Spreader Calibration

Calibrating a Granular Spreader

  1. Calculate the application rate per 1,000 sq. ft. for your site.
    • This can be found by taking your target nitrogen application, for this example we will use .8 lbs of nitrogen per thousand, and dividing it by the percent nitrogen content of the fertilizer you are applying.
    • If you are using a 25-0-4 fertilizer, you will divide .8 by .25 which yields 3.2. 
    • What this tells us is that in order to achieve our .8 lbs. of nitrogen that we want on 1,000 sq. ft. of turf we need to apply 3.2 lbs. of the blended fertilizer to the turf. This makes 3.2 pounds our target.
  2. Find an open area at least 50 feet long and gather the following materials:
    • your spreader
    • fertilizer to be calibrated
    • bucket
    • calculator
    • tape measure
    • a scale
  3. Estimate an amount of fertilizer you will need to do a short pass in the open area you have selected. 
    • Using your bucket, weigh this material, note the weight, and pour it into the spreader.
  4. After marking the beginning and ending points of your calibration area, set your spreader to the recommended setting on the product label. 
    • This is a suggested setting only. The condition of your spreader, your walking speed, and weather conditions could cause differences between the actual setting that this process will give us and the recommended setting on the label.
  5. Spread the material along the open area you have chosen.
    • Walk at a normal pace will while applying the product. 
    • The spreader's application rates are dependent on walking speed along with the spreader setting.
    • To get the most accurate measure start your calibration walk before the marked starting point of your test area to get you and the spreader up to speed and avoid any surges from the initial push. 
    • When you pass the starting point open the spreader gate and walk at an even pace until the marked ending point of your calibration area and shut the gate.
  6. Take the remaining product, dump it into your bucket and weigh it again. 
    • Subtract the weight after the pass from the weight before the pass.
  7. Measure the length and width of the area you covered.
    • Use these measurements to calculate the square footage you covered (length x width). 
      • Length is simply the distance you walked, width is found by measuring the spread of the applied product on the ground.
      • You can make this process easier by first finding the width of your spreader's application. This will vary depending on the product. 
        • An SGN 90 is a finer granule and will be lighter, thus will travel further than an SGN 220 product. Once you have the width, say 6 feet, you can use this to calculate an appropriate distance for your calibration pass to hit our ideal size of 1,000 sq. ft. For this example we take 1,000 and divide it by 6, giving us 166 feet. If we marked this distance and walked it with our spreader we know we're covering 1,000 sq. ft. exactly.
  8. Calculate the amount of product applied per 1,000 sq. ft.
    • Example: 10 pounds of fertilizer was put into the hopper. After the calibration pass there was 8 pounds of fertilizer left in the spreader. This means we applied 2 pounds of material in our pass. 
    • After measuring the area we used for our calibration pass we found that those 2 pounds were applied to 500 sq. ft. This means the rate at which we applied was 4 pounds per 1,000 sq. ft., more than our target rate of 3.2 from step 1, but not by much. 
    • Using this data we can make a best guess at what the setting should be and re-run the test until the appropriate setting for our 3.2lb/M rate is achieved.

Factors that affect spreader settings

  • walking speed
  • differences in granular size
  • moisture content of the granules
  • any coatings on the product
  • age & condition of the spreader
  • amount of overlap while applying
  • other environmental conditions.