Magnesium (Mg) is the ninth most abundant element in the universe and the fourth on our lovely blue planet (after iron, silicon, and oxygen). Chemically, it is an alkaline earth metal and occupies 12th place in the periodic table. This shiny gray substance makes up a significant chunk of Earth – about 13%. Aside from being widely found in seawater and various rocks, magnesium is also present in all living organisms, playing a crucial role in cell metabolism. From the perspective of plant physiology, it is a secondary macronutrient (along with calcium and sulfur) and the main structural component of the chlorophyll molecule.
A bit of history
Magnesium was first isolated at the beginning of the 19th century by an English chemist called Humphry Davy. He gave the newly found element the name magnium, based on a mineral (MgCO3) from Magnesia, Greece. However, scientists didn’t accept the name proposed by Davy, and the element became widely known as magnesium. Since its official discovery, magnesium has been widely studied, produced, and utilized in numerous ways. However, the importance of this element in plant production seemed to escape scientists and growers until recently. Some researchers even referred to it as “a forgotten element in crop production.” This title wasn’t suitable anymore in 2009 when magnesium finally grabbed the well-deserved attention of the scientific community. Since then, many researchers have focused on revealing more about the significance of this element for plant growth, stress tolerance, and photosynthetic activity.
Why is Mg important to plants?
Magnesium performs a wide plethora of important functions in all living organisms, but it is particularly essential for plants. Even though it makes up only between 0.1 and 1% of a plant’s dry mass, this element plays a vital role in its protein production and enzyme activity. As the main component of the chlorophyll molecule, magnesium is essential for light absorption and photosynthesis. Aside from having a role as a structural element in the chlorophyll molecule, it also activates various enzymes that make photosynthesis possible. To put it in numbers – about 20% of the total Mg content in plants is found in their chloroplasts (organelles that conduct photosynthesis). In case of an Mg deficiency or low light conditions, it can increase to a whopping 50%.
The importance of magnesium and many essential plant nutrients aside from NPK has been largely neglected until recently. Significant developments in technology and the combined effort of many researchers revealed a lot of exciting information over the previous six years. Current research continues to show information previously unknown to plant physiology and many intricacies of magnesium’s functions in plants. Let’s see what we have gathered so far.
Summary of magnesium’s most important functions in plants
- It is the critical structural component of the chlorophyll molecule and essential for its synthesis
- It is necessary for the production of thylakoids – compartments inside chloroplasts where light-dependent reactions happen
- Activator of many important enzymes – the ones necessary for photosynthesis (RuBisCO, PEP-carboxylase), cell metabolism (dehydrogenase, carboxylase, decarboxylase, etc.), phosphorylation, and RNA transcription
- It affects the structure of proteins and ribosomes
- Cofactor of enzymes which catalyze chemical reactions essential for transcription, translation, and replication of genetic material
- It plays a role in nitrogen metabolism, and as such, it can affect the availability of this nutrient
- Controls the water regime in plants – opening and closing of the stomata
- Increases iron (Fe) utilization
- Aids transport of products of photosynthesis (photosynthates) through the phloem and accumulation of reserve carbohydrates in tubers, bulbs
- Aids plant’s defense mechanisms against environmental stress (drought, high or low temperatures, high air humidity, etc.)
- It plays an essential role in stress signaling, biosynthesis of plant hormones, and leaf senescence
In a nutshell, adequate levels of magnesium ions in the soil secure active photosynthesis and reproduction of genetic materials. They also contribute to the production of different enzymes and hormones, availability of other nutrients, and plant’s resistance to diseases and stressful environmental conditions. Mg content also influences the transport and storage of carbohydrates, affecting the quality and taste of fruits, tubers, and other edible parts of the plant.
Magnesium and photosynthesis
We cannot fully understand the importance of this element without giving photosynthesis a little more attention. Magnesium is an essential part of many processes crucial for successful photosynthesis, both as a structural component of participating molecules and an activator of enzymes.
Let’s begin the story of photosynthesis and Mg with the molecule that enables plants to absorb the energy from the Sun in the first place – chlorophyll. There are two types of chlorophyll in plants, chlorophyll a and chlorophyll b. Both types consist of two main parts – a porphyrin head and a hydrocarbon tail. The central molecule in the porphyrin head is no other element than magnesium.
This element is essential for chlorophyll production. The structure magnesium forms with the porphyrin ring is why plants absorb certain parts of the electromagnetic spectrum and reflect others. Thanks to magnesium, plants absorb light most intensely in the blue and red portion of the spectrum and reflect the green portion. This is why we perceive plant’s photosynthetic tissues (leaves and young stems) as having a green color.
*picture of chlorophyll*
The most abundant enzyme on Earth is RuBisCO, the key enzyme responsible for CO2 fixation in plants. RuBisCO is abundant in chloroplasts – cell organelles specific to plants and photosynthetic algae. Mg ions regulate the activity of this crucial enzyme by increasing the affinity of RuBisCO towards CO2 molecules and thus enabling active photosynthesis. However, RuBisCO is not the only enzyme that aids CO2 fixation. PEP-carboxylase, SBPase, and FBPase are also an essential part of the process, and Mg ions activate all of them.
Magnesium also indirectly contributes to photosynthesis by activating enzymes that regulate photophosphorylation, i.e., ATP production in chloroplasts. ATP is an energy-rich molecule, the main “energy currency” in living organisms. Without effective photophosphorylation, plants cannot convert enough ADP to ATP and lack the necessary energy to function normally.
Considering magnesium is crucial for the activity of so many essential enzymes, it is easy to see why plants need this element in their regular diet.
Magnesium in the soil
Magnesium is a component of many naturally occurring minerals. Along with calcium, it is the most abundant element in Earth’s lithosphere, making up about 2.1%. As a result of this, soils that are naturally deficient in magnesium are pretty rare. However, just because magnesium exists in the ground doesn’t mean it is available to plants. Plants absorb nutrients in the form of ions, so if magnesium is present in the soil in the form of a carbonate or an oxide (which is often the case), it requires a bit of processing before becoming available. This processing is performed by soil microbes, which can convert various compounds with their activity and excrements. For this reason, a healthy soil microbiome is one of the key elements of successful plant cultivation. So, if you want to increase magnesium absorption, improving the biodiversity of beneficial soil microorganisms is the best long-term approach.
How do plants absorb Mg?
After the microbes have finished their work, plants take up magnesium ions passively through the root. The absorbed ions are transported through the xylem (vessels responsible for transporting water and nutrients) to other parts of the plant. Magnesium can also move through the phloem (vessels that transport organic substances), making it significantly more mobile than calcium. High mobility allows this element to quickly and easily reach active, young tissues and enable protein synthesis, chlorophyll production, and transport of photosynthates (products of photosynthesis).
The pH value of the soil solution is a significant factor for plant nutrient absorption. The optimal pH of the soil solution for plant-available magnesium is between 5.5 and 6.5.
Considering that magnesium is essential for many biochemical and physiological processes in the plant, its deficiency is highly detrimental to plant growth and development. When plants do not receive enough of this element, the production of chlorophyll significantly drops. The activity of many enzymes vital for successful photosynthesis also decreases. For this reason, symptoms of deficiency appear on older leaves first.
Aside from reduced photosynthetic activity, Mg deficiency also causes plants to lose more water. Because there aren’t enough Mg ions to regulate water transport and provide a sufficient amount to plant cells, the cells cannot create the pressure strong enough to close the stomata on leaves. By remaining open for prolonged periods, the stomata release too much water and slowly dehydrate the plant. Another problem with stomata staying open longer than they should is the increased risk of a pathogen attack. Many pathogenic bacteria and fungi use these natural entryways to get inside the plant tissue and infect it. Pathogens that cause leaf spotting are prime representative examples of this – each spot is where the microorganism got inside the plant through the stoma. The color change of the leaf tissue is a result of the pathogen’s activity.
An important thing to note is that the effects of Mg deficiency are exacerbated if the plants are grown under high light intensity. If the plants are also in the intensive vegetative growth phase, when leaves and shoots are actively produced, this physiological disorder can cause quite a lot of damage. Aside from reducing the plant’s photosynthetic activity, slowing its growth and water supply, Mg deficiency also makes it more susceptible to pathogens and negative effects of environmental stress. Plants that are sensitive to the negative effects of magnesium deficiency include beans, corn, and wheat.
Symptoms of magnesium deficiency:
- Pale leaves
- Yellowing of the leaves between the veins (also called interveinal chlorosis)
- Leaf margins turn brown-red or purple
- Necrosis on older leaves
- Older leaves start to drop
- Premature aging of the plant
Crops that are more susceptible to adverse effects of Mg deficiency include corn, grains, rutabagas, and many types of legumes.
Why does Mg deficiency happen?
Magnesium deficiency in plants rarely occurs as a result of a lack of this element in the soil. Mg is one of the most abundant elements on our planet, so when deficiency symptoms occur, they are likely caused by other factors. The two leading causes of magnesium deficiency in plant cultivation include:
- imbalance in the soil microbiome
- overuse of fertilizer
There are many species of fungi and bacteria in the soil. A good portion of those microbes are beneficial and make plant nutrients available by breaking down soil particles. They can process both the mineral and organic components. By actively participating in nutrient cycling and releasing different secondary metabolites, beneficial fungi and bacteria turn unavailable nutrients into plant-available ions. Similar to the microbiome in our gut, the soil microbiome “pre-digests” certain compounds and provides the plants with available food. So, if the microbial activity is poor or disrupted, nutrient cycling in the soil is slowed, and the plants end up with very few available nutrients.
Overfertilization with potassium and nitrogen can also result in magnesium deficiency. Because potassium (K+) and ammonium (NH4-) ions compete with magnesium ions (Mg2+), their high concentration in the soil solution can interfere with the absorption of magnesium. This problem is especially prominent on sandy and acidic soils.
If you’ve noticed the symptoms of Mg deficiency appearing after applying NPK fertilizer, the application rate or the ratio was likely too strong. An overly dominant presence of these elements in the soil solution causes an imbalance and reduces the availability of magnesium and other nutrients.
Magnesium toxicity in plants is very rare, hardly ever seen in nature. When it does occur, it usually happens to indoor plants that received too much fertilizer. This physiological disorder is not as dangerous as many others, but it can cause a disbalance within the plant. If left untreated, the disbalance can trigger more significant problems in the long term. Magnesium toxicity negatively affects the availability of other nutrients (calcium, potassium) and makes plants more sensitive to certain diseases. It visually manifests as typical salt toxicity.
Symptoms of magnesium toxicity include:
- Leaves become dark green
- Growth slows down or becomes stunted
Magnesium toxicity is especially harmful in tomato and pepper cultivation. Too much magnesium reduces the availability of calcium, and these crops are susceptible to calcium deficiency. The notorious blossom end rot (BER) is a common outcome of a lack of this element. If not treated promptly, blossom end rot can take an enormous toll on yields. Aside from causing more physiological disorders, magnesium toxicity also makes affected plants more susceptible to bacterial spots.
The moral of the story: Check the soil pH, note your fertilizer inputs and think twice before adding more Mg or any other type of fertilizer. Treating a deficiency is always easier than treating toxicity.
Fertilizing with Mg
Looking at Mg’s numerous functions in plants, it is not a big surprise that this element is an integral part of their diet. However, we shouldn’t forget that Mg is a secondary macronutrient (along with calcium and sulfur). These nutrients are “secondary” because plants require a lower amount of their ions to function normally compared to primary macronutrients (NPK). Just because an element is essential for a plant’s physiological and biochemical processes, it doesn’t mean that the plant needs to absorb a copious amount of it. Our green friends generally prefer to have a modest but constant supply of magnesium. That is why many experienced growers like to add Mg fertilizers that release the available ions slowly, over a prolonged period.
Note: Always check the pH of your substrate before deciding to apply fertilizer. More often than not, nutrient deficiency symptoms can be traced back to unfavorable pH levels instead of a lack of elements in the soil. The pH value also affects microbial activity, so adding more nutrients won’t benefit the plants if the pH is not within a favorable range.
Types of magnesium fertilizer
According to the speed of adoption by plants, there are two basic types of magnesium fertilizers – slow-release and quick-release fertilizers. The first type includes forms of magnesium that are insoluble in water. Having this quality, slow-release fertilizer is not susceptible to leaching or runoff. However, plants cannot absorb it immediately, requiring microbial processing before becoming available to them. Such fertilizers release the plant-available ions slowly and provide a long-term source of Mg. On the other hand, quick-release fertilizers are formulated to provide the available magnesium ions to plants almost immediately. They are water-soluble, which makes them great for hydroponic production. Because quick-release fertilizers are susceptible to leaching, it is not advised to apply them to sandy soils.
It is important to note that adding certain Mg fertilizers will increase the soil pH value. The best way to mitigate such effects is to apply them together with other nutrients. Preferably, this is done by combining natural sources of Mg with compost.
Slow-release Mg fertilizers
Dolomite [CaMg(CO3)2] – Dolomitic lime or dolomite is a naturally occurring mineral. It consists of calcium carbonate and magnesium carbonate. In its pure form, it contains about 21% of Ca and 18.5% of Mg. Widely used as fertilizer in plant cultivation, and probably the most commonly applied slow-release type of Ca and Mg fertilizer.
Magnesium carbonate (MgCO3) – When the substrate is already rich in calcium, applying dolomite isn’t necessary. That is why some growers like to boost their plants with pure magnesium carbonate instead of dolomite. The mode of application and the speed of ion release is the same as in dolomite. Magnesium carbonate contains about 28% Mg.
Magnesium oxide (MgO) – This is the most concentrated type of magnesium fertilizer. When added to soil, the high Mg content in MgO significantly increases its pH value. For this reason, farmers apply this type of Mg fertilizer in combination with other fertilizers. MgO contains about 39.7% Mg.
Calcium-magnesium phosphate – This mix of nutrients is becoming more and more popular, as it displays good performance in the production of various crops, especially when combined with organic fertilizers. When added to compost, calcium-magnesium phosphate reduces emissions of flammable compounds like ammonia (NH3), hydrogen sulfide (H2S), and other harmful sulfides. It contains about 15% Mg, 25% Ca, and 19% P.
Magnesium hydroxide [Mg(OH)2] – This form of Mg is most often used for manufacturing liquid fertilizers, and it is usually mixed with other plant nutrients. Its chemical properties are similar to magnesium oxide. It contains about 42% Mg.
Quick-release Mg fertilizers
Magnesium sulfate – Popularly known as Epsom salt, this is the most widely used water-soluble magnesium fertilizer. It is commonly added to water in drip irrigation systems and applied as a foliar spray. In its anhydrous form, magnesium sulfate contains about 20% Mg and 26% S. Commercially available Epsom salts often include about 10% Mg and 13% S. Fun fact: The discovery of Epsom salt took place before magnesium. According to the story, in 1618, a farmer from Epsom, England, took his cows to forage. He attempted to give them water from a nearby well, but the cows wouldn’t drink because the water was too bitter. However, the farmer noticed that washing his hands in this water seemed to help his scratches and rashes heal faster. The miracle ingredient became widely known as Epsom salt.
Magnesium chloride (MgCl2) – This type of Mg fertilizer is available in a liquid form, so it can be easily used as a foliar spray and applied in hydroponic systems. It contains about 25% of Mg.
Potassium magnesium sulfate – langbeinite [K2Mg2(SO4)3] – Langbeinite is a mineral used as a source of plant-available magnesium, potassium, and sulfur. Magnesium is present in its soluble form in this mixture, and as such, it can provide the plants with the necessary ions almost immediately after application.
Natural sources of Mg
Nature knows best when it comes to growing plants, so we should strive to imitate the processes we see in naturally rich soils. Aside from the dolomite and langbeinite we mentioned before, greensand is also an excellent natural source of this element. It is best to apply slow-release fertilizers (dolomite) with compost, while quick-release fertilizers (langbeinite) can be directly mixed with the substrate.
If you have any questions about magnesium in plant production, we encourage you to ask in the comments. Happy planting!
- Cakmak, I., Yazici, A.M. (2010). Magnesium: A forgotten element in crop production. Better Crops with Plant Food Vol.94 No.2 pp.23-25 ref.7.
- Guo, W., Nazim, H., Liang, Z., Yang, D. (2016). Magnesium deficiency in plants: An urgent problem. The Crop Journal Vol.4 pp.83-91.
- Huber, D.M., Jones, J.B. (2013). The role of magnesium in plant disease. Plant Soil 368, pp.73–85.
- Gerendás, J., Führs, H. (2013). The significance of magnesium for crop quality. Plant Soil 368, pp.101–128.