1. Core Position of Nitrogen
Nitrogen (N) is the primary macronutrient required by plants in the largest quantity, collectively known as the “Three Primary Nutrients” together with phosphorus and potassium.
Core Status: It accounts for 1% to 5% of a plant’s dry weight and serves as a fundamental component of cellular protoplasm and cell walls.
Basic Property: Hailed as the “life element”, it participates in the synthesis of over 90% of organic compounds within plants.
Key Functions: It acts throughout the entire plant life cycle—from seed germination and seedling development to flower bud differentiation and fruit expansion, directly determining crop yield and quality.
2. Building Block of Core Biological Macromolecules
Protein Synthesis: Nitrogen forms the backbone of amino acids. Enzymes (core regulators of metabolism), structural proteins (cell support frameworks), and transport proteins (nutrient transporters) all rely on nitrogen as their basic constituent.
Nucleic Acids & Genetic Material: Nitrogen is an essential component of DNA and RNA (found in purine and pyrimidine rings). It regulates gene expression and cell division; nitrogen deficiency suppresses cell division and results in stunted plants.
Chlorophyll Synthesis: Both chlorophyll a and chlorophyll b contain nitrogen. Nitrogen functions as the “light-harvesting carrier” for photosynthesis. Insufficient nitrogen reduces photosynthetic efficiency and triggers leaf yellowing.
3. Regulation of Physiological Metabolism & Growth Development
Energy Metabolism: It participates in the production of ATP and NADPH, supplying energy for cell division and substance transformation.
Phytohormone Synthesis: Boosts the generation of auxin (IAA) and gibberellin (GA3). Sufficient nitrogen promotes bud differentiation, while low nitrogen levels easily cause flower and fruit drop.
Substance Conversion: Accelerates the transformation and accumulation of carbohydrates, proteins and lipids. During fruit swelling, nitrogen facilitates the translocation of photosynthates to fruits.
4. Absorption & Metabolic Mechanisms
Absorbable Forms: Plant roots mainly take up nitrate nitrogen (NO₃⁻) and ammonium nitrogen (NH₄⁺), which account for more than 90% of total nitrogen uptake.
Environmental Influencing Factors:
- Temperature: Absorption efficiency peaks at 15–25°C.
- pH Value: Neutral soil (pH 6.0–7.0) delivers optimal absorption. Acidic conditions inhibit nitrate uptake, while alkaline environments suppress ammonium absorption.
- Soil Aeration: Adequate air circulation boosts root respiration and nitrogen absorption efficiency.
5. Symptoms & Identification of Nitrogen Deficiency
Visible Morphological Symptoms:
- Whole Plant: Stunted growth, thin weak stems, reduced branching.
- Leaves: Older leaves turn yellow first, with chlorosis gradually spreading upward; severe deficiency leads to leaf withering and shedding.
- Fruits: Low fruit set, undersized fruits, poor coloration, low sugar content and inferior quality. Physiological Impacts: Photosynthetic efficiency drops by 30%–50%, and plant stress resistance (drought, cold, pest & disease tolerance) is drastically weakened.
6. Hazards of Excessive Nitrogen
Visible Morphological Symptoms:
- Whole Plant: Excessive vegetative growth, soft fragile stems and elongated internodes (e.g., peach tree internodes longer than 5 cm).
- Leaves: Dark green, overgrown thick foliage that blocks ventilation and light penetration, making crops susceptible to powdery mildew, downy mildew and other fungal diseases.
- Fruits: Severe flower and fruit drop, delayed ripening by 7–15 days, uneven fruit pigmentation and poor storage tolerance. Physiological & Ecological Impacts:
- Creates nutrient antagonism that inhibits the uptake of phosphorus, potassium, calcium and magnesium.
- Degrades soil health; unabsorbed nitrogen easily leaches into groundwater or volatilizes into the air, triggering environmental pollution.
7. Scientific Principles for Nitrogen Fertilizer Application
- Apply according to crop demand: Adjust dosage based on crop varieties and growth stages.
- Balanced nutrient matching: Combine nitrogen with phosphorus, potassium and trace elements.
- Adapt application methods to conditions:
- Deep placement with soil covering (15–20 cm depth): Reduces nitrogen volatilization loss; surface broadcasting can cause over 30% nutrient wastage.
- Split topdressing: Avoid one-time heavy application; combine base fertilizer and supplementary topdressing.
- Water & fertilizer integration: Drip irrigation or sprinkler fertigation improves nutrient utilization efficiency by 20%–30% compared to manual broadcasting.
- Match application to environment: Reduce nitrogen input under low temperatures or rainy seasons; nitrate nitrogen suits acidic soils, while ammonium nitrogen fits alkaline soils.
