Magnesium Sulphate Fertilizer: Grades, Uses & Buying Guide

Magnesium sulphate fertilizer (also written magnesium sulfate fertilizer) is a water-soluble source of two secondary plant nutrients, magnesium (Mg) and sulphur (S). It is used in agriculture to supply magnesium and sulphur to crops and to correct magnesium deficiency, and in industry as a chemical raw material. The name covers four separate grades: magnesium sulphate anhydrous, Kieserite, magnesium sulphate monohydrate and magnesium sulphate heptahydrate, the last of which is widely sold as Epsom salt. The four are related chemically but are not interchangeable in formulations or in irrigation systems.

Problem Definition: One Search Term, Four Different Products

A buyer searching for magnesium sulphate fertilizer is usually looking for one of four products, and mixing them up is the most common buying error in this category. The difference is crystal water. Magnesium sulphate anhydrous contains no crystal water, magnesium sulphate monohydrate contains one water molecule per formula unit, and magnesium sulphate heptahydrate contains seven. Crystal water changes magnesium density per tonne, moisture behaviour in storage, and how quickly the material dissolves. It also changes how much of the shipped weight is actually nutrient.

Three practical problems follow from that:

• Comparing on price per tonne instead of price per unit of magnesium. A lower-cost heptahydrate and a higher-cost anhydrous grade are not equivalent on a per-tonne basis, because each tonne delivers a different amount of magnesium.
• Using a soil grade in a fertigation or foliar programme. Fertigation and foliar spraying need fast, complete water solubility and low water-insoluble matter, because insoluble residue is what blocks drippers, filters and spray nozzles.
• Ignoring moisture behaviour. Magnesium sulphate absorbs moisture from the air and can cake. Caking occurs under humid storage, with damaged packaging, or after long exposure of open material to moist air.

There is also a naming problem. Buyers use ‘magnesium sulphate’, ‘mgso4’, ‘Epsom salt’ and ‘Kieserite’ almost interchangeably in enquiry emails. Kieserite is a natural mineral form used for soil application, Epsom salt is the common name for the heptahydrate, and mgso4 is a formula that describes all four grades. Precision at the enquiry stage is what prevents a supplier from quoting the wrong material.

Industry Background: Market Signals Worth Knowing Before You Buy

The global magnesium sulfate market was reported at USD 1.8 billion in 2025 and projected at USD 1.9 billion in 2026 in an aggregated market research report. A second research house estimates the same category at roughly USD 1.0 billion for 2025. The gap reflects a scope difference rather than a contradiction: estimates change depending on whether technical, agricultural and pharmaceutical grades are counted together. Treat any single headline market figure as directional, and check which product scope it covers.

Demand structure is clearer than market size. Agriculture and fertilizers accounted for approximately 41.6% of global magnesium sulfate market share as of 2025, and heptahydrate (Epsom salt) accounted for 54% of global commercial production in the same year. The grade with the largest commercial volume is also the one most often purchased on price alone.

China is the reference supply origin for this category. In 2024, China was the leading exporter of magnesium sulphate with USD 149 million in export value under HS code 283321, according to OEC data based on UN Comtrade. Regulatory visibility exists in the European Union as well: magnesium sulphate is registered in the ECHA REACH database with an annual tonnage band of 100,000 to 1,000,000 tonnes.

Price references move with grade and region. US magnesium sulphate market prices were expected to stabilize near USD 674 per metric ton in early 2025 according to one industry source. That figure is a market-level reference, and it is not a quotation for a specific grade, packing or delivery term.

Data gap to keep in mind: customs statistics aggregate magnesium sulphates under one HS code (283321), which masks the split between Kieserite and Epsom salt. Grade-level trade flows are therefore not visible in public trade data. Buyers who need to compare sub-segments should request grade-specific specifications and certificates from suppliers instead of inferring them from customs totals.

Detailed Solution: The Four Magnesium Sulphate Fertilizer Grades

All four grades below belong to the same chemical family and are produced by Tianjin Xingyu Fertilizer Industry Co., Ltd. (XYF), a magnesium sulphate manufacturer established in 1993 in Chenguantun Town Industrial Park, Jinghai District, Tianjin, China. What separates the grades commercially is appearance, magnesium density, application mode and moisture behaviour.

1. Magnesium sulphate anhydrous

Magnesium sulphate anhydrous is a white powder or granular material used for soil application and as an industrial raw material. It carries the highest magnesium oxide content of the four grades, at approximately 33% MgO, and contains no crystal water. That combination makes it the natural choice for moisture-sensitive industrial formulations and for drying-agent applications, where residual moisture is a problem rather than a benefit.

For buyers, the commercial consequence is transport efficiency: low moisture means less shipping weight is spent on water, so the cost per effective magnesium unit is lower than for crystal-water grades. XYF supplies the anhydrous grade with low heavy metal and low chloride levels, in multiple grades for agriculture, feed and industrial use.

2. Kieserite

Kieserite is an off-white granular material used for soil application, and it is the grade to specify when the application is basal dressing rather than spraying or fertigation. It comes from natural mineral origin and is pH-neutral, so it will not acidify the soil. Its high MgO content of approximately 27% makes it a slow-release magnesium and sulphur source for long-term soil supply.

Kieserite also has favourable handling properties for bulk operations. Low hygroscopicity and anti-caking behaviour keep it free-flowing in storage and in blending lines, which is why it is used in bulk blending with other fertilizers. It is applied to correct magnesium and sulphur deficiency steadily over the growing cycle rather than in a single fast-release dose.

3. Magnesium sulphate monohydrate

Magnesium sulphate monohydrate is a white powder or granular material used for soil application, foliar spray and as an industrial raw material. Its MgO content is approximately 25–27%, giving it high nutrient density with lower freight cost per nutrient unit than heptahydrate on the same basis.

The grade is characterised by good thermal stability and low moisture, which supports long-term storage without caking. It is available in both powder and granular form, and it is suitable for soil application, feed use and compound fertilizer blending. Because it is synthetically produced with consistent quality and a stable heavy-metal index, monohydrate is often the grade chosen when a buyer needs one material that can serve agricultural and general industrial requirements at the same time.

4. Magnesium sulphate heptahydrate

Magnesium sulphate heptahydrate is a white granular crystal used for soil application, foliar spray, fertigation and as an industrial raw material. It is the grade commonly known as Epsom salt, and it is the grade best suited to fertigation systems because of its extremely fast water solubility.

Its advantages are speed and cost. Dissolved heptahydrate corrects magnesium deficiency quickly and supports chlorophyll formation and photosynthesis, which matters in horticulture, high-value crops and hydroponics. Crystal particles are uniform with low impurity, making it cost-effective and widely accepted in global fertilizer-grade trade. Its limitation is magnesium density: heptahydrate contains crystal water and therefore less magnesium per tonne than anhydrous, so it is usually the better choice for liquid application and the weaker choice where freight weight per nutrient unit dominates.

Where These Grades Are Produced

Tianjin Xingyu Fertilizer Industry Co., Ltd. was established in 1993 and operates from Chenguantun Town Industrial Park, Jinghai District, Tianjin, China. The manufacturing facility covers 20,000 square metres, employs approximately 36 staff and has an R&D team of 5 engineers. The site is 30 km from downtown Tianjin, 80 km from Tianjin Port and 15 km from the railway freight yard, which supports both sea and rail logistics.

The company runs automated production lines, implements an ISO quality management system and operates an independent standardized testing laboratory. Supported by a stable raw material supply base, it produces magnesium sulphate fertilizer across the anhydrous, monohydrate and heptahydrate ranges, together with magnesium oxide, sulfur magnesium fertilizer and various medium and trace element fertilizers. Products are available in fertilizer grade and industrial grade, covering agricultural planting, chemical production, building material processing and other fields. Export accounts for roughly 70% of business, with products shipped to more than 30 countries and regions across Southeast Asia, Europe and America; major markets include Pakistan, Japan, Korea and New Zealand. Full details are available at www.xyfertilizer.com.

Step-by-Step: How to Select a Magnesium Sulphate Grade

1. Start from the application method, not the price. Broadcasting and basal dressing point to Kieserite or granular grades; drip and fertigation point to a fully water-soluble grade such as heptahydrate; foliar programmes need a grade that dissolves completely in the spray tank; industrial dry formulations point to anhydrous. Foliar spray and fertigation are both supported by monohydrate and heptahydrate, while anhydrous and Kieserite are soil and industrial materials.
2. Compare nutrient density, not tonne price. Anhydrous delivers approximately 33% MgO, Kieserite approximately 27%, and monohydrate approximately 25–27%; heptahydrate contains crystal water and therefore less magnesium per tonne. Convert quotations to cost per unit of delivered magnesium before deciding.
3. Check moisture and caking behaviour against your warehouse. Anhydrous and monohydrate are low-moisture grades suited to long storage; heptahydrate is more moisture-sensitive. If your storage is humid or packaging is handled roughly, plan for sealed packaging and crush any caked material before application.
4. Confirm solubility and insoluble matter for irrigation use. Low water-insoluble content is a quality requirement for fertigation, because it is the direct cause of dripper and filter blockage.
5. Verify documentation and compliance. Ask for a COA and MSDS before shipment. Feed-grade material must meet feed-additive standards, and REACH or FAMI-QS documentation applies for EU export. Packaging must be sealed and moisture-proof, with shipping marks that comply with the destination market.
6. Validate with a sample before a bulk commitment. Test solubility, particle size and moisture against your own process conditions. Only then scale to a bulk order.

Use Cases: Where Each Application Method Fits

• Soil application and broadcasting. Granular product is spread directly on the field and dissolved by irrigation or rainfall. Kieserite is the typical choice where a steady, slow-release magnesium and sulphur supply is wanted and soil pH must not be pushed down.
• Drip irrigation and fertigation. Powder or crystal is dissolved in water and delivered through the irrigation system. Heptahydrate is suitable for fertigation systems because of its fast solubility; matched equipment includes drip lines and fertigation machines.
• Foliar spraying. The material is dissolved into solution and applied to the leaf surface for fast correction of magnesium deficiency. Monohydrate and heptahydrate are both used for this route.
• Fertilizer compounding and NPK blending. Magnesium sulphate is blended with other raw materials to produce compound and water-soluble fertilizers. Anti-caking and free-flow behaviour matter most here.
• Industrial raw material and water treatment. Anhydrous and monohydrate grades serve industrial production, including moisture-sensitive formulations.
• Livestock feed additive. Powder is dosed and mixed into animal feed; feed-grade material must meet feed-additive standards.
• Soil improvement. Magnesium sulphate is used to supply magnesium and sulphur in acid soil improvement programmes and in horticulture, supporting crop yield and quality.

Matching equipment across these applications includes fertilizer spreaders, drip irrigation systems, fertigation machines, sprayers, mixing tanks, blenders, crushers, and metering and batching equipment.

Comparison Table: The Four Grades Side by Side

Operating and storage conditions apply equally to all four grades: store in a cool, dry, well-ventilated place, keep away from moisture, sunlight and oxidants, and keep packages sealed to avoid caking. Do not mix the material with strong alkaline substances, and apply at the proper dosage to prevent fertilizer burn.

FAQ

Conclusion

Magnesium sulphate fertilizer is not one product but four, and the decision that matters most is matching the grade to the application method before comparing prices. Anhydrous suits industrial and moisture-sensitive dry formulations, Kieserite suits basal soil application and bulk blending, monohydrate covers both agricultural and general industrial needs, and heptahydrate is the practical choice for fertigation, foliar work and fast correction of magnesium deficiency.

Once the grade is fixed, the purchase criteria become comparable: magnesium content per tonne, moisture and caking behaviour, water solubility and insoluble matter, packing format, documentation, and the supplier's ability to repeat the same specification across shipments.