Small vegetable growers may find pesticide applications with a tractor driven sprayers impractical. Such growers may turn to hand sprayers. In a separate article, I argued that garden sprayers are not appropriate for most commercial pesticide applications. In this article, I will discuss the use and calibration of a backpack sprayer, an excellent alternative to a garden sprayer for small growers. The remainder of this article will discuss one method of calibrating a backpack sprayer.

Most pesticides are labeled for use on an area basis, typically an acre. Therefore, the first task in calibration is to measure the area to be treated. Let’s say that tomatoes are grown in a greenhouse where the production covers 30 X 100 feet = 3,000 square feet. For our staked tomato example, we will use a volume-based method of pesticide measurement. For this method:

- Determine the volume of water required to cover the fully-grown crop on an acre-basis. See discussion later.
- Mix the acre rate (or portion of an acre) of the particular pesticide to be applied in the volume of water determined in step 1.
- Apply the mixed pesticide to the plants.

For example, if Dithane F45 is labeled at 2.4 quarts per acre, and it takes 100 gallons of water per acre to adequately cover mature staked tomatoes, mix 2.4 quarts in 100 gallons for an acre of staked tomatoes. Now, let’s calculate the pesticide rate for our greenhouse example.

Since we calculated the size of our greenhouse production as 3,000 square feet, we can figure the proportion of acre of our production. 3,000 square feet/43560 square feet per acre = 0.07 acres.

Next, we must consult the label to determine how much product to apply per acre. Let’s say we want to apply Dithane F45, which is labeled for tomatoes in the greenhouse at 2.4 quarts per acre. How much should we measure for our tomato greenhouse example?

0.07 acres of greenhouse tomatoes x 2.4 quarts/acre labeled rate = 0.17 quarts for greenhouse example (Since there are 32 fl. oz in a quart, we have 0.17 x 32 = 5.5 fl. oz).

Next we must determine how much water to use with 5.5 fl. oz. of product in our backpack sprayer. Therefore, we must calibrate our sprayer. Let’s break this down into a few steps.

- Measure off a portion of the area to be treated. For our greenhouse example, we will measure 60 feet for calibration purposes. Since we have 6 rows of tomatoes at 100 feet each we have 600 linear feet of tomatoes. (We are using for calibration a tenth of the total area to be treated).
- Fill your sprayer with a known amount of water. We will put one gallon (128 fl. oz.) of water in our sprayer.
- Spray the water onto the calibration area.
- The person who will be doing the spraying should be the one doing the calibration.
- If the person or equipment changes, then the calibration needs to be re-done.
- The walking pace needs to be constant.

- After the 60 feet of tomatoes has been sprayed completely (did you spray both sides of your staked tomatoes?), completely empty the left-over water into a measuring device. In our example, we had 96 ounces of water left (128-96=32 fl. oz.). Therefore, 32 ounces of water was applied on 60 linear feet. How much water would we need for the entire greenhouse (600 linear feet of tomato)?

Multiply 32 fl. oz. of water that was applied on our calibration area x 10 to equal the total area to be treated. Therefore, we need to add our 5.5 fl. oz. of Dithane F45 in 320 fl. oz. of water or 2.5 gallons of water. That is, it will take 2.5 gallons to cover our tomatoes in the greenhouse example.

Here are a few other items to consider.

- It is best to repeat the calibration a few times to make sure that amount of water applied is accurate.
- The pace that is used in walking on the product must remain constant throughout the entire area and from application to application. It might help to keep in mind a particular piece of music or use a mechanical metronome to keep a constant pace.
- As the crop grows, it may be best to use a different number of nozzles on the boom. For example, at the start of the season one might use 2 nozzles while blocking off the remaining nozzles. When extra nozzles are used, the system must be re-calibrated.
- In the example above, we used 2.5 gallons of water on 0.07 acres. Since 3000 sq. ft. is about 1/14
^{th}of an acre that works out to be about 35 gallons per acre of water (2.5 gallons X 14). In general it is best to keep the water per acre amounts to 20 gallons or more. The idea is to thoroughly wet the plant tissue. In most cases, one does not want run off.

The example above is for a liquid fungicide. If instead the fungicide is a dry formulation, then the calculation might go like this: Penncozeb at 2 lb/acre x 0.07 acres = 0.14 lb. (2.25 oz).

How to measure out a dry oz? Don’t measure out the Penncozeb in a measuring cup designed for liquids. Weigh the 2.25 oz on a kitchen scale or on some other appropriate device. Pesticides that are given in fluid ounces should be measured in measuring cup-like devices; pesticides that are given in dry ounces must be measured on a scale. For example, a kitchen scale that may purchased at a well-known box store for about $10 can be used for weighing out dry pesticides. When we used a liquid measuring device for Penncozeb, the amount of Penncozeb was underestimated by 20%!

There are other methods of sprayer calibration. The important point is to spend time calibrating your sprayer equipment, whatever method you use. Proper calibration takes time, but must be done if pesticides are to be applied properly.