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Extension > Garden > SULIS > Maintenance > Sustainable Lawncare Information Series > Understanding and Using Lawn Fertilizers > Calibrating Your Spreader to Ensure Accurate, Cost-Effective Fertilizer Application

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Calibrating Your Spreader to Ensure Accurate, Cost-Effective Fertilizer Application

Many fertilizer spreaders are available and easy to use. Two types of spreaders for granular materials are available: "drop" spreaders and "rotary" spreaders.

Drop spreaders distribute the fertilizer directly below the hopper in a well-defined pattern. Rotary spreaders throw the fertilizer material to the front and side of the spreader and are satisfactory for most lawns. Although rotary spreaders give a less precise distribution, they are much faster and are less apt to leave a striped pattern on the lawn where areas were missed although lighter colored areas that did not receive the same amount of N fertilizer because of poor overlap during application may still be visible.

Drop Spreader
Drop spreader.
Rotary Spreader
Rotary spreader.

One way to ensure good coverage with either type of spreader is to fertilize one-half the desired application rate in one direction, then the second-half perpendicular to that direction. It may also be helpful to slightly overlap the "rows" in order to ensure adequate coverage. Many of the newer drop spreaders have indicators or guides on top of the hopper that are easily visible and help ensure proper overlap of fertilizer.

Fertilizer spreaders will apply different materials at different rates. Ideally, the spreader should be calibrated to your pace and the fertilizer used. To calibrate a drop spreader with a given fertilizer, adjust the spreader setting to a selected level, usually somewhere in the middle, and measure the ground area covered with fertilizer in the process. It may be convenient to do this on a sheet of plastic or hard surface area where the fertilizer particles can easily be collected and returned to the spreader or their original bag. Table 7.10 explains the calculation process for calibrating a drop fertilizer spreader.

Table 7.10. Calculations for Drop Fertilizer Spreader Calibration

To calibrate according to the pounds of N needed per 1000 ft2, use the following calculation:

(lbs of fertilizer x percent N as a decimal) / area to be covered by fertilizer =

lbs of N per 1000 ft2

Then calculate pounds N needed per square foot:

(lbs of N x 1000) / 1000 ft2 = lbs N per ft2

For example, when the spreader setting is at 8, you find that 0.6 pounds of fertilizer was needed to cover an area of 100 ft2. The fertilizer used in this example has an analysis of 23-0-3 or N = 23%, P = 0%, K = 3%. Determining the amount of N applied in the 100 ft2 area is as follows:

(0.6 lbs fert. x 0.23 lb N) / 100 ft2 = 0.14 lbs N/100 ft2

Then covert to pounds per square foot:

0.14 lbs N / 100 ft2 = 0.0014 lbs N/1 ft2

Then covert from pounds per square foot to pounds per 1000 square feet:

0.0014 lbs N/1 ft2 x 1000 = 1.4 lbs N/1000 ft2

Therefore, 1.4 pound N is applied per 1000 ft2.

Note: For other examples of fertilizer calculations, see the Extension publication Fertilizing Lawns.

In the above hypothetical example, this spreader at a setting of 8, applies this fertilizer at the rate of 1.4 pound nitrogen per 1000 square feet. The spreader setting can be adjusted for any desired application rate. To develop a calibration chart for this fertilizer with the rate of nitrogen application at various spreader settings, lower the setting to 7 and walk at the same pace covering the same square footage; determine amount applied. Next, lower the setting to 6 and walk at the same pace covering the same square footage, determine amount applied. Manufacturers of lawn fertilizers often recommend a setting for specific fertilizer spreaders. Remember to walk at the same pace you used to calibrate your spreader when you are actually applying the product to the lawn.

A photo guide for calibrating a drop spreader and a rotary spreader is below.

Preparing for Calibration

Authors would like to thank Andrew Hollman, Scientist, University of Minnesota, for the photographs and materials in the calibration section.

Materials needed to calibrate fertilizer spreader.
Materials needed to calibrate fertilizer spreader.
Pan and scale used to weigh fertilizer.
Pan and scale used to weigh fertilizer.
On a drop spreader, select a setting somewhere in the midrange.
On a drop spreader, select a setting somewhere in the midrange.

On a rotary spreader, the amount of fertilizer to be applied is regulated by adjusting the openings in bottom of the hopper.
A rotary adjustment dial may be found on either drop or rotary style spreaders.
A rotary adjustment dial may be found on either drop or rotary style spreaders.
Closed PositionOpen Position
Drop spreader fertilizer opening in closed position (left) and open position (right).

Drop Spreader Calibration

Large Scale Calibration

Large scale calibration is most commonly practiced outdoors where there is ample space to perform the fertilizer calibration test. Large scale calibration is not usually practiced in smaller areas such as home lawn situations.

Calculations for calibrating a drop fertilizer spreader are below in Table 7.11.

Table 7.11. Drop Fertilizer Spreader Calibration

To calibrate according to the pounds of N needed per 1000 ft2, use the following calculation:

(lbs of fertilizer x percent N as a decimal) / 1000 ft2 =

lbs of N per 1000 ft2

Then calculate pounds N needed per square foot:

(lbs of N x 1000) / 1000 ft2 = lbs N per ft2

In this hypothetical example, these are the givens:

  • Fertilizer to be applied: N-P-K 22-0-22 (N=22%, P=0%, K=22%)
  • Application rate: 1 lb N / 1000 ft2
  • Fertilizer spreader width = 2 feet
  • Length of test strip = 50 feet
  • Area of test strip 100 ft2 (2' x 50')
Drop Speader Calibration

Calculating the Fertilizer per 1000 Square Feet

First, convert % N to its decimal fraction:

22% N / 100 = 0.22 N

Then, calculate product rate by dividing the application rate (1 lb N per 1000 ft2) by the decimal fraction for N (0.22) to determine the amount of fertilizer product to be applied per 1000 ft2:

1 lb N/1000 ft2 / 0.22 lbs N /1 lb fertilizer = 4.5 lbs fertilizer/1000 ft2

To ensure adequate fertilizer coverage, plan on applying the fertilizer in two directions perpendicular to one another (i.e., crosshatch pattern). To calculate how much fertilizer is needed for each direction, divide the number of pounds of fertilizer needed for 1000 ft2 (4.5) by 2 (number of directions) to get the number of pounds needed for application in each direction:

4.5 lbs fertilizer / 2 = 2.25 lbs fertilizer/1000 ft2

Note: 2.25 pounds fertilizer/1000 ft2 is equal to 0.225 pounds fertilizer/100 ft2 per single direction

Therefore, 0.225 pounds of fertilizer is applied in only one direction in the 100 ft2 test strip. If the fertilizer is applied in two perpendicular directions (crosshatch pattern) in the test strip, the desired amount of 0.45 pounds of fertilizer would be applied.

The following picture series illustrates the process of applying the above amount (0.225 lbs. per 100 ft2) of fertilizer to the test strip.

Place 20 pounds of fertilizer in spreader.
Place 20 pounds of fertilizer in spreader.
Measure and mark 50 linear feet.
Measure and mark 50 linear feet.

Make sure all slots for fertilizer are open and not plugged to achieve uniform distribution. Apply fertilizer in one direction only.

Pour remaining fertilizer in spreader into measuring tub. Weighing remaining fertilizer; it should contain 19 lbs. 12 oz. or 19.77 lbs. (Table 7.12).

 

Table 7.12. Determining if Drop Fertilizer Spreader Calibration Calculations are Correct

For ease of calculation 20 pounds of 22-0-22 fertilizer is placed in drop fertilizer spreader. Fertilizer is applied in 100 ft2 test strip in one direction only. Fertilizer remaining in drop spreader is weighed after application. The remaining fertilizer weighs 19 pounds 12 ounces (19.75 lbs). So, the amount of fertilizer applied is calculated as follows:

Starting weight of fertilizer - amount left over after application

= amount applied to test strip:

20 lbs - 19.75 lbs = 0.25 lbs

The actual amount of fertilizer applied to the test strip was very close to the amount of fertilizer that was calculated in Table 7.10. (0.225 calculated vs. 0.25 actual). Often times the calculated amount and the actual amount will not be exactly the same. However, if the two numbers are within 10% or less of the desired amount, the drop fertilizer spreader is calibrated sufficiently for most home lawn applications. Remember that when applying fertilizer in only one direction with correct overlap, the amount applied in the test strip will need to be doubled: 2.25 pounds x 2 = 4.5 pounds. Also note that according to the fertilizer bag label, the amount calculated covers 1000 square feet. Therefore, to apply 1 pound of nitrogen it will take 4.5 pounds of fertilizer per 1000 square feet.

Small Scale Calibration

For most homeowners, access to large outdoor areas for calibration is non-existent or not practical. In those instances calibration can be done indoors on a level garage floor or outdoors on a hard level surface. In either case, the area should be covered with some form of a large tarp to ensure that the material is contained on that tarp and can easily be collected and placed back in the original container or into the spreader for distribution. Otherwise, the actual steps to calibrate the spreader (Table 7.13) are similar to those already described for calibration over an existing lawn surface.

Table 7.13. Small Scale Drop Fertilizer Spreader Calibration

Basic steps to small scale drop fertilizer spreader calibration.

  1. Place enough of the fertilizer into the spreader such that it will work correctly.
  2. Measure the width of the spreader such that you know how wide the application will be and thus how much overlap will be needed when the material is actually applied to the lawn.
  3. Mark out a length of travel such that the entire distribution of material from the spreader can be captured on the tarp or plastic sheet.
  4. From the length of travel and width of the spreader you can calculate the square footage to which the fertilizer was applied.
  5. At one end of the travel strip, begin walking and open the spreader allowing for material to drop onto the collection tarp or plastic sheet.
  6. Stop and close the spreader at the end of the predetermined length of travel.
  7. Gather up the fertilizer material on the tarp and place into an appropriate container for weighing.
  8. Weigh the material.
  9. From the square footage for the test strip determined above, and the weight of the material applied in that test strip you can calculate how much material was applied in that amount of square footage. Once determined, multiply that amount by whatever factor the test strip is of the 1000 square feet. For example, let's say that in a hypothetical example, 1.5 ounces was applied in a 2' by 10' test strip. Hence, 1.5 ounces was applied to 20 square feet. Thus, if we multiply our quantity by 50 we will have determined the amount that would be applied in 1000 square feet (i.e., 50 x 20 = 1000 square feet). For this example, 1.5 ounces times 50 is equal to 75 ounces or about 4.7 pounds of fertilizer would be applied to 1000 square feet. You can now determine whether that is more or less than needed per 1000 square feet. If that amount is higher or lower than needed, make the necessary adjustments to the spreader openings and recalibrate until the right amount of product is being applied.

What about rotary spreaders?

Note that a similar process could be done with a rotary spreader. However, a somewhat larger space and larger tarps are needed to contain the "width-of-throw" for the fertilizer. Accounting for proper overlap will also be necessary. Follow the basic steps outlined earlier in this section using an outdoor turfgrass area for calibrating a drop spreader except that it will now be scaled back to accommodate the smaller calibration area.

The picture series below illustrates the steps for small scale drop spreader calibration.


Apply fertilizer on a hard surface along a predetermined length (e.g., 2' x 10' strip). Sweep up fertilizer.

Pour fertilizer in spreader into measuring tub. Weighing fertilizer; it should contains 4 ounces (0.4 lbs) of fertilizer.

Rotary Spreader Calibration

As mentioned earlier, drop spreaders distribute the fertilizer directly below the hopper in a well-defined pattern, whereas rotary spreaders distributes the fertilizer in an arc, several feet wide, by means of a rotating disc below the fertilizer hopper. Although rotary spreaders give a less precise distribution, they are much faster and are less apt to leave a striped pattern on the lawn where areas were missed although lighter colored areas that did not receive the same amount of N fertilizer because of poor overlap during application may still be visible.

Calculations for calibrating a rotary fertilizer spreader using a large outdoor area are illustrated in Table 7.14.

Table 7.14. Rotary Fertilizer Spreader Calibration

To calibrate according to the pounds of N needed per 1000 ft2, use the following calculation:

(lbs of fertilizer x N as a decimal) / 1000 ft2 = lbs N per 1000 ft2

First, measure spreader broadcast width. Broadcast width is the total width of fertilizer distribution. Broadcast width is commonly 8 to 12 feet. Sufficient overlap is achieved by using wheel-track to wheel-track.

In this hypothetical example, these are the givens:

  • Fertilizer to be applied: N-P-K 22-0-22 (N=22%, P=0%, K=22%)
  • Application rate: 1 lb. N / 1000 ft2
  • Fertilizer spreader broadcast width = 12 feet
  • Length of test strip = 83 feet 4 inches
  • Area of test strip 1000 ft2 (12' x 83.3')

Steps to Calculating the Fertilizer per 1000 Feet Square

First, convert % N to its decimal fraction:

22% N / 100 = 0.22 N

Then, calculate product rate by dividing the application rate (1 lb N per 1000 ft2) by the decimal fraction for N (0.22) to determine the amount of fertilizer product to be applied per 1000 ft2:

1 lb N/1000 ft2 / 0.22 lbs N /1 lb fertilizer = 4.5 lbs fertilizer/1000 ft2

To ensure adequate fertilizer coverage, plan on applying the fertilizer in two directions perpendicular to one another (i.e., crosshatch pattern). To calculate how much fertilizer is needed for each direction, divide the number of pounds of fertilizer needed for 1000 ft2 (4.5) by 2 (number of directions) to get the number of pounds needed for application in each direction:

4.5 lbs fertilizer / 2 = 2.25 lbs fertilizer/1000 ft2

Therefore, 2.25 pounds of fertilizer is applied in only one direction in the 1000 ft2 test strip. If the fertilizer is applied in two perpendicular directions (crosshatch pattern) in the test strip, the desired amount of 4.5 pounds of fertilizer would be applied.

The following picture series illustrates the process of applying the above amount (2.25 pounds per 1000 square feet) of fertilizer to the test strip.


Weigh out 20 pounds of fertilizer and place in spreader.

Measure and mark 83 feet 4 inches.
Apply fertilizer in one direction only.
Apply fertilizer in one direction only.

Pour remaining fertilizer in spreader into measuring tub. Weigh tub; it should contain 17 lbs. 12 oz. or 17.7 lbs.

 

Table 7.15. Determining if Rotary Fertilizer Spreader Calibration Calculations are Correct

For ease of calculation 20 pounds of 22-0-22 fertilizer is placed in drop fertilizer spreader. Fertilizer is applied in 1000 ft2 test strip in one direction only. Fertilizer remaining in rotary spreader is weighed after application. The remaining fertilizer weighs 17 pounds 12 ounces (17.75 lbs). Therefore, the amount of fertilizer applied is calculated as follows:

Starting weight of fertilizer - amount left over after application

= amount applied to test strip:

20 lbs - 17.5 lbs = 2.5 lbs

The actual amount of fertilizer applied to the test strip was very close to the amount of fertilizer that was calculated in Table 7.14. (0.225 calculated vs. 0.25 actual). Often times the calculated amount and the actual amount will not be exactly the same. However, if the two numbers are within 10% or less of the desired amount, the rotary fertilizer spreader is calibrated sufficiently for most home lawn applications. Remember that when applying fertilizer in only one direction with sufficient overlap, the amount applied in the test strip will need to be doubled: 2.25 pounds. x 2 = 4.5 pounds. Also note that according to the fertilizer bag label, the amount calculated covers 1000 square feet. So, to apply 1 pound of nitrogen it will take 4.5 pounds of fertilizer per 1000 square feet.

Disclaimer: The information given in this publication is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by University of Minnesota Extension is implied.

Proceed to Fertilizers and the Lawn Care Program.

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