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Calibrating Your Fertilizer Spreader

Spreaders must be properly calibrated if they are to deliver granular fertilizers and pesticides to turf at correct rates. If calibration is done incorrectly, the product may be misapplied and either too much or too little of the product will reach the turf. Spreaders and be calibrated in several ways; but the following methods are relatively simple, fast, and accurate.

Types of Spreaders

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Figure 1. Rotary Spreader

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Figure 2. Drop Spreader

Rotary and drop spreaders are used for applying granular fertilizers and pesticides to turf (Fig. 1 and Fig. 2). Rotary spreaders may also be called centrifugal, spinner, cyclone, or broadcast. A rotary spreader is equipped with a hopper containing one to three holes in the bottom. The holes can be opened and closed by a lever located near the handle. When the holes are open, granules flow through and strike a gear-driven rotating plate, which distributes the granules by centrifugal force. The amount of product that falls through the holes is regulated by an adjustable lever, usually located on the back of the hopper. The adjustable lever can be set on numbers or letters that correspond to the size of the hopper openings (Fig 3).

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Figure 3. An Adjustable lever on the back of the fertilizer spreader that can be set on numbers or letters that correspond to the size of the hopper openings.

Rotary spreaders distribute granules in a band or swath that is greater than the spreader’s width. This feature allows you to cover large areas quickly. However, the product is not uniformly distributed over the entire swath, and fewer granules reach the outer margins of the swath than the area immediately around the spreader. To account for the lack of uniformity, you must overlap swaths. The amount of overlap varies among spreaders; therefore, you should measure the “effective swath width” of your spreader to determine spacing between passes. (See “Measuring the Effective Swath Width of Your Rotary Spreader”).

Drop spreaders have rectangular hoppers that taper to a series of equally spaced holes arranged in a row. Most models have a rotating agitator just above the holes to ensure uniform flow of the product. The holes in the hopper can be opened and closed with a lever near the handle; the size of the openings is regulated with an adjustable lever on the back of the hopper. A deflector or baffle is usually located beneath the discharge holes. Uniformity of application across the width of the spreader is improved when the stream of granules from a hole hit the deflector. The granules fall straight to the ground as they flow through the holes or off a deflector; thus, the swath width is only as wide as the row of holes at the bottom of the spreader hopper. Most drop spreaders range from 1.5 to 3 feet in width.

Drop spreaders are preferable to rotary spreaders for working in small, confined areas. They are not efficient for treating large areas. Drop spreaders are very accurate, and no overlap of swaths is needed. However, it is very important to make passes exactly adjacent to one another (like laying strips of wallpaper) and to avoid overlaps and skips. With drop spreaders, the product falls between the wheels. Therefore, you should overlap wheel paths with each pass to ensure uniform coverage.

Calibration Basics

Regardless of the spreader type or model you use, each individual spreader is slightly different from all others. Therefore, you should calibrate each spreader separately, even though it may appear identical to another spreader. You should also calibrate your spreader(s) for each granular product that you use because products vary in density, size, active ingredient, and nutrient content.

Fertilizer and pesticide manufacturers often provide recommended spreader settings on the product labels. These correspond to particular spreader models and application rates. Although these spreader settings are a useful starting point in the calibration process, a recommended setting should not be trusted to deliver the desired amount of product, primarily because of differences in ground speed among operators and differences among spreaders. Spreaders should be calibrated separately for each operator.

Even experienced applicators occasionally make skips and overlaps; and the result is alternating light and dark green stripes in the turf, inconsistent pest control, or foliar burn. To reduce these problems, make two coverage passes, the second pass at right angles to the first. The resulting grid pattern has the effect of masking skips or overlaps made in either coverage pass. If you choose this approach, calibrate the spreader to deliver half the desired rate of fertilizer, then cover the entire area twice.

Calibrating Rotary Spreaders

Step 1. Along with your fertilizer/pesticide product and spreader, gather the following materials (Fig. 4):

  • Tape measure
  • Scale — There are many types of scales, but you should choose one that will weigh small amounts of product accurately (preferably in ounces). Remember, a small error in calibration will be multiplied in treating large areas of turf.
  • Bucket
  • Chalk or flags
  • Calculator, pencil, paper

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Figure 4. Equipment needed to calibrate fertilizer spreaders

Step 2. Measure your spreader’s effective swath width. (See “Measuring the Effective Swath Width of Your Rotary Spreader”)

Step 3. Measure a 50 or 100 ft strip in a turf area that is not being used or where no one will object to striping or burning from the product application (Fig. 5). If a turf area is not available, you can mark off a strip on a paved area. (Be sure to sweep up the product from the pavement after your test runs to prevent runoff into turfgrass or storm sewers.) Use chalk or flags to indicate the starting and end points of the strip.

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Figure 5. A 50 to 100 foot strip of turf or pavement is needed to make calibration test runs.

Step 4. With the hopper-opening lever in the closed position, fill the hopper to about one-third to one-half full with a known weight of product (5 to 10 lb are usually adequate for most granular turfgrass products). Make sure you write down the weight of the product.

Step 5. Adjust the spreader to the recommended setting on the product label (if available) for the rate you desire. Choose the half-rate setting if you make two coverage passes when treating turf.

Step 6. Position the spreader several feet in front of the starting line and begin walking at a brisk, comfortable pace (the pace you will use to treat the lawn) (Fig. 6). When you reach the starting line, open the hopper holes and keep walking at the same pace without varying your speed. Close the hopper holes as you pass over the finishing or end point.

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Figure 6. When calibrating your fertilizer spreader, walk at a brisk, comfortable pace or the pace you will use when fertilizing lawns.

Step 7. Pour the remaining product in the hopper into a bucket, then weigh the product (be sure to subtract the weight of the bucket) (Fig. 7). The difference in weight between the original amount and the product left in the hopper after the calibration test run is the amount that was distributed.

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Figure 7 Make sure you write down the weight of the fertilizer before and after each calibration test run.

Step 8. Now you must determine if the amount of product distributed in the first calibration test run equals the rate you want to apply to the turf. Use the following example to help you determine how much product you need to collect in the calibration test run.

Example: You would like to fertilize a lawn with a 20-10-10 fertilizer at a rate of 1.0 lb nitrogen (N) per 1000 sq ft. You will be making two coverage passes, and each pass will deliver half the desired rate (0.5 lb N per 1000 sq ft).

First, use the following formula to determine how much fertilizer is needed to apply 0.5 lb N per 1000 sq ft to the turf:

lb of N per 1000 sq ft ÷ % of N in fertilizer (in decimal form) = lb fertilizer per 1000 sq ft.

Thus, in this example:

0.5 lb N per 1000 sq ft ÷ 0.20 = 2.5 lb fertilizer per 1000 sq ft

Second, determine the weight of fertilizer that must be applied in the calibration test run to have your spreader deliver 2.5 lb fertilizer per 1000 sq ft. Assuming that your rotary spreader’s effective swath width is 10 ft and your test strip is 50 ft long, use the following formula to determine the required fertilizer weight:

Weight of fertilizer per 1000 sq ft x [effective swath width of spreader x length of strip] ÷ 1000 sq ft = weight of fertilizer applied in test run

Thus, in this example:

2.5 lb fertilizer x [10 ft x 50 ft] ÷ 1000 sq ft = 1.25 lb of fertilizer needs to be applied in this test run

Next, compare the weight of fertilizer applied in your first calibration test run to the amount required (1.25 lb in this example). If the amount of fertilizer applied is less than 1.25 lb, then increase the spreader setting to provide larger hopper openings and make another run. Conversely, if you apply more than 1.25 lb, reduce the setting to make the hopper openings smaller. Repeat steps 4 through 8 until you reach the target weight of 1.25 lb, or come as close as possible to 1.25 lb.

Measuring the Effective Swath Width of Your Rotary Spreader

Your rotary spreader’s effective swath width can be determined by placing shallow cake pans or boxes (1 sq ft by 1 or 2 inches high) a foot apart in a row that is wider than the anticipated spreader swath (Fig. 8). Make sure there is enough space between the center pans to allow you and your spreader to pass. Fill the spreader about one-half full with the product that you plan to apply, and use the recommended setting on the product label for the desired application rate. Remember that the spreader swath will vary with the spreader model, the product used, and the ground speed of the operator. Therefore, the effective swath width should be determined for each spreader, product, and operator.

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Figure 8. Measuring the effective swath width of a rotary spreader

When all pans are in place and your spreader is one-half full of product, begin your pass well in advance of the row of pans. Travel in a direction perpendicular to the row of pans, and walk at a brisk, comfortable pace (your normal fertilizing speed) passing over the center of the row. Stop after you have passed over the pans. Make at least three more passes over the pans in the same direction. Record the location of each pan relative to its distance from the center pan (for example: pan #1: 1 foot from the center pan on the right side; pan #2: 3 feet from the center pan on the right side, etc.). Weigh the contents and record the weight of product from each pan. Since you will be dealing with small amounts of product, you may have to use a weighing scale that can measure in grams.

Next, compare the product weights from each pan. The weights are typically highest from pans nearest the center of the row, but will gradually decrease as you begin moving away from the center. When the weight of 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 these pans. This distance is your effective swath width. If you determine that the effective swath width of your rotary spreader for a particular granular product is 10 feet, then you should make your passes 10 feet apart.

Fig. 9 and 10 show the effective swath width of a rotary spreader and the appropriate distance between passes. Note that Fig. 9 represents a perfectly even product distribution. In actual practice, you will probably find that the distribution pattern is not perfectly even. More granules may collect in some of the outer pans than in the center and, depending on the model, more granules may be distributed to right side than the left side of the spreader. Some spreaders have devices that allow you to correct the skewing of granular product distribution.

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Figure 9. Chart of fertilizer weights from pans set at regular intervals on both sides of a pass of a rotary spreader.  The effective swath width is 16 feet.

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Figure 10. A 16 foot effective swath width as shown by parallel, adjacent passes of a rotary spreader. Note the overlap of fertilizer into the adjacent swath.

Calibrating Drop Spreaders

Step 1. Along with your fertilizer or pesticide product and spreader, gather the following materials:

  • Catch pan — A catch pan is a trough mounted underneath the spreader hopper. It collects granular product as you make a pass with the hopper holes open. If a catch pan is not sold with your spreader model, you can use the calibration method described for rotary spreaders. Spread the product onto the turf or pavement and determine how much was applied in your test run by subtracting what you have in your spreader at the end of the test run from the amount you started with.
  • Tape measure.
  • Scale — There are many types of scales, but you should choose one that will weigh small amounts of product accurately. Remember, a small error in calibration will be multiplied when large areas of turf are treated.
  • Bucket.
  • Chalk or flags.
  • Calculator, pencil, paper.

Step 2. Measure the swath width of your spreader (distance between the wheels).

Step 3. Measure a 50 or 100 ft strip in a turf area that is not being used or where no one will object to striping or burning from the product application. If a turf area is not available, you can mark off a strip on a paved area. (Be sure to sweep up the fertilizer from the pavement after your test runs.) Use chalk or flags to indicate the starting and end points of the strip.

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Figure 11. Calibrating a drop spreader

Step 4. Attach the catch pan to the spreader. If a catch pan is not available, weigh out 5 to 10 lb of product (or enough to complete the test run and still have some left over) and record the weight.

Step 5. With the hopper-opening lever in the closed position, pour the product into the hopper.

Step 6. Adjust the spreader setting to the recommended setting on the product label (if available) for the rate you desire.

Step 7. Position the spreader several feet before the starting line and begin walking at a brisk, comfortable pace. When you reach the starting line, open the hopper holes and keep walking at the same pace without varying your speed. Close the hopper holes as you pass over the finishing or end point.

Step 8. If you use a catch pan, remove it from the hopper, pour the product into a bucket, and weigh the product. If you did not use a catch pan, pour the product remaining in the spreader into a bucket and weigh it (don’t forget to subtract the weight of the bucket).

Step 9. Now you must determine if the amount of product collected in your calibration test run equals the rate you want to apply to the turf. Use the following example to determine how much product you need to collect in your calibration pass.

Example: You want to apply a 30-3-10 fertilizer at a rate of 1.0 lb nitrogen (N) per 1000 sq ft. You will be making two coverage passes, and each pass will deliver half the desired rate (0.5 lb N per 1000 sq ft).

First , use the following formula to determine how much fertilizer will be needed to apply 0.5 lb N per 1000 sq ft to the turf:

lb of N per 1000 sq ft ÷ % of N in fertilizer (in decimal form) = lb of fertilizer per 1000 sq ft

Thus, in this example: 0.5 lb N ÷ 0.30 = 1.6 lb fertilizer per1000 sq ft

Second , determine the weight of fertilizer that must be applied in the calibration run to have your spreader deliver 1.6 lb fertilizer per1000 sq ft. Assuming that the swath width of your spreader is 3 ft and your test strip is 100 ft long, use the following formula to determine the required fertilizer weight.

Weight of fertilizer per 1000 sq ft × [effective swath width of spreader × length of strip] ÷ 1000 sq ft = weight of fertilizer applied in test run

Thus, in this example:

1.6 lb fertilizer × [3 ft × 100 ft] ÷ 1000 sq ft = 0.5 lb (or 8 ounces) of fertilizer needs to be applied in this test run

Next , compare the weight of fertilizer collected in your catch pan or applied to the turf in the test run to the amount required (0.5 lb in this example). If the amount of fertilizer collected or applied is less than 0.5 lb, then increase the spreader setting to provide larger hopper openings and make another run. Conversely, if you collect or apply more than 0.5 lb, reduce the setting to make the hopper openings smaller. Repeat steps 4 through 9 until you reach the target weight of 0.5 lb, or come as close as possible to 0.5 lb.

Tips for Making Uniform Granular Product Applications

  • Applying products having different-sized granules with a rotary spreader can result in poor coverage. Try to use products containing granules with similar sizes and weights.
  • If running low on product in the hopper, don’t bounce or rock the spreader while the product is being applied. If you don’t have enough product to complete the pass, put more product in the hopper.
  • Always begin walking before opening the hopper holes. Always close hopper holes at the end of a pass while still walking and before turning to begin another pass.
  • Don’t operate spreaders traveling backwards.
  • If granules stick together in clumps, make sure you break them apart, or don’t use the product.
  • Don’t apply light weight materials on windy days.
  • Be sure to clean your spreader thoroughly after applying granular products to prevent build up of fertilizer or pesticide particles and corrosion on spreader parts. Also, lubricate gears and other moving spreader parts before storing.

Handy Conversions for granular products

Linear Measure
1 foot = 12 inches
1 yard = 3 feet
1 meter = 3.281 feet
1 meter = 1.094 yards
Square Measure
1 sq ft = 144 sq inches
1 sq yard = 9 sq feet
1 acre = 43,560 sq ft = 4,840 sq yards = 0.405 hectares
Rates of Application
1 lb/1000 sq ft = 43.6 lb/acre
100 lb/acre = 2.5 lb/1000 sq feet
Weights
1 ounce = 28.35 grams
1 pound = 16 ounces = 453.5 grams
1 ton = 2000 pounds
1 kilogram = 2.205 pounds = 1000 grams

Prepared by Peter Landschoot, professor of turfgrass science