How To Calibrate A Granular Drop Spreader

Determine the speed of application and amount of product to apply.

Drop spreaders operate by “dropping” or distributing granular products via gravity flow between the spreader’s wheels as it is pushed or pulled across the turfgrass. The rate of application is controlled by the width of the opening at the bottom of the drop spreader and the walking speed of the applicator.

To calibrate a drop spreader, the operator must determine the amount of product or granular material dispersed per unit area at a specific spreader setting and constant speed. The operator that will make the actual application should be pushing the spreader or driving the tractor, etc., so the speed during the actual application is the same as the speed used during the calibration.

Some drop spreaders have a calibration pan or tray that attaches beneath the opening at the bottom of the spreader. In this case, the operator simply connects the pan or tray to the spreader, fills the spreader with the actual granular product, and makes a pass over a given distance with the spreader open. The collected product in the pan or tray is then weighed. The rate of application per given area can then be calculated (amount of product collected per given distance) by dividing the amount of product collected by the given distance. This rate of application can then be easily converted to pounds of product per 1,000 square feet or acre.

In the absence of a calibration pan or tray, the operator must collect the granular product over a clean surface such as a plastic or concrete floor. The product can then be collected and weighed to determine the amount of product applied or the weight loss from the spreader when passing over a given area.

An alternative to calibration by pushing or pulling the spreader.

Another method to calibrate a drop spreader does not involve pushing or pulling the spreader. Instead, the diameter of the wheel is measured in feet, and the length or given distance is then determined by the number of revolutions that the wheel is turned to collect the product. For example, the wheel diameter for a drop spreader was 2 feet, and the width (swath) was 4 feet. Therefore, if the spreader wheels were turned for a total of 20 revolutions, then the distance would be 40 feet. The granular product can be collected while the wheels are being turned. Then, weigh the amount of product collected and calculate the rate of application over the given distance of 160 square feet (4 feet swath X 40 feet = 160 square feet).

Trial and error.

Calibration is a “trial and error” process and must be repeated until the desired results are obtained. If the spreader initially applies too much product, the setting or opening is reduced, and vice versa. If the setting applies too little product, then the setting or opening is increased.

Measurements required to calibrate a drop spreader include the distance the operator covers and the weight of the product applied. Obviously, a scale will be required to weigh the granular products.

Calculations should be kept simple and consistent to reduce the chances of inducing error. If the recommended rate for a given product is expressed in ounces per 1,000 square feet, measure the required distance needed to provide 100 or 200 square feet for ease of conversion to 1,000 square feet. For example, if the spreader has a three-foot width (swath), measure 33 and 1/3 feet or 66 and 2/3 feet for 100 or 200 square feet, respectively. Refer to the table below to determine the appropriate distance to measure for a spreader width (swath).

Distance to measure to cover 100 square feet or 1/100 of an acre:

Adapted from Michigan State University Extension Publication

After several passes over the distance to establish a consistent and practical speed, load the spreader with the actual product to be applied and make a pass over the starting and ending lines that mark the distance. Be certain to travel at a consistent speed when crossing the starting and ending lines. Do not begin walking at the beginning line and stop at the ending line.

While walking at a constant speed, simply open the spreader at the beginning line and close the spreader at the end line. Collect the product during the pass from the calibration pan or tray or from the surface between the beginning and ending lines. Weigh the product in ounces or pounds. Use one of the following equations to calculate the rate of application and compare that to the targeted rate.

Calibration Example

A lawn care operator wants to apply a 15-5-9 fertilizer at a rate of 10 pounds per 1,000 square feet using a three -foot drop spreader. He measures a distance of 33 and 1/3 feet based on TABLE 3 and marks the starting and ending lines with stakes. After establishing a consistent walking speed, he loads the spreader, moves the spreader setting to "5", attaches the calibration tray, and prepares to make a pass along the calibration distance. He begins walking several feet before the starting line stake so that he is “up to speed” when he opens the spreader at the beginning line stake. He closes the spreader at the ending line stake as he takes several steps past. The operator collects the product from the tray and weighs the contents. He finds that 12 ounces of the 15-5-9 fertilizer were collected. Using equation (1), he calculates that 7.5 pounds of 15-5-9 were applied per 1,000 square feet. Therefore, he increases the spreader setting to "6.5" and makes another pass across the trial path. This time he collects 16 ounces of 15-5-9. Using equation (1), he calculates that 10 pounds of 15-5-9 per 1,000 square feet

It is important to recognize that the operator must calibrate the spreader for each formulation or brand of granular product used. A different brand of 15-5-9 fertilizer may have granules of different particle sizing or density, thus requiring a different spreader setting.

The same calibration procedure is followed for rotary or centrifugal spreaders; however, it may be more convenient to weigh the product remaining in the spreader instead of trying to collect the widely dispersed product. Thus, the operator would weigh the product that was placed in the rotary spreader and subtract the product remaining in the spreader after a trial pass to determine the amount of product applied.

Key Takeaway:

Proper calibration of a drop spreader is critically important.

Only with a properly calibrated spreader can you ensure that you are applying the proper amount of fertilizer or pesticide.