Rotary spreaders use a rotating disc to spread material. Also known as broadcast spreaders, they are ideal for covering areas in a short amount of time. However, they are not the best choice when you're looking for accuracy. One of the most important steps in using a rotary spreader is calibrating it. Calibration will require some trial and error, but it is a fairly simple process.
Lay a plastic sheet or tarp over your lawn. The sheet should be at least 10 feet by 10 feet, but it can be larger.
Mark a 10 foot by 10 foot area of the sheet with the black marker. You may use other dimensions if you want, but 10 feet by 10 feet will make math easy.
Estimate which setting will provide the desired ratio of material. Set the spreader to your estimated setting. If the material you are spreading is coarse, set the spreader at its widest setting.
Start walking back and forth over the marked 10 foot by 10 foot area. Open the drop door of the spreader while you're walking. Use it as if you were spreading the material over your lawn.
Weigh a plastic container on a scale. Write down its weight.
Gather the material you spread over the tarp in the plastic container. Pour it into the container. Weigh the container. Subtract the weight of the empty container. You have the weight of the material you just spread.
Divide 1,000 square feet by the square footage of your test area. If you tested on a 10 foot by 10 foot area, the square footage would be 100 square feet. Dividing 1,000 by 100 gives you a conversion factor of 10.
Multiply the weight of the test material by the conversion factor to gain the application rate per 1,000 square feet.
For example, if you spread 0.68 Kilogram of material over the tarp, then you would multiply 1.5 by 10, which equals 15. This means that the spreader is set for 6.8 Kilogram per 1,000 square feet.
Adjust the spreader accordingly. If you want to apply more material per 1,000 square feet, you will need to open the spreader a bit more. If you want less, shut the spreader some.
Always refer to the material's specifications before calibrating the spreader.