April 4, 2008 

Sometimes the first hint of spring may be the start of trouble – weedy patches in your pasture or turf that need to spot-treated before they get out of hand. If you're planning to use a hand-held sprayer, try these hints for making use and sprayer calibration a little easier.

Determining How Much to Apply

Pesticides need to be applied according to label directions. Check the label for the recommended dosage of chemical to apply to a given site for the control of a specific pest. By calibrating the sprayer, you can ensure that the label rate will be applied.

You need to know two things:

1. The quantity of total spray output that you will be applying per unit of area, e.g., gallons per acre (GPA)
2. The recommended amount of product to apply per acre.

To calibrate the sprayer, use the following simplified method. This method works for calibrating either compressed air (pump up) sprayers or sprayers with a motor driven pump to create pressure (hand guns). Italicized text in the following directions apply only to compressed air sprayers.

1. Note the "full" mark on the spray tank. There will be space above the "full" mark for compressed air.

2. Determine the useful volume of the spray tank by measuring the volume of water needed to fill an empty tank to the full mark.

3. Using only water, fill the container to the full mark and test the spray pattern. Add pressure and/or adjust the spray tip until you are satisfied with the spray pattern.

4. Measure and mark a test area 18.5 feet x 18.5 feet.

5. Spray the test area in the manner you would normally do when controlling the target pest and record the time in seconds to treat the test area.

6. Pump up the pressure again and spray into a container for the same number of seconds it took to "treat" the test area (Step 5) and measure the fluid ounces (fl oz) caught in the container. The number of fluid ounces in the container will be equal to the gallons of spray you would have applied to a full acre (GPA).

Determine the Amount of Chemical to Add to the Tank

7. Divide the useful capacity of the tank (Step 2) by the spray output, GPA (Step 6) to determine the decimal fraction of an acre covered by each tank of spray solution.

8. Read the label to determine the volume of product recommended per acre.

9. Multiply the volume of product per acre (Step 8) by the fractional acre covered per tank (Step 7) to determine the amount of product to add per tank of spray.

Example:

• Don measured the useful volume of the spray tank and found it to be two gallons. (Step 2)
• He measured and marked a test area 18.5 x 18.5 feet. (Step 4)
• He sprayed the test area and found it took 32 seconds. (Step 5)
• He caught the output in a container for 32 seconds and measured 34 fluid ounces of water caught. This shows Don was putting out the equivalent of 34 gallons per acre. (Step 6)
• Each tank will cover 2 gallons / 34 gallons / acre = 0.059 acre (Step 7)
• The product label recommends 1 qt (32 fl oz) of herbicide per acre. (Step 8)
• The amount of product to add to a full spray tank is: 32 fl oz per acre x 0.059 acres per tank = 1.9 fl oz per spray tank. (Step 9)

Tom Dorn
UNL Extension Educator