Pesticide Efficacy Hinges on Spray Water pH

During a grower roundtable discussion at one of this summer’s FBGA meetings, we received questions about the effects of spray water quality on pesticide efficacy —specifically pH effects. This is a topic that we have not addressed in our own research, so we looked to the literature for answers. Here’s what we found: Most pesticides (insecticides, fungicides, and herbicides) come in a concentrated form that must be mixed with water before being injected into irrigation systems or applied with a tractor-mounted sprayer to plants or beds. Research has shown that the pH of the water used to dissolve or place a pesticide into suspension can impact its effectiveness. Some products are more sensitive to water quality than others. Below, we discuss where to look for information about pH effects on specific pesticides, and a brief overview of conditioning and buffering adjuvant products.    

The term pH refers to the concentration of hydrogen and hydroxide ions in a solution. If hydrogen dominates, then the solution is acidic, resulting in a pH below 7. If hydroxide is the dominant ion, the solution is alkaline, and the pH is above 7. A pH of 7 is neutral, and since pH uses a logarithmic scale, pesticide sensitivity to pH increases by a factor of 10 for each unit change of pH.  

Certain pesticides undergo a chemical reaction in alkaline water known as alkaline hydrolysis, which can reduce the effectiveness of a pesticide’s active ingredient and is irreversible. When alkaline hydrolysis occurs, the level of pH causes the pesticide molecules to separate and reassemble with different ions. These new combinations may have reduced or no pesticide efficacy. The rate of hydrolysis increases as the pH of the water rises. In general, pesticide efficacy decreases over time in water pH in excess of 7.  

The amount of time required for a pesticide’s strength to decrease by 50% is referred to as its half-life. A pesticide’s half-life is also impacted by water temperature (an increase of 18°F will double the speed of hydrolysis), the pesticide’s chemical properties, and the amount of time the mixture remains in the spray tank. The longer the pesticide is stored in the tank at an alkaline pH prior to application, the more potential there is for a reduction in efficacy.

Low pH (acidic) spray solutions can enhance the efficacy of certain herbicides, like the non-selective glyphosate. Acidic spray solutions can also increase the potential for phytotoxicity of products with copper as the active ingredient by increasing the solubility of copper ions. Many products with copper hydroxide warn against tank-mixes with acidic products like Aliette (fosetyl AL) for this reason.  

So how do we know which pesticides are sensitive to water pH and which are not? In general, insecticides — in particular organophosphates, pyrethroids, and carbamates — are more susceptible to alkaline hydrolysis than fungicides, herbicides, or growth regulators. However, certain older fungicides and herbicides can be very unstable in the presence of alkaline pH. The half-life of captan can be reached in as little as 3 hours at a pH of 7.1, but only 10 minutes at a pH of 8.2 and 2 minutes at 9.0. Other pesticides with short half-lives include Malathion, phosmet (Imidan), diquat, and paraquat. In many cases where pH effects are known by manufacturers, the product labels suggest maintaining spray solutions within a range of pH using a buffering agent or the use of a non-ionic surfactant will be recommended. Therefore, it is very important to read and follow all label instructions. 

General recommendations include applying pesticides immediately after mixing them rather than storing diluted products. To minimize the possibility of reduced pesticide efficacy through hydrolysis, water pH should be between 4 and 7, with an ideal range of 5.5 to 6.5 for most products. The most accurate method of testing water pH is by using a pH meter, and pH indicator test strips are also available. Florida deep well water that comes from the Floridan aquifer has a pH between 7 and 9. In most cases, the pH of this water should be lowered to a more favorable range. This can be accomplished using commercially available buffers or acidifying agents. These products should be added to the water before mixing in the pesticide according to the product label. 

If questions arise regarding the need for pH effects and the label is unclear, it’s never a bad idea to contact the pesticide manufacturer and ask a technical representative within the company. You can also reach out to us as your UF/IFAS Extension Specialists.  

References

Cloyd, R. 2015. Effect of Water and Spray Solution pH on Pesticide Activity. Kansas State University Cooperative Extension Service. 

Deer, H. and Beard, R. 2001. Effect of Water pH on the Chemical Stability of Pesticides. Utah State University Extension. 

Fishel, F. and Ferrell, J. 2019. Water pH and the Effectiveness of Pesticides. University of Florida EDIS Publication PI-156.

Halcomb, M. 2012. The pH of the Spray Water is Very Important. The University of Tennessee Extension.

CREDITS:

Dr. Philip Harmon, Professor, UF/IFAS

Dr. Oscar Liburd, Professor, UF/IFAS

Dr. Peter Dittmar, Associate Professor, UF/IFAS

Doug Phillips, Blueberry Extension Coordinator, UF/IFAS