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Magnesium fertilizers

Magnesium fertilizers are mineral fertilizers that meet the magnesium requirements of crops.

Raw materials for the production of magnesium fertilizers

The main source for the production of magnesium fertilizers are natural compounds and minerals of this element. Magnesium is part of more than 200 minerals, many of which are used directly as a source of magnesium or processed into magnesium fertilizers: sulfates, chlorides, carbonates, silicates, hydroxides, aluminosilicates.

Ways to provide magnesium to plants

Ways of providing magnesium to plants:

  1. Liming of soils with magnesium-containing lime fertilizers. This simultaneously achieves the supply of all crops of the rotation with magnesium and calcium and eliminates excessive acidity of the soil.
  2. Application of magnesium mineral fertilizers for crops in the rotation, taking into account their biological needs.
  3. The application of organic fertilizers, which contain magnesium in an amount of 0.01-0.09%.

Classification of magnesium fertilizers

Lime-magnesium and potassium-magnesium fertilizers account for the largest share in the range of magnesium fertilizers. Magnesium fertilizers are classified by solubility into:

  1. water insoluble – milled natural minerals and rocks, such as dunite, serpentinite, vermiculite, dolomite, magnesite, brusite and dolomitized limestone. When interacting with acidic soils, magnesium available to plants is released into the soil solution;
  2. water-soluble – raw salts and products of their processing – epsomite, kainite, carnallite;
  3. soluble in citric acid and available to plants – magnesium fused phosphate.

By composition, magnesium fertilizers are divided into:

  • simple – magnesite, dunite;
  • complex ones containing several nutrients:
    • nitrogen-magnesium – ammoshenite, dolomite-ammonium nitrate;
    • phosphorus-magnesium – magnesium fused phosphate;
    • Potassium-magnesium – potassium-magnesium concentrate, potassimagnesia, polyhalite, cainite, carnallite;
    • bormagnesium – magnesium borate;
    • lime-magnesium – dolomite, dolomitized limestone and products of their processing;
    • containing nitrogen, phosphorus and magnesium – magnesium-ammonium phosphate. 

Magnesium-containing lime fertilizers

Magnesium-containing lime fertilizers simultaneously enrich the soil with mobile magnesium compounds and neutralize excessive acidity, are practically the most effective and cheapest way to supply sandy and sandy loam soils with magnesium.

Dolomite flour (CaCO3⋅MgCO3) contains about 20% MgO and 30% CaO; carbonic calcium and magnesium account for at least 85%. It is used to lime acidic soils at a dose of 3-4 t/ha. In this case the soil is enriched with magnesium in amounts sufficient to provide the plants on one to two rotations of crop rotation. The most effective on the light soils.

Dolomites are insoluble in water, so their effect depends on the fineness of grinding. The largest increase in crop yields provides dolomite flour with a particle size of less than 1 mm.

Semi-fired dolomite (CaCO3⋅MgCO3) is a product of dolomite firing and contains approximately 27% MgO, 2% CaO, 57% CaCO3. Magnesium is well available to plants. It is used to lime the soil.

Magnesium carbonate, or magnesite, (MgCO3) contains 45% MgO – the most concentrated magnesium fertilizer. It is a natural mineral and burnt magnesite (up to 89% MgO), obtained in the manufacture of refractories. It has an alkaline reaction with a high neutralizing power superior to that of lime. However, high doses of magnesium carbonate cause calcium and boron starvation of plants. Therefore, its use is combined with the introduction of boron for crops that require it, such as sunflowers, beets, and clover, and for the neutralization of excessive acidity it is combined with calcium carbonate.

Burnt magnesia is packed in bags of waterproof material and stored in a dry room.

Main magnesium fertilizers

The Russian Federation industrially produces potassium magnesia and kainite. The share in the total assortment of potassium-magnesium fertilizers is insignificant.

Dunite flour and magnesium serpentine

Dunite flour and magnesium serpentinite, or serpentinite, are wastes of the mining and asbestos industry. According to their chemical composition they are magnesium silicates in a poorly soluble form, so they are used in advance in large doses. These magnesium-containing fertilizers are used as raw materials for the production of complex magnesium fertilizers, as well as a local fertilizer. They decompose slowly under the influence of soil acids. The finely ground dunite contains 41-47% MgO. Serpentinite consists of magnesium metasilicate and contains 32-43% MgO.


Ammoshenite ((NH4)2SO4⋅MgSO4⋅6H2O) is a double salt of ammonium sulfate and magnesium sulfate. It is a crystalline mineral of light brown to gray color. Used as a nitrogen-magnesium fertilizer; contains at least 7% N and 10% MgO. It is transported in multi-layer bags impregnated with bitumen.

Magnesium sulfate

Magnesium sulfate (eleonite and kieserite) is a one-sided magnesium fertilizer containing at least 84% MgSO4⋅7H2O and no more than 6% NaCl (17.7% MgO). It is well soluble in water. Used in intensive agriculture in a deficit of magnesium in weakly acidic and neutral soils. In this case, the high yield is a constant need for readily soluble sources of magnesium. It is also used in intensive meadows, in greenhouses, in vegetable growing in the open ground. Magnesium sulfate is used to eliminate acute magnesium starvation by foliar feeding. When entering the soil, most of the magnesium is converted into the metabolized state.


Calimagnesia (K2SO4⋅MgSO4⋅6H2O) is a semi-product in the production of potassium sulfate from kainite. It mainly contains the mineral schoenite.

It shows high efficiency on sandy sod-podzolic soils due to good solubility and potassium-magnesium ratio.

Table. Composition of granular species of potassium permanganate, in terms of dry product (%).

1st grade
2nd grade
potassium oxide
> 30
> 28
magnesium oxide
> 10
> 8
< 5
not regulated
< 2
< 2

Potassium-magnesium concentrate

Potassium-magnesium concentrate is obtained from cainite-langbeinite rock by flotation. The fertilizer mainly contains the mineral langbeinite (K2SO4⋅2MgSO4), in small amounts – polyhalite, halite, gypsum, etc. On average, it contains 30-38% K2SO4, 39-40% MgSO4, 4-5% KCl, and 8-10% NaCl.

Potassium-magnesium concentrate is produced in two grades: grade 1 contains at least 19% K2O and 9% MgO, grade 2 contains at least 17.5% K2O and 8% MgO. The content of chlorine is not regulated, but in the 1st grade – no more than 8%.

Polyhalite salts

Polyhalite salts (K2SO4⋅MgSO4⋅CaSO4⋅6H2O) contain 10-11% K2O, 8-12% MgO, are poorly soluble in water, but potassium and magnesium are available to plants. Polyhalite salts showed effectiveness on different crops, especially on meadows and pastures.


Kainite (KCl⋅MgSO4⋅3H2O) – with an admixture of sodium chloride up to 45-47% of the total mass. It contains 10-12% K2O, 22-25% Na2O; 6-7% MgO, 15-17% S and 32-35% Cl. It is a low-percentage fertilizer, so it is mainly used in grasslands and pastures, where it has advantages over potassium chloride because of the presence of magnesium.

Industrial waste

The wastes of potassium and magnesium plants can be used as potassium and magnesium fertilizers – dewatered carnallite containing 23-24% K2O, 18-20% – MgO, 0.9% – Na2O, 50-51% Cl, and potassium chloride electrolyte containing 39-42% K2O, 4% – MgO, 50% – Cl. The negative effect of chlorine is eliminated by advance application. Dehydrated carnallite is effective for various crops on sandy loam soils.

Phosphorus-magnesium fertilizers

Thermophosphates and tomato slag are phosphorus and magnesium fertilizers, a byproduct of metallurgy. The nutrients are contained in citric-soluble forms and are available to plants.

This group of fertilizers includes fused magnesium phosphate, which contains phosphorus and magnesium available to plants (Ca3(РO4)2 + MgSO4⋅SiO3). It is obtained by fusion of natural phosphates and magnesium raw materials (dunite, kieserite, serpentinite, olivinite) at 1350-1400 °С, followed by rapid cooling of the melt with water. It is vitreous transparent granules of different shapes and sizes. The color of the granules varies from bright green to black, depending on the initial raw material.

Fused magnesium phosphate contains 19-21% available citric-soluble P2O5 and 8-14% MgO. Phosphorus in fused magnesium phosphate is contained as a modification of tricalcium phosphate, soluble in 2% citric acid. The production does not require the use of sulfuric acid, and does not require large amounts of energy and water, and allows the use of low percent natural phosphates without prior enrichment. Fertilizer has good physical properties, does not flake, does not contain free acidity.

Finely ground molten magnesium phosphate shows high efficiency for main application on all types of soils. On acidic sandy and sandy loam soils fused magnesium phosphate to some extent neutralizes soil acidity. In humid tropical climates, the fertilizer is more promising than water-soluble forms, as it does not caked and loses less nutrients from leaching with precipitation.

Thermal phosphates are effective when they are finely milled, but in this state they are very dusty. One way to solve this problem is to pellet finely ground fused magnesium phosphate with potassium chloride.

Magnesium ammonium phosphate (MgNH4PO4nH2O) is a concentrated fertilizer containing phosphorus, nitrogen and magnesium. It is produced from phosphoric acid, ammonia and magnesium hydroxide or magnesium chloride, sulfate, carbonate. It can be in the form of crystalline hydrate, containing one (MgNH4PO4⋅H2O) or six (MgNH4PO4⋅6H2O) water molecules. The latter is unstable during storage, releasing ammonia at 30-50°. Single-water magnesium ammonium phosphate is non-hygroscopic, stable up to 230 °C, does not release ammonia during storage. Because of the smaller amount of water, one-water salt contains 35% more nutrients than six-water salt. Nitrogen in the magnesium-ammonium phosphate is poorly soluble in water, which reduces its leaching on light soils and does not increase the osmotic pressure of the soil solution. One-water magnesium-ammonium phosphate contains 45.7% P2O5, 10.9% N and 25.9% MgO.

Phosphorus in magnesium-ammonium phosphate is contained in a citric-soluble form, so this fertilizer is applied as a powder. When used at a dose of 45-60 kg of P2O5/ha, it introduces the amount of magnesium that can meet the needs of all crops on sandy and sandy loamy-sandy ashy soils. Magnesium-ammonium phosphate on such soils is used as the main pre-sowing fertilizer.

Magnesium-ammonium phosphate can also be used as a concentrated nitrogen-phosphate fertilizer. In this capacity it is used in irrigated agriculture, where before sowing phosphorus and nitrogen is added in small doses, and then in the form of top dressing.

Magnesium ammonium phosphate due to its good physical properties can be used to prepare concentrated fertilizer mixtures or compound fertilizers. It is enriched with nitrogen and potassium to the usual ratios.

Organic fertilizers

The source of replenishment of exchangeable forms of magnesium is organic fertilizers. Systematic application increases the accumulation of absorbed magnesium in the soil, especially on sod-podzolic sandy and sandy loam soils.

The application of manure reduces the effectiveness of mineral forms of magnesium fertilizers. On sandy loamy soils with magnesium deficiency, maximum yields can be obtained by the combined application of organic fertilizers and mineral forms of magnesium.

Importance of magnesium fertilizers

Increase of cereal crops yield from application of magnesium fertilizers is 0.2-0.6 t/ha, potato tubers – 1.5-3 t/ha, sugar beet root crops – 2-4 t/ha, corn green mass – 2-6 t/ha, perennial grass hay – 0.4-0.7 t/ha, tea leaf – 0.5-1.0 t/ha. Magnesium-containing fertilizers increase the level and quality of yield. The content of starch, sugar, protein, vitamin C increases in plant products. The quality of seed material is improved – germination and germination energy increase, resistance to adverse environmental conditions and fungal diseases strengthens.

Magnesium requirements of plants

To ensure optimal conditions of magnesium nutrition of crops in the crop rotation and application of rational doses of fertilizers is required:

  • determine the magnesium requirements of crops for the planned yield;
  • to provide soils with available forms of magnesium;
  • use of diagnostic methods;
  • determination of the state of the balance of magnesium in the system soil-plant rotation.

The need of plants for magnesium depends on the crop and the size of the crop. The table shows the average long-term data on removal of magnesium with crops of crops obtained on sod-podzolic loamy sand soils, where the lack of magnesium is the most common.

The total removal of magnesium by crops for rotation depends on the specialization. With an increase in the share of grain legumes, vegetables, potatoes and other row crops in the structure of sowing areas magnesium removal increases. Between the need of crops in magnesium and their responsiveness to magnesium fertilizers there is a relationship: more demanding to magnesium crops give a higher increase in yield when it is applied.

Таблица. Magnesium removal with crop yield, kg/t of main products

Winter rye
Spring wheat
Flax fiber
Winter wheat
Sugar beet
Vicia-oat mixture
Lupine (grain)

Cereal crops are characterized by less demand for magnesium compared to vegetable, technical and row crops. However, the lack of magnesium, especially at the beginning of the growing season, on cereals leads to magnesium starvation. This is due to the shallow root system of cereal crops at the beginning of the growing season, which can not use nutrients from deeper soil layers. Oats react strongly to magnesium deficiency, while wheat and barley are less sensitive.

Magnesium supply diagnostics

You can assess the availability of magnesium in plants by the appearance of the plant, which in deficiency or excess changes due to disruption of biochemical processes. The main outward sign of magnesium starvation is spotted necrosis: leaves become mottled, areas between veins are pale, veins retain their color. These manifestations are due to the fact that the tissues adjacent to the conductive system are richer in chlorophyll and have a more intense green coloration. Since magnesium moves from the lower leaves to the upper leaves, the signs of starvation from deficiency appear predominantly on the lower leaves. An excess of magnesium causes the leaves to become darker, and abnormal curling and wrinkling is noted.

Soil and plant diagnostics are used to assess magnesium nutrition more accurately and objectively and to optimize magnesium fertilizer doses.

Low magnesium content is often inherent in light soils of granulometric composition. Fertility of sandy soils in terms of magnesium reserves is determined by the degree of weathering of primary magnesium-bearing minerals – feldspars, biotite, serpentine, augite, etc.

The need for magnesium fertilizers is determined by the content of magnesium available to plants, which is determined in the soil extract of 1 n potassium chloride solution (KCl).

For most farming zones, the division of soils by magnesium content is proposed:

  • less than 1.0 mg/100 g of soil – very low;
  • 1.1-2.5 mg/100 g of soil – low;
  • 2.6-5.0 mg/100 g of soil – medium; 2.6-5.0 mg/100 g of soil – medium;
  • more than 5.0 mg/100 g of soil – good.

For simultaneous determination of magnesium and other cations, extracts with sodium chloride solution and 1 n acetic acid ammonium are also used. However, each extraction and element requires determination of a different scale of soil elements availability.

The degree of magnesium availability in plants during the growing season can be determined by plant diagnostics based on the magnesium content in individual plant organs (table).

Table. Levels of magnesium content in plants, %/dry matter

Vegetation phase of sampling, part of the plant
< 0,07-0,08
> 0,37
tubing (above-ground part)
< 0,05
> 0,20
tubing (above-ground part)
Winter rye
< 0,09
> 0,60
tubing (above-ground part)
Winter wheat
< 0,10
> 0,40
tillering (above ground)
< 0,13
0,13- 0,30
> 0,50
young plants
< 0,15
blooming (upper leaves)
Red clover
< 0,16
> 0,60
onset of blooming (above ground)
Sugar beet
< 0,05
> 1,0
row interlocking (leaves)
< 0,30
> 0,90
onset of fruiting (leaves)
< 0,13
> 0,77
fruiting (leaves)
< 0,44
blooming (middle leaves)
Apple trees
< 0,06-0,08
> 0,45
shoots end of growth (leaves)
< 0,18
> 0,30
berry ripening (leaves)
< 0,16
leaves at 4-6 months of age
Tea bushes
old leaves in the second half of the growing season

For some crops excessive magnesium level with visual signs of toxicity is established: for corn – more than 0.55% in pre-budding leaf, for alfalfa – more than 2.0% before flowering, for plum – more than 1.1% in leaves in July, for soybean – 1.5%.

For an objective assessment of the magnesium nutrient regime, a number of factors determining the amount, condition, and mobility of magnesium in the soil must be taken into account. An approximate model of these factors was proposed by Hungarian scientists.

Doses of magnesium fertilizer

To determine the needs of an individual farm in magnesium fertilizers, as well as for other elements, carry out balance calculations, taking into account the items of receipt, such as receipt with lime, mineral and organic fertilizers, precipitation and seeds, and consumption, such as removal with crops, losses from leaching and erosion.

Intensive chemicalization, in particular, the use of high doses of mineral fertilizers, leads to increased tension of the magnesium balance, primarily on light sod-podzolic and peat-bog soils, as a result of removal and leaching.

On soils of light granulometric composition with average magnesium content it is recommended to apply 30-40 kg MgO/ra for grain crops and 60-70 kg/ha for potatoes, corn and root crops. On soils with low and very low supply, the dose is increased, with high and high supply – reduced by 15-25%. The lower the magnesium content and the higher the soil acidity, the more the dose of magnesium fertilizer is increased.

Forms and timing of application

Soil liming with dolomites allows the plants to be fully supplied with magnesium.

Calimagnesia, potassium-magnesium concentrate, potassium salt on kainite, applied in doses of potassium fertilizers, provide the plants with the need for magnesium. Under root crops – sodium-loving crops – as potash fertilizers use kainit and potassium salt from kainit. Crops in this case are provided with potassium, magnesium, sodium and sulfur.

Soluble magnesium-containing fertilizers are applied in the spring during tillage. Low soluble magnesium-containing fertilisers are superior to highly soluble ones under conditions of excessive moisture, heavy precipitation and irrigation. Magnesium ammonium phosphate is promising for hydroponics. Magnesium sulphate is used indoors.

If magnesium fertilizers were not applied in the spring before sowing and if magnesium starvation is detected, supplementation is carried out. Well soluble magnesium fertilizer is used for this purpose. Half a dose of the basic fertilizer is given as a top-up, if it is carried out early and if magnesium starvation is severe, full doses are used.

Features of application of magnesium fertilizers for crops

Cereal crops

Features of application of magnesium fertilizers for cereal crops:

  1. Crops should respond well to magnesium and lime fertilizers. Doses of magnesium more than 40-60 kg/ha for winter cereal crops usually do not lead to further growth of yields.
  2. Signs of magnesium starvation in spring cereals appear at the beginning of growth, as the root system develops, magnesium nutrition improves and signs disappear. However, due to the low reutilization of magnesium, its deficiency at the beginning of the growing season can negatively affect the final yield and grain quality.
  3. When applying high doses of potassium fertilizer and liming, the ratio of calcium, potassium and magnesium must be maintained. Violation of this ratio leads to increased crop requirements for magnesium.
  4. Magnesium fertilizer increases grain yield and improves its quality: increases grain protein content, completeness and weight of 1000 grains.
  5. Sufficient supply of magnesium increases resistance to lodging, to fungal diseases such as rust.


Features of magnesium fertilizer application for potatoes:

  1. Potatoes respond well to the application of magnesium fertilizers, especially on sod-podzolic soils.
  2. The optimal forms for potatoes are magnesium sulphate, magnesium-containing potash and phosphate fertilizers. Magnesium carbonate can lead to boron starvation of potatoes. In this case, additional boric fertilizer is added.
  3. Magnesium fertilizers for potatoes are made at the same time as the basic fertilizer in rows during planting at 8-10 cm below the tubers, in the reserve for a number of years, such as a row of rotation or crop rotation, as well as top dressing during the growing season by spraying the tops during budding.

Sugar beet

Features of application of magnesium fertilizers for sugar beet:

  1. In determining the doses of magnesium fertilizers are based on the amount of magnesium needed to form the planned yield, and losses due to migration through the soil profile.
  2. The best form for sugar beet is dolomite. From mineral magnesium-containing fertilizers, sodium-containing ones are preferable, as beet is a sodium-voluble crop.
  3. If the soils are poorly supplied with magnesium, magnesium fertilizers increase the yield of sugar beet and sugar content of root crops.


Corn for silage with a lack of available forms responds well to magnesium. This is observed more often on light soils, despite a well-developed root system capable of consuming magnesium from the subsoil layers.

On light sandy loam soils, magnesium-containing carbon dioxide lime fertilizers are more effective for corn than pure lime.

Corn for silage is a valuable fodder crop, so the quality of green mass is no less important than the size of the crop. Systematic use of fertilizers worsens cationic composition of green mass, which affects quality of feed, its nutritive value and digestibility by animals. In addition to the usual methods of application of magnesium fertilizers, spraying corn leaves with 2% magnesium sulfate solution has a positive effect.

Grasses, hayfields and pastures

As a result of intensification of cultivation technologies of grasses, natural meadow hayfields and pastures there was a need to use magnesium fertilizers on these lands. When fertilizing perennial grasses to create a balanced content of nutrients in the green mass. Thus, the low content of magnesium in the feed leads to the disease of animals pasture tetany because of the delayed metabolism of mineral nitrogen into organic forms. Large doses of potassium fertilizers aggravate this process by preventing the entry of magnesium into plants due to ion antagonism.


Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific and Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.