If you apply liquid fertilizers and pesticides to large areas of your lawn, you need to be accurate. That means calibrating your sprayer to be sure you apply the correct amount of material for the area you’re treating, says Dr. Dick Parish, an engineer with the LSU AgCenter’s Hammond Research Station.
"Calibration of hand-carried sprayers is subjective, but it’s simple to do," Parish says, adding, "Calibration of vehicle-mounted, boom-type sprayers is simple and can be very accurate."
Most homeowners with small lawns are likely to use a hose-end sprayer or small hand pump sprayer. Parish suggests using the following procedure to calibrate your system, but he cautions the results will be subjective and depend on a consistent operator.
- Mark off a rectangular area of 10 feet by 43.6 feet – one-hundredth of an acre.
- Using only water, spray this area in your usual manner. Record the time it takes to make this application and be sure to calibrate for each person who may use the sprayer.
- With only clean water in the spray tank and the gun stationary, start spraying at the pressure you intend to use. Catch and measure the output from the gun for the same amount of time required to treat one-tenth of an acre.
- Calculate the output by multiplying 100 times the number of gallons or 0.78 times the number of ounces collected. This gives a spray rate in gallons per acre.
"Once you have determined your spray rate, check the product label to see if you are within the recommended rate," Parish says. "If not, you need to adjust travel speed, nozzle size or pressure and recalibrate until you’re within the label recommendations."
Parish says that after arriving at a suitable overall rate, you must need to mix the chemical properly for that rate. Multiply the recommend rate per acre by the area to be sprayed to determine the amount of material you need and mix the appropriate amount of material with the correct amount of water.
For larger lawns or small pastures, you might use a small tractor for applying your fertilizers and pesticides.
Parish says boom-type lawn sprayers that are mounted on vehicles are calibrated in the same way as agricultural sprayers. "The math involved is minimal," he says. Following these steps will give you a simple and effective procedure to find your rate of application.
- Measure the spacing between adjacent nozzles.
- Mark off a test-run area on the lawn 200 feet long by using flags, stakes or something else you can see.
- Drive the sprayer through the 200-foot test run at normal operating speed and time how long it takes to go between the marks. There’s no need to spray while driving. Be sure to get up to speed before the first mark and don’t slow down until after the second mark.
- Put clean water in the spray tank and with the spreader stationary, start spraying at the pressure you intend to use. Catch and measure the output from one nozzle for the same amount of time required to cover 200 feet in the previous step.
- Calculate the output by multiplying the nozzle output in ounces by 20.4 and then divide by the spacing between the nozzles in inches. This will give you a spray rate in gallons per acre.
"You can’t get much simpler than this," Parish says.
The engineer notes that using this method requires none of the calculations of sprayer ground speed in miles per hour or nozzle output in gallons per minute required by many other calibration methods. This method assumes all nozzles are equally spaced and of the same size.
"At least once a year, you should check the output from all nozzles," Parish says. "If all aren’t within 10 percent, replace the nozzles."
And he recommends recalibrating a boom-type sprayer every time you spray.
"Following these procedures will help you obtain the correct rate of pesticide or fertilizer on your lawn," Parish says.