Dropping pH can get you into a lot of trouble when spraying

Important tests to determine if water quality is suitable for spraying


Weeds, Pests, Diseases
This grower was told to drop the pH of his spray solution. He added citric acid to drop the pH and then another three products without measuring the pH of the solution before or after adding the citric acid. Two weak acid herbicides dropped the pH further, causing mix instability. PHOTO Rob Buttimor

This grower was told to drop the pH of his spray solution. He added citric acid to drop the pH and then another three products without measuring the pH of the solution before or after adding the citric acid. Two weak acid herbicides dropped the pH further, causing mix instability. PHOTO Rob Buttimor

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Poor water quality can greatly affect the control achieved with a range of pesticides.

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Key points

  • Poor water quality can adversely affect many products. Always consult product labels about water quality requirements
  • Test annually when using bore water, reticulated (piped) water sourced from ground water, water stored in unlined dams and concrete tanks. Water from streams and rivers should be tested after any major change in flow
  • Water tests should analyse for the following: total hardness, bicarbonate, pH and total dissolved salts (TDS) or salinity (electrical conductivity)
  • Deliberately dropping the pH of the spray solution can reduce the stability of tank mixes. The more products in the mix, the greater the risk

The quality of water being used as the carrier for pesticides can have a significant effect on how well those pesticides work.

Product effectiveness can be reduced by as much as 25 per cent when using poor-quality water - yet anecdotal evidence suggests few growers have had water quality tested or do so on a regular basis.

Product effectiveness can be reduced by as much as 25 per cent when using poor-quality water.

Growers need to be aware of a range of water quality variables that affect certain pesticide formulations. Water with suspended materials such as clay, algae and other debris will block filters and possibly nozzles, which delays spraying or gives uneven coverage across the width of the boom.

However, the two quality parameters 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 in solution with negatively charged products, such as 'weak acid' herbicides, which makes them less soluble.

Extreme cases can lead to the herbicide settling in the spray tank or, more commonly, reducing the ability of the active ingredient to be absorbed through the plant leaf.

Examples of weak acid herbicides include glyphosate and amine formulations of MCPA, clopyralid, diflufenican, 2,4-D and 2,4-DB.

While magnesium and calcium are the most common cations causing water quality problems, aluminium can be a problem if alum (aluminium sulphate) has been used to settle-out (flocculate) clay and other particles from the spray water.

Water hardness above 250 parts per million (ppm) (calcium carbonate - CaCO3 equivalents) should be treated before adding weak acid herbicides.

Water should be tested before and after the addition of pesticides because the pH will change.

Bicarbonates are not detected by standard water hardness tests and must be tested separately. Bicarbonates also affect herbicides such as Group A 'dims' (for example, clethodim) and 2,4-D amine at levels as low as 175ppm. .

Groundwater can be high in bicarbonates, but this can be overcome by substituting 2,4-D amine with MCPA or a low-volatile 2,4-D ester. Be mindful of potential off-target impacts when selecting herbicide products.

The efficacy of Group A 'dim' herbicides can be improved by modifying hard water with the use of ammonium sulphate. Use up to 1 per cent crystalline ammonium sulphate or liquid formulations of ammonium sulphate such as a Liase®.

Water pH

The pH of a liquid describes how acidic or alkaline it 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 acidic than a pH of 7, while a pH of 8 is 10 times more alkaline than 7 and 100 times more alkaline than 6. Table 1 gives examples of common materials and their pH.

While it is well recognised that pH above 8 will reduce the tank-life of certain pesticides, such as organophosphate insecticides, the effect on herbicides is largely overstated.

It is important to remember that water quality can vary over time depending on its source.

Acidic water (pH less than 5) can affect the stability of products and mixes and lead to gelling of salt-based products. It will also increase the volatility of herbicides such as dicamba.

Water above pH 8 can modify droplet contact on the leaf surface and reduce the stability of tank mixes.

Do I need to reduce the pH of my spray water?

Not if the pH is between 6 and 8.

Adding glyphosate will drop the pH of the tank mix two to three units, depending on initial pH, formulation and rate of glyphosate. The pH after addition of glyphosate is less than four - the yellow test strip. Scheme water right. PHOTO AGRONOMO

Adding glyphosate will drop the pH of the tank mix two to three units, depending on initial pH, formulation and rate of glyphosate. The pH after addition of glyphosate is less than four - the yellow test strip. Scheme water right. PHOTO AGRONOMO

In the image above, the test strip on the right is town water, which is normally pH 8.5, compared with the test strip to the left, which is 1 per cent (1L/100L) glyphosate 450 grams/L solution using the same town water - now showing less than pH 4.

Recent research in the United States has found drift damage from dicamba continued to be a problem, despite mandating the use of extremely coarse and ultra coarse spray quality. One cause identified was the addition of glyphosate to the mix, which reduced the pH of the spray solution (see Table 2 below).

Volatilisation of dicamba increases with decreasing pH.

Different formulations of dicamba were found to drop the spray solution pH from 7.8 to between 6.5 and 6.9. However, the addition of different formulations of glyphosate all dropped the spray solution to 5 or lower.

Water testing

Knowing the quality of the water you are using is essential for effective pesticide application.

Water should be initially tested by a qualified laboratory to establish an accurate baseline for your water quality.

It is important to remember that water quality can vary over time depending on its source.

Scheme or town water quality tends to vary little, while water from surface sources such as dams, tanks and rivers will vary depending on rainfall and other factors, and should be tested after any significant change in flow.

Groundwater (bores) can also vary over time depending on how much is being pumped and the recharge rates of the aquifer, and should be tested after any change in lift height.

Water used for spraying should be tested for:

  • total hardness;
  • bicarbonate;
  • pH; and
  • salinity (electrical conductivity) or total dissolved salts (TDS).

Something rarely discussed is that the addition of the pesticide will modify the solution pH. Water should be tested before and after the addition of pesticides because the pH will change.

Test strips can be used to quickly check water quality before and after the addition of pesticides and to monitor changes in water quality between laboratory tests.

Handheld meters are available, however, these need regular calibration to stay accurate.

Where can I get my water tested?

Check with your pesticide reseller or look for accredited laboratories in your state.

High-quality test strips can be purchased online from companies such as Hach (https://au.hach.com).

Water-testing kits for swimming pools will not be as accurate as those from a scientific supply company.

SOS Macquarie Valley

SOS Macquarie Valley decided to test water quality across the valley after an agronomist workshop survey revealed just 43 per cent of advisers 'sometimes' offered advice about water quality ahead of spraying, despite the group considering water quality often affected spray results.

The water quality survey of 180 samples, primarily from bores, showed:

  • nearly 90 per cent had high bicarbonate levels;
  • 20 per cent of samples were hard;
  • 32 per cent of samples were saline; and
  • nearly 80 per cent had a pH above 8.5.

The water quality survey also showed water north-east of a line from Coonamble through to Gilgandra and Warren to Nyngan was more alkaline, had higher bicarbonate concentration and contained few bores with hard and saline water than bores south and east of this line.

More information: GRDC Water Quality for Spraying Operations

SOS Macquarie Water quality analysis project - https://www.sosmacquarievalley.com.au/current-projects/water-quality-analysis

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