Department of Soil&Crop Sciences

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Calibrating a Rotary Spreader

Broadcast spreader with catch pans

First determine the width of the "throw pattern" for the rotary spreader - this is accomplished by walking at a normal speed and opening the hopper on the spreader while the desired fertilizer product is inside. How far in each direction do the pellets fly?

With this method we will be using catch pans laid in a perpendicular line to the line of travel. Lay out the catch pans (as many as possible to the edge of the throw pattern) with equal distances between each one.

Determine the square foot area of each catch pan (use the same size and style catch pans for each location).

Walk toward the line of catch pans and traverse directly over the center pan. Open the spreader before you reach the line of pans and close the spreader after you have passed the pans - this will ensure that all fertilizer thrown to the sides and behind is accounted for.

You will need to know your desired application rate and the analysis of your fertilizer product before you begin this process.

Example: You want to put down one pound of nitrogen per thousand square feet of a fertilizer with the analysis of 21-0-0 (21% nitrogen).

To determine how much of this particular FERTILIZER per thousand square feet you need to equate to one pound of nitogen per thousand square feet - divide the desired RATE by the ANALYSIS.

1 pound N per thousand / 0.21 (21%nitrogen)
1/0.21 = 4.76 pounds of this particular fertilizer

This means that you need to spread 4.76 pounds of this fertilizer over 1000 square to put out the desired 1 pound of nitrogen per thousand square feet.

Now determine how much of this fertilizer you need to drop within the catch pan (we will use the center catch pan only for this part of the process).

You need 4.76 pounds of 21-0-0 for 1000 square feet - now determine the area inside your catch pan which is in thes example 12 inches by 12 inches or 1 square foot area.

You will need an accurate scale to properly calibrate a rotary spreader using this method.

4.76 pounds of 21-0-0 X pounds of 21-0-0
1000 square feet 1 square foot

cross multiply (4.76)(1)=(1000)(x pounds)
x = 0.00476 pounds of 21-0-0 in the calibration area - this is equal to 0.076 ounces (16 ounces per pound) or 2.16 grams (454 grams per pound).

Now you go through the calibration process with the ultimate goal of finding the correct setting on the spreader to drop this amount of fertilizer within the calibration area.

Designed by Jason L. Gray.