Even spreading, why does it deserve your attention?

Dual disc centrifugal spreaders have distinct spreading patterns.

They vary as a function of construction principles and adjustable parameters such as pale position, length, inclination and rotational speed. In practice, however, it is the variation coefficient that results from superposition of several spreading patterns as the tractor moves back and forth in the field that determines spreading quality. How to ensure predictable results?

A close look at spreading patterns

A typical spreading pattern profile of a modern centrifugal spreader.

Spreading patterns of modern high performance spreaders are measured and optimized using computerized test benches. The exact repartition of the fertilizer alongside the tractor pathway and perpendicular to it can be measured and simulated for different parameters. this figure shows a snapshot of a typical spreading pattern. The red arrow indicates the tractor movement. Fertilizer deposit varies from low (blue) to high (red).

Measuring variation coefficients

The overlapping of individual spreading patterns creates a more or less even distribution of fertilizer over the field. The variation coefficient measures the variability of observed application rates.

The variation coefficient can be determined by field test. However, modern test benches for spreaders enable much more detailed computer simulations of spreading profiles and variation coefficients. Once the spreading pattern of a given fertilizer and spreader setting is recorded, the computer overlays these patterns as the (virtual) tractor moves forward (see figure above). Then the profiles from successive tractor paths are combined to establish the transversal spreading profile as well as the corresponding variation coefficient (see figure to the left).

Spreadability of fertilizer

Comparison of spreading profiles for identical working width of 32 meters. The triangular profile is much more robust against perturbations than the trapezoidal profile.

Different fertilizer qualities create different spreading profiles. Under optimum conditions, even spreading with low variation can be achieved with both, triangular and trapezoidal spreading profile. However, both profiles are not equally robust against perturbations such as tramline deviations. This figure shows a comparison between a triangular and a trapezoidal spreading profile, as well as the corresponding variation coefficients as a function of working width.

  • In the case of the triangular profile, spreading parameters have been adjusted to a working width of 28 meters. The variation coefficient is at a minimum for this width. Even with deviations of the actual working width of +/- 3 meters, variations do not exceed 5 %.
  • In the case of the trapezoidal curve, the working width has been adjusted to 28 meters, too. However, the same deviation of tramlines result in a much higher variation, exceeding 10 %.

A high spreading distance alone is not sufficient to ensure good results. The spreading pattern has a major impact on results. It depends on spreader configuration and on the mechanical characteristics of the fertilizer. For identical working widths, low density fertilizers require spreader settings that favor trapezoidal spreading profiles, while high density fertilizers enable triangular profiles. High density fertilizers therefore result in lower variations.