Pour fertilizer into the spreader. Turn the flow dial to the number recommended on the bag of fertilizer. For example, if the bag suggests 20 pounds per 1,000 square feet, turn the dial to 20.
Lay a 12-foot-square tarp on the ground. Measure a block in the center of the tarp that is 10 foot square and mark off the square.
Start pushing the spreader down the marked block to the other end, then turn the spreader and work back and forth in rows that do not overlap. Push the spreader while walking behind it at a pace of about 3 miles per hour. This is an approximate brisk walking speed.
Push the spreader off the tarp. Fold two sides of the tarp inward, rolling them as you go to move the fertilizer into the center and make a channel in the center. Roll the tarp toward one end to push the fertilizer into the channel in a size that is easy to handle. Pour the fertilizer from the tarp into a jar.
Place the jar on a bathroom scale. Record the weight of the jar and fertilizer. Pour the fertilizer back into the bag and weigh the empty jar on the scale. Subtract the jar weight from the first weight to obtain the seed weight that was distributed. For example, if the total weight is two pounds, less the jar weight of ½ pound equals 1.5 pounds of fertilizer.
Multiply 1.5 pounds times 10 to obtain 15 pounds per 1,000 square feet of fertilizer spread on the rating of 20 on the dial. The number 10 is used because the tarp size is 1/10th of the distribution size of 1,000 square feet.
Turn the dial on the spreader to the next highest number and repeat the test to achieve the spread rate of 20 pounds per 1,000 square feet.