Magnesium Sulphate Monohydrate vs Heptahydrate

Magnesium Sulphate Monohydrate vs Heptahydrate

Magnesium sulphate is produced in several hydrated forms, and the two most common are monohydrate (MgSO4·H2O, often called Kieserite) and heptahydrate (MgSO4·7H2O, Epsom salt). They deliver the same nutrients - magnesium and sulfur - but differ sharply in concentration, handling and cost, which decides where each belongs.

Water Content and Nutrient Density

Heptahydrate is about 48% water by weight, so a tonne carries far less actual MgO-equivalent nutrient than monohydrate. Monohydrate is the denser, more economical choice when freight and storage dominate the equation, while heptahydrate's high solubility suits dissolved or foliar applications where rapid uptake matters.

Kieserite in Fertilizer Blends

Kieserite (monohydrate) is the workhorse of solid fertilizer and NPK blending. Its lower solubility means magnesium is released more gradually in soil, supporting sustained correction of deficient croplands. HOLY supplies magnesium sulfate monohydrate (Kieserite) powder with W. MgO 20% / 23% / 25% min for agricultural, feed and industrial use.

Heptahydrate for Soluble and Feed Use

Where a fully water-soluble magnesium source is needed - fertigation, foliar spray, or certain animal feeds and bath/feed supplements - heptahydrate's quick dissolution is advantageous. The trade-off is higher transport cost per unit of nutrient and caking risk in humid storage.

Handling and Storage

Both grades are hygroscopic to varying degrees; monohydrate is the more robust in humid tropical warehouses. Properly packed, palletized and kept dry, Kieserite performs well through long sea voyages to South Asian and Southeast Asian distributors.

Choosing the Right Grade

Blenders and distributors should decide by end use: bulk soil correction and NPK blending favor monohydrate; soluble and foliar programs favor heptahydrate. Specify the required MgO minimum, particle size and any heavy-metal limits for feed-grade material.

HOLY Perspective

HOLY (CNMGO Group Limited) provides both Kieserite and complementary magnesium sources for crop-nutrition programs across 50+ countries. Our recommendation is to match the hydrate to the application: monohydrate for blended solids, heptahydrate for soluble systems.