Fertilizer International 510 Sept-Oct 2022
30 September 2022
Calcium and magnesium: secondary nutrients of prime importance
SECONDARY CROP NUTRIENTS
Calcium and magnesium: secondary nutrients of prime importance
The drive for ever higher crop yields is shifting the focus onto secondary nutrients such as calcium and magnesium. In this article, we look at the role of magnesium and calcium in crop nutrition and the main fertilizer product options.
Although calcium and magnesium, alongside sulphur, are classed as secondary nutrients, they are as important for balanced crop nutrition as any of the other 14 essential plant nutrients. The calcium requirement of some crops can even exceed that of the primary nutrient phosphorus.
Yet the most commonly applied commodity fertilizers – urea, DAP/MAP (diammonium phosphate/monoammonium phosphate) and MOP (muriate of potash) – notably lack the presence calcium and magnesium. The resulting crop deficiencies can be yield limiting.
Other fertilizers such as polyhalite can supply all three secondary nutrients – Ca, Mg and S – in a plant available form.
As well as being necessary for balanced fertilization, calcium can improve the storage properties and shelf life of produce, while magnesium is vital for photosynthesis.
Calcium – the strength nutrient
Calcium (Ca) is generally needed in moderate amounts by plants, being typically present in growing shoots at between 0.1 to 5 percent dry weight1 .
Described by some as “the premier of the soil’s nutrient elements,” Ca is necessary in plants for protein production, cellular growth and improves disease resistance by protecting against microbial pathogens such as fungi and bacteria. Specifically, Ca maintains the stability and rigidity of plant cell walls and is essential for root growth and function2 .
According to The Mosaic Company: “Calcium is a low-key essential nutrient that carries a heavy load in plant growth. Too often, it takes a backseat as soil fertility programmes developed for many high-yield and high-quality crops. Peanut and tomato growers are probably exceptions in their emphasis on good calcium nutrition.”
High Ca demanding crops include alfalfa, almonds, apples, peanuts, potatoes and tomatoes. Canola is also reported to have around twice the Ca demand of wheat. Typical Ca removal rates by crops range from around 1.1 kg/ha for barley and wheat (2.7 t/ha yield) to 125 kg/ha for alfalfa (9.8 t/ha yield).
Increasing the calcium concentration in onions, apples, pears and potatoes has been shown to improve2 :
- Crop quality
- Resistance to disease and environmental stress
- Storage of produce.
Yara International suggests that calcium fertilizers such as calcium nitrate can improve plant health and deliver more valuable crops and produce by:
- Extending shelf-life and preserving quality. Fruit and vegetables stay fresher looking for longer, resulting in high quality produce with the potential for higher prices.
- Strengthening cell walls. Calcium binds cells together and plays an important role in the cell structure.
- Increasing stress resistance. A good supply of calcium helps crops tolerate a wide range of stresses making them less susceptible to disease and minimising physiological disorders.
Some of the main crop benefits of Ca are shown in Figure 1.
The Ca content of non-calcareous, temperate soils normally ranges from 0.7-1.5 percent, while highly weathered tropical soils typically have a much lower Ca content (0.10.3 percent). Calcareous soils, in contrast, can contain as much as 25 percent calcium.
The concentration of Ca in soils and its availability to crops is influenced by pH, cation exchange capacity (CEC) and soil type. In general, Ca deficiency is more likely in crops grown on sandy soils with a low CEC2 .
Costly Ca-deficiency disorders in fruit and vegetables include:
- ‘Tipburn’ and ‘brown heart’ in leafy vegetables
- ‘Black heart’ in celery
- ‘Blossom end rot’ in watermelon, pepper and tomato fruit
- ‘Bitter pit’ in apples
- ‘Empty pod’ in peanut
- Cracked skin in tomato, cherry and apple fruit.
Fertilizing with calcium
Common calcium sources are shown in Table 1. These include relatively insoluble liming materials, such as calcitic limestone, used to correct acid soil pH – generally known as aglime (agricultural lime). Most acidic soils which have been treated with aglime will have an adequate plant-available supply of Ca. Gypsum is also commonly applied as a soil amendment in countries such as Brazil to improve the chemical and physical properties of tropical soils.
Soluble Ca fertilizers such as calcium nitrate are applied at specific growth stages in high-value tree and vegetable crops to boost calcium uptake2 . These can be soil-applied as a dry fertilizer or applied via fertigation in water-soluble or fluid form. Periodic foliar applications of soluble Ca fertilizers during the growing season are also common for fruits and vegetables to correct deficiencies and improve crop quality.
Some popular calcium fertilizers and their properties are highlighted below.
Calcium ammonium nitrate (CAN)
The delivery of nitrogen in nitrate form can deliver superior yields and quality in arable, fruit and vegetable crops. Because of this, the popularity of nitrate fertilizers – including calcium ammonium nitrate (CAN) and calcium nitrate (CN) – continues to grow.
Global CAN production reached 15.9 million tonnes in 2019. Distinct regional markets for nitrates reflect both grower preferences and regulations. In Europe, for example, restrictions on the sale of straight AN – due to its potential for misuse – has created a sizeable market for CAN. Consequently, EU countries together with Turkey are now responsible for more than half of global CAN consumption (Figure 2) (Fertilizer International 503, p28).
Calcium nitrate
Agricultural calcium nitrate (CN, 15.5-00+26.3CaO) products are typically manufactured from nitric acid and calcium carbonate (limestone). They are available as both liquid fertilizers [45% Ca(NO3 )2 ] and in solid crystalline form [Ca(NO3 )2 .4H2 O]. Global output is estimated at around 2.32.5 million tonnes p.a. (Fertilizer International 503, p28).
Yara through its YaraLiva range is the largest CN producer globally. The company’s market-leading products include soil-applied YaraLiva TROPICOTE and the water-soluble greenhouse-grade YaraLiva CALCINIT.
CN is widely used in fertigation and hydroponic systems. Soil-applied products, meanwhile, due to their calcium content, can improve the texture of clayey soils, improve soil water retention and soil oxygenation, and help release exchangeable nutrients held by the soil.
Standard CN is relatively impure, containing around seven percent of total nitrogen in ammonium form. This level of ammonium, linked to the presence of ammonium nitrate, can be deleterious to the yield and quality of fertigated greenhouse crops. Uralchem and Prayon have both responded to this perceived problem by bringing high-purity anhydrous CN products (17-0-0+33CaO) to market.
Belgium’s Prayon added Calcium Nitrate EXTRA to its Hortipray® range at the end of 2016. This highly-concentrated, water-soluble product boosts calcium content from 25 percent to 33 percent (CaO), compared to standard calcium nitrate. It also guarantees that at least 17 percent nitrogen content is available as nitrate.
Calcium carbonate – combatting soil acidity
Globally, acid soils with a pH below 5.5 occupy about 30 percent of the land surface and are estimated to reduce crop productivity by around 40-50 percent. Traditionally, liming has been the most common method for neutralising widespread soil acidity and keeping soil pH within a range that is favourable for crop production. This typically involves the addition of calcium-and magnesium-rich materials to soils such as chalk, limestone, burnt lime or hydrated lime. It is common for farmers to apply large quantities of limestone (aglime) to their land every 5-10 years – with the general aim of improving soil productivity.
Omya recently launched the soil amendment product Calciprill® . This highquality form of natural calcium carbonate is designed for precise correction of soil acidity. Calciprill consists of micronised ultrafine particles (0.7-100 μm) that have been granulated into 2-6 mm diameter prills for easy application. Calciprill’s purity means its neutralising value is equal to or better than traditional aglime.
Calciprill granules can be applied in precision agriculture with standard fertilizer spreaders before, during or after planting, at times when the change in pH most benefits crop growth. Once in the soil, the granules disintegrate easily, increasing pH where it is needed, especially in the crop root zone.
By correcting soil pH, Calciprill improves the plant availability of soil nutrients. It also provides crops with essential calcium throughout their growth cycle, enhancing crop health and quality, suggests Omya (Fertilizer International 509, p22).
Calcium thiosulphate
Tessenderlo Kerley International markets the calcium thiosulphate fertilizer CaTs® as a versatile source of liquid calcium and sulphur. This clear liquid fertilizer contains six percent calcium and 10 percent sulphur. It is offered as a chloride- and nitrate-free liquid calcium source for fertigation, foliar spaying and direct application to soil.
As well as supplying two key nutrients, calcium thiosulphate also behaves as an activator – improving overall fertilizer efficiency by enhancing the uptake of other nutrients. In addition, it functions as a soil conditioner and amendment, helping to reduce the harmful effects of sodium on soils, for example.
Normal soils are well structured and contain aggregates able to resist erosion. The pore spaces between these aggregates hold water, as well as providing it with a pathway to drain, and offer a good medium for healthy crop roots. Such soils are described as ‘flocculated’. The aggregates in soils containing too much sodium, in contrast, are unstable and separate to become ‘dispersed’. The resulting poor structure of these degraded ‘sodic’ soils make it more difficult for plants to root and for water to penetrate.
Conventional Ca sources such as limestone or gypsum can be used as soil conditioners to treat sodic soils. They function by displacing sodium with calcium, although their effectiveness is hindered by their limited solubility. Calcium thiosulphate, in contrast, being a liquid product, is more effective at displacing sodium, because its calcium ions can quickly enter the soil solution.
Applying calcium thiosulphate to sodic soils helps to reduce crusting problems, improves water infiltration, and leaches out sodium, according to Tessenderlo. The thiosulfate present in products like CaTs® can also liberate additional calcium from any aglime present. These valuable properties suggests that calcium thiosulphate has a role to play in regenerating degraded soils by helping to improve soil health and structure.
Magnesium – essential for plant growth
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. The element is a key component of chlorophyll, 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 (Fertilizer International 502, p34).
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 fertilizers, an optimal supply of Mg improves the stress tolerance of crops and increases the yield and quality of harvested products3 .
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. This causes the main symptom of Mg deficiency, chlorosis – shown by yellowing between leaf veins which remain green (Fertilizer International 455, p19).
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 availability.
Magnesium fertilizers and crop requirements
A number of different Mg fertilizer options are available to meet crop demands (Table 2). 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 crops4.
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 also been shown to be beneficial3 (Fertilizer International 502, p34).
Mg fertilizer applications are necessary to ensure balanced nutrient supply to both pasture and animals. The removal of soil Mg without sufficient resupply generally causes significant falls 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.
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 produces around 600,000-700,000 tonnes of K-Mag® annually from around 3.0-3.4 million tonnes of mined ore, while Intrepid produced 228,000 tonnes of finished product in 2019 from 935,000 of mined ore.
Magnesium nitrate
Magnesium nitrate (Mg(NO3 )2 .6H2 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.
ICL offers the fully water-soluble magnesium nitrate product Select Magnific (110-0+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 operates the world’s largest magnesium chloride production plant at its Dead Sea Works (DSW) complex in Israel. 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) (Fertilizer International 502, p34).
According to the company, magnesium is an undervalued yet irreplaceable macronutrient for crop production due to its many beneficial effects on plant growth, yield and quality. Mg is an essential crop nutrient, according to K+S, due to its roles in root growth, and carbohydrate production and transport, all of which influence the uptake of nutrients from the soil. Agronomic evidence collated by K+S also suggests that Mg is an especially valuable plant nutrient under climate change conditions– when plants are subjected to drought, strong radiation and heat stress.
The basic Mg and S needs of crops can be covered by soil fertilizers such as ESTA® Kieserit, Korn-Kali® or Patentkali® , according to K+S. Additionally, crops can be supported during different growth stages by applying a foliar fertilizer such as of EPSO Top® . This contains Mg and S in combination with micronutrients such as 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 grain, roots or tubers.
Sinomagchem (Yingkou Magnesite Chemical Ind Group Co Ltd) is based in Dashiqiao – the so-called ‘magnesite metropolis’ – in China’s Liaoning Province. It 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.
The company 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).
Polyhalite
Unlike the other Ca and Mg sources highlighted in this article, the natural mineral polyhalite (K2 Ca2 Mg(SO4 )4·2H 2 O) combines all three secondary nutrients (Ca, Mg and S) in a single fertilizer.
ICL, for example, manufactures the established and popular FertilizerpluS product range using polyhalite extracted from its Boulby mine in the UK. Anglo American is also investing heavily in the nearby under-development Woodsmith Mine near Whitby on England’s North Sea coast. This massive deep mine project will extract the company’s polyhalite product POLY4 from the same UK mineral deposit (see accompanying article on p33).
ICL recently expanded its FertilizerpluS product portfolio by adding two new product lines – ICL NPKpluS and Polysulphate Premium (Fertilizer International 509, p22).
NPKpluS is a new NPK line developed in response to rising demand for magnesium and calcium and the increasing importance of sulphur as a nutrient. The product’s nutrient composition – with three soluble sulphates of magnesium, potassium, and calcium – guarantees more complete and efficient crop fertilization and improves yields compared to traditional fertilizer practices, according to ICL.
NPKpluS is produced by ICL at plants in China (prilling) and Ludwigshafen, Germany (blending) and is available in a variety of formulations. Blends can incorporate zinc and/or boron, for example, if required.
Polysulphate Premium contains uniform, robust spherical granules formed by granulating powdered polyhalite. These have an attractive appearance and blend easily with other granulated fertilizers. The smooth surface of granules protects against abrasion, humidity and damage, while their spherical shape guarantees a steady flow and a consistent broad spread during field application.
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