Your Crops Like Corn Is Asking for Zinc

Plants determine whether an element is essential based on their own physiological and developmental needs. Elements in plant tissues are classified as essential and non-essential. According to the required amount, essential nutrients are divided into macronutrients — nitrogen (N), phosphorus (P), and potassium (K); secondary nutrients — sulfur (S), calcium (Ca), and magnesium (Mg); and micronutrients — boron (B), iron (Fe), copper (Cu), zinc (Zn), manganese (Mn), molybdenum (Mo), chlorine (Cl), and nickel (Ni).

Zinc is an indispensable micronutrient for both animals and plants. In humans, zinc regulates immune function, maintains normal physiological activity, supports normal development in children, and helps treat anorexia and malnutrition. In plants, zinc participates in carbohydrate conversion, promotes enzyme synthesis, and improves crop quality. However, when soil zinc levels exceed a certain threshold (i.e., contamination), zinc exhibits heavy-metal toxicity that decreases yield and may even cause total crop failure. So, what is the role and significance of zinc as a nutrient in maize growth?

I. Nutritional Value of Zinc

Zinc is a component or activator of many plant enzymes. It promotes protein metabolism, reproductive organ development, auxin metabolism, and carbon dioxide hydration in photosynthesis, and enhances plant stress resistance.

1. Zinc influences photosynthesis and respiration.

When zinc is deficient:

• Photosynthetic rate, chlorophyll content, and nitrate reductase activity decrease.

• Protein synthesis is inhibited.

• Carbonic anhydrase activity declines, further reducing photosynthesis.

• Free radicals and sucrose accumulate in chloroplasts, leading to structural damage, functional disorders, thicker cuticles, reduced stomatal opening, and lowered CO₂ fixation capacity.

2. Zinc participates in nitrogen metabolism.

Zinc is closely related to protein metabolism. It is required for:

• RNA polymerase (essential for protein synthesis)

• Proteases involved in nitrogen metabolism

• Glutamate dehydrogenase (for glutamate synthesis)

Zinc deficiency disrupts RNA metabolism, thereby affecting protein synthesis and causing accumulation of free amino acids in the plant.

3. Zinc is one of the most important micronutrients affecting protein synthesis.

4. Zinc deficiency reduces auxin levels.

Even before visible symptoms appear, auxin content declines. After zinc is applied, auxin concentrations increase correspondingly.

5. Zinc improves stress resistance.

Adequate zinc enhances the plant’s tolerance to adverse environmental conditions.

II. Zinc Deficiency in Maize: Symptoms and Characteristics

Under adequate supply, most absorbed zinc is transported to the aboveground parts. Zinc can be redistributed from leaves, stems, and roots to seeds during certain stages. Excess zinc tends to accumulate in roots, part of which is luxury consumption.

Zinc accumulation patterns:

• Highest in roots

• High in shoot tips and young leaves

• Lower in older tissues

• Lowest in grain

Because zinc influences auxin synthesis, zinc-deficient plants show:

• Stunted growth

• Poor leaf differentiation

• Deformed growth

• “Small-leaf disease” in seedlings

• Interveinal chlorosis

• Brown spots that expand into necrotic patches

In maize:

• Zinc-deficient seedlings develop “white seedling disease”, where the lower and middle parts of young leaves turn pale yellow or white.

• After jointing, maize leaves show pale yellow or white stripes that are semi-transparent and tear easily in the wind — known as streaked leaf disease or white streak withering.

• In mid–late stages, interveinal chlorosis appears on upper leaves, forming light yellow-green stripes or patches.

• Plants become dwarfed, ears are small, and barren tips develop.

Zinc deficiency significantly reduces yield.

III. Toxic Effects of Excess Zinc

Excess zinc causes zinc toxicity and interferes with the uptake of other nutrients, resulting in chlorosis, physiological disorders, and even plant death.

Symptoms of zinc toxicity include:

• Root damage and inhibited root growth

• Brown spots and necrosis on leaves

• Pale leaves at 2.25 mg/kg Zn in solution

• Growth inhibition at 4.5 mg/kg

• Brown leaf spots at 11.4 mg/kg

Excess zinc also indirectly induces iron deficiency symptoms.

Anatomical studies show:

• Destroyed cellular structure

• Severe mesophyll cell shrinkage

• Reduced chloroplast numbers

Visually, plants appear stunted with yellowed leaves.

IV. Zinc in Soil

Zinc exists mainly as Zn²⁺ in nature. Major zinc minerals include:

• Sphalerite (ZnS)

• Zincite (ZnO)

• Smithsonite (ZnCO₃)

Decomposed zinc minerals dissolve well, forming Zn²⁺ or complex ions such as [ZnCl]⁺, [Zn(OH)]⁺, [Zn(NO₃)]⁺, which can be absorbed by plants.

However, zinc solubility rapidly decreases due to:

• Soil pH

• Adsorption and fixation

• Organic matter interactions

• Competition or synergy with other ions

Generally:

• Nitrogen and potassium fertilizers contain little zinc.

• Zinc contamination mainly comes from phosphate fertilizers, compound fertilizers, and fertilizers derived from municipal waste or sludge.

With long-term cultivation, topsoil concentrations of Zn, Cu, and Pb tend to rise.

V. Types of Zinc Fertilizers

1. Zinc sulfate heptahydrate (ZnSO₄·7H₂O)

   • Zn: ~21%

   • White or light orange crystals

   • Highly soluble; becomes white powder after losing crystal water in dry air

     
     

2. Zinc sulfate monohydrate (powder) (ZnSO₄·H₂O)

   • Zn: ~35%

   • White powder

   • Highly soluble; hygroscopic

     
     

3. Zinc sulfate monohydrate (granular)

   • Zn: 21%, 25%, 31%, 33% (various grades)

   • White granules

   • Soluble; less hygroscopic than powder

     
     

4. EDTA-chelated zinc (C₁₀H₁₂N₂O₈ZnNa₂·3H₂O)

   • Zn: 12–14%

   • White crystals

   • Very water-soluble; neutral to slightly acidic

     
     

VI. Challenges in Zinc Fertilization

1. High phosphorus reduces zinc uptake.

   Although moderate P increases Zn content, excessive P supply depresses Zn levels in plants.

2. Organic matter has dual effects on zinc.

   • It can increase zinc availability and correct deficiency.

   • Or, it may bind zinc and reduce its availability.

3. Soil pH strongly affects zinc availability.

   • Zinc deficiency is common in alkaline soils.

   • In acidic soils, zinc is more available.

   • As pH rises, zinc adsorption increases and available Zn decreases.

VII. How Should Drip-Irrigated Maize Be Supplemented With Zinc?

1. Apply moderate amounts, split into multiple small doses.

   Zinc easily precipitates with carbonates, especially in alkaline irrigation water (common in Xinjiang).

   Therefore, frequent small doses are more effective.

2. Prefer chelated zinc.

   Chelated forms (e.g., EDTA-Zn) are more available and stable.

3. Lower soil pH to increase zinc availability.

   Acidic fertilizers or acids via drip irrigation can reduce rhizosphere pH, improving Zn, Ca, and Mg solubility.

4. Use foliar spraying to quickly correct deficiency.

   Field crops can be sprayed with:

   • 0.1% zinc sulfate solution, or

   • 0.3% zinc sulfate + 0.2–0.3% quicklime, or

   • Lime-sulfur mixture + 0.3% zinc sulfate

     Apply at early growth stages once deficiency symptoms appear.

Conclusion

Zinc plays an essential role in maize production. From the jointing stage onward, growers should monitor zinc status and supplement appropriately through drip irrigation and foliar feeding to increase yield and support healthy crop development.