The white, sugar-like crystals of Epsom salts contain magnesium (Mg) and sulfur (S) atoms--two elements that facilitate healthy growth and metabolic processes in plants. Scientifically speaking, Epsom salt is known as magnesium sulfate. It is highly soluble in water and can be mixed into irrigation to supply plant roots with a source of magnesium and sulfur that may be lacking in the garden soil. A basic fertilization formula is 1 tsp. of Epsom salt crystals in 1 gallon of water.
Production of Chlorophyll
The magnesium ion is the central structure of a chlorophyll molecule--the green pigment that drives the conversion of sunlight energy into food in photosynthesis. If magnesium is lacking, new chlorophyll production is hampered and often the leaves of a plant begin to turn yellow, especially older leaves. Magnesium is readily transported around the plant to regions that require the ions. This is the reason why magnesium-deficient plants abort their lower, oldest leaves in order to supply the new, young growth with ample magnesium ions.
Epsom salts provide magnesium and sulfur ions that also are key building blocks for production of enzymes and other protein molecules. Enzymes trigger cell reproduction, among other processes, and stem elongation and production of new leaf tissues. Gerald Kidder and Sydney Park Brown write in "Your Florida Landscape" that sulfur is a constituent of the amino acids cystine, cysteine and methionine as well as in some vitamins. According to the North Carolina Department of Agriculture and Consumer Services, sulfur aids seed development and production of new roots. Sulfur facilitates the nodules in legume plant roots for nitrogen fixation. A lack of sulfur causes new growth stems to be stunted and brittle leaves to be smaller than usual and rolled up.
Other Potential Nutrient Deficiencies
When magnesium is over-abundant in soils (for instance, due to an over-application of Epsom salts into soils that don't necessarily need it) the uptake of other important nutrients occurs. Too much magnesium prevents easy uptake of calcium (Ca) and/or potassium (K) by plant roots. Calcium ions ensure plant cell walls are sturdy and potassium plays an essential role in plant metabolism. Noticeable results are seen when either nutrients are blocked from a plant--either brittle stems and especially scrawny-looking plants with poor or inadequate new growth and diminished flowering, fruiting or seed formation, according to Kidder and Brown in "Your Florida Landscape."