The importance of spreader calibration

Effective applications starts with the end of the last job. Machines need to be thoroughly cleaned and, if possible, dried before they are put away.

Fertiliser absorbs moisture from the air and if any is left in the spreader this will quickly corrode metal parts.

Before starting work, operators should give the applicator a thorough inspection: Ensure the rate setting mechanism is free and adjusts easily, check the on/off works and the distribution rotor or spinning discs are not worn and functioning properly.

It is vital to then set and test the machine with the actual fertiliser it will be applying. The bulk density of the material effects the spread pattern and flow rate. The same application rate will need fewer denser granules to flow through than less dense material, even though they could be the same physical size. Denser particles will be spread further – e.g. consider how far a cricket ball can be thrown compared with a tennis ball.

This is a very important consideration when setting pedestrian and tractor-mounted rotary spreaders, which rely on centrifugal force to achieve the spreading width, with larger, denser granules travelling further than lighter, less dense ones.

With these applicators it is crucial to check the distribution pattern, with a full-width tray test, as well as the rate calibration. This will ensure you know how much is being applied as well as ensure it is being spread evenly across the whole width.

There are four main spreaders used to apply fertiliser – pedestrian drop spreader, pedestrian rotary, tractor-mounted spinning disc or oscillating spout. All have different calibration and settings procedures.

With pedestrian operated machines settings are usually made for a ‘normal walking pace’ of 5km/hr (3mph). It is important to maintain the speed for which the calibration settings have been made to prevent under- or overdosing. Walking speed also influences the spread width on spinning disc applicators. Also keep spreaders parallel to the ground to maintain an even distribution.

Calibration & testing procedures

Pedestrian drop spreader

Application width is set (usually a little narrower than the hopper), rate is adjusted by opening or closing the outlet and will vary with walking speed.

• Adjust spreader outlet to setting in operator’s manual for product type and application rate
• Apply fertiliser over a measured distance (e.g. 10m) at normal walking pace (5km/hr)
• Determine quantity spread over distance by one of the following methods:
  – Lay sheet on ground and then collect and weigh fertiliser distributed on top
  – Place a known weight in hopper, apply fertilizer, empty and weigh remaining hopper contents and subtract from original weight.
  – Adjust machine setting for desired application rate if necessary. Note this setting will apply only for this fertilizer and operator’s walking pace. Recalibrate for different operators and materials.

Use following formulas to calculate rate in g/m²

Spreading width x distance = area covered
Weight of fertiliser collected ÷ area covered = kg/m²

e.g.
Weight spread over distance = 125g
0.5m (width) x 10m (distance) = 5m²
Application rate = 125 ÷ 5 = 25g/m²

Pedestrian rotary

Application width and dose rate will vary with walking speed and fertiliser characteristics and opening adjustment.

• A spreading width test with the actual material should be carried out, preferably across special collection trays, before any other calibration. This will determine the spread width for the particular fertiliser and operator’s pace (aim for 5km/hr) and how far apart you should walk to maintain an even spread
• Adjust spreader for desired application rate. Place known quantity e.g. 2kg in hopper.
• Walk set distance (10m) at normal walking pace
• Empty and weigh remaining hopper contents
• Calculate area by multiplying the application width by the distance walked

To calculate application rate use following procedure:

Original hopper content minus contents after spreading e.g:
2,000g – 1,000g = 1,000g spread in 10m.

Area: (distance) 10m x (width) 4m = 40m²

Weight ÷ area = application rate/m²
1,000 ÷ 40 = 25g/m²

Distribution Pattern Check

• Lay out testing trays to full application width
• Use fertiliser to be applied and actual operator doing the job walking at set pace (5km/hr)
• Spread fertiliser through trays, which will collect material being spread
• Place contents of each tray in test tubes to check the evenness of the spread pattern. Alternatively this could be done by weighing contents of each tray and plotting on a bar chart.

Tractor-Mounted Spinning Disc

Application width determined by fertiliser characteristics, machine type and PTO drive speed. Forward speed and opening setting will influence application rate.

• Follow application rate calibration procedure in operator’s manual. Usually this means opening a chute and collecting the amount of fertiliser that runs out in a set time. Weigh amount collected and compare with recommendation in manual. Make necessary adjustments to gain correct application rate. This must be done using actual fertiliser to be applied.
• Spreading width will be set by the machine – usually 6-12m. But this will be achieved only at the correct PTO speed (e.g. 540rpm) and this requires the tractor engine speed to set at the correct rpm.
• The forward speed for the application rate will be shown in the manual. It is crucial to select the right gear with the engine rpm to achieve the PTO speed to drive at the forward speed

Use the following formula to check the application rate:
Application rate in kg/ha =

Total weight collected in one minute (kg) x 600
Spread width (m) x tractor forward speed (km/hr)

Spread Pattern Accuracy Test:

• A full-width tray test is essential to ensure the fertiliser is being distributed evenly across the whole width
• Place special collection trays across width
• Tractor is driven in the gear and set engine speed to achieve the PTO and forward speed
• Spreader is operated through the line of trays and contents of each are placed in individual test tubes
• Measure the contents of each tube to assess spread pattern
• Make the machine adjustment to correct any highs or lows to achieve even pattern.