How to Calibrate a Fertilizer Spreader

Fertilizer application is only effective if you ensure uniform coverage. Dry fertilizers can be applied with either a drop (gravity) spreader or a rotary (centrifigal) spreader.

A drop spreader has the advantage of applying a fairly exact pattern since the area of application is limited to the distance between the wheels. This exact delineation allows a "tight" pattern (line) to be cut but requires that each pass meet precisely with the previous one or skips will be noticeable. Wide (>6 feet) drop spreaders can be cumbersome in the landscape by limiting access around trees, shrubs, and gates. The grinding action of the agitator in the bottom of a drop spreader may break the coating of some slow-release fertilizers, such as sulfur-coated urea. Friction generated by the agitator may cause a reduction in the desirable physical properties of some fertilizer mixtures, which cause spreading and application problems.

The cyclone (also known as rotary or centrifugal) spreader has a wider and less uniform distribution pattern compared to a drop spreader and thus can cover a larger area. The uniformity of the application pattern of the cyclone spreader gradually diminishes as the distance from the spreader increases, reducing the probability of an application skip. The uneven, wide pattern of the cyclone spreader is initially more difficult to calibrate and heavier fertilizer particles tend to sling farther away from the machine. However, proper calibration and experience can minimize these influences.

A recent improvement in fertilizer spreader technology is the use of air to apply the material to the turf. This produces a fairly wide pattern (like the cyclone spreader) that is somewhat exact (like the drop spreader) without damaging the granules or slinging heavier particles farther. Wind and rain effects also are reduced using the technology but initial equipment expense and application expertise are higher.

Spreader calibration involves measurement of the fertilizer output as the spreader is operated over a known area. One way to ensure uniform application of a material is to divide the material into equal portions. Use a spreader calibration that will deliver one-half of the desired amount of material. Make an application over the entire area, turn the spreader direction ninety degrees (90°) from the initial application, and make a second application. This eliminates skips in coverage. Accordingly, calibration of the spreader should be based on one-half desired application rates. A flat surface, a method of collecting the material, and a scale for weighing the material is needed for calibration. The following sequence of steps will aid in calibrating a fertilizer spreader.

Calibrating a drop-type (gravity) spreader

Follow these steps, in order, to calibrate your drop-type (gravity) spreader (Figure 1).

1. Check the spreader to make certain all the parts are functioning properly.
2. Mark off a distance, which when multiplied by the width of the spreader will give 100 square feet of area. For example, the distance required for a 1.5-, 2- and 3-foot spreader is 66.67, 50 and 33.33 feet, respectively.
3. Fill the spreader with the material you wish to apply (fertilizer, seed, herbicide, lime, other).
4. Make several trial runs over the distance and practice opening the spreader as you cross the starting line and closing it at the finish line. Opening the spreader before it is in motion will result in a non-uniform distribution. Walk at a pace which will be used when actually applying the actual material. Open and close the spreader gradually, not in a fast, jerky method.
5. The weight of the material applied by the spreader must be determined. It can be swept up from a hard surface or caught on a large piece of paper or plastic. The easiest method is to attach a catch pan (cardboard works nicely) under the spreader openings and catch the material in the catch pan during the test run to determine how much was applied.
6. Begin calibration at the lowest setting and proceed to progressively higher settings (larger openings). The more trials at a given setting, the better will be the average rate of application. Usually three trials at a given setting are sufficient to obtain a reliable application rate. Weigh the material and record the information on each trial run for future use.
7. One of the calibrated settings will approximate the correct rate of material. Example: You wish to calibrate a spreader to apply 1 pound of nitrogen per 1000 square feet using a 10-10- 10 fertilizer. This calculates to 10 pounds of fertilizer per 1000 square feet since the material is 10% nitrogen (10% x 10 pounds = 1 pound nitrogen). Since the area for calibration trials is only 100 square feet, apply one-tenth of 10 pounds or one pound of fertilizer per 100 square feet. Complete calibration is suggested for the complete spreader range. Settings are not necessarily linear, therefore, half of a particular application range may not necessarily be obtained by using a setting number half the original, thus it is necessary to calibrate the spreader over the entire range of application settings.
8. The same calibration procedure is used for any material you want to apply. Since the quantity applied depends upon the physical properties of the material, the same settings cannot be used for different materials, even if the ratios are the same. Once the spreader is calibrated and set for the proper rate, any size area can be treated accurately.

Calibrating a rotary (centrifugal) spreader

It is important that the "effective" width of application be determined first. Follow these steps, in order, to calibrate the rotary (centrifugal) spreader.

1. Check the spreader to make certain all parts are operating properly.
2. Fill the spreader about half full with the material you plan to apply and run it with the spreader setting about half open (medium setting). Make the application on bare ground or on a hard surface where the width of the surface covered by the material can be measured.
3. Rotary spreaders (Figure 2) do not apply a constant amount of material across the entire width of application. More material is applied toward the center and less at the edges. For this reason, the width of application is accurate for a constant application rate only at about 2/3 (60 to 70%) of the actual width measured. Example: If the application width is 12 feet, only about 8 feet or 4 feet from the center on both sides of the spreader, within the band of application, is receiving approximately the same application rate. The other 2 feet on each edge respectively receive much less material than the center area. Once this "effective" width is determined, calibration is fairly simple.
4. Mark off a distance, which when multiplied by the effective width will give you a 1000 square foot area. For this example, assume that the "effective" width is 10 feet. Then the test strip will be 100 feet long since the width times length is 10 x 100 or 1000 square feet.
   Note: This calculation is based on "effective" width of application and not the total width.
5. Determine the amount of material to be applied. Example: To apply 1 pound of nitrogen per 1000 square feet using a 16-4-8 fertilizer, 6.25 pounds of material should be applied per 1000 square feet.
6. Fill the hopper with a known weight of fertilizer and adjust the spreader to the lowest setting which will allow the material to flow. Push the spreader down the center of the test area, opening the hopper at the starting line and closing it at the finish. Weigh the material left in the spreader and subtract that amount from the starting weight to determine the amount used per 1000 square feet. The beginning weight minus the ending weight tells how much material was applied per 1000 square feet.
7. Repeat step 6 at successively greater settings (openings) and record the material applied at each setting.
8. Select the spreader setting which most closely applies the desired rate of material, set the spreader accordingly, and use it on any size area. To obtain uniform spread of material, remember to set the spreader at half the desired rate of application and make two passes at 90° to each other. Strive for proper spread overlap during application. Example: If the "effective" width is 10 feet, after each pass, move the spreader over 10 feet from the center of the tire tracks. This will give a fairly constant rate of application over the entire area. Also, it should be remembered that each time you change the grade (or analysis) of the materials being applied it is necessary to calibrate the spreader to the material being applied.

For more information about fertilizer spreaders, refer to the Florida Lawn Handbook.