Fertilizer Caking: Causes, Differentiation, and Solutions

What is Fertilizer Caking?

Fertilizer caking refers to the phenomenon in which granules or powders of fertilizer adhere and fuse together, forming solid lumps. Typically, caking does not alter the fertilizer’s chemical composition or nutrient content. However, it creates difficulties in application, reducing efficiency and causing uneven distribution, which may hinder crop growth. In modern agriculture—where mechanical application is standard—fertilization demands high efficiency and precision. Even mild caking can directly affect performance, and thus tolerance is much lower. In Japan, the presence or absence of caking is considered a key quality indicator, and caking-related returns or disposals are common.

Causes of Fertilizer Caking

Internal Factors

1. Moisture Within the Fertilizer

   The primary cause of caking includes:

   • Free moisture: adsorbed by capillary or diffusion action on particle surfaces

   • Crystalline water: structural moisture formed during chemical reactions within fertilizer compounds

     
     

2. Raw Material Composition

   High-nitrogen fertilizers based on nitrate or urea are more prone to caking than sulfate- or phosphate-based products. Fertilizers containing ferrous ions are less prone to caking. Materials like urea and ammonium nitrate are highly hygroscopic, and when mixed, they absorb moisture and liquefy, leading to crystal bridge formation and recrystallization during storage.

   
   

3. Physical Properties

   Fertilizers with small particle sizes and high surface area tend to cake more easily. Products with high granule strength and smooth surfaces are less likely to cake.

Caking Tendency Reference

• Straight fertilizers:

  Ammonium nitrate > Urea > Ammonium chloride > Ammonium sulfate > Diammonium phosphate > Potassium chloride > Single superphosphate = Triple superphosphate > Potassium sulfate

• Compound fertilizers:

  High-concentration NPK (N+P+K ≥ 30%) > Standard NPK (<30%)

• Nitrogen type:

  Nitrate-based > Urea-based > Ammonium chloride/sulfate-based

• Granule shape:

  Fine particles > coarse; rough/irregular > smooth/rounded

External Factors

1. Air Humidity

   High humidity increases the water content of fertilizer particles, lowers vapor pressure, and enhances capillary condensation, leading to caking.

   
   

2. Temperature Fluctuations

   Storage temperature variations trigger dissolution–crystallization cycles of salts, promoting crystal bridges. High temperatures lower the critical relative humidity and increase hygroscopicity.

   
   

3. Storage Pressure & Time

   Excessive stacking increases contact area and deformation risk, accelerating caking. Longer storage increases the frequency of recrystallization, strengthening the tendency to cake.

Impact & Case Studies

Caking affects not only fertilizer efficiency but also storage, transport, and application. For instance, ammonium bicarbonate stored at 26–28 °C for 10 days can lose up to 75.4% of its nitrogen content. During mechanical spreading, caked fertilizers often clog nozzles, drastically reducing application efficiency.

False Caking Exists

Not all caking is irreversible. Some clumps are physical and temporary—known as false caking.

Definition

False caking refers to loosely aggregated clumps formed via static electricity, temporary moisture, or light pressure. These can easily be broken apart by hand or vibration without tools.

Main Causes

1. Static Electricity

   Fine or small-particle fertilizers (e.g. MAP) generate static charge during transport, causing particles to cluster.

   
   

2. Short-Term Moisture Exposure

   Temporary increases in humidity (e.g., rainy season) cause surface moisture adsorption, leading to loose adhesion.

   
   

3. Mild Compression

   Overfilling or vibrations during transport cause slight compression, leading to loosely bound clusters (e.g., top layer of stacked bags).

   
   

Characteristics of False Caking

• Low hardness: breaks easily by hand

• Reversible: disperses with ventilation or vibration

• Low impact: does not hinder machine use or nutrient distribution

Handling Tips

• No special treatment required: simply break apart by hand or tool

• Improve storage: move to dry area if temporary moisture is involved to avoid transformation into true caking

  
  

Quick Identification Table

Category	True Caking	False Caking
Appearance	Hard blocks, tight adhesion	Loose clusters, visible particle gaps
Texture	Cannot break by hand	Breaks apart with light pressure
Response to drying	Remains hard	Disperses after drying or shaking
Particle condition	Rough surface, may show crystals	Smooth, intact granules

Solutions to Prevent True Caking

Process Optimization

1. Formula Design

   Select raw materials with higher critical relative humidity and reduce system hygroscopicity after blending.

   
   

2. Moisture Control

   • Standard NPK: ≤1.5% moisture

   • High-N (urea/ammonium nitrate-based): ≤1.0%

   • Ideal: ≤0.5%

     
     

3. Granulation

   Ensure uniform particle size, smooth surface; remove fines and undersized particles.

   
   

4. Packaging & Storage

   Use low-permeability bags, degassed packaging, limit stack height, avoid prolonged outdoor storage.

Use of Anti-Caking Agents

Mechanism of Action

• Crystal Phase Inhibition: prevents solid bridging due to recrystallization

• Moisture Management: absorbs external moisture or inhibits internal water release

• Surface Coating: forms a barrier film to prevent adhesion

Product Types

Type	Application Scenarios	Typical Products
Specialty Anti-Caking Agents	Straight fertilizers (e.g. phosphate, nitrogen)	Phosphate-specific anti-caking agent
Compound Fertilizer Agents	High-N, nitrate-rich NPK	High-nitrogen NPK anti-caking agent
Water-Soluble Fertilizer Agents	Drip & foliar application fertilizers	Anhydrous magnesium sulfate, ammonium sulfate, boric acid

Recommended Raw Materials

Inorganic anti-caking agents such as magnesium sulfate, boric acid, etc., are widely used due to their availability, low cost, good compatibility, and environmental safety. For example, at 30 °C, the hygroscopic point of pure ammonium nitrate is 59.4%; by forming a double salt with ammonium sulfate, this increases to 62.3%.

Application Method

Add 0.5%–2% by weight uniformly using mixers or frequency-controlled screw feeders. Suitable for powdered water-soluble fertilizers and standard NPK.

Our Products

Our company has years of experience in anti-caking solutions. We offer high-purity, low-impurity anti-caking agents including:

• Anhydrous Magnesium Sulfate: Strong moisture absorption; forms an isolating film to suppress crystal bridge formation

• Ammonium Sulfate: Excellent compatibility with ammonium fertilizers; double salt formation enhances stability

• Boric Acid: Dual function of crystal phase control and drying effect; ideal for formulations sensitive to metal ions

We maintain a rigorous quality control system and offer customizable particle size and packaging solutions to support better fertilizer quality and market competitiveness.