William T. Hlubik, Middlesex County Agricultural Agent
Joseph R. Heckman, Extension Specialist Soil Fertility
Calibration is the process of determining the exact amount of fertilizer that is applied to a given area of lawn when a spreader is opened to a particular setting under specific conditions.
Calibration helps to prevent the misapplication of fertilizers based on the variability among spreaders when using various formulations of fertilizers. Excessive applications of fertilizers can harm plants and the environment, and increase the cost of application. To account for the variability among fertilizer spreaders under various conditions, it is wise to calibrate each spreader with the fertilizer you will be using. Factors which can influence application rates include:
- type (formulation) and physical properties of the fertilizer used
- speed of application
- mechanical differences of each spreader
Remember that all fertilizer and lime applications should be based on soil test results. See Rutgers Cooperative Extension fact sheets FS33 Fertilizing the Home Lawn and FS635 Managing Soil pH for Turfgrass for more information.
The purpose of the following calibration trial is to determine the rate (amount) of fertilizer your spreader is applying per 1000 square feet of lawn when adjusted to a specific setting. The spreader setting is then adjusted according to the calibration results.
- Tape Measure
- Bucket and Scoop
- Hand Calculator
- Measure spreader application width. For Rotary spreader, place a small amount of fertilizer in the hopper and test the width of application at normal walking speed.
- Select an area suitable for calibration trials. Measure off 50 feet and mark the start and finish points.
- Make sure the fertilizer hopper is closed and fill the spreader with a pre-weighed amount of fertilizer - about 5 to 10 pounds. Add enough fertilizer to cover the moving parts on the inside floor of the hopper. If available, attach a collection box or bag to your drop spreader. Refer to *alternative calculation in step 7 if you use this method.
- The initial setting for the fertilizer spreader can be based on information available on the spreader manual or, if none is available, then begin with a medium setting and adjust accordingly after each calibration trial.
- Begin walking approximately 10 feet before the starting line and open the spreader hopper as you pass over the starting line. Continue over the 50 ft. distance at your normal walking speed. Close the hopper on the spreader as you pass over the finish line.
- Removing the fertilizer left in the spreader by pouring the remaining fertilizer into a previously weighed container. Weigh the amount of fertilizer removed from the spreader. Skip step six if you are using a collection bag or box.
- Subtract the amount of fertilizer left in the spread from the original amount added. This gives the application rate for the area covered in the calibration trial. This is Calculation (A). *Alternative calculation (if a collection bag or box is used): Weigh the amount of fertilizer in the collection bag or box. This is also Calculation (A).
- Calculate the area covered in the calibration trial. Multiply the spreader width (convert to feet) by 50 feet covered in the trial. For example: a 30 inch spreader is 2.5 feet multiplied by 50 feet equals 125 square feet area in the calibration trial. This is Calculation (B).
- Solve the following equation to calculate "X" for the amount of fertilizer applied per 1000 square feet:
(A) Weight of fertilizer applied in trial (from #7)
(B) Area in the calibration trial (from #8)
1,000 sq feet
To solve for X, Divide A by B and multiply by 1,000.
The last step is to compare the fertilizer applied in the calibration trial with the target application rate. The target application rate is based on soil tests recommendations and is limited by New Jersey law. Adjust spreader setting to deliver more or less material if necessary. Repeat steps 3 through 9 until the target rate is achieved.