Calibration should be done at least once every season.  It is critical to calibrate your spreader before applications, especially from one product to the next.    The distribution of different products will vary because of characteristics such as particle shape, size, uniformity, humidity, etc.  Not taking the time to calibrate equipment can cause your application to be inaccurate, which can lead to repeat applications, damaged plants, excess cost, and contamination of the environment.

There are two basic types of spreaders

Drop spreaders deliver a uniform pattern with precise control around edges, however because the edge of the pattern is abrupt even small errors in application can result in under or over-treated and missed areas.  OPH does not recommend the use of drop spreaders for fertilizer products.  Improper application will result in green striping of your lawn.

Rotary spreaders are the preferred type of spreader because they broadcast the product to a wider area and allow for a more evenly dispersed product, greatly reducing the risk of striping.

Applications for both spreaders are affected by spreader design, product characteristics, surface & air environment, and operator walking speed.

To calibrate a spreader: 

For calibration tests, always weigh out 2-3 times the amount of product required to allow for even weight distribution in spreader.  For example, product application rate is 5lbs per 1000 sq. ft.

Weigh out 10 lbs. of product and put in spreader.  Be sure to take into account the weight of the container being used for this step.

Mark off a 20 X 50 or 10 X 100 area.  This can be done over grass area or preferably on a hard surface area where the product can be cleaned up after test.

Set spreader setting at a moderate opening and walk entire area at a moderately brisk pace. (3 mph)

Weigh remaining product left in spreader to determine how much was applied.

In this example where the goal was to apply 5lbs for 1000 sq. ft. and you only applied 3 lbs, you’ll need to open the spreader setting slightly wider and walk another 1,000 sq. ft. section to test again.  Continue testing until spreader is calibrated to accurately apply the correct amount of product.   Make sure to record this data for future reference.  Re-calibration will need to be done on a regular basis due to adjustments being loosened, parts wearing, etc.  Pay close attention to your equipment to notice any changes that may occur.  Re-calibration will also need to be done with each change in product type because of variations in particle size, shape, etc.

Proper application technique & tips:

Do not spread products on days when wind speeds are over 15 mph.

Close spreader lever before filling the hopper.

If your spreader has a screen, make sure it is in place to catch any clumps before they reach the spreader opening.  To prevent clogging in spreaders that do not have a screen, break up or remove any product that sticks together or forms clumps.

Always push the spreader, do not pull or go in reverse.

Start walking before entering measured area, opening the lever once inside area. As you near the end of the measured area, close the lever before stopping.

Walk in straight lines noting your course and width of disbursement.   Use the wheel marks as a reference. 

Knowing the swath width of your rotary spreader is essential to determining the distance between spreader passes.

For example: if your spreader applies product over an 8 foot width, after each pass you must move the spreader over 8 feet from the outside tire tracks to begin next pass.   Apply the product in parallel passes at a full rate setting or half rate setting as pictured below.  Until you become a skilled applicator and can accurately apply products while in the turn of your pattern, you will want to apply product to banner strips at both ends of a rectangular area. Once this is done, you can walk straight lines to apply product.  Before entering the already applied end zone, turn off spreader.  While in the zone, turn, and prepare to walk your next parallel pass re-opening the spreader as you enter.

NOTE: 

The spreader settings that are listed on bagged products are done by skilled technicians with properly calibrated equipment.  The recommended rates are the results of multiple tests and are accurate, taking into account many factors such as speed, width, wind, particle size, etc. 

If the setting on your spreader distributes the product correctly, your spreader is properly calibrated and you are meeting all of the conditions required.  If you run out of product before completing your entire area, your spreader may not be calibrated correctly, your walking speed may be too slow, or one of many factors could be influencing the disbursement rate.  You will need to adjust your spreader based on your individual conditions.