Assessing performance 1: Application Rate

Fertiliser spreader performance is generally assessed through the use of trays placed across the path of a spreader as it travels along the field or in a testing hall. International protocols vary slightly in their layout, including the number of trays used, tray size, and tray spacing.

These guidelines are designed to allow a farmer to rapidly determine performance of a spreader as it is operating on a given day, with a given product in prevailing weather conditions. If any factor changes, the results are likely to be different. There are two aspects to be assessed: the rate of application (kg/ha) and the evenness of the spread pattern (coefficient of variation, CV).

Because of the vast range of fertiliser application equipment types and technologies, a simple guide to calibration is not possible. Even manufacturers cannot fully guide calibration because the product used, terrain covered and weather conditions all have influence on the final result. Ensure equipment is correctly maintained and serviced and set up according to manufacturer’s instructions. This is an essential part of good practice.

If the performance checks show actual application varies from the intended rate or distribution pattern, follow manufacturer’s recommendations to make appropriate adjustments. Machine settings that influence spread pattern are listed in the previous section “Factors affecting spreading performance”. These give a guide to options for calibration to enhance performance. Re-check performance after such adjustments are made.

Application Rate (kg/ha)

A final check of the application rate can be made by comparing the amount of fertiliser actually applied to a known area. If accurate paddock size is known (many are incorrectly assumed) and the total fertiliser applied is known, then a simple division provides the answer.

A quick check when only part of the paddock is covered is recommended. Area can be calculated from bout width and distance travelled. If a known amount of fertiliser is put into the spreader, the amount applied can be determined by measuring the amount remaining after application.

The fertiliser application rate AR (kg/ha) is determined by the product bulk density BD (kg/L), product discharge flow DF (L/min), bout width (swath spacing) BW (m) and spreader travel speed TS (km/h). A factor of 600 is needed to adjust for mixed units; kilometres and metres, minutes and hours.

In practice, the product density and discharge flow can be combined as the product discharge rate; the mass of product leaving the hopper each minute (kg/min).

Many new machines have electronic equipment that controls application rate. This equipment must be calibrated to ensure accurate results. Follow manufacturers’ instructions. The discharge rate should be checked by weighing the amount of product collected in a set time (suggest minimum of 30 seconds) with the spreader operating.

Determine Product Discharge Rate (DR)

Static test
This can be a static test (not driving) if the spreader allows. It may require particular techniques such as manually turning a drive wheel. Collecting a sample from a broadcast spreader may require shrouding the discharge. This may be difficult and care is essential to avoid injury.

• Ensure the hopper is about half full, and in any case contains sufficient product to ensure discharge at the correct rate for the setting.
• Arrange a shroud or containers to collect discharged fertiliser
• Run the applicator for a timed period (at least 30 seconds)
• Weigh the caught fertiliser in the measured time period

Moving test
If the machine must be moving to work, a variation is required.

• Load a known weight of product into the spreader.
• Drive along the paddock and discharging it fertiliser for a measured period of time (ideally a minimum 30 seconds)
• Measure the amount of fertiliser remaining in the spreader
• Subtract the amount remaining from the amount loaded to determine the amount discharged in the measured time period

Determine Product Bulk Density (BD)

Product bulk density is the mass of a known volume of product and is recorded as kg per litre of fertiliser. If the fertiliser appears to have variable grain size, take a few samples and compare the density values determined. If product bulk density is very variable, it is likely field application rates will also be variable.

Some applicators use the product density to determine correct machine settings. For example, density is a key factor in determining discharge door openings.
Some manufacturers supply calibrated scales and standard volume sample collectors. Follow instructions to determine product density. If not, collect a known volume sample of product (minimum of 2 Litres) and weigh it as accurately as possible (to the nearest 10g or better). Divide the weight by the volume.

Determine Bout Width (BW)

The bout width is the working distance between adjacent runs (m). Measure the distance from tractor centre to tractor centre on adjacent runs.

New GPS enabled equipment can follow accurate travel paths and keep adjacent passes parallel. The Bout Width can be very constant and can be easily determined. Older manually steered systems without guidance may have considerable variation in the distance between adjacent passes. This can have a severe impact on overall field uniformity. For the purpose of this test the farm’s usual expected Bout Width is used.

Determine Travel Speed (TS)

The speed of the spreader, truck or tractor (m/min). It may not be safe to rely on tractor speedometers. GPS speeds are reliable, but should be recorded to parts of a km/h. A field check is easily made by record the time taken to cover a set distance (minimum 50m).

Calculate the Application Rate (AR)

Calculate the application rate according to Equation 2.