Fertilizer International 502 May-June 2021
31 May 2021
Essential magnesium: fertilizer sources
CROP NUTRIENTS
Essential magnesium: fertilizer sources
Magnesium, a key constituent of chlorophyll, plays a vital role in plant photosynthesis and carbohydrate production. Commercial sources of this essential crop nutrient include Epsom salt, kieserite, langbeinite, magnesium chloride and magnesium nitrate.
Magnesium (Mg) is an essential, if frequently overlooked, plant nutrient. It is instrumental in many plant functions, being present in vegetative dry matter at between 0.2-0.4 percent concentration.
Mg is a key component of chlorophyll (Figure 1), with each molecule containing 6.7 percent magnesium. It is therefore vital for photosynthesis, as well as playing an important role in the synthesis of starches, sugars, amino acids, proteins, vitamins and oils.
Essential for plant growth
Mg increases plant productivity by boosting photosynthesis, mobilising carbohydrates, promoting the uptake of other nutrients including phosphorus (P), and functioning as an enzyme activator. Additionally, it acts as a P carrier in plants and regulates cellular respiration. Mg also helps create strong root systems, and increases resistance to pests, fungal attack and disease in leaves, seeds and fruit. Overall, Mg creates stronger, healthier and nutrient-rich crops with increased yields (Fertilizer International 455, p19).
According to a recent review of the role of magnesium fertilizers1 : “Magnesium is involved in many physiological and biochemical processes. It is an essential element for plant growth and development and plays a key role in plant defence mechanisms in abiotic stress situations.”
Mg release from soil minerals is generally small compared with the amounts needed to sustain crop yield and quality. This makes the application of Mg fertilizers crucial in many farming systems (see box). An optimal supply of Mg improves the stress tolerance of crops and increases the yield and quality of harvested products1 .
Mg deficiency
As already indicated, Mg is an activator for many critical enzymes. Without sufficient Mg, photosynthesis and enzyme activity within the plant decreases and chlorophyll begins to degrade in the older leaves of plants (see photo). This causes the main symptom of Mg deficiency, chlorosis – shown by yellowing between leaf veins which remain green (Fertilizer International 455, p19).
As The Mosaic Company notes: “Magnesium is mobile within the plant and easily translocated from older to younger tissues. When deficiencies occur, the older leaves are affected first. Loss of colour occurs between the leaf veins, beginning at the leaf margins or tips and progressing inward.”
This can give the leaves of Mg-deficient plants a striped or marbled appearance.
“Magnesium (Mg) is an essential, if frequently overlooked, plant nutrient. A key component of chlorophyll, it is vital for photosynthesis”
Other signs of Mg deficiency include:
- Leaves that becoming more brittle and thinner than normal and show cupping or curve upwards l Reddish-purple leaf tips and edges in cases of severe deficiency, especially in cotton
- Older leaves die (necrosis) and drop off after prolonged deficiency.
Magnesium deficiency occurs most frequently in low pH, sandy soils where Mg can be easily leached away. Soil testing to identify potential deficiencies is therefore recommended. Mg assimilation by plants is also depressed in the presence of high levels of soil aluminium (Al3+ ) which have a detrimental effect on root growth. Excess potassium (K) can also negatively affect Mg availability2 :
“The competition between these two cations for root uptake appears to be the primary cause, although high K may also impair Mg translocation [movement] within the plant. Low forage Mg concentrations following K fertilization have been linked with low Mg in the grazing animals (grass tetany) where it is essential for certain enzyme and metabolic reactions.”
When Mg deficiencies are detected via soil or plant tissue analysis, soluble magnesium sources can be applied to crops via the soil or through fertigation. Relatively small doses of Mg can also be applied to growing crops through foliar fertilization to prevent or correct deficiencies. In soils, the application of Mg before crops are planted or begin active growth is generally the preferred approach.
Magnesium fertilizers and crop requirements
A number of different Mg fertilizer options are available to meet crop demands (Table 1). These are typically divided into semi-soluble and soluble sources. For semi-soluble Mg fertilizers, particle size largely determines their dissolution rate in soils.
Magnesium removal by crops depends on soil Mg supply, growing conditions, crop type and target yields. At the top end, high-yielding sugarbeet and forage crops can remove Mg at rates of 90 kg/ha and 56 kg/ha, respectively. In general, cereal crops remove smaller amounts of Mg at harvest compared with root crops and many fruit crops2 .
While Mg is essential for all plants, the following crops are said to be especially responsive to Mg fertilization: alfalfa, blueberry, beet, broccoli, cabbage, cauliflower, celery, clover, conifers, corn, cotton, cucumber, eggplant, lettuce, onion, pepper, potatoes, pumpkin, spinach, squash, tobacco, tomato, and watermelon. Applying Mg fertilizers to grassland, wheat and potatoes has been shown to be particularly beneficial1 .
Mg fertilizer applications are necessary to ensure balanced nutrient supply to both pasture and animals. The removal of soil Mg without sufficient resupply generally cause significant decline in available Mg in grassland soils.
Mg is important for both the quality and yield of wheat and other cereals. Carbohydrate translocation and therefore optimal grain filling is supported by available Mg. Thousand-grain weight, one of the most important wheat-grain quality parameters, is generally negatively affected in Mg-deficient soils. Processing properties, such as milling behaviour during flour production, are also known to be directly related to plant Mg content.
Mg is known to be a key determinant of potato quality and yield. It has a direct effect, for example, on starch content and therefore the ‘mealiness grade’ of cooking potatoes. Tuber firmness is a major quality parameter – because it prevents bruising and various forms of discolouration during harvest, transport and storage – and is improved by Mg supply. Mg is also thought to decrease the incidence of black spot and prevent the discolouration of pulp during potato processing.
Leading magnesium fertilizer products and major producers are highlighted below.
Potassium magnesium sulphate (SOPM)
Potassium magnesium sulphate (K2 Mg2 (SO4 )3 , SOPM, 11% Mg) is manufactured by extracting and processing naturally-occurring deposits of the mineral langbeinite. SOPM is valued as a fertilizer for its magnesium and sulphur content, as well as being a chloride-free source of potassium.
SOPM has been mined in the United States for over 70 years from what The Mosaic Company calls “the world’s largest and purest deposits of langbeinite ore” at Carlsbad, New Mexico. Two producers, Mosaic and Intrepid Potash, mine and manufacture SOPM at Carlsbad and market this under the brand names K-Mag® and Trio® , respectively.
Mosaic offers K-Mag ® in three formulations:
- K-Mag Premium ® (0-0-22.5+10.5Mg+21S) is suitable for high-quality blending due to its uniform particle size
- K-Mag Granular ® (0-0-22.5+10.5Mg+21S) is typically used in bulk blends
- K-Mag Standard ® (0-0-22+10.8Mg+22S) is suitable for direct application and as a key ingredient for granulation.
The granulated premium grade is less abrasiveness, has a lower bulk density, more uniform particle size, and a larger SGN number, making it ideal for blending.
K-Mag ® is recommended for chloride-sensitive vegetable and fruit crops requiring high fertilizer application rates. It typically benefits crops with a high potassium and magnesium demand that are also sensitive to chloride, such as tobacco.
According to Mosaic, K-Mag ® is an excellent source of non-chloride potassium, water-soluble magnesium and sulphur. With less than three percent chloride content, the risk of fertilizer ‘burn’ is also minimal. The combination of magnesium and potash in K-Mag ® improves plant resistance to winter kill and insects. The product’s magnesium content also activates the enzymes that synthesise chlorophyll, while the sulphur it supplies boosts amino acid formation.
Mosaic produces around 600,000700,000 tonnes of K-Mag ® annually from around 3.0-3.4 million tonnes of mined ore. Traditionally, about half of the finished K-Mag ® products are sold within the US, with the other half destined for international markets where the product is also known as Sul-Po-Mag or S-P-M. Reserves at the company’s Carlsbad mine are sufficient to yield a further 32 million tonnes of langbeinite concentrate (average grade 22% K 2 O), equating to a mine life of 46-47 years at projected rates of production.
Intrepid Potash markets its SOPM brand Trio ® as: “A long-lasting, readily-available source of low-chloride potassium, magnesium and sulphur ideal for fruit, vegetable and row crops grown in magnesium-deficient soils.” Trio ® is offered in four main grades:
- Premium (0-0-21.7+10.9Mg+21.8S, 3.0% Cl)
- Granular (0-0-22.1+11.2Mg+22.2S, 1.4% Cl)
- Standard (0-0-22.1+11.2Mg+22.2S, 1.5% Cl)
- Fine standard (0-0-21.7+11.0Mg+22.2S, 2.0% Cl).
These are all OMRI-listed and approved for organic farming.
Intrepid’s East Mine at Carlsbad has the capacity to produce around 400,000 t/a of Trio ® , after being converted into a Trio ® -only operation with the cessation of potash production in 2016. The company produced 228,000 tonnes of finished product in 2019 from 935,000 of mined ore, according to Intrepid’s latest annual report. Trio ® is mainly sold into the domestic US market with only 19-28 precent being exported on average over the last three years. The product generated around $28 million in sales for the company in 2019.
Magnesium nitrate
Magnesium nitrate (Mg(NO 3 ) 2 .6H 2 O, 9% Mg) is widely used in the horticultural sector to supply water-soluble Mg alongside nitrogen (N).
Commercial grades are typically man-made products synthesised by reacting nitric acid with magnesium metal or magnesium oxide. Magnesium hydroxide and AN also react to form magnesium nitrate, releasing ammonia as a by-product (Fertilizer International 455, p19).
Haifa Chemicals of Israel supplies water-soluble magnesium nitrate (11-00+16MgO) under the Magnisal ™ brand-name. The product is marketed at a range of crops, including barley, citrus, cucumber, deciduous fruit trees, grapes, mango, olive, potato, tomato and winter wheat. According to Haifa, Magnisal ™ : l Prevents and cures magnesium deficiencies.
- Is free of chloride, sodium and other detrimental elements.
- Dissolves quickly and completely in water.
- Is ideal for fertigation and foliar spray.
Magnisal ™ is extremely soluble in water. The white or yellowish-green flakes dissolve quickly and do not precipitate, even at low temperature.
Providing magnesium in nitrate form offers distinct advantages, claims Haifa: “Plants absorb the magnesium from Magnisal ™ more readily, due to the interaction between the magnesium and the nitrate anion. Magnisal ™ is up to three-fold more effective than magnesium sulphate in preventing and curing magnesium deficiencies, and therefor enables considerably lower application rates.”
ICL Specialty Fertilizers offers the fully water-soluble magnesium nitrate product Select Magnific (11-0-0+15.5MgO). This crystalline fertilizer, provided in whitish flakes, is designed for fertigation and foliar use, and is recommended for field crops, orchards, flower crops, as well crops grown under protection in polytunnels or greenhouses.
Magnesium chloride
Magnesium chloride (MgCl2 , 25% Mg) is sold as both a water-soluble and liquid fertilizer.
ICL Group operates the world’s largest magnesium chloride production plant at its Dead Sea Works (DSW) complex in Israel.
About 170 grams of magnesium chloride is present in every litre of Dead Sea water. These waters are concentrated by solar evaporation to yield a solution containing around 420 grams of MgCl2 per litre, equivalent to 33 percent concentration. This concentrated liquid is then converted into pellets or flakes using a heating process. The end-product contains about 47 percent magnesium chloride and is supplied as uniform quality white flakes or pellets. These are used in the following applications:
- Special fertilizer mixtures
- Animal feed additive
- De-icing and dust control
- The formulation of abrasives
- The manufacture of Sorel cement for industrial ‘spark-free’ flooring
- As a catalyst in textile finishing processes
- The treatment of sewage and industrial effluents.
Compass Minerals is the only US producer of naturally occurring magnesium chloride, harvesting this from the Great Salt Lake in Utah. The company markets this as an early-stage plant nutrient for wheat crops. It is also sold in North America as stand-alone de-icer or de-icing additive, and as a dedusting/stabilising agent for gravel roads.
Kieserite and Epsom salt
Germany’s K+S is the leading international producer of magnesium fertilizers derived from naturally occurring kieserite (MgSO4 .H2 O, 17% Mg) and Epsom salt (MgSO4 .7H2 O, 9% Mg)
ESTAKieserit ® (25% MgO, 50% SO3 ) contains water-soluble magnesium and sulphur in plant-available form. It is effective for all crops and all soil types and certified for organic farming. The kieserite-based product is suitable for use in agriculture, horticulture, special crops, plantations and forestry. It is recommended for amelioration of soils with poor magnesium status and is typically applied after harvest or prior to sowing.
ESTAKieserit ® is available as either a granulated or fine product. For annual crops, it is applied as a basal dressing in autumn or late winter. Early spring application as a basal or top dressing is recommended on sandy soils and/or under high rainfall conditions. Sub-surface fertilization, alongside nitrogen and phosphate fertilizers, has proved to be effective for maize.
The granular variant, ESTAKieserit gran, ® has excellent spreading properties due to its close particle-size distribution and granule hardness. It can be applied accurately and efficiently with all modern fertilizer spreaders. The best application time for perennial crops is prior to the main growth period.
Fertilizer specialities with magnesium
Prof Jóska Gerendás, Dr Matthias Rott and Dr Heike Thiel of K+S Minerals and Agriculture GmbH highlight the role magnesium plays in crop nutrition. They share insights on the benefits of magnesium fertilization, based on extensive experience gained with the company’s unique portfolio of magnesium- and sulphur-containing speciality fertilizers.
K+S is mainly known as a potash producer. But there is much more to the company than potash-containing products. The unique deposits in Germany also contain valuable magnesium and sulphur in the form of kieserite. This has allowed K+S to create a speciality fertilizer portfolio with a wide range of plant nutrient formulations, most of them certified for organic farming, as described in the main article. These nutrients are highly concentrated, fully water-soluble and directly available for uptake by the roots and leaves of plants. In this article, we focus on the function of magnesium, its role in plant growth and its necessity for high yielding and best quality crop production.
Essential for chlorophyll, vital for photosynthesis
Magnesium (Mg) belongs to the group of macronutrients like N, P, K, S and Ca. Yet its importance for a high-quality yield is not nearly as well-known as its counterparts.
In fact, Mg is one of the essential components of chlorophyll, and thus plays a pivotal role in the first step of photosynthesis – the absorption of sunlight. In subsequent steps of photosynthesis, Mg is also essential for the conversion of light into the biochemical energy used to synthesise carbohydrates from CO2 and water. Additionally, Mg is the catalyst for energy transformations in general, and plays an important role in protein and carbohydrate metabolism (Figure 2).
The capacity for assimilates in storage organs (e.g. grain, beet, tuber, etc.) is significantly affected by the Mg content of the plant. Because magnesium deficiency is directly associated with a lower photosynthesis rate during the growing phase, it reduces the yield and quality of the harvest products. Furthermore, Mg promotes root growth and enables the plant to access more water and nutrients in the soil.
While Mg should be a central part of the fertilizer regime in all crops, special attention should be paid to heavily Mg-dependent crops like oil seed rape, oil palm and maize – or to acidic soils where Mg is less efficiently retained.
Helping crops combat climate change
The factor with the largest impact on crop yield which cannot be controlled by growers is the weather. Extremes of weather, such as drought conditions, are increasing due to climate change. Under these circumstances, Mg becomes even more important.
Plants suffering from Mg-deficiency react much more sensitively to high solar radiation and temperatures than plants well supplied with magnesium. This is because energy from photosynthesis not used for sugar production when Mg is lacking, but instead forms reactive oxygen species, which ultimately kills cells and lead to visible necrosis. Damage by sunlight is therefore a foreseeable consequence if the Mg content of leaves is not maintained within the optimum range.
Overall, an insufficient supply of Mg will disturb and disrupt crop growth. Such deficiency effects are also intensified by heat. Root growth is greatly inhibited on the one hand, and shoot growth is reduced on the other. Consequently, magnesium deficiency – particularly when combined with heat-stress – results in yield losses by impairing water uptake and restricting nutrient uptake and transport.
Magnesium enhances root growth
Optimising root growth offers the best security for plant production. Yet Mg deficiency often remains undetected until it is too late – as less visible impacts on root growth occur much earlier than the more visible effects on shoot growth. This makes it very difficult to recognise the early symptoms of reduced root growth in the field.
Fortunately, deficiency can be prevented through the pre-emptive application of magnesium-containing soil fertilizers. Products such as ESTA ® Kieserit, Korn-Kali® , and Patentkali® promote root length and improve the root mass of plants from the start by improving nutrient and water uptake from the lower soil levels.
Magnesium and sulphur for better nutrient use efficiency
The longer roots and bigger root systems promoted by Mg also increase the uptake of other nutrients such as nitrogen and phosphorus from the soil. Nevertheless, optimal nitrogen uptake and utilisation can only be ensured by combining Mg with plant-available sulphur (S).
Sulphur is particularly crucial for nitrogen use efficiency. If plants are S deficient, the formation of (S-containing) proteins is impaired and consequently the amount of soluble nitrogen compounds in the plant increases, including nitrate levels. This, in turn, inhibits further nitrate uptake from the soil. The resulting accumulation of amino acids also makes the plant highly susceptibility to pathogens.
The adequate supply of sulphur to crops is important, as the failure of the plant to convert nitrate, so-called nitrate stagnation, occurs very early on during latent S deficiency. Nitrate accumulation in plants does need to be guarded against, as it is a subtle and non-visible symptom that can only be identified in the field using nitrate test strips on sap.
Magnesium for successful crop production
Magnesium and sulphur have an impact on both crop yield and quality. For example, in cereals, with the beginning of flowering, Mg is translocated from the green plant organs (mainly flag leaf and spelt) into the grain. Here, Mg is essentially needed for grain filling with assimilates, and is also held in reserve for the seedling.
For this reason, foliar application of Mg to the flag leaf with the EPSO® family of products helps to prevent deficiencies in cereals – due to suboptimal translocation of assimilates to the storage organs – by prolonging photosynthesis output until maturation. This results in higher thousand grain weights and increased carbohydrate and protein contents. Similarly, magnesium beneficially affects starch content in maize, and oil content in oilseed rape and oil palm.
Basic Mg and S needs should always be included in the fertilization strategy. Crop needs can be covered by their respective soil fertilizers like ESTAKieserit, Korn-Kali® ® or Patentkali® .
Additionally, crops can be supported during different growth stages by applying a foliar fertilizer such as of EPSO Top® – which contains Mg and S in combination with micronutrients like manganese, zinc or boron. Early applications of EPSO Top® promote root growth, while applications during later growth stages support the transport of assimilates from the leaves to the storage organs such as grain, roots or tubers. In this way, multiple EPSO Top® applications offer the best insurance for maintaining optimal Mg and S supply – and helping guarantee the highest crop yields as well as the best quality harvested produce. Foliar EPSO® applications are also ideal as a fast-acting Mg and S supplement in situations where deficiency symptoms are observed in the crop.
Conclusions
Magnesium is an undervalued yet irreplaceable macronutrient for crop production due to its many beneficial effects on ecological and physiological processes that control plant growth, yield and quality. Mg is both essential and of central importance due to its roles in carbohydrate production and transport and root growth, all of which influence the uptake of nutrients from the soil. It is an especially valuable plant nutrient under climate change conditions too – when plants are subjected to drought, strong radiation and heat stress, etc.
In summary, magnesium clearly helps plants cope with unfavourable environmental conditions. In our experience, balanced magnesium fertilization – when supplied together with sulphur and micronutrients in combination with potassium – also leads to the best crop production results.
Patentkali® (30% K2 O, 10% MgO, 42.5% SO3 ) is a speciality potassium fertilizer with a high magnesium and sulphur content due to its kieserite content. It is certified for organic farming. Nutrients are present in sulphate form and are fully water-soluble. The product is unaffected by pH and therefore suitable for all soil types. It also has good spreading properties due to its uniform particle size distribution.
Patentkali® can be safely applied until shortly before the drilling or planting date. On light soils, it should be applied in the spring to avoid potential nutrient losses over the winter. The product is particularly well-suited for chloride-sensitive agricultural, horticultural and forestry crops due to its low chloride content (3% Cl maximum). It is recommended for application to potato, vegetables, fruits, grape and sunflower.
Korn-Kali® is a potash and magnesium fertilizer that combines potassium chloride (40% K2 O), kieserite (6% MgO) and sodium chloride (4% Na2 O). It also supplies sufficient sulphur for most crops due to its high sulphate content (12.5% SO3 = 5% S). All these nutrients are fully water-soluble and therefore directly available to plants independent of soil pH.
The product also has good spreading properties in the field due to its uniform particle size distribution.
Korn-Kali® is recommended as a potassium fertilizer for chloride-tolerant crops which also have a requirement for the other nutrients supplied – such as sulphur for oil crops and magnesium and sodium for sugar beet. Suitable crop types include cereals, oilseed rape, sugar beet, maize and forage crops. The product is generally applied during the autumn.
Magnesia-Kainit® (9% K2 O, 4% MgO, 35% Na2 O, 9% SO3 ) is a crude kieserite-containing potash salt. It is marketed by K+S as a cost-effective organic fertilizer for grassland and forage crops. The product significantly improves the palatability of pastureland due to its high sodium content. By increasing magnesium and sodium accumulation of the fodder, it contributes to good animal health, growth and fertility. It should be applied in early spring before vegetative growth begins.
K+S offers a range of four organic-approved fertilizers containing Epsom salt. All nutrients are fully water-soluble and are present in sulphate form
EPSO Top® (16% MgO and 32.5% SO3 ) is a quick acting magnesium and sulphur fertilizer for foliar application. It dissolves rapidly without a residue in water, making it well-suited for foliar spray applications and for fertigation through irrigation systems. These can supplement soil applications during periods of peak demand, particularly if magnesium deficiency is observed. The product is recommended for a range of crop types, including cereals, oilseed rape, sugar beet, potatoes, hops, asparagus and softwood.
EPSO Microtop® (15% MgO, 31% SO3 , 0.9 % B, 1% Mn) is micronutrient-enriched foliar fertilizer recommended for sugar beet, oilseed rape, brassica, potatoes, sunflower, maize and grapevines. The product is designed to rapidly alleviate magnesium, sulphur, boron and manganese deficiencies in growing crops. It is particularly suitable as a preventive application, helping to avoid deficiencies before they occur.
EPSO Combitop® (13% MgO, 34% SO3 , 4% Mn, 1% Zn) is a quick-acting, micronutrient-enriched foliar fertilizer containing magnesium, sulphur, manganese and zinc. It has been specifically developed to meet the nutrient needs of cereals, maize and leafy crops, being able to satisfy high demand for manganese in combination with magnesium and sulphur. The product is particularly suitable as a preventative application.
One or two applications should completely meet magnesium and sulphur demands and provide a full maintenance dressing of manganese and zinc. Autumn application to winter barley has been shown to improve winter hardiness.
EPSO Bortop® (12.6% MgO, 25% SO3 , 4% B) is a new foliar product from K+S. It targets crops with medium-to-high boron demand such as rapeseed, sugar beet, sunflower and corn. The combination of sulphur, magnesium and boron helps with efficient nutrient uptake by the plant. This can be further improved by split applications. A special feature of the product is its ability to lower the pH of spray solutions by up to two units. It quickly alleviates boron deficiencies, aiding plant physiological processes such as starch and sugar formation, cell development and winter hardiness.
Chinese producer
Founded in 2000, privately-owned Sinomagchem (Yingkou Magnesite Chemical Ind Group Co Ltd) is based in Dashiqiao – the so-called ‘magnesite metropolis’ – in China’s Liaoning Province. The company extracts magnesite from Dashiqiao’s large-scale deposits to manufacture up to 500,000 t/a of magnesium sulphate and 200,000 t/a of magnesium oxide. Its fertilizer, feed and industrial product lines are offered in more than 100 different specifications.
Unsurprisingly, given the scale of its operations, Yingkou Magnesite is China’s largest manufacturer and exporter of chemically-produced magnesium sulphate fertilizers. Its product range includes fertilizer-grade:
- Magnesium sulphate (kieserite).
- Magnesium oxide.
- Boron-and zinc-enriched magnesium fertilizers (Mg plus B, Mg plus Zn).
As China’s leading producer, Yingkou Magnesite drafted the country’s national standard for magnesium sulphate and ‘Mg plus B’ fertilizers. Operating under the Sinomagchem brand internationally, the company successfully exports high quality magnesium products to more than 60 countries globally, complying with the EU’s REACH certification and operating to ISO9001.
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