Fertilizer quality – which parameters really matter?

Such high accelerations put severe mechanical strain on the granules. Fertilizers that do not withstand the extreme conditions of centrifugal spreaders will compromise yield and crop quality. The physical characteristics of fertilizer have a decisive impact on spreading performance and precision. Which are the physical characteristics that affect spreading?

9 Key Parameters for optimum spreadability

Fertilizer spreading performance depends on a number of parameters that fertilizer manufacturers need to control closely.

1. Density

For the same diameter, the densest particles are projected at a wider distance. Density therefore affects application rate and working width. The density of a fertilizer shall be known and shall not vary. Density is measured as bulk density. It represents the mass of product contained in a given volume and includes the air between the granules. The higher the density, the better the spreading.

2. Size

Size is measured by two parameters:

• The mean size should be above 3,2 mm. For identical density, bigger particles will be projected farther as they are heavier.
• The size distribution (granulometric spread) indicates the variation of diameter between the smallest and biggest granules. If the variation is too big or too low, the distribution width is not optimal. A variation of 0,8 mm is recommended.

3. Shape

The more the granules are perfectly rounded and smooth, the better the aerodynamics and fertilizer spreading trajectory. Aerodynamic resistance is measured as the Cx value.

4. Hardness

The harder the granules, the less likely they crush during handling and under the impact of spreader blades. Crushed granules create unwanted dust and deteriorate granulometric spread. The spreading pattern becomes unpredictable. Hardness is measured by the force or weight required to crush the granules. Hardness shall exceed 3 kg (30 N).

5. Flowability

The flowability indicates how easily granules move under the impact of gravity, for example from the spreader hopper to the discs. Good quality fertilizers have a smooth surface and high flowability. Flowability is measured by the time needed for a sample of fertilizer to flow through a standardized funnel (EN 13299). Standard values range from 4 to 8 kg per minute.

6. Fertilizer dust content

Fertilizer dust compromises spreadability, increases environmental burden and impacts working conditions in bulk storage areas. Dust results in over-fertilization along the tractor pathway. Dust also increases the risk of caking. The dust content of fertilizer is measured by separating dust from regular particles through airflow. Dust shall represent less than 0,1% of weight.

7. Abrasion resistance

Friction and shocks cause abrasion of granules during handling. The mechanical resistance depends on the surface structure and particle strength. Fragments below 1,6 mm resulting from abrasion shall represent less than 5% of weight.

8. Caking

Under the effect of moisture, temperature and ongoing chemical reactions during handling and storage, fertilizer granules can stick together (caking). Caking impedes even flow and spreading. Surface treatment of fertilizer granules, low dust content and low humidity reduce the risk of caking.

9. Composition

Blended fertilizers are composed of different nutrients and each of these nutrients is represented as a different granule. Blended fertilizers tend to segregate during storage and in the spreader hopper. In addition, each granule is projected according to its specific mechanical characteristics. High precision, uniform spreading therefore cannot be achieved with blended fertilizers. Compound fertilizers with all nutrients contained in a single granule are always the first choice.