Physical properties of granular fertilisers

There have been many developments in the field of fertilisers over the years.
In general, the quality has improved considerably. Fertilisers have improved not only in terms of chemical composition but also in terms of physical properties.
The physical properties of fertilisers largely determine the behaviour of the granules when spread. The larger the spread pattern, the greater the impact of the physical properties. Several physical properties are important with regard to fertilisers.

Fraction composition

The fraction composition of the fertiliser indicates the spread in granule size.
In a “homogeneous” composition, the size of each granule is the same. In that case, the fertiliser is uniform. This makes the flow of fertiliser constant. In a “homogeneous” composition, the granule travels a path that the spreader manufacturer believes is required to achieve a good spreading pattern.

Fertiliser consisting of many large and small granules is referred to as “heterogeneous”.
A wide range of fractionation results in segregation. In this case, the flow is not constant, so smaller granules leave the spreader first. This means that small granules are not spread as far as large granules.
If the variation in granule size is too large or too small, the distribution width is not optimal. This means that the granules are not distributed evenly across the plot.
The fraction distribution of fertilisers can be determined using a sieve fraction tester.


Hardness is an essential property of a fertiliser granule.
In practice, this gives rise to a lot of problems. The harder the granules are, the less likely they are to be broken or pulverised by vanes. Granules that are too soft may snap on contact with the vane due to the considerable force it exerts. In addition, soft granules lose their shape quality during transport and storage. Broken granules create dust and cause poor fractionation, leading to an unpredictable spreading pattern. Hardness is particularly important when using a large working width. Due to the high speed at which the discs rotate, the risk of broken granules is higher than with a smaller working width.

Granule hardness is measured with a hardness tester, which exerts force on the granule. The measure of hardness is the force in kilograms at which the granule breaks. A range of readings is displayed on the hardness tester.
If necessary, contact the fertiliser manufacturer/supplier to check the measured value.

Bulk weight

Density affects the application rate and spreading path.
At the same setting and hardness, a higher-density fertiliser will be spread further than a lower-density fertiliser. This is because a high-density fertiliser has a higher flow rate. The density of a fertiliser should not vary. A large working width requires a fertiliser with a high bulk weight to achieve a correct spreading pattern.

The density of a fertiliser is measured as bulk weight.
The bulk weight is expressed in kg/l and can be measured by filling a measuring cup of exactly one litre capacity with the mineral fertiliser. The weight of the contents constitutes the bulk weight.


A fertiliser that is hygroscopic readily absorbs moisture. Fertilisers very sensitive to hygroscopicity absorb moisture in all conditions, while slightly hygroscopic fertilisers absorb moisture only at high soil or air humidity. Highly hygroscopic fertilisers have poor flow, which can itself cause blockages in the spreader. Hygroscopic fertilisers pose a high risk of clod formation.
Clod formation is caused by granules sticking together during the distribution and storage of fertilisers. This happens under the influence of moisture, temperature, pressure and post-production chemical reactions. The clods can cause problems with spreading if they do not readily disintegrate.
The risk of clod formation can be reduced by coating the granules and reducing moisture and dust levels during careful storage.

Average granule size

The granule size determines the spreading properties. The larger the granule, the further and more accurately the fertiliser can be spread. At the same density and settings, larger granules will be spread further away than smaller ones. With too many large granules in over-fertilisation, there is a risk of the granules causing damage to the crop.

Granule shape

The shape of the fertiliser affects flow. The smoother and rounder the surface of the granules, the less friction there is between them. As a result, the flow of smooth and round granules is higher than for granules with a rough and angular surface. The perfect shape for movement through the air is the spherical shape. The rounder and smoother the granules, the lower the resistance to the air. And the better the spreading path. The lower the air resistance, the better the spreading properties. Angular and rough-shaped granules exhibit slower and anomalous spreading behaviour. Fertilisers with the same fraction distribution, density and hardness have different spreading patterns when they are different shapes.

Flow factor

The flow factor is the time it takes for a sample to flow through a standardised funnel. This indicates how easily the granules move under the influence of gravity . This factor is determined by the density, average granule size, homogeneity and shape of the fertiliser. The flow behaviour of the fertiliser is important for dosing, but also plays a part in spreading the granules.
With a wide fraction distribution, segregation can occur in the flow, as large granules roll to the outside edge more easily than the smaller granules.

Wear resistance

Friction and impact can cause wear on the granules during distribution and handling. Wear resistance depends on the surface texture and strength of the granules. So never drop a big-bag too hard on the ground

Dust content

The amount of dust (dust content) can affect the spreading process. The build-up of dust in fertilisers has a negative impact on spreading properties. Moreover, it is bad for the environment and working conditions during storage. Dust increases the likelihood of clod formation, causing areas along the rut to be over-fertilised. Dust content is measured by separating the dust from granules and then weighing it.

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