When the application of a pesticide fails to deliver the desired control of a pest, the immediate thoughts are that something is wrong with the pesticide or there is pesticide resistance. However, the cause may be due to a different reason, water quality.

Water is the most common ingredient in pesticide applications. Water is an effective solvent for many pesticides and enables small amounts of pesticides to be applied uniformly over large areas.

Water makes up over 90% of most pesticide spray mixtures and is often taken for granted. Water quality is not usually considered given that water used for mixing pesticides comes from a variety of sources including rivers, lakes, ponds, and wells. The quality of water from these sources can vary widely in three critical areas: water pH, water hardness, and water turbidity.

Water pH

• pH is a value that describes the relative acidity or alkalinity of a solution. The pH scale runs from 0 – 14 where:
  • A pH less than 7 is considered acidic
  • A pH of 7 is neutral
  • A pH greater than 7 is considered alkaline.
• Most pesticides perform best in water with a pH between 4 and 6.5. When the spray solution is outside the ideal range, the pesticide will be hydrolyzed and degraded, and will not work as desired.

Water hardness

• Water hardness is a measure of the total concentration of positively charged calcium, magnesium, iron, sodium, and aluminum ions in water.
• Water hardness is measured in milligrams per liter (mg/l), parts per million (ppm), or grains per gallon (grains/gal).
• Hard water can be found in over 85% of US water resources.
• When hard water is used to mix pesticides, negatively charged pesticide molecules will combine with positively charged ions in hard water to create molecules that will either precipitate out of the solution, enter target pests at a slow rate, or cannot enter the target pest at all.

Water turbidity

• Turbidity is a measure of the total suspended solids in water. It is the haziness of a liquid caused by suspended solids.
• Turbidity is usually caused by soil and organic matter, which can reduce the effectiveness of the active ingredientsof some pesticides.
• Positively charged pesticide molecules are attracted to negatively charged particles found in water, making them unavailable for plant uptake.
• In addition, soil particles and organic matter will plug nozzles and screens leading to uneven spray patterns and time lost repairing equipment.

How to check water quality

• Determine the pH of your spray water before adding the pesticide. Check it several times a year.
• Determine the hardness of your water. Knowing only the hardness of the water being used for spraying may not be adequate. The pH level determines the amount of ions in the solution which can be tied up by hard water minerals. For an accurate hardness reading, take a water sample to a local lab.
• Always use clear water in spray tanks. Applicators can easily test turbidity by dropping a quarter into a five-gallon bucket filled with water. If the water is too cloudy to see the quarter, seek an alternative source of water for the spray mixture.

Some rules to follow to overcome issues with water quality

• Use pesticides that are least affected by water quality. For example, if an applicator wants to apply 2,4-D and the water pH is high, using 2,4-D ester instead of 2,4-D amine will yield the best result.
• Use nonionic adjuvants to solve water quality problems. For example,
  • AquabupH™ with Nitrogen is a nonionic buffering and conditioning agent that can:
    • Lower the pH of the spray solution to less than 4.
    • Combat all hard water cations that negatively affect pesticides.

• Water Conditioner™ is a nonionic buffering and conditioning agent that can:
  • Lower the pH of the spray solution to less than 4.
  • Sequester cations like iron, calcium, and magnesium salts.

• Spray as soon as possible after adding the pesticide to the spray tank or include an adjuvant that can maintain the spray mixture pH desired by an applicator. For example:
  • AquabupH™ with Nitrogen and Water Conditioner™can maintain pesticide spray solution at a low pH for over 8 hours.

Further Reading

• Brewer 90-10 Surfactant for Excellent Vegetation Management
• Optimize Your Weed and Brush Control with Silnet 200 – A Superior Nonionic Surfactant
• Best Practices of Utility Vegetation Management
• Control of Woody Vegetation in Rights-Of-Way using Basal Applications
• Basal Bark Treatment: What is it and How is it Used?

References

• Whitford, F. et al. The Impact of Water Quality on Pesticide Performance: The Little Factor that Makes the Difference. Purdue Univ. Ext. Bull. PPP-86. https://www.extension.purdue.edu/extmedia/PPP/PPP-86.pdf#:~:text=Water%20qual-%20ity%20parameters%20such%20as%20acidity%20and,in%20inferior%20performance%20and%20the%20need%20for%20re-treatment. [Accessed, July 27, 2022].