The swath width of a rotary or centrifugal spreader must be measured for each product to be applied since it will vary according to the particle sizing and density of the individual particles.
How To Calibrate A Granular Rotary or Centrifugal Broadcast Spreader
February 01, 2023
Determine the swath width of your rotary spreader.
The swath width of a rotary or centrifugal spreader must be measured for each product to be applied since it will vary according to the particle sizing and density of the individual particles. The swath width is controlled by the speed of the centrifugal disk and can be altered by ground speed in the case of ground-driven spreaders or motor speed in the case of powered centrifugal spreaders. Coverage for these spreaders is, therefore, variable and must be determined for the speed to be used in actual pesticide or fertilizer application.
To determine the swath width of rotary spreaders, place a series of collection trays on a 1-foot spacing perpendicular to the line of travel of the spreader. These containers can be any type of box, pie tins or similar container. It is recommended to find some calibration trays that are 1 square foot in their dimension. Each container used, however, should be of the same dimensions.
Load the rotary spreader with the actual granular product to be applied and set the opening on a medium setting. Run the spreader, at the speed to be used in actual operation, over the collection trays with the spreader open.
Check the collection trays for the granular product. By observing the amount and distribution of the product in the collection trays, a reasonable estimate of the spreader swath can be made. If the product in each container is also weighed, distribution can be checked, and adjustments can be made if necessary. Coverage is determined by multiplying the swath width by the calibration distance. You may use the following table to determine the calibration distance to travel. Since rotary spreaders cover a relatively larger area, a 250 to 500 square feet trial area may be used instead of the 100 square feet recommended for drop spreaders. For example, a rotary spreader with a swath width of 10 feet traveling a distance of 25 feet has a coverage of 250 square feet (10 feet X 25 feet = 250 square feet).
Distance to measure to cover 250 square feet or 500 square feet:
| Width of Spreader (In Feet) | Distance to Measure to Cover Specified Area (In Feet) |
| 5' | 250 sq. ft.: 50 |
| 6' | 250 sq. ft.: 42 |
| 7' | 250 sq. ft.: 35 ½ |
| 8' | 250 sq. ft.: 31 |
| 9' | 250 sq. ft.: 28 |
| 10' | 250 sq. ft.: 25 |
| 500 sq. ft.: 50 | |
| 12' | 500 sq. ft.: 42 |
| 13' | 500 sq. ft.: 35 ½ |
| 16' | 500 sq. ft.: 31 |
| 18' | 500 sq. ft.: 28 |
| 20' | 500 sq. ft.: 25 |
Adapted from Michigan State University Extension Publication
Next, the amount of product required to deliver the rate of pesticide or fertilizer is required. For example, if the labeled rate of a product is 180 pounds of product per acre. To facilitate calibration and avoid unnecessary waste of pesticide or fertilizer, this quantity is reduced to a much smaller area. This is done by setting up a ratio or proportion problem and solving for the unknown/
| 180 Pounds |
X Pounds |
| ————————— | |
| 43,560 sq. ft. |
250 sq. ft. |
| In this case, X equals 1.03 pounds (43,560 X = 180 X 250 therefore X = 1.03). When X is solved correctly, this is the amount of product required to deliver 180 pounds per acre, based on the previously determined coverage. |
| ounces product applied per 250 square feet ———— 16 |
*4 = lbs. product per 1,000 sq. ft. |
| ounces product applied per 500 square feet ———— 16 |
*2 = lbs. product per 1,000 sq. ft. |
| ounces product applied per 500 square feet ———— 16 |
*87 = lbs. product per acre |
Calibration Example:
An operator has a rotary spreader that covers a 6-foot wide swath when applying a granular fungicide. She wants to apply the product at 7.5 pounds per 1,000 square feet of turfgrass. She measures a trial swath that is 42 feet long based on Table 4, puts 5 pounds of product into the spreader, and makes a pass across the trial path. After making a pass, she weighs the product remaining in the spreader and determines that it is 3.9 pounds. Using equation (3), she calculates the application rate of 4.4 pounds of product per 1,000 square feet. Since her target rate is 7.5 pounds per 1,000 square feet, she can lower the spreader setting and make twice as many passes over the turfgrass area or increase the setting and make a single pass. In either case, she must adjust the spreader setting and make more calibration runs.
To avoid skips or overlaps at the full rate of product, it is recommended to calibrate the spreader to apply ½ the recommended use rate and make two passes across the turfgrass at right angles to one another or parallel to one another at intervals of ½ of the swath width. This will ensure better distribution and uniformity of the granular product over the treated area. This example would require applying 3.75 pounds of product per 1,000 square feet per pass at intervals of 3 feet.
Key Takeaway:
Particle sizing and density of the individual particles play a vital role in the calibration of a rotary spreader.
The swath width is controlled by the speed of the centrifugal disk and can be altered by ground speed in the case of ground-driven spreaders or motor speed in the case of powered centrifugal spreaders.
