In the January 1997 issue we determined (1) how to find out how much product you may need to apply to a lawn, (2) how to measure the square feet of a lawn and, (3) how to calibrate a drop spreader.

In this issue we will learn how to calibrate a "rotary" or "centrifugal" spreader. Centrifugal spreaders are used by landscapers because they cover more ground in less time, but, seldom are they calibrated correctly. Here's why!

Rotary spreaders discharge granular products in an unequal, but measurable pattern in front of the spreader. If a rotary spreader flings a fertilizer fourteen feet across, the distribution of the fertilizer will not be equal across the fourteen foot wide path. In fact, it should appear as a broad arc pattern, in which the greatest amounts of fertilizer are released closest to the spreader itself. Lesser amounts should appear as you move towards the ends of the application width. It should ideally release fertilizer (or any other granular product) in a bell shaped curve. This is not apparent from visual observation.

Large discrepancies in both application amounts, and uniformity of application, will occur if the lawn applicator simply applies the fertilizer from the edge of the application width, and moves fourteen feet away on the return path. Therefore, there are two steps necessary to perform when using a rotary (centrifugal) spreader.

The first is to determine the effective swath width. This will tell the operator how much he needs to overlap, in order to apply a uniform application.

The second is to calibrate for the application rate for the product.

Effective Swath Width:
(1) Line up in a straight row 10 or 12 shallow trays, or shallow cardboard boxes.
(2) At a moderate setting opening, walk towards the trays, which are spread out from left to right.
(3) Walk by, 5 or 10 times, so you get enough material to compare between trays. We want to know the relative amounts of material in the trays, according to their position from the center of the spreader.
(4) Weight out (in grams) the material in each pan separately.

LEFT

10 8 6 4 2 0 2 4 6 8 10

1g 2g 4g 6g 8g 9g 8g 7g 4g 3g 1g

(5) The Effective Swath Width is calculated by determining at which point(s) the rate has dropped approximately ½ the amount of the center (or 3 center) pans.

From the above, you see that the center of the patterns averaged about 8g (8,9,8). The overlaps should occur where the drop rate is about half this amount (4g). This occurs at about 6' left and 6' right (4g, 4g). The total distance between the two pans is 12 feet. Therefore the EFFECTIVE SWATH WIDTH is 12 feet. The operator should then make the walking rows (passes) 12' apart.

Note that a small volume container can be used to determine fertilizer amounts instead of a scale. Small bottle caps, mouthwash caps, or something similar can be used as a base measurement. Simply record how many "times" the caps are filled from each collection can. Record this data, just as was done above (gram weight example).

Rate Calibration for Rotary Spreaders

We now know from the above information that the (ESW) is 12 feet.

(1) Mark off a distance of 100 feet in a straight line.

(2) Pre-weigh the granular product, or place a catch bag around the impeller.

(3) Noting the opening setting number (or letter), walk at a normal walking speed for 100 feet.

(4) Determine the amount of product delivered to the 1200 ft² area.

(a) Either subtract the amount of material left from the initial amount placed in the hopper, or (b) weigh the amount in the catch bag.

(5) Determine the rate of application as follows.

(a) weight of material applied = amount of material applied per square foot
test application area
 

Example: 50 lbs. material = 0.04166 lbs./square foot

1200 sq. ft. test area

(b) Multiply this value above by "1000," to get "lbs. of product applied per 1000 ft²."

Example: 0.04166 lbs. product/square foot x 1000 ft =

41.6 or 42 lbs. of product applied per thousand square feet

... this was achieved for this specific setting (opening size or number) for this specific product, for this specific centrifugal (rotary) spreader, which has an Effective Swath Width of 12 feet.

Repeat the process at additional setting(s) to achieve the recommended or selected product rate. You can walk in one direction (full calibration) or walk (make 2 passes) in two (opposite) directions at ½ of the calibration rate.