Magnesium in Plant Physiology and Fertilization- A Key Secondary Nutrient for Plant Health

In modern agricultural production systems, the theories and practices surrounding the application of nitrogen (N), phosphorus (P), and potassium (K)-the “three primary nutrients”-have become highly mature. However, magnesium (Mg), a critically important secondary nutrient, has long been overlooked and remains a “nutritional bottleneck.”

As an essential element for plant growth and development, magnesium plays an irreplaceable central role in maintaining crop health, enhancing yield potential, and improving quality parameters. With the advancement of precision agriculture and balanced fertilization concepts, systematically understanding the physiological functions of magnesium and mastering its scientific application has become a key step in unlocking crop productivity and achieving high-quality agricultural development.

I. Core Physiological Functions of Magnesium

Magnesium is the only highly mobile secondary nutrient within plants. It is extensively involved in energy metabolism, biosynthesis, and structural stability, with functions spanning from the cellular level to the whole plant:

1. Central Atom of Photosynthesis (Most Critical Function)

Magnesium is the central atom in the porphyrin ring of chlorophyll a and chlorophyll b (accounting for approximately 2.7% of chlorophyll mass), forming the structural basis for light energy capture.

• Energy conversion: Four nitrogen atoms coordinate around Mg²⁺ to form a stable complex, enabling chlorophyll to absorb, transfer, and convert light energy.

• Enzyme activation: Magnesium is a specific activator of key photosynthetic enzymes such as RuBisCO, directly influencing CO₂ fixation efficiency.

• Assimilate transport: It regulates membrane potential and promotes efficient translocation of photosynthetic products (sucrose, starch) from “source” leaves to “sink” organs such as fruits and roots.

Deficiency impact: Chlorophyll synthesis is inhibited, with content dropping by 30-50%, leading to a sharp decline in photosynthetic efficiency, leaf chlorosis, and severe growth retardation.

2. Structural Backbone of Protein Synthesis

Magnesium acts as a bridging ion between ribosomal subunits and is essential for maintaining ribosomal structural stability.

• Approximately 75% of magnesium in plant leaves is directly or indirectly involved in protein synthesis, ensuring accurate translation of mRNA into polypeptides.

• It stabilizes the binding of tRNA to ribosomes, enabling orderly amino acid polymerization-fundamental for cell division and growth.

3. Universal Activator of Enzyme Systems

Magnesium functions as a cofactor or activator for more than 300 enzymes, regulating nearly all key metabolic pathways.

• Energy metabolism: Forms Mg-ATP complexes, the “universal energy currency” within cells, supplying energy for all energy-dependent reactions.

• Carbon metabolism: Activates key enzymes in glycolysis and the TCA cycle (e.g., pyruvate dehydrogenase), ensuring respiration and material transformation.

• Stress resistance: Enhances the activity of antioxidant enzymes such as SOD (superoxide dismutase) and POD (peroxidase), scavenging reactive oxygen species and improving tolerance to drought, cold, and salinity.

4. Other Key Physiological Roles

• Lipid synthesis: Works synergistically with sulfur to promote oil body formation in the endoplasmic reticulum, increasing oil content in crops such as rapeseed, peanuts, and soybeans.

• Ion balance: Coordinates with potassium (K⁺) and calcium (Ca²⁺) to regulate osmotic pressure and membrane potential, maintaining root nutrient uptake and water transport.

• Seed development: Around 70% of magnesium in seeds is stored as magnesium-calcium phytate, serving as a nutrient reserve for germination and early seedling growth.

II. Typical Symptoms of Magnesium Deficiency

Due to its high mobility, magnesium is translocated from older leaves to younger tissues, so deficiency symptoms first appear on lower, older leaves.

1. General Symptoms

• Interveinal chlorosis in older leaves, while veins remain green, forming a distinct “green veins with yellow tissue” pattern.

• In severe cases, chlorotic areas turn brown and necrotic, leaves senesce and fall prematurely, plant vigor declines, and yield and quality drop sharply.

2. Crop-Specific Symptoms

• Dicotyledonous crops (soybean, cotton, potato): Patchy interveinal chlorosis spreading from leaf margins inward; leaves may curl and scorch in later stages.

  
  

• Gramineous crops (rice, wheat, maize): Yellow-green parallel striping; wheat may show bead-like green spots, maize often develops reddish-purple leaf margins.

  
  

• Fruit trees: 

  • Citrus: “Yellowing disorder” with V-shaped orange-yellow chlorosis between veins on lower leaves, leading to severe defoliation.

  • Grapes: Interveinal yellowing on older leaves, progressing to reddish-brown necrosis; brown scars on fruit stems.

    
    

• Vegetables:

  • Tomato: Interveinal chlorosis on older leaves, orange-red margins; poor fruit coloration, soft texture, reduced quality.

III. Soil and Management Factors Causing Magnesium Deficiency

1. Soil Conditions

• Poor parent material: Red soils, yellow soils, and granite-derived soils often have low total and available magnesium.

• Sandy soils: Low water and nutrient retention, leading to magnesium leaching.

• Strongly acidic soils (pH < 5.5): High H⁺ and Al³⁺ activity competes with Mg²⁺, reducing magnesium availability.

• Nutrient imbalance: 

  • High potassium: Each 1 mmol/L increase in K⁺ can reduce Mg uptake by ~40% (“K-Mg antagonism”).

  • Excess calcium or ammonium: Competes with Mg²⁺ uptake.

2. Agronomic Practices

• Over-application of potassium fertilizers (especially during fruit expansion stages).

• Low organic matter content, reducing buffering capacity.

• Continuous cropping, depleting soil magnesium and increasing acidification.

• Climate stress (heavy rainfall, drought, low temperature), reducing root activity and Mg uptake.

3. Diagnostic Indicators

Indicator	Critical Value	Interpretation
Leaf Mg content (dry weight)	< 0.15%-0.25%	Potential/clear deficiency
Soil exchangeable Mg	< 50 mg/kg	Insufficient Mg supply
Soil K/Mg ratio	> 3:1	Potassium suppresses Mg uptake
Soil Ca/Mg ratio	> 10:1	Calcium suppresses Mg uptake

IV. Types of Magnesium Fertilizers and Scientific Application Strategies

1. Water-Soluble Magnesium Fertilizers (Fast-Acting)

Type	Mg Content	Key Features	Application
Magnesium sulfate (MgSO₄·7H₂O)	9.5%-10%	Most common, high purity, fast response	Basal: 150-225 kg/ha; Foliar: 0.5%-1.0%
Magnesium nitrate	15%-16%	Supplies nitrogen + Mg	Foliar: 0.3%-0.5%; fertigation
Magnesium chloride	~25%	High Mg, low cost, contains chloride	Fertigation: 5-10 kg/ha; avoid in chloride-sensitive crops

2. Slightly Soluble / Slow-Release Fertilizers

Type	Mg Content	Application	Advantages
Calcium magnesium phosphate	8%-12%	Acid soils, basal application	Supplies Ca + Mg, pH regulation
Dolomite (CaCO₃·MgCO₃)	10%-13%	Strongly acidic soils	Long-term Mg supply, raises pH
Magnesium oxide (MgO)	50%-60%	Acid soils	High purity, slow-release

3. Chelated Magnesium Fertilizers (High Efficiency)

• Sugar alcohol magnesium: 3-5× higher absorption efficiency than inorganic Mg; stable under stress conditions; foliar spray dilution 1000-1500×.

• Humic acid Mg / EDTA-Mg: Improves Mg availability in soil, suitable for fertigation or soil application.

4. Key Application Principles

1. Basal application as the foundation, foliar spray as supplementation 

   • Acid soils: prioritize calcium magnesium phosphate or dolomite with organic fertilizers.

   • Neutral/alkaline soils: use magnesium sulfate.

   • Deficiency during growth: apply foliar sprays at 7-10 day intervals (2-3 times).

2. Target critical growth stages 

   • Fruit trees: spring flush, fruit expansion, post-harvest

   • Vegetables: flowering and fruiting stages

   • Field crops: tillering/jointing and grain filling

3. Avoid nutrient antagonism 

   • Maintain K/Mg ratio at approximately 2:1-3:1

   • Avoid mixing magnesium sulfate with calcium fertilizers or alkaline materials to prevent precipitation and clogging

V. Conclusion and Outlook

Magnesium is not only a structural component of chlorophyll and proteins but also a functional driver of energy metabolism, enzyme activation, and stress resistance. It serves as a critical link connecting soil nutrients, plant metabolism, and yield quality.

In the context of modern agriculture-focused on high yield, superior quality, and efficiency-magnesium is shifting from a “forgotten nutrient” to a central regulator in balanced fertilization systems.

Future agricultural practices must establish an integrated magnesium management system based on:soil testing → plant diagnosis → precision fertilization.

By determining application rates through soil analysis, adjusting timing based on crop symptoms, selecting appropriate fertilizer types, and coordinating Mg with K and Ca, magnesium can be elevated from a hidden limiting factor to an actively managed nutrient-unlocking its full physiological potential and supporting sustainable agricultural development.

Magnesium in Plant Physiology and Fertilization- A Key Secondary Nutrient for Plant Health