Calibration 101

Here are some tips to determine effective swath and properly calibrate a rotary spreader.

March 28, 2016
Kristen Hampshire

Careless calibration can cause costly mistakes. Follow these steps to do the job right for a rotary spreader, provided by Peter Landschoot, professor of turfgrass science at Penn State University.

#1 Gather Materials:
Rotary spreader (professional grade)
Fertilizer or other granular product
Tape measure
Scale (measures in ounces)
Chalk or flags
Calculator, pencil and paper
Shallow cake pans or boxes (for measuring spreader swath)

#2 Determine Spreader Swath
Rotary spreaders do not evenly disperse material like drop spreaders do. More product is distributed in the middle (center) of a pass, with less product distribution on either side. An overlapping pass is necessary to achieve uniform application.
Test spreader swath by placing a row of pans 1 feet apart. Fill spreader with product using manufacturer recommendations. Make a pass over the line of pans. Weigh the product collected in each of the pans. Make at least three passes in the same direction.
When the weight of the product from pans on the left and right sides of the swath equals half the weight obtained from the center pan, measure the distance between those pans. This distance is your effective swath width.

#3 Mark Off a Test Area
Measure off a test area of lawn to test calibration. Choose an area size that’s easy to work with, such as 50 by 100 feet. You can also mark off a paved area for this test.

#4 Test Calibration  
•    Pour enough product in the bucket to fill one-third of the spreader. Weigh this product. Pour product in spreader.
•    Adjust spreader to recommended setting on product label for the rate you desire.
•    Walk at a comfortable pace while releasing product in the marked-off area. Make a few passes, minding your swath (overlapping as necessary).
•    Pour remaining product back into bucket and measure on scale. (Be sure to subtract the weight of the bucket.) Figure out the difference between the original amount and the product left over.
•    Calculate whether the correct amount of product was distributed. Your numbers should be at least within 10 percent of the products recommended rate. Use the example below.

Calculation Example
You want to fertilizer a lawn with a 20-10-10 fertilizer at a rate of 1.0 lb nitrogen (N) per 1,000 square feet. You will make two coverage passes, and each pass will deliver half of the desired rate (0.5 lb N per 1,000 square feet).
1.    Use this formulate to determine how much fertilizer to apply 0.5 lb N per 1,000 square feet.
2.    lb of N per 1,000 square feet divided by % of N in fertilizer (in decimal form) = lb fertilizer per 1,000 square feet
3.    Example: 0.5 lb N per 1,000 square feet divided by 0.20 = 2.5 lb fertilizer per 1,000 square feet
4.    Determine amount of fertilizer for test run so spreader will deliver 2.5 lb fertilizer per 1,000 square feet. Your rotary spreader’s swath width is 10 feet, and your test strip is 50 feet long. So, weight of fertilizer per 1,000 square feet x (swath width of spreader x length of strip] divided by 1,000 square feet = weight of fertilizer applied in test run
5.    Example: 2.5 lb fertilizer x [10 ft x 50 ft] divided by 1,000 square feet = 1.25 lb of fertilizer for test run
6.    Compare weight of fertilizer in first calibration test run to the amount required (1.25 lb in this example). If you applied too much, decrease the spreader setting. If you did not apply enough, increase the setting. Repeat steps until you reach the target weight.