When a pesticide is applied to control a pest according to label instructions it may not work as expected. When it doesn’t work, the assumption may range from a highly complex one, such as in the occurrence of pesticide resistance, toa simple one, such as the pesticide spray water being at the wrong pH.
Pesticide resistance is complex because numerous studies must be conducted to affirm the resistance of the pest to the pesticide. Only thereafter can pesticide resistance be addressed properly. On the other hand, water pH can be easily confirmed using simple tests. Once confirmed, the pH can be adjusted using buffering agent, which makes the pesticide more effective.
What is pH?
The pH of a solution is a measure of acidity or alkalinity of a solution and refers to the number of hydrogen (H+) and hydroxyl (OH-) ions in the solution. The lower the pH, the more acidic the solution, while a higher pH indicates the solution is more alkaline.
- Water at a pH of 7 is neutral, meaning there are equal numbers of hydrogen and hydroxyl ions in the solution.
- Water at a pH less than 7 is acidic, whereas water at a pH greater than 7 is alkaline.
- Many water sources used for pesticide application have pH values between 7 and 9.
- A pH of 4 to 7 is recommended for mixing most pesticides while a pH of 4.5 to 6.5 being the ideal.
What factorsaffect pesticide activity?
Water pH, soil pH, and mixing conditions can affect pesticide activity.
Water pH
- Some pesticidesundergo a chemical reaction in the presence of alkaline water (water that has a pH value greater than 7), for example carbamate and organophosphate insecticides. Known as alkaline hydrolysis, the effectiveness of the pesticide’s active ingredient is reduced at high pH.
- Most pesticides are sold in concentrated form and have to be dissolved or suspended in water before they can be applied. Water used for pesticide application can come from a variety of sources, e.g., municipal water supply, ponds, rivers, or wells.
- Water naturally varies in the amount of dissolved minerals and pH, depending on the source. The pH of water can negatively affect the stability of some pesticides.
- Under alkaline conditions, pesticides can be degraded to non-toxic levels which renders them inactive resulting in poor pesticide performance.
- Most natural waters have high mineral content which causes them to be alkaline.
- Check the pH of the water used for spraying pesticides frequently throughout the season.
How to check the pH of water to be used to mix pesticide
- The fastest way to determine pH level of water is to use a pH meter or test strip. A pH meter is more reliable than a test strip.
- Alternatively, a sample of the water can be sent to a soil and plant testing laboratory to conduct water pH analyses.
Soil pH
When soil pH is low it can affect the persistence of some pesticides.
For example, if the pH of the soil drops below 6.0, triazine, sulfonylurea, and imidazilinone herbicides can become increasingly bound to soil particles which reduces their availability for breakdown by soil microorganisms, the primary mechanism for their degradation. The bound herbicide can be released several months later becoming available for plant uptake and potentially injuring a sensitive crop.
Mixing Conditions
The effect of pH on pesticides varies from product to product and is also moderated by buffering solutions contained in the pesticide formulation. In addition, tank-mixing multiple pesticides can modify the pH of the spray solution in the sprayer tank.
What chemicals are used to adjust pH?
If the pH of the water used to make the pesticide spray solution is high or low, you may need to use a buffering agent.
What is a buffering agent?
A buffering agent is classified as a utility modifier that widens the range of conditions under which a given pesticide formulation is useful. Thus, a buffering agent increases the dispersion and/or solubility of a pesticide under conditions of extreme acidic or alkaline waters.
When should you use a buffering agent?
If you know that your water pH is 7.5 or greater, consider lowering the pH.
Product labels will provide statements regarding the reduced effectiveness of the pesticide if it is mixed with alkaline water. Additional directions may indicate the need to add a buffering or acidifying agents in the spray tank.
Example of buffering agents
- AquabupH®
- Regulates pH of spray solutions.
- Sequesters cations in hard water.
- Enhances uptake of herbicides in plants via the addition of nitrogen.
- Water Conditioner®
- Buffers and lowers pH of water.
- Sequesters cations such as calcium, magnesium, and iron.
Read the label
For the best results read and follow all recommendations on the labels of the pesticide and buffering agent.
Further Reading
- Best Water Conditioners for Irrigation/Agricultural Use.
- pH Adjustments – Optimizing the Performance of Weak Acid Herbicides.
- The ABCs of Tank Mixing
- Adjuvants 101: Understanding What’s in Your Spray Tank
References
- Fishel, F.M. and J.A. Ferrell. 2010. Water pH and the Effectiveness of Pesticides. Univ. FL IFAS Extn, PI-156. https://edis.ifas.ufl.edu/pdf/PI/PI19300.pdf [Accessed, June 27, 2022].
- Schilder, A. 2008. Effect of water pH on the stability of pesticides. Michigan St. Univ. Extn., Dept. Plant Pathology. https://www.canr.msu.edu/news/effect_of_water_ph_on_the_stability_of_pesticides [Accessed, June 16, 2022].
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