Fertilizer Particle Size Distribution Testing: Principles, Methods, and Applications

Fertilizer particle size distribution (PSD) testing quantifies the mass fractions of particles within defined size ranges and is a decisive quality parameter for granular, prilled, and powdered fertilizer products. Determined primarily by calibrated sieve analysis in accordance with ISO 2591-1 and EN 1235, accurate PSD results support uniform field spreading, blend stability, dust control, and compliance with national and international product specifications. This article outlines the sample types, testing items, methods, quality controls, and applications of fertilizer particle size distribution determination.

Technical Overview

Particle size distribution (PSD) describes the relative proportions, by mass, of particles falling within specified size intervals in a solid fertilizer. For a granular product, PSD is typically expressed as the percentage of the test portion retained on each sieve of a calibrated nest of test sieves, together with the cumulative percentage passing or retained below or above a stated aperture. The measurement is a fundamental release test in fertilizer manufacturing because particle size directly governs how a product behaves in transport, storage, blending, and field application.

The practical significance of PSD determination includes the following aspects:

• Spreading performance: uniform particle size ensures an even distribution pattern and consistent spread width from disc, pneumatic, and rotary spreaders.
• Blend stability: mismatched size distributions cause nutrient segregation in bulk blends, leading to uneven nutrient delivery across the field.
• Handling and storage: excessive fines generate dust, while broad distributions influence flowability and caking tendency during storage.
• Agronomic performance: particle size affects dissolution rate, nutrient availability, and, for coated products, the integrity of release-controlling coatings.
• Specification compliance: most product standards and commercial contracts define explicit limits for the undersize, oversize, and target size fractions.

The determination is performed using test sieves conforming to ISO 565 and ISO 3310-1 (woven metal wire cloth) or ISO 3310-2 (perforated metal plate), with sieving conducted according to ISO 2591-1 and the fertilizer-specific sieve analysis procedure in EN 1235. For fine and powdered products, instrumental methods such as laser diffraction (ISO 13320) and static or dynamic image analysis (ISO 13322 series) may be applied. In China, corresponding sieve requirements are specified in the GB/T 6003 series, and testing is routinely performed by accredited third-party laboratories.

Testing Samples

Nitrogen Fertilizers

Prilled and granular urea, ammonium sulfate, ammonium nitrate, calcium ammonium nitrate, and ammonium bicarbonate are tested to verify size fractions against product grades. Prilled urea is typically evaluated over a narrower fine-to-medium range, whereas granular urea is assessed over coarser intervals relevant to bulk blending.

Phosphate and Potash Fertilizers

Granular diammonium phosphate (DAP), monoammonium phosphate (MAP), single superphosphate (SSP), triple superphosphate (TSP), granular muriate of potash (MOP), and sulfate of potash (SOP) are analyzed to confirm blend-compatible size distributions, since these materials are the principal components of bulk blends.

Compound, Blend, and Specialty Fertilizers

NPK compound fertilizers, bulk blend (BB) fertilizers, organo-mineral fertilizers, and polymer- or sulfur-coated controlled-release fertilizers are tested both for specification compliance and for coating integrity, because broken or undersized coated particles can release nutrients prematurely.

Powdered and Fine Products

Powdered water-soluble fertilizers, micronutrient carriers, fillers, and raw materials are characterized with finer sieve series or by laser diffraction, where the sub-millimeter fractions control dusting, solubility, and mix homogeneity.

Testing Items

Depending on the product specification and customer requirements, a typical PSD test report includes the following measurable items:

• Mass percentage retained on each sieve aperture of the selected series (for example 8.00, 5.60, 4.75, 4.00, 3.35, 2.80, 2.36, 2.00, 1.70, 1.40, 1.18, 1.00, and 0.50 mm).
• Cumulative percentage passing and cumulative percentage retained at each aperture.
• Target fraction content, such as the percentage between 1.00 mm and 4.75 mm for many granular NPK grades.
• Fines content and oversize content against specification limits.
• Characteristic diameters D10, D50, and D90, and the span of the distribution.
• Size Guide Number (SGN), expressed as the median particle size in millimeters multiplied by 100, which is widely used in the blending trade.
• Uniformity Index (UI), which compares the particle sizes at the fine and coarse ends of the cumulative curve, commonly the 5 % and 95 % points, expressed as a percentage.

Testing Methods

Sample Preparation

Laboratory samples are reduced to representative test portions by riffle splitting or cone-and-quartering, taking care to avoid particle breakage and segregation. Hygroscopic materials such as urea and ammonium nitrate are conditioned in a controlled-humidity environment and tested promptly, or gently dried at low temperature where the method permits, so that surface moisture does not cause particles to agglomerate and distort the fine fractions.

Dry Sieve Analysis

The reference procedure uses a nest of certified test sieves stacked in descending aperture order on a collecting pan. A weighed test portion, commonly 200 g to 500 g depending on top size, is placed on the coarsest sieve and shaken with a mechanical sieve shaker for a validated period, typically 5 to 10 minutes, at a defined amplitude. Each retained fraction and the pan fraction are weighed to 0.1 g. The sum of the fraction masses is compared with the initial mass, and a mass balance within approximately 1 % is required for a valid run. Sieving follows ISO 2591-1 and EN 1235, with sieves selected from the ISO 565 nominal aperture series and verified against ISO 3310-1 or ISO 3310-2, or the corresponding GB/T 6003 requirements.

Air-Jet and Wet Sieving

Air-jet sieving is used for individual fine sieves where blinding and electrostatic adhesion affect dry sieving. Wet sieving is generally avoided for water-soluble fertilizers because the particles dissolve during the operation; where a wet method is justified for agglomerated powders, it is validated with a non-dissolving liquid and is followed by careful drying before weighing.

Laser Diffraction and Image Analysis

Fine powders and specialty products may be characterized by laser diffraction in accordance with ISO 13320, using dry dispersion or a validated liquid carrier compatible with the sample. Static and dynamic image analysis per the ISO 13322 series additionally provide particle shape information, which is valuable for evaluating prill sphericity and coating quality. Instrumental results must be validated against the sieve-based reference method for the specific material.

Data Evaluation and Reporting

Fraction masses are converted to percentages of the total recovered mass, from which cumulative curves, D-values, SGN, and UI are calculated in line with ISO 9276-1. Replicate determinations are performed and results accepted when the difference between major fractions does not exceed the agreed repeatability limit, commonly 1 % absolute. Reports state the sieve series, shaking parameters, conditioning steps, fraction table, cumulative data, and a clear pass or fail conclusion against the applicable specification.

Applications

Particle size distribution determination is applied throughout the fertilizer value chain:

• Production quality control: routine release testing of granulators, prilling towers, and screening circuits to keep the product within specification.
• Blend formulation: matching SGN and UI between urea, DAP, and potash components to prevent nutrient segregation in bulk blends during handling and spreading.
• Spreader calibration: providing the size data needed for accurate vane settings, spread width verification, and uniform field application.
• Trade and customs: independent verification of contractual size specifications, dispute resolution, and certification by accredited third-party laboratories.
• Research and development: evaluating coating processes, anti-caking treatments, dust suppressants, and alternative granulation technologies.
• Storage and logistics studies: assessing dust generation, flow behavior, and caking risk for long-term storage and overseas shipment.

FAQ

Which standards govern fertilizer particle size distribution testing?

Sieve analysis is performed according to ISO 2591-1 and the fertilizer-specific procedure in EN 1235, using test sieves conforming to ISO 565 and ISO 3310-1 or ISO 3310-2 (GB/T 6003 series equivalently). Laser diffraction is conducted per ISO 13320 and result representation per ISO 9276-1. The applicable product standard or contract defines the required size fractions and limits.

How much sample is needed and how long does testing take?

A test portion of roughly 200 g to 500 g is normally sufficient for granular products, and a laboratory sample of 1 kg to 2 kg allows replicate determinations. The sieving itself takes under an hour, and accredited third-party laboratories typically issue a full report within one to three working days.

Why is dry sieving preferred for water-soluble fertilizers?

Most fertilizer salts dissolve rapidly in water, so wet sieving would remove fine particles by dissolution rather than by size and bias the distribution. Dry sieve analysis, supported by air-jet sieving of fine fractions or dry-dispersion laser diffraction, preserves the particle system as it exists in the product.

What are SGN and UI, and why do blenders monitor them?

SGN is the Size Guide Number, the median particle diameter in millimeters multiplied by 100; a median of 3.35 mm corresponds to SGN 335. UI, the Uniformity Index, expresses the spread between the fine and coarse ends of the distribution as a percentage. Blenders match these indices across components because large mismatches cause segregation, resulting in uneven nutrient application across the field.

How is measurement accuracy ensured?

Accuracy is assured through certified and periodically verified sieves, validated shaking time and amplitude, mass balance checks within approximately 1 %, replicate determinations with defined repeatability limits, and participation in interlaboratory comparisons within accredited third-party laboratories.

What are typical acceptance criteria for granular fertilizers?

Acceptance limits depend on the specific product standard and contract. Many granular NPK specifications require at least 90 % of the mass within a range such as 1.00 mm to 4.75 mm, with tight limits on fines below 1 mm and oversize above 5.6 mm, while prilled products are assessed over correspondingly finer ranges.