Using a knapsack sprayer on a big farm

Knapsack sprayers have tanks of fertilizer, insecticide or herbicide that farmers wear on their backs to spray gardens or trees. The farmer controls a nozzle with his right hand and a pump handle with his left hand to keep up the pressure in the tank. Knapsack sprayers are more convenient than the sprayers they replaced, which had to be carried by hand and set on the ground to pump.

Many farmers think that using a knapsack sprayer is taking a step backwards as large spray rigs are fast and effective. However, knapsack sprayers still have their uses on big farms.

The first is for spot spraying. Cabbage farmers often find a plant here and there that is infested with aphids. These are easy to identify, as cabbage aphids make the leaves curl and leave distinct white marks.

Instead of using the tractor-mounted sprayer to treat the land, go through it with a knapsack and spray only the infected plants. You will be able to spray them far more thoroughly, kill the entire colony and save on tractor costs and insecticide.

Aphids sometimes enter the land and colonise young cabbages. When the transplants are about a month old, the aphids settle on the growth tip and are difficult to control with a boom sprayer. In this case, move from plant to plant with a knapsack, concentrating the spray onto this area only.

Penetration will be far better than with a tractor-mounted sprayer, the boom of which will also cover the open ground between plants, leading to waste.

It is not even necessary to wet the outer section of the leaves. Concentrate on the inner part, as this can remain a source of continual re-infection if not properly sprayed.

This method is highly economical, as you can average one plant per second and cover 1ha in a day.

Remember, though, to spray only one row on the same pass; spraying the rows to left and right slows the process down. To save more time, provide your worker with a drum of mixed product.

Because you’re not wasting chemicals, you may even be able to afford more effective products.

Spot-spraying is also useful in the case of a weed such as nutgrass that requires glyphosate. Here, regrowth generally occurs in patches, or as a plant emerging here and there; with a knapsack you can concentrate on spraying these.

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Service and maintenance

Modern knapsacks and compression sprayers are designed for ease of service and maintenance. Only a few simple rules are needed to keep the equipment in top condition. These include:

• reading the service manual to check for any specific service requirement of your piece of equipment
• at the end of each spraying job, thoroughly clean the inside and outside of the sprayer, disassemble it, and clean each part meticulously
• check for wear and correct fitting of “O” rings (it may even be advisable to have a few spare “O” rings on hand)
• complete the service by reassembling the unit and returning it to storage. Plastic sprayers should not be left out in the sun, as the ultra-violet radiation harms the plastic and shortens its useful life.

Calibration

Calibrating a knapsack or compression sprayer is a simple task which takes little time. A sprayer with a pressure gauge is the best one to use because the pressure gauge assists in ensuring an even output is maintained. There are a few points to remember before calibrating the sprayer:

• it must be clean, serviced, and operating according to the manufacturer’s instructions
• only use water to calibrate the sprayer – calibration is never carried out using chemicals
• the accuracy of the calibration depends on whether you are able to use the pump to produce a constant pressure, and walk at a constant rate. This is why a pressure gauge is so valuable. If your sprayer is not fitted with a pressure gauge, you will need to operate the pump at a constant rate – so choose a rate you find comfortable.

Calibration method

1. Carry out a pre-operation service and prepare the sprayer for the job.
2. Mark out a 25m2 calibration plot in the area to be sprayed with a peg in each corner (the 25m2 can be any configuration – e.g. 5m x 5m, 12.5m x 2m, 25m x 1m or 16.6m x 1.5m, or any other combination to give 25m2).
3. Fill the sprayer to a known level, or use a permanent marker to mark the tank and fill it to that line.
4. Spray the calibration plot at the pressure and speed you will use when you do the job. Be sure to avoid over sprays, spills, missed areas, and any other spray faults.
5. When you have finished spraying the calibration plot, carefully take the sprayer off, take it back to a level area and measure the amount of water it takes to the fill the sprayer back to the original mark (from Step 3). The amount of water taken to refill the tank to the mark must be in measured in litres.