The quality of water being used in the spray tank to act as the carrier for your pesticides can have significant effects on how well those pesticides will work. It is surprising, however, to find that very few growers have had water quality tested.
Water with suspended materials such as clay, algae and other debris will block filters and possibly nozzles which makes spraying very frustrating. However, there are a range of water quality variables unseen to the naked eye that can affect specific pesticide formulations. The two that cause the most confusion are hardness and pH.
Water hardness

Water that is considered hard has high levels of calcium, magnesium or bicarbonate ions. Calcium and magnesium ions have positive electrical charges that enable them to bind with negatively charged products such as ‘weak acid’ herbicides in solution making them less soluble. Extreme cases can lead to the herbicide settling out in the spray tank or most commonly reducing the ability of the active ingredient to be absorbed through the plant leaf. Example of weak acid herbicides includes glyphosate and amine formulations of 2,4-D, MCPA, 2,4-DB, clopyralid and diflufenican.
Water hardness above 250 to 350 parts per million (ppm) (calcium carbonate – CaCO 3 equivalents) should be treated before adding weak acid herbicides.
The cations that can cause the most trouble for pesticides include:

  • aluminium (Al +++ )
  • iron (Fe +++ , Fe ++ )
  • magnesium (Mg ++ )
  • calcium (Ca ++ )
  • sodium (Na + )

With magnesium and calcium being the most common cations causing water quality problems, aluminium can be a problem if alum (aluminium sulphate) has been used to remove (flocculate) clay and other particles from the spray water.
Bicarbonates can also affect herbicides such as Group A ‘dims’ (e.g. clethodim) and a 2,4-D amine at levels as low as 175 ppm. Bicarbonates are not detected by standard water hardness tests and must be tested for separately, to total hardness. Groundwater from areas with lots of limestone can be high in bicarbonates. Water high in bicarbonates can be overcome by substituting MCPA or a low volatile ester formulation of 2,4-D for the 2,4-D amine. Group A ‘dim’ herbicides can be helped with the use of ammonium sulphate.
To treat water for hardness and bicarbonates, use up to 1% crystalline ammonium sulphate or liquid formulations of ammonium sulphate such as a Liase®.
Water pH

The pH of a liquid is a method of describing how acid or alkaline the fluid is. A neutral pH is about 7 whereas a pH of 2 is very acidic and a pH of 14 is very alkaline. It is important to remember that the pH scale is a log scale. This means that a value of 6 is 10 times more acid than a pH of 7 while a pH of 8 is 10 times more alkaline than 7 and 100 times more alkaline than 6. The table below gives some examples of common materials and their pH.
Table 1. pH values of some common substances.

pH Substance
14 Sodium hydroxide (caustic soda)
12.6 Sodium hypochlorite (bleach)
11.5 Ammonia
10.2 Magnesium hydroxide (antacids)
9.3 Sodium borate (borax)
8.4 Sodium bicarbonate (baking soda)
8.1 Seawater
7.4 Human blood