October 2012 Newsletter (vol.11)   >>   Water Quality and Pesticides

Blue Water Wave

There are a lot of factors that go into spraying pesticides successfully. However, water quality is usually not even considered. It is important to know that the quality of spray water can have an effect on your pesticide efficacy. Suspended solids, hard water, alkalinity, and pH can affect the amount of control you get with your pesticide application. This article will describe what these water quality factors are and what you can do to remedy problems.


Suspended solids include clay, silt, algae, iron, and other solids that will not pass through a filter. Some pesticide molecules can adsorb to suspended solids making them unavailable to execute their mode of action on the target pest. Glyphosate, diquat, paraquat are especially affected by suspended solids. These solids can also block nozzles, lines, filters & can potentially reduce sprayer life. Suspended solids can also contain plant pathogenic fungi (i.e. pythium), bacteria, and human pathogens. Sedimentation in a tank or reservoir and/or the use of coagulants can be used to precipitate out the solids making it safe for use in a pesticide application. The best precaution is to use water that has a low amount of suspended solids. If it looks turbid it probably shouldn’t be used as a pesticide carrier.


Hard water is water that contains high concentrations of dissolved minerals including magnesium, calcium, sodium & iron. Simple indicators of hard water are that soap won’t lather and it leaves spots on glass. Hard water measurement units are either ppm or grains per gallon. Hard water according to the World Health Organization is any water that tests above 342 ppm. If the sum of the concentration (ppm) for all of the cations exceeds 400 ppm, corrective measures will be needed to ensure good results. Cations cause problems because they bind with the pesticide molecules and form a precipitate. This reduces the pesticide efficacy and can also form scale inside the sprayer and clog it.


High pH and hard water act together to reduce herbicide effectiveness. High pH causes the herbicide to dissociate and the high concentrations of cations bind with the dissociated herbicide to reduce its efficacy. In general, pesticides prefer a pH of 4 to 7. Some insecticides and fungicides are susceptible to alkaline hydrolysis (Table 1.)


Table 1. Insecticide half-life of some common landscape insecticides
Insecticide Optimum pH Alkaline (pH > 8) Neutral (pH = 7) Acidic (pH , < 6)
Permethrin EC
7 Unstable Stable Stable
Permethrin WP 4 42 Days Stable Stable
Cypermethrin 4 35 Hours Stable Stable

Malathion (01-1108)

5 5 Hours 3 Days 8 Days
Orthene (01-1290) 7 16 Days 46 Days 40 Days
Imidacloprid (01-1129) 7 Stable Stable Stable
Carbaryl (01-1364) 7 24 Hours 24 Days 100 Days

Always read the label for instructions such as this: Azadirachtin (Azaguard) label “Always use this product promptly after mixing with water. AZAGUARD will break down in the spray solution if not used within 8 hours. Never allow tank mix to stand overnight. AZAGUARD will break down in spray tank mixtures that have pH values exceeding 7.0. The recommended pH range is between 5.5 and 6.5. Use a buffering agent.”


Water pH guidelines should be followed when mixing pesticides. A pH of 3.5 to 6.0 range for a short-term storage of 12-14 hours. Solubility of sulfonylurea herbicides (Corsair, Katana (04-0780), Revolver, Prosedge) can be increased by increasing pH. A pH of 6 to 7 is great for immediate spraying (1-2 hours). Products mixed with alkaline water should be sprayed immediately. When water pH exceeds 7.0, consider using adjuvants to lower pH.


Alkalinity is another water quality measurement that needs to be considered. Alkalinity is expressed as ppm of calcium carbonate and gives water the power to neutralize acids. Have your water tested for alkalinity to determine what you need to consider. Some pesticides precipitate when alkalinity levels exceed 300ppm. Adding ammonium sulfate to your alkaline water before adding the pesticide will prevent precipitation.


Insecticides and herbicides are influenced the most by water quality. Fungicides are less affected by alkaline water. Read the label to find out if water quality has an effect on pesticide efficacy. Herbicides can be affected by several different aspects of water quality, see Table 2.


Table 2. Water quality and herbicides
Herbicide Muddy Hard pH > 8 pH < 5
metsulfuron methyl (Blade) + + Marginal X
Dicamba + NR    
diuron (Karmex) + + +  
fluazifop (Fusilade, Ormec) + + NR X
chlorsulfuron (Corsair) + + Marginal X
glysophate (Roundup) X X +  
diclofop-methyl (Illoxan) + + NR +
Clopyralid (Lontrel & Confront) + X X  
Sethoxydim (Post, Grass Getter) + + X +
Simazine (Princep, Caliper) + + NR  
Paraquat & Diquat (Spray Seed) X + + +

X=do not use; NR=not recommended; Marginal=may or may not affect efficacy


When choosing a 2,4-D herbicide be sure to know your water quality. The two different 2,4,-D formulations (amine & ester) are affected differently by hard water and alkalinity. Amines of 2,4-D are more sensitive to hard water >600 ppm and alkalinity >500 ppm. Switch to an ester if your water is close. Or you can add ammonium sulfate, organic acids, and a non-ionic surfactant to get more out of your application.


The basic protocol in dealing with water quality problems is to first test your water, then correct it with Ammonium sulfate, organic acids and a non-ionic surfactant. Hummert has an exclusive product called Sur-Tec (01-1693) that has all THREE ingredients! Mix Sur-Tec in the water carrier first before adding the pesticide. Consider using the high rate and spray the tank mixture immediately.


How long can pesticides be stored in a tank? Pesticides breakdown over time through hydrolysis. Today most pesticides are formulated to stay in slightly acidic water for 24 hours. Municipal water usually has a pH of 7.8 and 8.5 due to addition of chlorine. Check the label for a mention of pesticide half life and heed the instructions. And a good rule of thumb is to not let spray mixtures stand overnight.


It is imperative that you know your water quality to ensure that your pesticide applications are successful. Suspended solids, hard water, alkalinity, and pH can all affect the amount of control you get with your pesticide application. Get your water tested today.


Deer & Beard. 2001. Effect of water pH on the chemical stability of pesticides. Utah State University Cooperative Extension Petroff. Water effects on pesticide performance. Montana State University Purdue University Extension. 2009. The impact of water quality on pesticide performance. PPP-86


Effect Of Water PH On The Chemical Stability Of Pesticides

Water Effects On Pesticide Performance

The Impact Of Water Quality On Pesticide Performance