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Physical Properties of Fertilizers

I am writing this article to share with you a general description of the physical properties of fertilizers.

Physical properties of fertilizers are important in processing, handling, transportation, bagging, storage, and application. Several laboratory methods are available to perform physical quality evaluations of commercial and experimental fertilizer products and raw materials.


Hygroscopicity is the degree to which a material will absorb moisture from atmosphere. Hygroscopicity of fertilizers is important when considering conditions under which a bulk pile can be stored, and material flowability during handling and field application. Fertilizers materials vary in their ability to withstand physical deterioration when exposed to humid atmosphere

Critical Relative Humidity

Critical relative humidity (CRH) is that humidity of the atmosphere above which a material will absorb a significant amount of moisture and below which it will not.

For each fertilizer compound or mixture, there is a maximum relative humidity to which the fertilizer can be exposed without absorbing moisture from the air. Determining this value is important when controlled humidity storage areas are being designed for a material. the value is of interested also as an indication of the degree of protection that is likely to be required during handling. In the case of mixtures, the blended materials can have an intolerably low CRH, much lower than the CRH of each component alone.


flowability is the ability of a material to remain flowable under humid conditions. flowability is important when considering the movement of material in conveyor systems and fertilizers applicators.

Chemical Compatibility in Blends

Chemical compatibility in blends is the ability of two or more materials to remain dry, chemically stable, and free flowing when blended together. Incompatibility is observed by wetting, caking, gas evolution, and/or particle disintegration. compatibility of materials is important in any bulk-blending or NPK granulation system.

Physical Compatibility in Blends

Physical compatibility in blends is the ability of two or more materials to remain well mixed during handling, storage and application. Segregation of materials in a bulk blend is normally caused by mismatched particle sizes.

Caking Tendency

Caking is the formation of a coherent mass from individual particles in either bulk or bag storage and is affected by one or more of the following : moisture content, particle size, particle hardness, presence of conditioners, storage temperature, storage time, curing time, material composition. Excessive caking can cause problems in handling and field application.

Particle Size Distribution

Particle Size of fertilizer products and/or raw materials is defined as the particle diameter ranges of the granules. Particle size affects agronomic response, granulation techniques, storage, handling, and blending properties.

Angle of Repose

The angle of repose is the angle at the base of the cone of fertilizer obtained by allowing a sample to fall onto horizontal baseplate. it is of interest when considering storage capacity and the design of hoppers, conveyors, and sloped roofs of bulk storage buildings.

Bulk Density (Loose Pour)

Bulk density (loose) is the mass per unit volume of a material, after it has been poured freely into a container under clearly specific conditions. Bulk density is a measure of the material density, material porosity, and the voids between the particles of the material. Loose pour bulk density is of interest in bag sizing, in calibration of volumetric feeders, and when considering capacity of storage bins and transport vehicles for example.

Bulk Density (Tapped)

Bulk density (tapped) is the mass per unit volume of a material, poured into a container and then compacted under specific conditions. Bulk density is a measure of the material density, material porosity, and the voids between the particles of the material. Tapped bulk density represents the maximum density to which a material might be reduced by vibration during processing or in transport.

Apparent Density

Apparent density is the mass per unit volume of a material. Apparent density is a measure of the material density and material porosity and excludes the voids between the particles. The apparent density of individual granules often is of interest in connection with development of new fertilizer processes. For a given product, variations in granule density can result in variations in hardness, moisture holding capacity, and storage properties.

True Density

True density is the mass per unit volume of a material, excluding voids between particles and all porous space.

Crushing Strength

Crushing strength is the minimum force required to crush individual particles. crushing strength is of interest in estimating the expected handling and storage properties of a granular material and determining the pressure limits applied during bag and bulk storage.

Abrasion Resistance

Abrasion resistance is the resistance to the formation of dust and fines as a result of granule-to-granule and granule-to-equipment contact. It is useful in determining material losses, handling, storage, and application properties, and also pollution control equipment requirement.

Impact Resistance

Impact resistance is the resistance of granules to breakage upon impact against a hard surface. Impact resistance is of interest when material is discharged from an overhead point into a bulk pile, and when bags of material are dropped during handling.


Sphericity is a measure of particle roundness of granular (or prilled) fertilizers.

Coating adherence

Coating is a substance added to a fertilizer to maintain good physical quality during storage and handling. Granular fertilizers vary in surface smoothness and other characteristics that affect the adherence of coatings. also, coating agents themselves vary in adherence properties. to study these variations, coating tests and measurement of adherence are carried out by special procedures.

Specific Surface Area

Specific surface areas of a solid material is the surface are per unit weight and gives an indication of the fineness of a material. It is particularly useful in determining the fineness of phosphogypsum created during phosphoric acid production but can also used for other powders. these data can be used to evaluate reaction efficiencies and filtration rates.


Porosity is a measurement of the pore space within fertilizer granules. External and, for the most part, internal pores can be detected. Pores that have openings to the granule surface are considered external pores, pores that are completely enclosed within a granule are considered internal pores. excessive porosity is often the reason for weak granules. Also, in fertilizer compacting systems, porosity is an indication of the degree of compaction.

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