Magnesium fertilizers
Magnesium fertilizers — mineral fertilizers that meet the magnesium requirements of agricultural crops.
Yield increases of cereal crops from the use of magnesium fertilizers are 0.2–0.6 t/ha, of potato tubers — 1.5–3 t/ha, of sugar beet roots — 2–4 t/ha, of maize green mass — 2–6 t/ha, of perennial grass hay — 0.4–0.7 t/ha, and of tea leaves — 0.5–1.0 t/ha.
Magnesium-containing fertilizers increase the level and quality of the yield. In plant products, the content of starch, sugar, protein, and vitamin C increases. The quality of seed material improves — germination and germination energy increase, and resistance to adverse environmental conditions and fungal diseases is enhanced.
Magnesium content in soil and availability
Scheme of magnesium transformation in soil:
Magnesium deficiency is observed on many soils, primarily on deluvial sandy, strongly leached, acidic soils of raised bogs, as well as on soils after liming due to calcium–magnesium antagonism. A deficiency of magnesium nutrition at optimal soil content may be caused by antagonism with hydrogen, potassium, ammonium, calcium, and manganese ions (Bergman, 1983).
For an objective assessment of the magnesium nutrient regime, it is necessary to take into account a number of factors determining the amount, state, and mobility of magnesium in soil. An approximate model of these factors was proposed by Hungarian scientists.
Raw materials for the production of magnesium fertilizers
The main source for the production of magnesium fertilizers is natural compounds and minerals of this element. Magnesium is a component 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.
Methods of supplying plants with magnesium
Methods of supplying plants with magnesium:
- Liming of soils with magnesium-containing lime fertilizers. This simultaneously provides all crops in the crop rotation with magnesium and calcium and eliminates excess soil acidity.
- Application of magnesium mineral fertilizers to crop rotation crops taking into account their biological requirements.
- Application of organic fertilizers, in which magnesium is contained in amounts of 0.01–0.09%.
Classification of magnesium fertilizers
In the range of magnesium fertilizers, the largest part consists of lime–magnesium and potassium–magnesium fertilizers. By solubility, magnesium fertilizers are classified into:
- water-insoluble — ground natural minerals and rocks, for example, dunite, serpentinite, vermiculite, dolomite, magnesite, brucite, and dolomitized limestones. Upon interaction with acidic soil, plant-available magnesium is released into the soil solution;
- water-soluble — crude salts and their processed products — epsomite, kainite, carnallite;
- citric acid-soluble and plant-available — fused magnesium phosphate.
By composition, magnesium fertilizers are divided into:
- simple — magnesite, dunite;
- complex, containing several nutrients:
- nitrogen–magnesium — ammonium schoenite, dolomite-ammonium nitrate;
- phosphorus–magnesium — fused magnesium phosphate;
- potassium–magnesium — potassium-magnesium concentrate, kalimagnesia, polyhalite, kainite, carnallite;
- boron–magnesium — magnesium borate;
- lime–magnesium — dolomite, dolomitized limestones and their processed products;
- 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 excess acidity; they 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; calcium and magnesium carbonates account for at least 85%. It is used for liming acidic soils at a rate of 3–4 t/ha. The soil is thereby enriched with magnesium in an amount sufficient to supply plants for one or two crop rotations. It is most effective on light soils.
Dolomites are insoluble in water, so their effect depends on the fineness of grinding. The greatest crop yield increases are provided by dolomite flour with a particle size of less than 1 mm.
Partially burnt dolomite (CaCO3⋅MgCO3) — a product of dolomite calcination, contains about 27% MgO, 2% CaO, 57% CaCO3. Magnesium is readily available to plants. It is used for soil liming.
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 refractory production. It has an alkaline reaction with a high neutralizing capacity exceeding that of lime. However, high doses of magnesium carbonate cause calcium and boron starvation in plants. Therefore, its use is combined with boron application for crops demanding it, such as sunflower, beet, clover, and when neutralizing excess acidity it is combined with calcium carbonate.
Burnt magnesia is packed in bags made of waterproof material and stored in a dry room.
Main magnesium fertilizers
In Russia, kalimagnesia and kainite are produced industrially. Their share in the total range of potassium–magnesium fertilizers is insignificant.
Dunite flour and magnesium serpentine
Dunite flour and magnesium serpentine, or serpentinite, are wastes of the mining and asbestos industries. Chemically, they are magnesium silicates in a poorly soluble form, so they are applied in advance in large doses. These magnesium-containing fertilizers are used as raw materials for producing complex magnesium-containing fertilizers, as well as a local fertilizer. Under the action of soil acids, they slowly decompose. Finely ground dunite contains 41–47% MgO. Serpentinite consists of magnesium metasilicate and contains 32–43% MgO.
Ammonium schoenite
Ammonium schoenite ((NH4)2SO4⋅MgSO4⋅6H2O) is a double salt of ammonium sulfate and magnesium sulfate. It is a crystalline mineral ranging in color from light brown to gray. It is used as a nitrogen–magnesium fertilizer; it contains at least 7% N and 10% MgO. It is transported in multilayer bags impregnated with bitumen.
Magnesium sulfate
Magnesium sulfate, or sulfate of magnesia, (eleonite and kieserite) is a single-nutrient magnesium fertilizer; it contains at least 84% MgSO4⋅7H2O and no more than 6% NaCl (17.7% MgO). It is highly soluble in water. It is used in intensive farming under magnesium deficiency on slightly acidic and neutral soils. In this case, at high yields, there is a constant need for readily soluble magnesium sources. It is also used on intensive grasslands, in greenhouse farms, and in open-field vegetable growing. Magnesium sulfate is used to eliminate acute magnesium starvation by foliar feeding. When it enters the soil, most of the magnesium passes into an exchangeable state.
Kalimagnesia
Kalimagnesia (K2SO4⋅MgSO4⋅6H2O) is an intermediate product in the production of potassium sulfate from kainite. It mainly contains the mineral schoenite.
It shows high effectiveness on sandy soddy-podzolic soils due to good solubility and the potassium-to-magnesium ratio.
Table. Composition of granulated types of kalimagnesia, calculated on a dry basis (%).
| Grade 1 | Grade 2 | |
|---|---|---|
| potassium oxide | > 30 | > 28 |
| magnesium oxide | > 10 | > 8 |
| chlorine | < 5 | not standardized |
| moisture | < 2 | < 2 |
Potassium-magnesium concentrate
Potassium-magnesium concentrate is obtained from kainite-langbeinite rock by flotation. The fertilizer mainly contains the mineral langbeinite (K2SO4⋅2MgSO4), and 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, with moisture up to 5%. Chlorine content is not standardized, but in grade 1 it is no more than 8%.
Polyhalite salts
Polyhalite salts (K2SO4⋅MgSO4⋅CaSO4⋅6H2O) contain 10–11% K2O and 8–12% MgO; they dissolve poorly in water, but potassium and magnesium are available to plants. Polyhalite salts have shown effectiveness on various crops, especially on meadows and pastures.
Kainite
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 used mainly on meadows and pastures, where it has advantages over potassium chloride due to the presence of magnesium.
Industrial wastes
Wastes from potassium and magnesium plants can be used as potassium-magnesium fertilizers — dehydrated carnallite, containing 23–24% K2O, 18–20% MgO, 0.9% Na2O, 50–51% Cl, and chlorine-potassium electrolyte, containing 39–42% K2O, 4% MgO, 50% Cl. The negative effect of chlorine is eliminated by early application. Dehydrated carnallite is effective for various crops on sandy loam soils.
Phosphorus-magnesium fertilizers
Thermophosphates and Thomas slag are phosphorus-magnesium fertilizers, a by-product of metallurgy. The nutrients are contained in citric-soluble forms and are available to plants.
This group of fertilizers includes fused magnesium phosphate (FMP), containing plant-available phosphorus and magnesium (Ca3(PO4)2 + MgSO4⋅SiO3). It is produced by fusing natural phosphates and magnesium raw materials (dunite, kieserite, serpentinite, olivinite) at a temperature of 1350–140 °C, followed by rapid cooling of the melt with water. It consists of vitreous transparent granules of various shapes and sizes. The color of the granules varies from bright green to black depending on the raw material.
Fused magnesium phosphate contains 19–21% available citric-soluble P2O5 and 8–14% MgO. Phosphorus in FMP is contained as a modification of tricalcium phosphate soluble in 2% citric acid. Production does not require the use of sulfuric acid and is not associated with high consumption of energy and water; it allows the use of low-percentage natural phosphates without preliminary beneficiation. The fertilizer is characterized by good physical properties, does not cake, and contains no free acidity.
Finely ground FMP shows high effectiveness when applied as a basal fertilizer on all soil types. On acidic sandy and sandy loam soils, FMP to some extent neutralizes soil acidity. Under humid tropical climate conditions, the fertilizer is more promising than water-soluble forms, since it does not cake and loses fewer nutrients to leaching by atmospheric precipitation.
Thermal phosphates are effective in finely ground form, but in this state they are very dusty. One way to solve this problem is granulation of finely ground FMP with potassium chloride.
Magnesium ammonium phosphate (MAP) (MgNH4PO4⋅nH2O) is a concentrated fertilizer containing phosphorus, nitrogen, and magnesium. It is produced from phosphoric acid, ammonia, and magnesium hydroxide or magnesium chloride, sulfate, or carbonate. Magnesium ammonium phosphate can be in the form of a crystalline hydrate containing one (MgNH4PO4⋅H2O) or six (MgNH4PO4⋅6H2O) water molecules. The latter is unstable during storage and releases ammonia at 30–50 °C. Monohydrate magnesium ammonium phosphate is non-hygroscopic, stable up to 230 °C, and does not release ammonia during storage. Due to the smaller amount of water, the monohydrate salt contains 35% more nutrients than the hexahydrate. Nitrogen in 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. Monohydrate 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 applied at a rate of 45–60 kg P2O5/ha, it introduces an amount of magnesium capable of meeting the requirements of all crops on sandy and sandy loam podzolized soils. On such soils, magnesium ammonium phosphate is used as the main pre-sowing fertilizer.
Magnesium ammonium phosphate can also be used as a concentrated nitrogen–phosphorus fertilizer. In this capacity, it is used under irrigated farming, where phosphorus and nitrogen are applied before sowing in small doses and then as top dressings.
Due to its good physical properties, magnesium ammonium phosphate can be used to prepare concentrated fertilizer mixtures or complex fertilizers. In this case, it is enriched with nitrogen and potassium to usual ratios.
Magnesium in organic fertilizers
Organic fertilizers are a source of replenishing exchangeable forms of magnesium. Systematic application increases the accumulation of absorbed magnesium in the soil, primarily on soddy-podzolic sandy and sandy loam soils.
Application of manure reduces the effectiveness of mineral forms of magnesium fertilizers. On sandy loam soils with magnesium deficiency, maximum yields can be obtained with the combined application of organic fertilizers and mineral forms of magnesium.
Doses of magnesium fertilizers
To determine the need of an individual farm for magnesium fertilizers, as for other elements, balance calculations are carried out taking into account input items, for example, input with lime, mineral, and organic fertilizers, atmospheric precipitation, and seed material, and output items, such as removal with the harvest, losses from leaching, and erosion.
Intensive chemicalization, in particular the use of high doses of mineral fertilizers, leads to increased tension in the magnesium balance, primarily on light soddy-podzolic and peat-bog soils, as a result of removal and leaching.
On soils of light granulometric composition with an average magnesium content, it is recommended to apply 30–40 kg MgO/ha for cereal crops and 60–70 kg/ha for potato, maize, and root crops. On soils with low and very low availability, the doses are increased; with increased and high availability, they are reduced by 15–25%. The lower the magnesium content and the higher the soil acidity, the more the dose of magnesium fertilizers is increased.
Forms and timing of application
Liming the soil with dolomites makes it possible to fully supply plants with magnesium.
Kalimagnesia, potassium-magnesium concentrate, and potassium salt based on kainite, applied at potassium fertilizer rates, meet the plants' need for magnesium. For root crops — sodium-loving crops — kainite and potassium salt from kainite are used as potassium fertilizers. The crops are thereby supplied with potassium, magnesium, sodium, and sulfur.
Soluble magnesium-containing fertilizers are applied in spring during soil cultivation. Under conditions of excessive moisture, heavy precipitation, and irrigation, slightly soluble magnesium-containing fertilizers have an advantage over highly soluble ones. For hydroponics, magnesium ammonium phosphate is promising. Magnesium sulfate is used in protected ground.
If magnesium fertilizers were not applied in spring before sowing and magnesium starvation is detected, top dressing is carried out. For this, highly soluble magnesium fertilizers are used. For top dressing, half the dose of the main fertilizer is applied; if it is carried out early and with severe magnesium starvation, full doses are used.
Effectiveness of magnesium fertilizers
To ensure optimal conditions for magnesium nutrition of crops in a crop rotation and to apply rational fertilizer doses, the following is required:
- determination of crop magnesium requirements for the planned yield;
- provision of soils with available forms of magnesium;
- use of diagnostic methods;
- determination of the state of the magnesium balance in the soil–plant system of the crop rotation.
The magnesium requirement of plants depends on the crop and yield level. The total magnesium removal by crops over a crop rotation depends on specialization. With an increase in the share of grain legumes, vegetables, potato, and other row crops in the sown area structure, magnesium removal increases. There is a relationship between crop magnesium requirements and their responsiveness to magnesium fertilizers: crops more demanding for magnesium give a higher yield increase when it is applied.
Adequate potassium plant nutrition increases the magnesium content in seeds and fruits; high doses of potassium fertilizers, by contrast, suppress this process. With a wide potassium-to-magnesium ratio, the manifestation of chlorosis increases even if magnesium is sufficient in the soil. High doses of ammonium have a similar effect.
References
- Mineev V. G., Sychev V. G., Gamzikov G. P., et al. (2017). Agrochemistry. Textbook. Edited by Mineev V. G. Moscow: D. N. Pryanishnikov VNIIA Publishing House.
