1 Core Definition & Classification Positioning
1.1 Basic Properties of Magnesium
Magnesium (symbol Mg, atomic number 12) has an abundance of approximately 2.1% in the Earth’s crust and is an essential nutrient element for plants.
With a dry weight proportion of ≥0.1%, it is categorized as one of the primary macro & secondary nutrients alongside nitrogen, phosphorus, potassium, calcium and sulfur.
Functional Positioning: A “multi-functional regulator” for plant metabolism, the core constituent of photosynthesis, and a key activator of enzyme activity.
1.2 Functional Differences from Other Nutrients
- Primary macronutrient (N): Mainly responsible for structural composition (proteins, chlorophyll).
- Secondary macronutrient (Ca): Focuses on structural support (cell walls) with low mobility.
- Micronutrient (Fe): Assists single specific physiological functions. Characteristics of magnesium: Dominated by metabolic regulation, reusable inside plants, with large demand and involvement in nearly all metabolic processes.
1.3 Nutrient Priority Ranking
Growth Stage Priority: Photosynthetic tissue development stage > critical reproductive growth stage (fruit expansion stage) > nutrient accumulation stage (post-harvest)
Organ Allocation Priority: Leaves (core photosynthetic organs) > fruits (quality formation) > roots (nutrient absorption base) > stalks (nutrient transport channels)
2 Natural Occurrence Forms & Transformation Pathways
2.1 Natural Existing Forms
Mineral magnesium (over 90% of total soil magnesium): Magnesium oxide (MgO), magnesium sulfate (MgSO₄), magnesium carbonate (MgCO₃), dolomite, etc.
Three forms of magnesium in soil:
- Exchangeable magnesium: Adsorbed on soil colloids, readily available for plant uptake (accounts for 60%–70% of available magnesium).
- Water-soluble magnesium: Exists as Mg²⁺ and can be absorbed directly by crops.
- Fixed magnesium: Insoluble compounds that require transformation before absorption.
2.2 Transformation & Environmental Influencing Factors
The optimal soil pH range for magnesium availability is 6.0–7.5. Magnesium leaches easily when pH < 6.0 and precipitates when pH > 7.5.
Sandy soil has weak magnesium retention capacity leading to severe loss, while clay soil holds magnesium effectively.
High temperature plus heavy rainfall accelerates magnesium leaching; drought inhibits root magnesium absorption.
3 Plant Absorption & Transport Mechanisms
Absorption site: Primarily the mature zone of root tips; foliage can also take up magnesium.
Absorption form: Mg²⁺, absorbed mainly via ATP-consuming active transport.
Transport pathways:
- Xylem: Upward transportation along the transpiration stream (main route).
- Phloem: Bidirectional transport enables magnesium redistribution inside plants. Characteristic: Higher mobility than calcium, boron and other nutrients. Allocation ratio: Leaves 60%–70% (concentrated in chloroplasts) > fruits 10%–15% > roots 8%–12% > stalks 5%–8%
4 Existing Forms & Traits of Magnesium Inside Plants
- Bound magnesium (70%–80%): Chlorophyll-bound magnesium (dominant form), protein-bound magnesium, nucleic acid-bound magnesium.
- Free magnesium (20%–30%): Ionic Mg²⁺ in cell sap for physiological regulation. Core Traits:
- Reusable nutrient: Magnesium stored in old leaves can translocate to new tissues; magnesium deficiency symptoms first appear on mature old leaves.
- Synergy & antagonism: Synergistic with phosphorus and sulfur; competitive antagonism with potassium and calcium.
5 Core Physiological Functions
5.1 The “Core Engine” for Photosynthesis
Magnesium forms the central core of chlorophyll molecules. It participates in light reactions (light energy capture and conversion) and dark reactions (activating RuBisCO enzyme for CO₂ fixation).
5.2 “Activator” of Enzyme Activity
It acts as an activator for more than 300 types of enzymes, participating in carbohydrate metabolism, nitrogen metabolism and phosphorus metabolism (energy conversion).
5.3 “Booster” for Substance Metabolism & Nutrient Translocation
Promotes translocation of photosynthates from leaves to fruits and roots.
Regulates intracellular ion balance and alleviates ion antagonism between elements.
5.4 “Optimizer” of Fruit Quality
Internal quality: Raises contents of sugar, vitamin C and organic acids to improve fruit flavor.
External appearance: Maintains dark green foliage, boosts uniform fruit pigmentation and glossiness.
5.5 “Enhancer” of Plant Stress Resistance
Boosts drought resistance (regulates stomatal movement), cold resistance (raises cell sap concentration) and disease resistance (strengthens cell wall structure).
6 Rules of Magnesium Demand at Different Crop Growth Stages
表格
| Growth Stage | Magnesium Demand Intensity |
|---|---|
| Seed Germination Stage | Low |
| Seedling & New Shoot Stage | Medium to Relatively High (promote root & leaf growth) |
| Flower Bud Differentiation Stage | Relatively High (guarantee high-quality flower buds) |
| Blooming & Fruit Setting Stage | High (improve pollination & fertilization) |
| Fruit Expansion & Ripening Stage | Extremely High (accelerate dry matter accumulation) |
| Post-Harvest Recovery Stage | Medium (nutrient reserve for next cycle) |
7 Hazards & Root Causes of Magnesium Deficiency
7.1 Identification of Magnesium Deficiency Symptoms
Leaves: Symptoms first emerge on old leaves with interveinal chlorosis (reticular yellowing), while leaf veins remain green; brown necrotic spots develop in severe cases.
Whole plant: Slow growth and thin, weak stalks.
Flowers & fruits: Sparse blossoms, small deformed fruits, uneven pigmentation and degraded fruit quality.
7.2 Main Causes of Magnesium Deficiency
Soil factors: Low native magnesium content in sandy soil, inappropriate pH value, nutrient antagonism from excess potassium or calcium.
Environmental factors: Magnesium leaching under high temperature and heavy rainfall, or blocked root absorption caused by drought.
8 Scientific Management Program for Plant Magnesium Nutrition
8.1 Comparison of Common Magnesium Fertilizer Sources
- Fast-acting inorganic magnesium fertilizers: Magnesium sulfate (excellent water solubility, rapid efficacy), magnesium chloride.
- Chelated magnesium (EDTA-Mg): High stability, resistant to nutrient antagonism, high absorption efficiency.
- Slow-release inorganic magnesium fertilizer: Magnesium oxide (high magnesium content, long-lasting fertilizer effect).
- Organic magnesium sources: Humic acid magnesium, seaweed fertilizer.
8.2 Standard Application Techniques
Foliar Spraying
- Magnesium sulfate solution: 0.3%–0.5% concentration
- EDTA chelated magnesium solution: 0.2%–0.3% concentration
Soil Application Management
Maintain soil moisture at 60%–70%; apply magnesium oxide or magnesium carbonate for acidic soil, and magnesium sulfate for alkaline soil.
