Foliar Fertilizer Application Technology: Precision Application Guide for Modern Agriculture

In the agricultural production system, fertilizers are like the "food" for crops, serving as the core elements to ensure crop growth, yield, and quality. From traditional farming to modern agriculture, fertilization technology has always been a key link in agricultural techniques — the balanced supply of soil nutrients can meet the needs of crops at different growth stages, and scientific fertilizer application can effectively enhance crop stress resistance and reduce the occurrence of diseases and pests. 

However, with the improvement of intensive land use, problems such as soil compaction and nutrient imbalance have become increasingly prominent, and the limitations of traditional root fertilization have gradually emerged, prompting agricultural production to continuously explore more efficient fertilization methods. 

As a new type of fertilizer absorbed directly through leaves, foliar fertilizer has become an important supplement to root fertilization in modern agriculture. Compared with traditional fertilization, the significance of foliar fertilizer lies not only in "efficiency" — directly absorbing nutrients through leaf stomata and cuticles — but also in "precision" — it can quickly supplement specific nutrients for crop nutrient deficiency symptoms. In addition, when the soil environment is harsh (such as saline-alkali land, acidified soil) or crop roots are damaged (such as waterlogging, diseases, and pests), foliar fertilizer becomes a "first-aid measure" to maintain crop growth. 

Despite the significant advantages of foliar fertilizers, their effectiveness highly depends on the scientific application technology. In actual production, phenomena such as leaf burning, flower and fruit drop, or poor effects caused by improper application timing, excessive concentration, or wrong methods are not uncommon. Therefore, systematic mastery of foliar fertilizer application technology is the key to exerting its role in yield increase and quality improvement. This article will comprehensively analyze the core technologies of foliar fertilizer application from basic principles to practical key points, providing scientific guidance for agricultural producers.

Basic Concepts of Foliar Fertilizers

I. Definition of Foliar Fertilizers

Foliar fertilizer refers to a type of fertilizer applied by spraying on the surface of crop leaves to supplement nutrients required for plant growth. Different from traditional root fertilization, it breaks through the limitations of the soil environment on nutrient absorption, using leaves as a "nutrient inlet" to directly transport nutrients into plants through cuticle penetration and stomatal absorption. With the increasing demand for precision fertilization in modern agriculture, foliar fertilizers have developed into an independent fertilizer system comprising various nutritional types and functional characteristics.

II. Main Component Classification of Foliar Fertilizers

(1) Macronutrient Foliar Fertilizers

Taking nitrogen (N), phosphorus (P), and potassium (K) as the main components, they are used to quickly supplement the basic nutrients required for crop growth.

For example:

• High-nitrogen foliar fertilizers: Containing urea, ammonium nitrogen, etc., suitable for promoting leaf growth during the seedling stage of crops. For instance, spraying during the tillering stage of rice can increase the number of effective leaves.

• High-phosphorus foliar fertilizers: With potassium dihydrogen phosphate and ammonium polyphosphate as the core, application during the flowering period can promote flower bud differentiation, improving the fruit setting rate in crops such as tomatoes and strawberries.

• High-potassium foliar fertilizers: Often containing potassium nitrate and potassium sulfate, spraying during the fruit expansion period can accelerate sugar accumulation.

(2) Medium and Trace Element Foliar Fertilizers

Designed for nutrient deficiency symptoms of medium and trace elements such as calcium (Ca), magnesium (Mg), boron (B), zinc (Zn), and iron (Fe):

• Calcium-magnesium type: orbitol calcium, chelated magnesium, etc., which can prevent blossom end rot in tomatoes and bitter pit in apples, while enhancing leaf photosynthesis.

• Boron-zinc type: Compound of boric acid and zinc sulfate, spraying during the flowering period can promote pollen tube elongation and reduce the phenomenon of "flowering without fruiting" in citrus.

• Iron-manganese type: EDTA chelated iron and manganese sulfate, used to correct leaf chlorosis in crops grown in alkaline soils.

(3) Organic Foliar Fertilizers

Made from natural organic materials such as amino acids, humic acids, and seaweed extracts:

• Amino acid foliar fertilizers: Hydrolyzed from animal and plant proteins, containing 18 kinds of amino acids. Spraying in low-temperature environments can enhance crop cold resistance, such as application to greenhouse vegetables in winter.

• Humic acid foliar fertilizers: Extracted from weathered coal or lignite, which can improve leaf cell membrane permeability and reduce water evaporation when sprayed in arid areas.

• Seaweed foliar fertilizers: Rich in seaweed polysaccharides and auxins, which can promote crop root development and are often used during the fruit expansion period of fruit trees to improve fruit quality.

(4) Other Functional Foliar Fertilizers

Composite products with both nutrient supplementation and special functions:

• Stress-resistant foliar fertilizers: Added with components such as abscisic acid and salicylic acid. Spraying after disasters like drought or flood can reduce crop transpiration and promote post-disaster recovery

.• Regulating foliar fertilizers: Containing plant growth regulators (such as brassinolide), which can control excessive growth when compounded with foliar fertilizers. For example, spraying during the bud stage of cotton can reduce the growth of unnecessary buds.

• Biostimulant foliar fertilizers: Incorporated with microbial metabolites, which can activate leaf enzyme activity and enhance the crop's nutrient absorption efficiency.

III. Characteristics of Foliar Fertilizers

(1) Rapid Absorption

The absorption rate of nutrients by leaves is far higher than that by roots. Taking potassium fertilizer as an example, it takes 7-10 days for roots to transport it to leaves after fertilization, while potassium ions can be detected in mesophyll cells within 2-4 hours after foliar spraying. This rapid absorption characteristic makes foliar fertilizers particularly important for emergency treatment of crop nutrient deficiencies.

(2) Direct Action

Nutrients enter the plant's metabolic system directly through the leaves, avoiding losses such as soil fixation and leaching. This "directness" is especially suitable for use in saline-alkali soils. When the soil pH is too high, causing elements like iron and zinc to be fixed, foliar fertilization can break through soil barriers and directly meet crop needs.

(3) Low Dosage and High Efficiency

Taking Kelewell's product—potassium dihydrogen phosphate as an example, the dosage for foliar spraying is only 100-200 grams per mu, equivalent to 1/5 to 1/10 of the root fertilization amount, but it can achieve a similar yield increase effect. The input-output ratio is significantly higher than traditional fertilization.

(4) Strong Targetedness

Nutrient requirements can be precisely matched according to the crop growth stage: focusing on nitrogen fertilizer for growth during the seedling stage, boron fertilizer for pollination during the flowering period, and potassium fertilizer for ripening during the fruit expansion period. This "targeted" fertilization greatly reduces nutrient waste and the risk of soil pollution.

Principles of Foliar Fertilizer Action

As the name suggests, foliar fertilizers act on leaf surfaces. Let's first understand the structure of plant leaves and how nutrients are absorbed by plants through the foliage.

I. Structure and Absorptive Function of Plant Leaves

(1) The "Nutrient Channel" Role of Stomata

Stomata distributed on the leaf surface serve as the main gateway for foliar fertilizer absorption — each square millimeter of leaf contains approximately 50-500 stomata, and their opening and closing states directly affect nutrient absorption efficiency. Stomata are surrounded by two guard cells. When the environmental humidity is suitable (60%-80%) and the temperature is 20-28℃, stomata open to the maximum extent. Spraying foliar fertilizers at this time allows nutrients to quickly enter mesophyll cells through stomata.

(2) Permeability Mechanism of the Cuticle

In addition to stomata, foliar fertilizers can also be absorbed through micro-pores in the leaf cuticle. The cuticle is composed of lipid substances, which have a low permeability to water-soluble nutrients but absorb lipid-soluble substances (such as chelated trace elements and amino acid-based fertilizers) more smoothly. Studies have found that foliar fertilizers with added surfactants can reduce solution surface tension, expand cuticle pores, and enhance nutrient permeability.

II. Absorption and Transport Mechanisms of Foliar Fertilizers

(1) Transfer from Leaf Surface to Cell Interior

After foliar fertilizer is sprayed, it first forms a liquid film on the leaf surface. As water evaporates, nutrient concentration increases, entering cells through the following pathways:

• Stomatal absorption pathway: Water-soluble ions (such as potassium ions and nitrate ions) diffuse with water through stomata into mesophyll cell gaps, then enter the cytoplasm through the cell wall.

• Cuticle penetration pathway: Lipid-soluble molecules (such as humic acid and alginic acid) directly penetrate the lipid bilayer of the cuticle to enter epidermal cells.

Taking potassium dihydrogen phosphate as an example, within 30 minutes after spraying, phosphate ions can enter mesophyll cells through stomata; obvious accumulation is detected in leaf vein vascular bundles after 6 hours, and transportation to the vigorous apical meristem is completed within 24 hours.

(2) Conduction Pathways in Plants

Nutrients entering the leaves are transported throughout the plant via the following systems:

• Symplastic transport: Nutrients are transferred within living cells through plasmodesmata between cells, mainly transporting small organic molecules (such as amino acids and sucrose).

• Apoplastic transport: Transport occurs through xylem and phloem vessels in cell walls and intercellular spaces, serving as the main pathway for mineral elements (such as nitrogen, phosphorus, and potassium).

Notably, the mobility of different elements varies: Potassium, nitrogen, etc., can be bidirectionally transported in the phloem, moving from old leaves to new ones; while calcium, iron, etc., have poor mobility in the phloem and mainly rely on unidirectional transport in the xylem. Therefore, when calcium is deficient, symptoms first appear in new leaves, and direct spraying of calcium fertilizer on new leaves is required for effective relief.

III. Synergy and Complementarity between Foliar and Root Fertilization

(1) Functional Synergy: Building a Three-Dimensional Nutritional Network

Root fertilization acts as a "basic supply," providing continuous and stable nutrient sources for crops, while foliar fertilization serves as "precise supplementation," forming a complementarity:

• Long-acting nature of root fertilization: After being absorbed by roots, organic fertilizers, compound fertilizers, etc., can slowly release in the soil to meet the basic nutritional needs of crops throughout the growth period.

• Emergency response of foliar fertilization: During the critical nutrient demand period of crops (such as the booting stage of rice and the fruit expansion period of fruit trees), foliar fertilizers can quickly supplement peak-demand nutrients, avoiding yield reduction caused by delayed root absorption.

(2) Environmental Complementarity: Breaking through Soil Limitations

When the soil environment is unfavorable for root absorption, foliar fertilizers can exert unique advantages:

• Saline-alkali soil environment: High soil pH fixes elements like iron and zinc, reducing root absorption efficiency. Foliar spraying of chelated micronutrients can bypass soil barriers to directly meet crop needs.

• Damaged roots: When root absorption function is weakened due to waterlogging, diseases, pests, or pesticide damage, foliar spraying of amino acid foliar fertilizers can maintain partial nutrient requirements of the plant, buying time for root recovery.

(3) Efficiency Optimization: Reducing Nutrient Loss

Combining the two can mitigate the drawbacks of single fertilization methods:

• Root fertilization is prone to loss due to leaching and fixation, while foliar fertilizer nutrients reach leaves directly, significantly improving utilization efficiency.

• Foliar fertilization has a short single-application duration and needs to be combined with root fertilization to achieve continuous nutrient supply.

Through this synergistic mechanism, modern agriculture is gradually constructing a three-dimensional fertilization system of "root foundation + foliar enhancement," maximizing nutrient use efficiency while ensuring yield.

Key Techniques for Foliar Fertilizer Application

I. Timing of Application

1. Critical Growth Stages of Crops

• Seedling Stage: During early seedling development, root systems are not yet well-formed. Foliar application of fertilizers containing amino acids or humic acids can promote root differentiation.

  
  

• Flowering Stage: This stage sees a spike in demand for micronutrients like boron and zinc. Spraying compound foliar fertilizers with boron and zinc 7–10 days before flowering can enhance pollination rates.

• Fruit Set Stage: It is crucial to supplement secondary nutrients such as calcium and magnesium to reduce physiological disorders.

• Fruit Expansion Stage: High-potassium foliar fertilizers are essential. For instance, spraying monopotassium phosphate (MKP) on grapes during the fruit expansion phase can increase sugar content by 1.5–2 degrees Brix.

• Maturity Stage: Spraying calcium fertilizers about 20 days before harvest can improve fruit firmness and extend storage life.

2. Optimal Environmental Conditions

• Temperature: The optimal temperature for foliar application is 15–28°C. High temperatures (>30°C) can cause the spray to evaporate rapidly, increasing concentration and causing leaf burn. Low temperatures (<10°C) reduce leaf physiological activity and nutrient uptake efficiency.

• Light Conditions: Avoid spraying under intense sunlight. The ideal time is in the morning (9–10 AM) after dew has dried, or in the evening (4–6 PM) when light intensity is moderate and stomata are open.

• Humidity: The best range is 60%–80% relative humidity. During dry conditions, pre-wetting with water can enhance absorption efficiency.

II. Selection of Fertilizer Types

Based on Crop Type

• Grain Crops: For rice and wheat, use balanced N-P-K foliar fertilizers. Apply MKP during the panicle initiation stage.

• Cash Crops: For cotton, apply boron + potassium fertilizers during the bud and boll stage to reduce boll drop. For tea, foliar application of amino acid fertilizers during the bud stage increases amino acid content in fresh leaves.

• Fruit Trees: For citrus, apply high-nitrogen fertilizers during spring flush to promote shoot growth, and high-potassium fertilizers during fruit expansion. Spraying potassium phosphite on grapes during color change enhances anthocyanin synthesis.

• Vegetables: Leafy vegetables require high-nitrogen fertilizers during growth, while fruiting vegetables benefit from potassium and calcium fertilizers during the fruit expansion phase.

• Flowers: For foliage plants, use nitrogen-magnesium fertilizers during growth. For flowering plants, apply phosphorus-potassium fertilizers before blooming—for example, spraying MKP on roses during the bud stage promotes flowering.

III. Application Methods

1. Determining Spray Concentration

• Macronutrient Fertilizers:

  
  

  • Urea: 0.5%–2% (0.5%–1% during seedling stage, 1%–2% for mature plants)

    
    

  • MKP: 0.2%–0.5% (lower range during flowering, higher range during fruit expansion)

  
  

• Secondary and Micronutrient Fertilizers:

  
  

  • Calcium (e.g., calcium sugar alcohol): 1%–2%

  
  

  • Boron (e.g., boric acid): 0.1%–0.2%

  
  

  • Iron (e.g., EDTA chelated iron): 0.05%–0.1%

  
  

2. Target Spray Areas

• Leaf Surfaces: The underside of leaves has 30%–50% more stomata than the upper side, making it more effective for absorption. Prioritize spraying the undersides.

  
  

• Young vs. Old Leaves: Young leaves have high metabolic activity and strong absorption but smaller surface area. Older leaves have thicker cuticles and lower absorption efficiency. During active growth stages, focus on young leaves (e.g., citrus spring shoots); in later stages, include older leaves to increase photosynthetic product accumulation (e.g., spraying rice flag leaves during grain filling).

• Fruits and Other Organs: Some foliar fertilizers can be sprayed directly on fruit surfaces. For example, spraying anthocyanin promoters on grape clusters during color change can improve color uniformity by up to 40%. Applying calcium fertilizers directly to tomato fruits during expansion can reduce blossom-end rot.

3. Frequency of Application

• By Crop Type:

  
  

  • Leafy Vegetables: With short growth cycles, 2–3 applications suffice. For example, spray once during the seedling stage and once during peak growth.

    
    

  • Fruit Trees: Require 4–6 applications throughout the growth cycle. For apples, apply before and after flowering, during fruit expansion, and before harvest.

    
    

• Interval Between Applications:

  
  

  • Most foliar fertilizers: every 7–10 days (e.g., spray MKP every 7 days during cucumber fruiting).

    
    

  • Micronutrient fertilizers: every 15–20 days (e.g., spray boron on grapes once per month).

Precautions for Foliar Fertilizer Application

I. Climatic Considerations

1. Avoid Spraying During High Temperature and Intense Sunlight

When temperatures exceed 30°C, water in the fertilizer solution evaporates quickly, increasing the concentration and risking leaf burn. The optimal spraying time is 9–10 AM or 5–7 PM, when leaf surface temperatures are 8–10°C lower than at noon. This prolongs the retention time of the spray to 2–3 hours and increases nutrient absorption efficiency by over 40%.

2. Avoid Spraying Before Rain

Spraying foliar fertilizers 1–2 hours before rainfall is strictly discouraged, as rainwater will wash away nutrients from the leaf surface. If unexpected rain occurs, a supplementary spray should be applied 2–3 days after the rain, using half the original dosage.During prolonged overcast and rainy periods, high leaf moisture levels may cause fertilizer solutions to mold. In such cases, foliar application should be suspended and replaced with soil-based fertilization.

3. Avoid Spraying During Low Temperatures

When the temperature falls below 10°C, leaf cell activity declines and stomata tend to close, leading to poor nutrient absorption. Moreover, prolonged retention of fertilizer on leaves may encourage bacterial diseases.

II. Compatibility of Fertilizer Mixtures

1. Compatibility Between Different Foliar Fertilizers

• Avoid Mixing Acidic and Alkaline Fertilizers: Mixing monopotassium phosphate (acidic) with calcium-magnesium fertilizers (alkaline) results in calcium phosphate precipitates, rendering the nutrients ineffective and potentially clogging the spray nozzle.

• Cation-Anion Antagonism: For example, mixing zinc fertilizers (cations) with phosphate fertilizers (anions) can lead to zinc phosphate precipitation, reducing the availability of both nutrients. The correct approach is to apply them separately with a 5–7 day interval, or use chelated zinc (e.g., EDTA-Zn), which can be mixed with phosphates without significant antagonism.

2. Principles of Mixing Foliar Fertilizers with Pesticides

• Match pH Levels: Foliar fertilizers and pesticides should have similar pH values. For instance, imidacloprid (acidic) can be mixed with monopotassium phosphate (acidic), whereas Bordeaux mixture (alkaline) must not be mixed with any acidic foliar fertilizer.

• Mixing Sequence: Always dilute the foliar fertilizer first, then add pesticides in the following order: water → foliar fertilizer → wettable powders → suspensions → emulsifiable concentrates → adjuvants.

• Incorrect Combinations to Avoid: Mixing copper hydroxide with magnesium-containing foliar fertilizers may form magnesium hydroxide precipitates; mixing lime sulfur with any nitrogen-containing foliar fertilizer can release ammonia gas and cause leaf burn.

3. Hazards of Improper Mixing

• Reduced Fertilizer/Pesticide Effectiveness: Mixing urea with Bordeaux mixture causes copper ions to bind with amide groups in urea, lowering both the fungicidal effect and nitrogen utilization efficiency.

• Phytotoxicity: Mixing emulsifiable pesticide formulations with foliar fertilizers at high concentrations can damage the leaf cuticle. For example, a farmer mixing foliar fertilizer with organophosphate pesticides at recommended dosages caused large oil-like spots on cotton leaves due to cuticle damage.

III. Common Misconceptions

1. Foliar Fertilizer Cannot Replace Soil Fertilization

Some farmers believe foliar fertilizers act quickly and thus abandon soil fertilization. In reality, foliar fertilizers have a short duration of effect (7–10 days) and are applied at only 1/10 the dosage of soil fertilizers per hectare, which cannot meet the crop’s full-season nutritional needs. The correct approach combines base fertilizer + topdressing + foliar spray.

2. Higher Concentration Does Not Mean Better Results

Excessive concentrations can lead to fertilizer damage. Different crops have different tolerance thresholds, so application concentrations must be adjusted accordingly.

3. Not All Growth Stages Are Suitable for Foliar Application

Crops are particularly sensitive to foliar fertilizers during flowering and early fruit stages. Improper use during these stages can lead to flower or fruit drop.

4. Importance of Spray Targeting

Spraying only the upper leaf surface overlooks the fact that the underside absorbs nutrients more efficiently. During application, aim the nozzle upward to spray from below, ensuring the underside of the leaf is evenly covered.

Conclusion

As the core technology of precision fertilization in modern agriculture, foliar fertilizer has effectively broken through the bottleneck of traditional root fertilization being restricted by the soil environment with its characteristics of "direct nutrient delivery, high efficiency and controllability", especially showing unique advantages in the key growth period of crops and adverse environments.

From the perspective of technical practice, its effectiveness must strictly follow the "three precisions" principle: accurately grasp the spraying period, accurately control the concentration ratio, and accurately locate the action site. Improve nutrient utilization, reduce the risk of soil salinization, and provide core technical support for "weight reduction and efficiency increase, green agriculture". For producers, mastering foliar fertilizer application technology is not only an inevitable choice to deal with the degradation of cultivated land quality, but also a key springboard to the new agricultural era.