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Man using a hand pesticide sprayer.

AUBURN UNIVERSITY, Ala. – With spring fully upon us, many people will be spending much of their time planting and maintaining their gardens. Part of this maintenance is spraying to control weeds and pests. Calibrating sprayers before applying pesticides is extremely important.

How Much Pesticide To Use

Chip East, an Alabama Extension commercial horticulture regional agent, said to determine how much pesticide to put in a tank, starts with determining how many gallons of water you are applying per acre.

“In this example we will use the 1/128th acre method, but other methods could be used,” East said. “This method discussed is for the handgun or wand applicators. It does not matter if your sprayer is a handheld, backpack, ATV or a larger sprayer mounted on a tractor.”

Determining Gallons of Water Per Acre

Step One: Measure 128th of an acre, which is about 340 square feet or 18.5 feet by 18.5 feet. You can use flags, string, spray paint etc. to mark the area.

Step Two: With only water in the tank, measure the time required, in seconds, to spray the marked area.

East said the goal is to apply the water consistently. Try applying the water several times until you determine your average time.

“Wetting the area more or less will change the calibration rate,” he said. “The goal is to spray the same way in the field as you did while calibrating.”

Step Three: Spray water in a container for the length of time you determined in step two. Measure the water caught in ounces. The ounces caught in the time required to spray the 340 square foot area equals the gallons of water the applicator is applying per acre.

Let’s say it took 23 seconds to spray the 340 square feet area,” East said. “When sprayed for 23 seconds and measured, if 50 ounces were caught, the applicator would be applying 50 gallons of water per acre.”

Adjusting Gallons of Water Per Acre

To adjust the gallons of water per acre, you may change pressure at the pump, change or adjust spray tips or adjust the speed. Once these adjustments are made, repeat the calibration process until you are applying the desired amount per acre. Once you are applying the desired volume of water per acre, you do not need to adjust the pressure, tips, or speed.

Calculating Amount of Pesticide

Once you calculate the area that can be covered with one tank, you can determine the amount of pesticide needed per tank.

“The pesticide label will give a range of desired gallons of water per acre that is needed to be applied along with the recommended rate of pesticide,” he said. “Remember to read and follow the label directions before applying pesticides.”

More Information

The Alabama Extension website has information on pesticides that are labeled for certain crops. For more information on sprayer calibration, contact your county Extension office.

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