The role of the fertiliser spreader is often underestimated in the delivery of fertilisers (N, P & K) as evenly and as accurately as possible.

Fertilisers are a significant cost in grassland and tillage farming systems, representing between 20-30% of total production costs for either a cereal or grass silage crop.

There are many steps involved in determining the actual rate of fertiliser from soil sampling to preparing a fertiliser plan.

To profit from fertiliser planning it is essential that fertilisers are applied precisely and accurately. There are a number of factors to consider before spreading fertiliser such as:

  • Selecting the correct machine for the bout width and fertiliser to be used.
  • Using good-quality fertiliser.
  • Correct setting of the machine.

    • Choosing a fertiliser spreader

    The most important aspect to consider when choosing a fertiliser spreader is its ability to spread the types of fertiliser to be used at the desired bout widths evenly. This is determined by the spread pattern of the machine. The spread pattern is assessed by a full-tray test where every 0.25m or 0.5m width of the spread pattern is measured. The weight of fertiliser from each 0.25m or 0.5m wide tray is weighed and the evenness is assessed by the shape of the plotted graph and the calculated coefficient of variation (CV%).

    Fertiliser characteristics

    The physical characteristics of the fertiliser will affect how evenly it spreads and the settings required. The key factors are:

  • Density of the fertiliser – low density fertiliser such as urea (80% normal density), is more difficult to throw.
  • Size of the granules – generally larger particles will throw further. Particle size distributions with the majority of particles (80%) in the 2-4mm range will usually allow a good spread to be achieved, but larger diameters (3-4.5mm) will probably make it easier achieve wider spread widths. The component parts of blends should ideally have similar size distributions.
  • Shape of the granule – rounded particles will roll off the disc more predictably than irregular granules or angular particles.
  • Strength/hardness of granule – strong particles will resist break-up on the disc.

    • Setting the fertiliser spreader

    There are two elements to getting fertiliser spread evenly at the correct rate in the field:

  • Setting the spreader to spread evenly with a particular fertiliser at a specific bout width.
  • Calibrating the spreader to spread the correct rate of fertiliser.

    • Setting for even spreading

    Most modern spreaders need some level of setting or adjustment to enable them to spread different fertilisers at different bout widths. The elements that may be set depending on the machine design include:

  • Disc type and speed.
  • Vane type and position.
  • Fertiliser drop point on the disc.
  • Height above crop.
  • Spreader angle eg tilting discs upwards at back to throw further.

    • The setting needed is machine specific and is based on the type of fertiliser being spread and the bout width chosen. Basic information is given in the instruction manual, but increasingly either spreader manufacturer websites or phone apps are used to allow the operator easily access up-to-date setting information.

    Tray testing

    The evenness of fertiliser spreading (spread pattern) can be assessed by collecting spread material (fertiliser) in a series of trays laid out across the bout width. For a complete spread analysis a full set of 0.25m or 0.5m wide trays laid out across the bout width for a total width of between two and three times the expected bout width is required. Level ground and wind free conditions are also essential. Following spreading over the trays, fertiliser is collected and weighed and from this single-pass, the overlapped pattern can be generated and CVs calculated. This level of testing is rarely carried out in the field.

    A faster but less-accurate tray test is to place one tray every 2m between the bouts and to measure the overlapped pattern by pouring the fertiliser from the trays into a number of small cylinders which reflect the spread pattern. A less-accurate CV% can be calculated from these figures.

    Calibration of fertiliser spreader

    Calibration of fertiliser spreaders is essential for accurate application rate of fertiliser. Fertiliser application rate is affected by:

  • Bout width.
  • Forward speed.
  • Regulator setting.

    • Bout width

    Bout width is the distance from the centre of one tractor pass to the centre of the previous tractor pass. While spreaders can be set for different bout widths, the bout width is a key element for application rate setting.

    Forward speed

    It is essential to have an accurate forward speed measurement, whether your tractor gives a direct speed reading or is read from a chart in conjunction with a tachometer (rev counter). The tractor’s speed measurement should be calibrated as tyre sizes and other factors will impact on accuracy. This is carried out by measuring a precise distance (eg 100m) and timing the tractor as it passes the start and finish of that distance.

    Regulator setting

    Regulator setting of the shutter controls the amount of fertiliser leaving the hopper to the discs or spout. The wider the shutter opening, the higher the application rate will be.

    Fertiliser calibration formula

    App rate (kg/ha)=600 x flow rate (kg/min)

    Forward Speed (km/hr) X Bout width (m)

    Technology and calibration aids from manufactures have greatly aided this process. Check manufacturers online support and mobile phone apps when calibrating your fertiliser spreader.

    More hi-spec machines at the top of the market offer self-calibrating options, which involve weigh-cells and forward speed sensors to monitor and adjust the regulator setting on the machine while in work.

    In short

  • Select a fertiliser spreader capable of even spreading at the desired bout width with the fertiliser types to be used. Check spread pattern and CV% values from independent or manufacturers tests.
  • Select fertiliser that has good spreading characteristics with at least 80% of the granules in the 2-4mm size range and preferably smooth round shapes.
  • To ensure an even spread across the bout width, use the spreader manufacturer’s resources such as instruction manuals, internet material or phone apps, to set the spreader as accurately as possible for the fertiliser being used. To apply the correct rate of fertiliser, the spreader should be calibrated, using the manufacturer’s resources as a starting point.
  • Clean after every working day and protect from corrosion. Check all the spreading components for wear frequently.