with Epsom salts mixed into their fertilizer feed at a ratio of 2 tbsp. of salt per 2 cups of granular fertilizer. Epsom salt has long been associated with better blooms in rose bushes and other flowering shrubs such as azaleas. Other plants prone to magnesium deficiency include potatoes, cherries, grape vines and rhododendrons.
- Epsom salt is available in two main forms -- a pre-mixed spray and the traditional salt granules. Spray generally offers a temporary solution for any plants that show signs of magnesium deficiency in summertime. You can direct the spray straight at the leaves. More commonly, adding Epsom salts to the soil in fall or winter helps enrich the magnesium and sulfur content in time for the next growing season. For individual plants, a teaspoon of Epsom salts for each inch of plant height added at the start of spring is enough.