Rotary spreaders, also called broadcast spreaders, use a rotating disk to spread fertilizer over an area. They are ideal for covering areas in a short amount of time, but they are not considered very accurate. While actually spreading material is fairly simple, the major step in using a rotary spreader is calibrating it for the desired rate of application.
Calibrating a Rotary Spreader
Lay down a plastic sheet or tarp. The sheet should be at least 10 feet by 10 feet, but it can be bigger.
Mark a 10 feet by 10 feet area, or use the sheet, itself, if it is 10 feet by 10 feet.
Estimate the setting will provide the application you're seeking. Set the spreader to that setting. Select the widest setting if the material to be spread is coarse and grainy; exact settings will be determined later.
Walk back and forth over the sheet, pushing the rotary spreader, while spreading the material just like you would over your lawn.
Weigh a plastic container on a scale. Write down its weight.
Gather the material you spread over the tarp. Place it in the plastic container. Weigh the container. Subtract the weight of the container to find the weight of the spreaded material.
Find the conversion factor for 1,000 square feet. Divide 1,000 square feet by the square footage of your test area. Using a test area of 10 feet by 10 feet, the square footage would be 100 square feet. Dividing 1,000 by 100 equals 10, providing a conversion factor of 10.
Multiply the weight of the material that was spread over the sheet by the conversion factor. This will tell you how much material is spread per 1,000 square feet.
For example, if you spread 1.5 lbs of material over the tarp, then multiply 1.5 by 10. This equals 15 pounds of material per 1,000 square feet.
Adjust the spreader based on your results. Open the spreader a bit more to apply more material per 1,000 square feet, Close the spreader to apply less.