How to Calibrate a Drop-Spreader

Introduction

Drop-spreaders are used to spread, pesticides, pelletized lime, fertilizer, and grass seeds on a lawn.  Since these materials have different shapes and sizes and are spread at varying rates, the setting generally must be changed each time the spreader is used.

Figure 1

Even when the label on the bag of material suggests a setting, that setting may not be correct for your particular spreader.  The purpose of this page is to present a relatively simple method for you to calibrate your drop-spreader. 

Overview

The lawn care books and instructions on the bag label usually tell you to spread a specified quantity in pounds per 1000 square feet of lawn.  Here is a 3-Step plan for figuring out the how many pounds per 1000 square feet when you set your spreader.  We use a formula that looks like this:

Spreader Rate equals K times WEIGHT

Step 1 -

A. Measure the circumference of the wheel in inches.  (I used a seamstress tape measure.  My wheel measured 31 inches.)

B. Measure the width of the inside bottom of the spreader.  (My spreader measured 20 inches)

C. Multiply the number from A times the number from B.

D. Divide 900 by the number from C.  (I got 1.45 for my spreader.) This is your "spreader constant," which we will call "K".

Step 2 -

A. Look at Figure 4.  Lay something down to catch material as it drops from the spreader.  I used a shallow plastic tray that I bought at a garden store for leaky flower pots.  It is about 1 foot by 2 feet.  As shown Figure 4, place two bricks about 4 inches apart at one end and the other brick outside the tray. 

B. Lift your spreader one side at a time and turn the lifted wheel.  You will observe that one wheel turns the toothed gadget inside the spreader.  Call this the driver wheel.  The other wheel just rolls idly along.  Call this the idler wheel.  On my spreader, the wheel shown in Figure 5 is the driver.  Place the spreader over the tray with the idler wheel on the two bricks.  On the third brick, place the support leg that is near the driver wheel.  Notice in Figure 5 that if you apply a little pressure to the spreader handle with your left hand, you can easily lift the driver wheel until the spreader is level.  Now with your right hand, you can turn the driver wheel.  

C. Mark the driver wheel so that you will be able to know when it has gone around a complete turn.  As you can see in Figures 2 and 5, I marked the wheel with a piece of duct tape.  While you have the duct tape handy, look at Figure 3.  Put a little piece of duct tape on the spreader at the exact point where you will read spreader settings so that you will be sure to read the setting at the same position each time.

D. Fill the spreader about half full of the lawn product for which you wish to calibrate.  Set the spreader at the number recommended on the bag label, or if none is suggested start at a setting of about 5 or 6.  Open the spreader as if to roll it across your lawn.  As shown in Figure 5, press down on the handle with your left hand to raise the spreader to a level position.  With your right hand, turn the driver wheel in the direction it would roll across your lawn.  Turn 10 complete rotations.  Gently lower the driver wheel to the ground and close the spreader.

Step 3 -

Here is that simple formula I promised:

Spreading Rate = K x WEIGHT    pounds/1000 sq ft

In this formula

As stated in Step 1, the value of K for my spreader is 1.45.  Here are two examples, the first is for starter fertilizer and second is for pelletized lime.  

*This filled several Ziploc bags, which I weighed separately and added the weights.