In my last blog, I discussed the relative advantages and disadvantages of different types of pesticide sprayers that are powered by humans. That is, a hand sprayer is any sprayer that depends on humans to move the sprayer from plant to plant. This is in contrast to pesticide sprayers that are moved along a field by a tractor.
In a tractor borne sprayer, it is possible to set a constant speed and pressure. This fact facilitates calibration. In the space below, I would like to discuss one method whereby it might be possible to calibrate a hand sprayer.
As stated in my last blog, make sure that the product is labeled for the host and disease(s) that are of concern. If the area to be sprayed is a greenhouse, make sure the product can legally be applied to a greenhouse (or high tunnel). In this example, the fungicide to be applied has a label that requires 1 lb/acre of tomatoes.
In our example, we will be applying the fungicide to a greenhouse. The greenhouse is 30 ft x 90 ft. The total number of square feet in the greenhouse is 2700. 2700 sq. ft. per greenhouse divided by 43560 sq. ft per acre = 0.062 acres/greenhouse. That is, our greenhouse is 0.062 acres. Since we know our product is applied at a rate of 1 lb/acre, then we know that we must apply 0.062 lbs of product to our greenhouse. To make it a bit easier, we will use ounces. If a pound equals 16 oz., 0.062 lbs of product equals very close to 1 oz.
But how much water is needed to apply 1 oz. of our fungicide to the greenhouse tomatoes? Remember that the greenhouse is 0.062 acres. We need to know how much water will be needed to cover 0.062 acres of greenhouse tomatoes.
To determine how much water to apply to our greenhouse of tomatoes, apply water to a portion of the greenhouse and then calculate how much water it will take to apply a spray to the entire greenhouse.
In this greenhouse example, there are 6 rows of tomatoes each 90 feet long on 5 foot centers. If we apply water to one row of tomatoes, we will be able to determine how much water to apply to the entire greenhouse. We place 5 gallons of water in our back pack sprayer and apply it to our tomatoes as if we were actually applying fungicide. Use a slow and steady motion. Carefully apply water to each plant. Avoid stopping in the middle of the row and have the person who will do the applications be the same person to conduct this sprayer calibration.
After we have applied water to one row, we will measure the contents of the sprayer. We find out that we have 4.5 gallons left in the sprayer. That means that we applied 0.5 gallons to one row. Since there are 6 rows, 3 gallons of water will be needed to cover the entire greenhouse.
Place the amount of product calculated above for the greenhouse, 1 oz. of fungicide, in 3 gallons of water.
There are other ways to calibrate a hand sprayer. For example, if you know accurately how much time it takes to apply fungicide to one row of tomatoes (or to the entire greenhouse), one could measure how much water the sprayer puts out in a minute and thus calculate the total amount of water.
Perhaps the most important factor is consistency. Follow a steady motion across all the plants. Use a constant pressure. Nozzle wear will affect calibration as will new nozzles. Recalibrate the sprayer if someone else will do the spraying.
If you have questions or concerns, please let me know. Or maybe you would like to share an idea in the comments below.