Often you'll see an application rate on bags of fertilizer--10 pounds per 1,000 square feet, for example. To ensure proper application, you need to determine how much fertilizer your drop spreader lays down. (If your model of spreader is listed on the fertilizer bag, use that setting.)

Fill the spreader with fertilizer, set the opening to the number indicated on the bag (different spreaders have different settings), and then operate it over 50 feet. You'll need to collect the fertilizer, so either attach a collection pan under the spreader or run it over a tarp.

Collect the distributed fertilizer and weigh it.

Calculate the square footage covered by multiplying the actual width the holes cover on the spreader (in feet) by 50.

To calculate how much your spreader drops, divide the area covered by the weight collected. A spreader that drops half a pound over 50 square feet, for example, would have an application rate of 1 pound per 100 square feet or 10 pounds per 1,000 square feet. If that is the amount intended, you've confirmed that your spreader is accurate. If not, adjust the setting on the dial up or down and repeat until the amount delivered matches the amount recommended.