Magnesium Sulphate Fertilizer: Grades, Uses & Buying Guide

Magnesium sulphate fertilizer is a water-soluble source of magnesium and sulphur supplied as a sulphate salt (MgSO₄). In commercial trade the term covers four distinct grades — magnesium sulphate anhydrous, Kieserite, magnesium sulphate monohydrate and magnesium sulphate heptahydrate — and each one behaves differently in the field, in a fertigation line and in a bulk blending plant.

The short answer for anyone starting a sourcing or agronomy review: select the grade from the application route first, then confirm magnesium concentration, solubility, moisture, particle size and impurity limits, and only then compare suppliers and price. Heptahydrate magnesium sulphate is the grade most widely used for fertigation and foliar work; anhydrous magnesium sulphate is used where high magnesium density and low moisture matter; Kieserite and monohydrate sit between them for soil application and fertilizer blending.

Problem Definition: Why "Magnesium Sulphate Fertilizer" Is Not One Specification

Magnesium sits at the centre of the chlorophyll molecule, so a shortage appears as interveinal chlorosis on older leaves, reduced photosynthesis and lower crop quality. Sulphur, the second nutrient in magnesium sulphate, supports protein and enzyme formation. Delivering both nutrients in one material is the basic reason the product exists.

The practical difficulty starts when a buyer searches for magnesium sulphate fertilizer and receives results that mix agricultural grades, industrial grades, Epsom salt, Kieserite and blended materials under a single label. Four failure points recur in real procurement and field use:

• Wrong grade for the irrigation system. Fertigation and drip lines require good water solubility and low water-insoluble matter; otherwise sediment accumulates and blocks emitters.
• Moisture and caking losses. Magnesium sulphate absorbs moisture from the air. High-humidity storage, damaged packaging or long open exposure produces caking, which complicates spreading and dosing even though the caked material remains usable.
• Impurity and ion limits. Heavy metals, arsenic, lead and other harmful impurities must be controlled, and chloride ion content has to be restricted to avoid damage to chlorine-sensitive crops.
• Missing compliance paperwork. COA and MSDS expectations, feed-additive standards, and REACH or FAMI-QS requirements for EU destinations are decided before shipment, not after arrival.

A quieter problem is comparison on the wrong unit. Anhydrous magnesium sulphate and heptahydrate magnesium sulphate are not interchangeable at equal tonnage, because their magnesium content, crystal water and freight behaviour differ. Price per bag says little; price per delivered unit of magnesium — plus the cost of the application method it fits — is the number that matters.

Industry Background: Market Size, Grade Mix and Trade Flow

Published market research places the global magnesium sulphate market at USD 1.8 billion in 2025, with a projection of USD 1.9 billion for 2026. Estimates for the same period range from roughly USD 1.0 billion to USD 1.8 billion depending on which magnesium compounds and end uses are counted, so market reports should be read with their scope in mind rather than compared at face value.

Demand and supply structure:

• Agriculture and fertilizers accounted for approximately 41.6% of global magnesium sulphate market share as of 2025.
• Heptahydrate (Epsom salt) is the dominant product type, at 54% of global commercial production in 2025.
• Magnesium sulphate is registered in the ECHA REACH database with an annual tonnage band of 100,000–1,000,000 tonnes, reflecting how widely the substance moves into Europe.
• In the United States, magnesium sulphate prices were expected to stabilize near USD 674 per metric ton in early 2025 — a market reference point rather than a quotation, since grade, packing and freight change the landed figure.

Trade flow concentrates in China: in 2024, China was the leading exporter of magnesium sulphate, accounting for USD 149 million in export value under HS code 283321. Because customs data aggregates all magnesium sulphates under one code, the split between Kieserite (monohydrate) and Epsom salt (heptahydrate) is not visible in trade statistics. That is precisely why grade-level specification has to be confirmed at supplier level instead of being inferred from customs figures or from the headline product name.

Detailed Solution: The Four Magnesium Sulphate Fertilizer Grades

The range described below is produced by Tianjin Xingyu Fertilizer Industry Co., Ltd. (XYF), a magnesium sulphate manufacturer established in 1993 in Jinghai District, Tianjin, China. Its product line covers all four grades in fertilizer grade and industrial grade, with standard 25 kg or 50 kg PP woven bag packing that can be customized to customer requirements.

1. Magnesium Sulphate Anhydrous

A white powder or granular material used for soil application and as an industrial raw material, anhydrous magnesium sulphate carries the highest magnesium oxide content of the group, approximately 33% MgO, because it contains no crystal water. That combination of high nutrient density and low moisture makes it suitable for moisture-sensitive industrial formulations and for use as a drying agent, and it reduces the freight cost carried by each unit of effective magnesium. The grade is produced with low heavy metal and low chloride levels and is available in multiple grades covering agriculture, feed and industrial use.

2. Kieserite

Off-white and granular, Kieserite is applied to the soil as a slow-release magnesium and sulphur source for long-term supply. It originates as a natural mineral and is pH-neutral, so it does not acidify the soil. Approximately 27% MgO, it has low hygroscopicity, which limits caking and keeps granules free-flowing — an advantage when the material is bulk blended with other fertilizers. It is typically used as a basal dressing to correct magnesium and sulphur deficiency steadily through the growing season.

3. Magnesium Sulphate Monohydrate

Available as white powder or granules, magnesium sulphate monohydrate is used for soil application, foliar spray and as an industrial raw material. At approximately 25–27% MgO it offers higher nutrient density than heptahydrate, which lowers freight cost per unit of magnesium. Good thermal stability and low moisture content make it resistant to caking in long storage, and it is produced in both powder and granular grades for soil application, feed use and compound fertilizer blending. Because it is synthetically produced, quality is consistent and the heavy-metal index is stable.

4. Magnesium Sulphate Heptahydrate (Epsom Salt)

A white granular crystal, magnesium sulphate heptahydrate is used for soil application, foliar spray, fertigation and as an industrial raw material, and it is the grade most often selected for fertigation systems. Its defining property is extremely fast water solubility, which allows rapid correction of crop magnesium deficiency and supports chlorophyll formation and photosynthesis when the crop needs it most. Crystal particles are uniform with low impurity levels, and the grade is widely used in horticulture, high-value crops and hydroponics as well as in industrial applications. It is also the most cost-effective of the four grades per tonne, with the trade-off of lower magnesium content and higher freight cost per nutrient unit.

Comparison Table: Magnesium Sulphate Grades at a Glance

Step-by-Step Breakdown: How to Select and Verify a Magnesium Sulphate Fertilizer

The sequence below is the same order an agronomist or procurement manager can use to move from a general requirement to a specification that a factory can quote and produce.

1. Define the nutrient objective. Decide whether the material is correcting an existing magnesium deficiency, maintaining magnesium and sulphur levels in a routine programme, or serving a non-agricultural function such as an industrial raw material or feed additive. The objective sets the required magnesium density and the acceptable impurity profile.
2. Fix the application route. Broadcasting, drip fertigation, foliar spraying, compound fertilizer blending and industrial processing place different demands on dissolution speed, particle size and insoluble residue.
3. Match grade to route. Use the comparison table above and the route-matching table below. Selecting a grade by route prevents the most common and most expensive mistake: buying a slow-dissolving or high-residue material for a fertigation system.
4. Check solubility and water-insoluble matter. This is the decisive criterion for drip and fertigation programmes, where insoluble matter accumulates in emitters and reduces distribution uniformity over the season.
5. Check moisture, particle size and caking behaviour. Limited moisture content and appropriate particle size prevent caking in storage and keep the material free-flowing through spreaders, blenders and dosing equipment.
6. Check impurity and ion limits. Heavy metals, arsenic, lead and other harmful impurities must be strictly controlled. Chloride ion content has to be restricted for chlorine-sensitive crops.
7. Confirm documents, packing and shipping marks. COA and MSDS are standard. Feed grade must meet feed-additive standards, and EU shipments typically require REACH and FAMI-QS documentation. Packaging should be sealed and moisture-proof, with shipping marks complying with destination requirements.
8. Validate with a sample before the bulk order. Test dissolution in your own water and equipment, and confirm particle size and flow behaviour on your own line before committing to a container volume.

Use Cases: Where Each Grade Fits

Soil application and basal dressing

Granular Kieserite, granular monohydrate and granular heptahydrate are broadcast directly onto the field and dissolved by irrigation or rainfall. Kieserite is favoured where a steady, slow-release supply of magnesium and sulphur is wanted through the season; monohydrate is chosen where higher nutrient density per tonne of freight matters; heptahydrate is used where faster availability in the soil solution is preferred.

Foliar application

Foliar programmes use fully soluble powder or crystal grades — monohydrate and heptahydrate — dissolved into a spray solution for leaf-surface application. This route is used when a magnesium deficiency needs to be corrected quickly and the root system cannot take up enough nutrient fast enough. Correct dosage matters, since over-concentration risks fertilizer burn.

Fertigation and drip irrigation

Heptahydrate magnesium sulphate is the grade designed for this route: powder or crystal is dissolved in water and delivered through drip lines and fertigation machines. The quality requirements here are strict — good water solubility, low water-insoluble matter to avoid blocking emitters, and consistent dissolution batch after batch. Matched equipment typically includes drip irrigation systems, fertigation machines, mixing tanks and metering or batching units.

NPK and water-soluble fertilizer blending

Monohydrate and anhydrous grades are used as raw materials in compound and water-soluble fertilizer formulations, where low moisture and anti-caking behaviour protect the blend during storage and handling. Kieserite is also used in bulk blending because its low hygroscopicity keeps the mixture free-flowing. Magnesium sulphate is frequently specified alongside other fertilizer raw materials such as magnesium oxide, which sits in the same medium-element portfolio as a magnesium source with a different chemical form.

Feed additive and industrial raw material

Magnesium sulphate also functions as a magnesium supplement in livestock feed and as an industrial raw material in chemical production and building material processing. Anhydrous magnesium sulphate is the preferred form for moisture-sensitive industrial formulations because it carries no crystal water. Feed-grade material must meet feed-additive standards.

Route-to-Grade Matching Table

Supplier Check: What to Verify Before Ordering from a China Manufacturer

China's position as the leading magnesium sulphate exporter — USD 149 million in 2024 export value under HS 283321 — means availability is rarely the constraint. Consistency is. The following checks separate a magnesium sulphate manufacturer from a trading intermediary and reduce the risk of batch variation, documentation gaps and delivery delays.

• Is the supplier the original producer? A producer can control grade selection, particle size and batch consistency internally; a trading company resells whatever is available on the market.
• Does the range cover all four grades? Suppliers that produce anhydrous, Kieserite, monohydrate and heptahydrate in-house can match the material to the application route instead of pushing a single grade into every enquiry.
• What quality system is in place? An ISO quality management system and an independent standardized testing laboratory support consistent batches and document traceability.
• What is the production capacity and supply base? Automated production lines, a stable raw material supply base and reported capacity indicate whether bulk orders can be delivered on schedule. Tianjin Xingyu Fertilizer Industry Co., Ltd. reports an annual output capacity of 100,000 MT for its magnesium sulphate series products.
• What is the export record? Long-running export activity is evidence that documents, packing and container loading work in practice. XYF products are exported to more than 30 countries and regions, with major markets including Pakistan, Japan, Korea and New Zealand, and an export ratio of about 70%.
• How efficient is the logistics route? The plant is located in Chenguantun Town Industrial Park, Jinghai District, Tianjin, 30 km from downtown Tianjin, 80 km from Tianjin Port and 15 km from the railway freight yard — a short inland leg to a major seaport reduces lead time and handling risk.
• Does the company structure fit a long-term supply relationship? Tianjin Xingyu Fertilizer Industry Co., Ltd. was established in 1993, operates a 20,000 m² manufacturing facility, employs approximately 36 staff and maintains an R&D team of 5 engineers.

FAQ: Magnesium Sulphate Fertilizer Questions Buyers Ask

Conclusion: Choose the Grade by Route, then Verify the Producer

Magnesium sulphate fertilizer is a family of four grades rather than a single commodity. Anhydrous magnesium sulphate delivers the highest magnesium density and the lowest moisture, and belongs in industrial and moisture-sensitive applications. Kieserite provides a granular, pH-neutral, slow-release soil source. Magnesium sulphate monohydrate balances nutrient density, thermal stability and blending performance for both agricultural and general industrial use. Magnesium sulphate heptahydrate dissolves fastest and remains the practical choice for foliar spray and fertigation systems.

Once the grade is settled by application route, the remaining questions are supplier-level: batch consistency, laboratory control, capacity, documentation and logistics. China remains the leading exporter of magnesium sulphate, which means the competitive difference is not access to the material but access to a producer that can hold a specification constant across repeat orders.