A practical guide to magnesium oxide, why calcination temperature decides what it is good for, and when to choose it over a soluble magnesium source, from a US-based distributor.
Magnesium oxide (MgO, CAS 1309-48-4) is the most concentrated common magnesium source, at roughly 60% magnesium when pure. It is used as a slow-release soil amendment, a feed supplement and rumen buffer, and an industrial raw material. It is alkaline, so it limes as well as feeds. Stratus Chemical supplies agricultural, feed, and technical grades across the United States.
| Property | Value |
|---|---|
| Chemical name | Magnesium oxide |
| Synonyms | Magnesia, calcined magnesia, burnt magnesia, caustic calcined magnesia, periclase |
| CAS number | 1309-48-4 |
| Molecular formula | MgO |
| Molecular weight | 40.30 g/mol |
| EC / EINECS | 215-171-9 |
| Appearance | Grey granule, or white to buff powder depending on grade |
| Grades supplied | Agricultural, feed, technical |
| Packaging | Bags, supersacks, bulk |
| Documentation | COA & SDS provided on request |
The figures below are representative of granular fertilizer grade magnesium oxide. They are indicative rather than guaranteed: specifications vary by source, grade, and lot, and the certificate of analysis for your material governs.
| Parameter | Typical value |
|---|---|
| Magnesium oxide (MgO) | 65.0% min |
| Magnesium (Mg) | 39.0% min |
| Silicon oxide (SiO) | 3.0% max |
| Calcium oxide (CaO) | 2.0% max |
| Iron (III) oxide (Fe2O3) | 2.0% max |
| Loss on ignition | 4.0% max |
| Appearance | Grey granule |
| Particle size | 2 – 4 mm, 95% pass |
| Packing options | 50 lb bags, big bags / totes, bulk |
Magnesium oxide is produced by calcining magnesite ore or magnesium hydroxide recovered from seawater and brines, driving off carbon dioxide and water to leave the oxide. In mineral form it occurs as periclase.
Its commercial advantage is concentration. Pure magnesium oxide is about 60% magnesium by weight, roughly six times the magnesium content of Epsom salt. For any application where magnesium has to be moved in quantity, that ratio dominates the freight bill and usually the buying decision.
Magnesium itself sits at the center of the chlorophyll molecule, which makes it non-negotiable for photosynthesis. Deficiency shows as interveinal chlorosis on older leaves, with the veins staying green while the tissue between them yellows.
Magnesium oxide is not one product. The temperature at which it was calcined determines its reactivity, and a grade that is right for one use is useless for another.
Light-burned magnesia, also called caustic calcined, is fired at moderate temperature. It retains high surface area and reacts readily with water and acids. This is the grade for agriculture, animal feed, environmental treatment, and chemical manufacture, because all of those depend on the material actually reacting.
Dead-burned or hard-burned magnesia is fired at much higher temperature until it sinters into a dense, largely inert form. That is exactly what refractory linings need and exactly what a soil or a rumen cannot use.
Fused magnesia, melted in an electric arc furnace, is denser still and used for electrical insulation and specialist refractories. When ordering, state the application rather than only the assay: two materials can both be 65% MgO and behave completely differently.
Magnesium oxide corrects magnesium deficiency on sandy, acidic, and heavily leached soils, and on ground where years of high potassium application have induced magnesium shortage through cation competition. Because it dissolves slowly, it behaves as a season-long amendment rather than a rescue treatment.
Feed grade magnesium oxide serves two functions in ruminant diets. It supplies magnesium, guarding against grass tetany in cattle grazing lush spring pasture, and it acts as a rumen buffer, moderating pH in high-concentrate rations where acidosis is a risk. Feed use requires material meeting the applicable feed-ingredient specification, which is a different product from technical grade.
Magnesium oxide neutralizes acidic wastewater and acid mine drainage, and is often preferred over lime or caustic soda because it buffers to a moderate pH rather than overshooting, and because the resulting sludge dewaters better.
Dead-burned and fused grades go into refractory brick and furnace linings. Light-burned grades are used in rubber and polymer compounding as an acid acceptor, in fertilizer and pharmaceutical manufacture, and as a feedstock for other magnesium compounds.
| Magnesium source | How it behaves | Trade-off |
|---|---|---|
| Magnesium oxide | Highest analysis, slightly soluble, alkaline, slow release | Cheapest magnesium per ton and limes as it feeds, but too slow for an in-season correction |
| Magnesium sulfate heptahydrate (Epsom salt) | Fully soluble, immediately available, pH neutral, supplies sulfur | Ideal for foliar and fertigation, but low analysis means high freight per unit magnesium |
| Magnesium sulfate monohydrate (kieserite) | Soluble, higher analysis than the heptahydrate, pH neutral | Good middle ground for soil application, dissolves more slowly than the heptahydrate |
| Dolomitic lime | Supplies magnesium and calcium with strong liming action | Very cheap per ton, but low magnesium analysis and unsuitable where pH is already adequate |
How to choose: start with soil pH. On acidic ground needing both lime and magnesium, magnesium oxide or dolomitic lime does two jobs and is hard to beat economically. Where pH is already at target, an alkaline source will push it too far and a neutral sulfate is the right answer. Then ask about timing. Magnesium oxide releases over months, which is fine when building soil reserves ahead of a season but useless against a deficiency visible in the crop today; for that, a soluble sulfate applied foliar is the tool. See our crop micronutrients guide for how magnesium interacts with potassium and calcium uptake.
Store magnesium oxide dry in closed original packaging, off damp floors. The material reacts slowly with atmospheric moisture and carbon dioxide, converting at the surface to hydroxide and carbonate, which reduces reactivity and causes caking. Light-burned grades are the most affected because their high surface area is exactly what makes them useful.
Magnesium oxide is alkaline and dust is irritating to eyes, skin, and the respiratory tract. Use the eye and respiratory protection specified in the Safety Data Sheet and avoid dry sweeping of spills.
Hydration of light-burned magnesia is exothermic. Adding water to a large quantity of fine, highly reactive material generates heat, so wet spills and mixing operations deserve more care than the low hazard rating suggests.
Magnesium oxide is used as a concentrated magnesium source in soil amendment and crop nutrition, as a magnesium supplement and rumen buffer in animal feed, and in industrial applications including refractories, environmental treatment of acidic water and soil, and as a raw material for other magnesium compounds. Stratus Chemical supplies agricultural, feed, and technical grades across the United States.
The CAS number for magnesium oxide is 1309-48-4. Its molecular formula is MgO and its molecular weight is 40.30 g/mol. Its EC number is 215-171-9. It is also known as magnesia, calcined magnesia, burnt magnesia, and in its mineral form as periclase.
Pure magnesium oxide is about 60% magnesium by weight, the highest analysis of any common magnesium source. Commercial agricultural and feed grades typically run 65% MgO minimum, corresponding to around 39% actual magnesium, with the balance being silica, calcium oxide, and iron oxide from the ore. This concentration is why magnesium oxide costs far less to freight per unit of magnesium than sulfate sources.
The difference is calcination temperature, and it determines reactivity. Light-burned or caustic calcined magnesia is fired at lower temperature, retains a high surface area, and reacts readily, which is what agricultural, feed, and environmental applications need. Dead-burned or hard-burned magnesia is fired much hotter, becoming dense and largely unreactive, which is what refractory use requires. Specifying the wrong one produces material that either will not react or will not last, so state the intended use when ordering.
Magnesium oxide carries far more magnesium per ton and costs less to freight, but it is only slightly soluble and releases magnesium slowly as soil acidity acts on it, which makes it a soil amendment rather than a quick correction. Magnesium sulfate is fully soluble and available immediately, which is what foliar sprays and fertigation require, but its lower analysis means more tons moved for the same magnesium. Use the oxide for building soil magnesium over a season, the sulfate for correcting a deficiency now.
Yes. Magnesium oxide is alkaline and has a liming effect, so it raises soil pH as well as supplying magnesium. On acidic soils that need both, this is an advantage and the product does two jobs at once. On soils already at or above target pH, it is a reason to choose a neutral magnesium source such as magnesium sulfate instead. Account for the liming value when planning application rates.
Stratus Chemical supplies magnesium oxide in bags, supersacks, and bulk, in agricultural, feed, and technical grades and in granular and powder forms, with a certificate of analysis provided for each shipment. Tell us the grade, MgO assay, particle size, and volume and we will confirm pricing and availability within one business day.
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