Epsom salt

Epsom salt is a hydrated magnesium sulfate. Unlike kieserite, which is the monohydrate and dissolves more slowly, the heptahydrate form dissolves rapidly, making epsom salt the usual choice for solution and foliar use.

Chemical formula

MgSO₄·7H₂O

Magnesium content

About 10% magnesium

Sulfur content

About 13% sulfur, as sulfate

Effect on soil pH

Neutral; does not acidify or lime the soil

Epsom salt supplies magnesium, a component of chlorophyll and an activator of many plant enzymes, together with sulfate-sulfur. Because both nutrients are supplied in soluble ionic form, they are available quickly, but magnesium can also leach on light soils under heavy rainfall or irrigation.

• Magnesium and sulfate are immediately plant-available on dissolving
• Foliar application can correct visible magnesium deficiency quickly
• Soluble magnesium is prone to leaching on coarse-textured soils

Epsom salt is commonly used as a foliar spray to correct magnesium deficiency in crops such as tomato, potato, grape, and citrus, where a rapid response is wanted. It is also applied to soil, though for large-scale magnesium correction on acid soils, dolomitic lime or kieserite is often more economical.

• Foliar sprays give a fast response to magnesium deficiency symptoms
• Neutral reaction allows use without changing soil pH
• For bulk magnesium building, cheaper sources are often preferred

Epsom salt is a corrective for magnesium and sulfur, not a general fertilizer; applying it where those nutrients are already adequate provides no benefit and adds cost. Foliar sprays applied too concentrated can scorch leaves, so recommended dilution should be followed.