3/30/2015 - By Dr. Raymond Snyder

Getting the most out of a pesticide application requires proper diagnosis, appropriate timing, and a well-trained applicator. Unfortunately, in some cases a successful pesticide application never has a chance because of poor or inappropriate quality of water used to fill the spray tank. Learning to determine and understand the effect of spray tank water conditions on your pesticide of choice can significantly improve efficacy and reduce overall pesticide use.

The two main water quality factors that determine the stability and overall efficacy of pesticides are water hardness/softness and pH. Addressing these two factors when tank mixing will help increase the success of a pesticide application.

Hard Water

Water hardness is a measure of the total concentration of calcium (Ca²⁺) and magnesium (Mg²⁺), which often are expressed as parts per million (ppm) or milligrams per liter (mg L^-1)(Figure 1). Table 1 can be used as a reference when categorizing water hardness with values received in a water quality analysis. Hard water ions react with certain pesticides, especially herbicides, causing them to precipitate out of solution, thereby reducing overall pesticide efficacy when water hardness is excessive.

The effect of hard water on isopropylamine salt formulations of glyphosate is a classic example of a pesticide that can be affected by hard water. Herbicides formulated as salts, such as glyphosate, dissociate in water forming a positively charged species (cation) and a negatively charged species (anion). Hard water contains elevated concentrations of positively charged Ca and Mg which compete with the isopropylamine cation for association with the glyphosate anion. Overall glyphosate efficacy is reduced because the glyphosate anion combination with Ca and Mg precipitates out of solution and is less likely to be adsorbed into the plant leaf. Other pesticides that form salts in water are similarly affected by the calcium and magnesium in water.

Reducing the antagonistic effects of hard water can be achieved several ways. The addition of ammonium sulfate (AMS) to the tank solution is one of the most commonly used techniques. In water, AMS dissociates into the ammonium cation and the sulfate anion. The ammonium cation can compete with Ca and Mg to reduce the formation of undesirable glyphosate Ca and Mg salt combinations. The following equation can be used to calculate the amount of AMS required:

AMS (lb/100 gal) =
(0.005 x sodium ppm) + (0.002 x potassium ppm) +
(0.009 x calcium ppm) + (0.014 x magnesium ppm)

For most water quality conditions the use of 8.5 lbs AMS / 100gal water is suitable. Adjuvants that contain AMS, such as Delux, which are specifically formulated to enhance herbicide efficacy can be utilized to combat the negative effects of hard water conditions.

Organic acids can also be used to reduce the antagonistic effects of hard water. For example, citric acid is commonly used to remove hard water cations from solution. The negative portion of the organic acid sequesters the positively charged cations, removing them from solution. Use 2.2 lbs of citric acid per 100 gallons to treat water with 250 ppm of Ca²⁺. Many user friendly products, such as New Balance, are citric acid based products that are specifically designed modify spray tank solution conditions and do not require the handling of citric acid specifically.

Water pH

Water pH plays an important role in the stability and efficacy of pesticides. A pesticide can begin degrading from the moment it is introduced to a spray tank. Hydrolysis, a process that breaks the bonds holding pesticides together, can reduce the life of a pesticide in solution and is significantly affected by water pH. Extremes in spray tank pH can render a pesticide mixed in the morning useless in the afternoon. Getting the most out of your pesticide application requires one to determine their water pH level and adjust spray tank pH as necessary.

Pesticides are formulated for optimum performance at specific pH ranges. Most pesticides are stable at pH 7 or in slightly more acidic conditions between pH 5-7. Orethene (acephate), Chipco 26GT (iprodione), Daconil (chlorothaloni), and Sevin (carbaryl), are examples of pesticides that resist hydrolysis at pH values 5-7. Some pesticides such as Signature (fosetyl-Al) are stable at a wider range of pH values and are not as susceptible to alkaline hydrolysis until pH exceeds about 9. Table 2 provides some specific examples of commonly used pesticides and the effect of water pH on the rate of alkaline hydrolysis.

The product label instructions will generally provide guidelines as to specific recommendations for water pH. For example, the Subdue maxx (mefonoxam) label specifically states that pH level must be less than 7.5. The Revolver (Formasulfuron) herbicide label recommends a pH near 7 and suggests the use of a buffer if water pH is less than 6. In general, most sulfonyl urea herbicides, such as Revolver and Monument, are susceptible to degradation below pH 7.

The following suggestions can help to minimize pesticide efficacy due to pH:

1. Get to know your water. Have it analyzed periodically for hardness (calcium and magnesium) and pH. You can check water pH yourself regularly throughout the year. Seasonal changes can occur from sources such as ponds and reclaimed water facilities. Inexpensive meters and pH strips are readily available for measuring pH.
2. Read the pesticide label and adjust the spray tank water pH when necessary. Buffers and acidifying agents are available for adjusting the spray tank water before the addition of a pesticide if water pH is not in favorable range according to the label.
3. Use a jar test to help adjust the spray tank water pH. Example:
   • Measure out one pint of water to be used.
   • Using an eyedropper, add three drops of the buffer or acidifier that will be used to adjust the water pH.
   • Stir well with a clean, non-porous utensil.
   • Check the pH with test strips or a pH meter.
   • Repeat above steps if further pH adjustments are needed, recording the total number of drops used.
   • For every 100 gallons of water in the spray tank, add 2 ounces of the buffer or acidifying agent for 3 drops that were used in the jar test.
   • Mix the water in the spray tank thoroughly.
   • Test a sample of the spray tank water for pH.
   • Add more buffer or acidifying agent if necessary, and re-check the pH.
   • When the pH is correct, the pesticides can be added to the spray tank.

In conclusion, many factors affect the efficacy of pesticides applied to turf and ornamentals. Two factors, water hardness and pH, can be easily determined and accounted for when making pesticide applications. In these days of rising chemical costs it is becoming increasingly more important to take the necessary steps that help insure a proper pesticide application is made.

Figure 1.

Hardness (mg/l) = 2.5[conc. of Ca²⁺ (mg/l)] + 4.1[conc. of Mg²⁺ (mg/l)]

Table 1.

Table 2.