Magnesium and Plants
- KIWITA
- Jan 15
- 5 min read
"Knowledge on Chemical Fertilizers"
BSI Biological Science Research Institute
File No. 59 – Magnesium and Plants
Magnesium (Mg) is an alkaline earth metal element with an atomic number of 12 and an atomic weight of 24.3. Its abundance in the Earth’s crust is 1.93%, making it the 8th most abundant element after oxygen, silicon, aluminum, iron, calcium, sodium, and potassium.
Magnesium is an essential element for both animals and plants. In mammals, it plays an important role in the formation of bones and teeth, the maintenance of ribosome structure, protein synthesis, and the functioning of enzymes related to energy metabolism. In plants, magnesium is present at the center of chlorophyll, the photosynthetic pigment, where it absorbs light and converts light energy into chemical energy. It is also a necessary cofactor for the activation of many enzymes, is involved in carbohydrate and phosphate metabolism, and is indispensable for the synthesis of amino acids, proteins, and carbohydrates. In agriculture, magnesium is often referred to as “magnesia.”
The best-known presence of magnesium in plants is in chlorophyll. Chlorophyll is a chemical compound responsible for absorbing light energy during the light reaction of photosynthesis and is found in the chloroplasts of leaves. In plant photosynthesis, chlorophyll efficiently absorbs light energy and converts it into chemical energy, synthesizing carbohydrate organic compounds from carbon dioxide and water. Photosynthesis also produces oxygen through water decomposition, supplying it to the atmosphere and supporting the survival of animals, including humans. Chlorophyll is classified into six types based on the type of tetrapyrrole ring forming its main structure and the substituents attached to it. All chlorophyll molecules contain a magnesium ion at the center of the tetrapyrrole ring as a metal complex element. Figure 1 shows the structure of chlorophyll α, which is the most abundant in plants.
Figure 1. Molecular structure of plant chlorophyll α
Besides chlorophyll, magnesium mainly exists in chloroplast stroma and intracellular organelles in the form of ions or salts bound to organic acids or ATP. Magnesium ions present within cells play a crucial role as cofactors for enzymes. In living organisms, including plants, many enzymes are involved in life activities such as metabolism. While some enzymes function alone, many require cofactors (coenzymes or cofactors) besides the protein component to function. Magnesium is said to assist a very large number of enzymes among metal ion-type cofactors.
The reason magnesium functions as a cofactor for many enzymes is that it has a high affinity for oxygen in phosphate groups and can bind to phosphate compounds through coordination bonds, thus participating in many enzyme reactions involving phosphate compounds. For example, enzymes that involve magnesium as a cofactor include RNA polymerase, ATPase, protein kinase, phosphatase, glutathione synthetase, and carboxyl group transferase. All of these use phosphate compounds as substrates or products and are indispensable for biochemical reactions including protein synthesis, glycolysis, the TCA cycle, and nitrogen metabolism. Magnesium also binds to phosphate groups on cell membranes and ribosome surfaces, playing an important role in maintaining their three-dimensional structure.
Magnesium content in plants is 0.2–0.4% of dry weight. Roughly 10–25% of magnesium in plants is in chlorophyll, about 5% is in intracellular organelles such as cell membranes and ribosomes, and more than 70% exists as ions inside cells.
When magnesium is deficient in plants, physiological activities are disrupted, and deficiency symptoms appear. The main symptom of magnesium deficiency is chlorosis, where leaves turn yellow due to insufficient chlorophyll concentration. Initially, only the veins of lower leaves remain green, with interveinal areas turning yellow. As deficiency worsens, the entire leaf yellows, but it does not immediately fall off. This occurs because magnesium is a component of chlorophyll, and when magnesium is deficient, chlorophyll loses magnesium and is degraded, causing the leaf to turn yellow. Magnesium deficiency symptoms first appear in older, lower leaves because magnesium ions in the plant are preferentially transported from physiologically less active older leaves to young leaves with higher photosynthetic efficiency. If magnesium deficiency persists, symptoms expand from lower leaves to upper leaves (Figures 2 and 3).
Once chlorosis caused by magnesium deficiency occurs, the affected leaves lose their photosynthetic and carbohydrate-synthesizing capacity, and recovery is impossible. Magnesium deficiency symptoms resemble iron deficiency, but they can be distinguished because iron, which is poorly mobile within the plant, causes chlorosis in young leaves first.
Figure 2. Magnesium deficiency in tomato
Figure 3. Magnesium deficiency in cucumber
Magnesium deficiency not only reduces leaf photosynthetic capacity but may also induce physiological declines due to reduced enzyme activity and destabilization of intracellular ribosome structures. Long-term magnesium deficiency can cause lower leaf chlorosis and leaf drop, slow plant growth, small fruit size, and hardened, woody tissues.
Magnesium in plants, like other nutrients, is absorbed by roots from the soil. The amount of magnesium in soil varies greatly depending on the parent material. For example, soils derived from volcanic ash or weathered granite often have insufficient available magnesium, whereas soils derived from serpentinite rock often have excessive magnesium. Soil type also significantly affects magnesium availability. Sandy soils with few clay minerals often suffer magnesium loss due to leaching of exchangeable base cations from rainfall.
Even when the soil has sufficient available magnesium, deficiency can occur. Excessive use of lime, potassium chloride, and chemical fertilizers often induces magnesium deficiency. This is because potassium and calcium antagonize magnesium, and excessive K⁺ and Ca²⁺ in the soil solution strongly inhibit root magnesium absorption.
Magnesium-containing fertilizers include magnesium sulfate, lightly burned magnesia, and water magnesia (magnesium hydroxide). Dolomitic lime is also widely used as a soil amendment. Additionally, many chemical fertilizers have magnesium added as a nutrient component. Appropriate selection and application of these fertilizers can prevent magnesium deficiency.
In hydroponic cultivation, especially without solid media, magnesium must always be added because no magnesium supply can be expected from the soil, and all magnesium must be absorbed from the nutrient solution. Normally, the magnesium ion concentration in the nutrient solution should be about 1/3 to 1/2 of the calcium concentration. For example, in the standard horticultural test formulation, magnesium concentration is 4 me/L, half of calcium, and in Otsuka Chemical A formulation, magnesium concentration is 3 me/L, one-third of calcium. In soil-based hydroponics, magnesium deficiency is sometimes observed due to the antagonistic effect of potassium accumulation in the medium.
Magnesium deficiency has distinctive symptoms, making it easy to detect and minimize damage with proper intervention. When chlorosis is observed in lower leaves and magnesium deficiency is diagnosed, applying a dilute magnesium sulfate solution (20–30 g/L) as a side dressing to the soil can suppress symptoms within a few days.
The fastest way to treat magnesium deficiency is foliar application of magnesium nitrate. Magnesium nitrate contains 15% water-soluble magnesium and over 10% nitrate nitrogen, is highly soluble in water, and is easily absorbed by leaves. Magnesium absorption is promoted synergistically with nitrogen. Foliar application shows effects within a few hours and is far more effective than magnesium sulfate.
Excess magnesium in soil generally does not cause direct toxicity but may inhibit the uptake of potassium and calcium due to nutrient antagonism, leading to deficiencies of these elements. Excessive dolomitic lime application can also cause exchangeable bases in the soil colloid to be monopolized by calcium or magnesium ions, preventing other cations from being retained, leading to leaching or precipitation, and promoting micronutrient deficiencies.
Note: This paper is translated from the following URL. The content is provided for reference on the scientific research of the raw material only. Whether APA raw materials are used or not, we hope this research will help increase understanding and awareness of body minerals.
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