Mechanisation: How to effectively use knapsack sprayers

The effective use of pesticides and herbicides is important for good yields in any crop, and could save you money and/or increase yields.

To understand the methodology of chemical spraying one has to look at the principles of the spraying technique.


  • Droplets are formed when a water or oil base substance is forced through a narrow opening under pressure.
  • Spray nozzles produce a mixture of large and small droplets.
  • More large droplets are produced at low pressure.
  • Droplets tend to become smaller when the pressure is increased.
  • Droplets also tend to drift in strong wind and when they become too small, they will miss the target totally.
  • By using a fan on a sprayer (as used in orchards) droplets can be forced to reach a target.


  • Two types of chemicals are used in the industry.
  • The one is a systemic substance, which means that the chemicals are absorbed by the plant when sprayed and then works from the inside of the plant.
  • The other type of chemical is called a contact substance, which means the chemical only works effectively where it touches the target.
  • When spraying a systemic substance, spray coverage is not so important, but when spraying a contact substance, it is of vital importance to get good spray coverage.
  • The same volume of liquid divided into many small droplets gives better coverage than fewer large droplets of the same volume.
  • It is therefore better to spray at a higher working pressure, which produces small droplets, for good overall leaf coverage.
  • Drift, as a result of wind, must be taken into account because smaller droplets tend to drift more easily than bigger droplets.


The following weather conditions and external factors will have an influence on the efficiency of spraying:

Wind. Spraying in very windy conditions is not recommended because the wind causes the droplets to drift from the target. Wind can be tolerated to some extent by using nozzles that produce bigger droplets, but spraying is not recommended to spray in wind with speeds of more than 6 km/h.

Day temperature. Do not spray during the hottest time of the day because the smaller droplets tend to evaporate into the air. The sprayed chemical will not reach the target, which will result in a loss of money.

Crop stage. Crop stage and crop density has an influence on the choice of volume rate applied. The bigger the crop, the higher the spray volume rate in l/ha that must be applied to get good spray coverage.

The recommended spray volume is indicated on the chemicals container.


  • Just as one can change the spray pattern on a hose pipe, different nozzles are shaped to produce specific spray patterns.
  • A variety of nozzles and nozzle sizes are available.
  • 2 nozzle types may even look alike but their spray patterns will differ.
  • Only by knowing the marking code on the nozzle, can one be sure what kind of nozzle it is. For this one needs to consult the manuals of the manufacturer of the specific nozzle.
  • Your extension officer may be able to advise you in this regard.
  • You could also do test sprays onto a dry surface to get an idea of what the nozzle does. Carefully observe the patterns you get and judge whether a fine spray or course spray is produced.


The knapsack sprayer is widely used in various types of applications.

A lever-operated sprayer consists of a 15 to 20-litre tank, which will stand erect on the ground and when in use fit comfortably on the operator’s back like a knapsack, a hand-operated pump, a pressure chamber and a lance with an on/off tap and one or more nozzles.

The volume of liquid in the tank is indicated by graduated marks moulded onto the plastic tank.


  • When operating the sprayer, liquid is drawn through the inlet valve into the pump chamber on the up stroke.
  • On the down stroke, liquid in the pump chamber is forced through the outlet valve into the pressure chamber.
  • The inlet valve between the tank and the pump is closed during this operation to prevent the return of liquid to the tank.
  • Air trapped in part of the pressure chamber and compressed as liquid is forced into the chamber.
  • The compressed air forces the liquid from the pressure chamber through the hose to the nozzle.


  • Clean and flush the sprayer after each use with clean water to prevent chemical corrosion and cross contamination.
  • Decompress the sprayer after use to release all the fluid out of the sprayer without further pumping.
  • Clean the nozzle assembly thoroughly by rinsing it in hot water after use to prevent blockages.
  • Clean all available filters.
  • Always mix the chemicals in the knapsack tank, never in clean water containers.
  • Make sure the air vent in the screw cap is unblocked. The sprayer will not work properly if the vent is blocked.
  • Adjust knapsack straps when fitted to ensure comfort before starting. Let someone help you.

Also read:
Mechanisation: A short guide for new farmers
Crop production: Are your weeds herbicide resistant?

  • This article was written by Johan van Biljon and first appeared in Farming SA.

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