Calibrate Fertilizer Spreaders Prior to Application

Broadcast fertilizer spreaders are an excellent way to apply low rates of highly concentrated fertilizers to large areas quickly and effectively. However, they are only accurate if the operator takes time to verify spreader settings and material application rates. Fertilizers are always one of the most expensive crop production inputs, and over- or under-application of nutrients can create economic losses to your operation. Taking a few minutes to calibrate a spreader before each fertilizer application will ensure nutrients are used economically. Consult your machine's operator manual to guide your initial set-up.

1. Gather Machine Information

Begin by gathering information for the equipment used to make the application. Be sure to note the operator, date of calibration, tractor and spreader (or spreader if self-propelled), machine ground speed, and engine/PTO RPM settings. Also verify spreader gate, flow, and spinner vane settings.

2. Verify Working Width

Next, measure the width of material application. This is the farthest point from left to right that you can detect material. Working width varies by PTO speed, wind speed, material bulk density, material uniformity, and particle shape/size. Working width should be measured each time material changes in conjunction with the fertilizer pan test (see below).

3. Conduct Pan Test

Continue by conducting the fertilizer pan distribution test. A detailed description of testing procedures can be found in this Ohio State University fact sheet. Set a pan under the dead center of your calibration pass, skipping the first rows for your tire tracks, and repeating every 5 feet across the working width of the machine. Pans can be as simple as aluminum trays, but must contain baffles to prevent granules from bouncing out. Calibration should be conducted as close to in-field conditions as possible.

4. Measure Pan Contents

After making your calibration pass, measure the contents of each individual pan across the working width of the application pass. Record the net weight of each pan in the attached record sheet in grams, which allows for greater accuracy in small-scale work compared to ounces or pounds. Make a successive pass on either side of the initial pass to determine the application rate of your overlap areas.

5. Calculate Rate

Calculate the area of your pans by multiplying the length by the width. Use the following equation to calculate your actual application rate per acre.

Pan Length (feet) × Pan Width (feet) = Pan Area (square feet)

(Pan Contents (grams) / Pan Area (square feet)) × 96 = Application Rate (pounds per acre)

6. Adjust Settings

Adjust your fertilizer spreader settings to increase or decrease your application rate per acre. Your operator's manual should provide some useful input in fine-tuning application rate and distribution. You may also need to adjust your spreader vanes if material distribution is uneven across the working width.

Note the distribution pattern in the example figure above: The blue line represents the actual application rate at each pan location, and the yellow shaded area represents a 10% variance from the actual application rate. The gray line at the edges represents pan contents from successive overlapping passes on either side of the initial calibration pass. Although material is being broadcasted 45 feet in either direction, the desired application rate ends after about 35 feet. The operator should overlap their passes slightly to achieve their full target application rate from pass to pass. In this case, the actual application rate falls to about 50% of the target application rate of 225 pounds per acre at around 42 feet, so the operator would set their application width to 84 feet.

Some calibration kits may contain a series of vials to pour each pan's contents into for a visual representation of broadcast pattern. In some cases, material distribution may vary by over 25% across the application width of the spreader. You may see a "W" or "M" shape in the pattern, or the pattern may favor the left or right sides of the application width. In these cases, consult the spreader's operator manual to determine if spreader vanes must be adjusted or replaced, or if material isn't flowing correctly out of the spreader gate.