Magnesium sulfate, known in most households as Epsom salts, is a combination of magnesium and sulfur, both nutrients beneficial for plant health. Magnesium is involved in photosynthesis, so deficiency of this nutrient affects leaf color. Sulfur activates plant enzyme systems. A deficiency of sulfur results in growth and color changes. Plant requirements for sulfur are equal to those for phosphorus, although sulfur is not considered one of the primary plant nutrients. Observation of plant leaves provides information needed to diagnose nutrient deficiencies. However, because magnesium and sulfur symptoms look very similar to symptoms related to other nutrients, it is necessary to distinguish between the symptoms of various nutrient deficiencies.

Although magnesium (Mg) is an essential element for plant growth, its use in a fertilizer program receives only minor emphasis in Minnesota. For most of the state, this lack of emphasis is justifiable because when management properly, most soils in Minnesota contain sufficient Mg to meet crop needs. If Mg is limited in the diet, animals can develop grass tetany. Therefore, some special consideration is given to the Mg status of forage crops.