Epsom salt, chemically known as hydrated magnesium sulfate, is often discussed in gardening circles for its potential benefits to plants, including popular flowering varieties like geraniums (Pelargonium genus). Whether this simple mineral supplement is beneficial depends entirely on the current nutrient status of the plant and the soil. This naturally occurring salt contains two secondary macronutrients necessary for plant function, but its application must be precise to avoid unintended harm.
The Essential Nutrients in Epsom Salt
Epsom salt is composed of magnesium (Mg) and sulfur (S), bound together as magnesium sulfate (MgSO₄). Magnesium is a central component of the chlorophyll molecule, making it necessary for photosynthesis. Without adequate magnesium, a plant cannot effectively convert sunlight into energy required for growth and flowering. Magnesium also assists the geranium in efficiently absorbing other primary nutrients from the soil, such as phosphorus and nitrogen. Sulfur is needed for the synthesis of plant proteins and essential vitamins. Both elements are classified as secondary macronutrients, meaning plants require them in smaller quantities than nitrogen, phosphorus, and potassium, but in larger amounts than micronutrients.
How Geraniums Respond to Magnesium and Sulfur
Geraniums not deficient in magnesium will not show dramatic improvement from an Epsom salt application. If a deficiency is present, the visual symptoms are specific and easily recognizable. A lack of magnesium causes interveinal chlorosis, where the tissue between the leaf veins turns yellow or pale green while the veins themselves remain distinctly green. This symptom appears first on the older, lower leaves because magnesium is mobile within the plant, moving to support new growth. If the deficiency persists, the chlorotic areas may progress to necrosis (death of the leaf tissue), causing brown spots and a downward curling of the leaf margins. Supplying magnesium sulfate quickly corrects this issue, restoring the deep green foliage color by boosting chlorophyll production. Sulfur deficiency, which is less common, would also be corrected, supporting robust stems and healthy root systems necessary for abundant flowering.
Practical Application: Dosage and Methods
Epsom salt should only be applied to geraniums if a magnesium or sulfur deficiency is suspected or confirmed via soil test. When application is necessary, two primary methods are used: a soil drench or a foliar spray.
Soil Drench
For a soil drench, mix one to two tablespoons of Epsom salt into one gallon of water. Apply this solution directly to the soil around the base of the plant, ensuring the magnesium is delivered to the root zone for slower, longer-term uptake. The soil drench method provides a more sustained supply of the nutrients as the roots gradually absorb the magnesium and sulfur.
Foliar Spray
A foliar spray offers a quick, temporary fix because the leaves absorb the magnesium immediately. Use the same dilution ratio (one to two tablespoons per gallon of water) and spray the foliage until runoff. Gardeners generally apply this treatment once a month during the active growing season. Avoid spraying the leaves during the hottest part of the day or when the sun is brightest, as this can cause the foliage to scorch. For container-grown geraniums, which deplete nutrients faster than those in the ground, a regular, cautious application is often beneficial.
Signs Your Geraniums Do Not Need Epsom Salt
Applying Epsom salt to a geranium that is not magnesium-deficient can create a complex problem. Magnesium ions compete with other positively charged nutrients, such as calcium and potassium, for absorption by the plant roots. Excessive magnesium levels in the soil can thus induce a secondary deficiency in calcium or potassium, leading to nutrient antagonism. A healthy geranium displays deep green foliage, strong stems, and vigorous flower production, indicating a balanced nutrient profile. If your plant exhibits these signs, additional Epsom salt is unnecessary and could lead to problems like stunted growth or chlorosis caused by the lockout of other essential elements. The best practice is to confirm a specific need through a professional soil test before adding any supplement. If a test is not feasible, observe the plant for the specific interveinal chlorosis on older leaves before considering an Epsom salt application.