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Effects of pH on Pesticides and Growth Regulators

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Have you ever had trouble controlling a pest with a pesticide? You may have thought that the problem was due to the rate used or pest resistance, or maybe the chemical itself. Did you ever think to check the pH of the water used to mix the pesticide? The pH of the water that goes into your spray tank influences how effectively many pesticides and growth regulators work.

A review of pH and alkalinity

A pH reading is a measurement of the hydrogen ion concentration of a solution (how acidic or basic a solution is), and readings range from 0 (most acidic) to 14 (most basic). Alkalinity is a measure of the capacity of water to neutralize acids. It is the concentration of soluble alkalis in a solution. Dissolved bicarbonates such as calcium bicarbonate (Ca(HCO 3 ) 2 ), sodium bicarbonate (NaHCO 3 ), and magnesium bicarbonate (Mg(HCO 3 ) 2 ); and carbonates such as calcium carbonate (CaCO 3 ) are the major contributors to alkalinity in irrigation water. Alkalinity establishes the buffering capacity of water and affects how much acid is required to change the pH. Both alkalinity and pH must be considered when adjusting the pH of water.

Water test results

Forty-eight greenhouse businesses in Massachusetts participated in a two-year water quality project that includes having their irrigation water tested. The first test results performed in spring 04, showed that thirty-seven growers had water with pH greater than 7.0 and nine had water pH above 8.0, yet the alkalinity of most samples tested was within an acceptable range. Growers that had irrigation water with alkalinity that measured within the target range of 40 -160 ppm were able to manage the pH of their growing media by using fertilizers that had either a basic or acidic reaction depending on the needs of the crops. Acid injection for managing growing media pH was not recommended for these growers. The water tests helped growers to customize their fertilizing practices. Although the pH and alkalinity of the water was acceptable for irrigation, the pH of the water was too high to be used for mixing some pesticides and growth regulators.

Effects of pH on pesticides and growth regulators

Water pH is a critical factor in the effectiveness of many pesticides and growth regulators. Many materials work best when mixed in water that is acidic while a few perform best in a neutral or higher pH. P esticides such as organophosphates, synthetic pyrethroids, carbamates, chlorinated hydrocarbons, the growth regulator ethephon and others undergo a chemical reaction, called hydrolysis that causes them to break down in water with a pH greater than 7.0. The more alkaline the water, the more rapidly the pesticide breaks down. The hydrolysis can be very fast when the pH of the water is greater than 8 and 9. For every unit increase in pH, the rate of hydrolysis increases 10 times. Some pesticides begin to break down as soon as they are combined with water with high pH.

The rate and severity of the reaction is determined by how susceptible the pesticide is to hydrolysis, the amount of time the pesticide is in contact with water (pH >7) and the temperature of the diluted pesticide mixture. For example, if a spray tank is allowed to stand several hours or overnight before the contents are used, as much as 50 percent of the active ingredient may be lost.

Which pesticides and growth regulators are affected

Options for finding the optimum pH range is by reading the pesticide or growth regulator label or contacting the company technical representative. If the product label says to avoid alkaline water or materials, the spray mixture will benefit by adjusting the pH to 6.0 or slightly lower. Many manufacturers provide information on the rate at which their products hydrolyze. The rate is usually expressed as "half-life" or the time it takes for 50% of the product to breakdown (hydrolyze). Below are some examples of pesticides and optimum pH range of the water used for mixing.

Pesticide Optimum pH range Pesticide/Growth Regulators Optimum pH range
azadirachtin
(Azatin)
3.0 - 7.0 thiophanate methyl
(Cleary's 3336)
6.0 - 7.0
abamectin
(Avid)
6.0 - 7.0 chlorothalonil
(Spectro)
6.0 - 7.0
acephate
(Orthene)
5.5 - 6.5 fenhexamid
(Decree)
5.5 - 6.5
pymetrozine
(Endeavor)
7.0 - 9.0 (degrades at low pH) ancymidol
(A-Rest)
5.5 - 6.5
    ethephon
(Florel Pistill)
Less than 5.0

How to correct water pH

If you do not need to inject acid into your irrigation system and your water pH is above 7.0, adding a buffering agent is an easy way to change the pH of the water for mixing pesticides that require a low pH. Buffering agents prevent pesticide hydrolysis during mixing and storage in the tank. Buffering agents should not be used with pesticides containing fixed copper or lime such as copper sulfate, or lime sulfur. Copper is more soluble under acid conditions, and will damage plants . How much buffering agent to use will depend on the water pH. Too much of a buffer will cause the water to be too acid, and it can be phytotoxic to your plants, so be sure to test the water pH using the guidelines on the next page.

Water pH is affected by temperatures, sunlight, rainfall, drought and many other factors and is seldom the same from one spraying to the next.  Therefore, check water pH before each pesticide or growth regulator application.  Samples should be collected in a clean, non-reactive container, such as a glass bottle or jar. The water should be representative of the water used for spraying, so let the water run long enough to flush out the water that was standing in the hose and pipes. The pH should be tested soon after collection, because it can change if it is stored too long. The most accurate way to measure pH is to use an electronic pH meter. However, soil test kits and pH paper are adequate. Once the water pH has been tested, use a commercial buffering agent such as Buffer P.S., Buffer Extra Strength or  Brandt Indicate5, available from greenhouse and chemical supply companies. Read and closely follow the directions on the label of the buffering agent. The pH of the water should be adjusted before adding the pesticide. A pH of 6.0 is satisfactory for many pesticides. Some buffering agents such as pHase5 will have a color indicator when the correct pH is achieved. Growers can add this product into the water until it reaches the color that indicates a given pH. For example, 5 = pink or red; 6 = orange; etc. Not all pesticides react the same to the pH of the spray water solution and some products should not be used with pHase5:

Fungicides: Camelot, Chipco 26019 or 26GT, Daconil Ultrex, Junction DF, Kocide 2000, Medallion, Milstop, Pathguard, Phyton 27, Systhane.

Insecticides: Citation, Conserve, Endeavor, Thiodan (Thiodex) WP and EC.

Herbicides: Basagran T&O, Scythe

 For buffering agents that do not use a color indicator, the following jar test can be used as a guide to determine how much buffering agent to use. The test is first performed in a pint sized jar, and then the results applied to a 100 gallon spray tank.

pH adjustment procedure in sample jar:

  • Using a standard eyedropper, add 3 drops of buffer or acidifier to a measured pint of water.
  • Stir well with a clean, non-porous utensil.
  • Check pH using a pH meter.
  • If further adjustment is needed, repeat steps, until the pH is satisfactory.

Record the number of drops that were added to bring the solution to the proper pH.

pH adjustment in spray tank:

  • Before adding pesticides to the sprayer, fill the tank to the level required for the application.
  • For every 100 gallons of water in the spray tank, add 2 ounces of buffer for each time 3 drops of buffer were used in the jar test above. Add buffer or acidifier to water while agitators are running. If tank is not equipped with an agitator, stir or mix well.
  • Collect a sample of the water in the sprayer and check the pH to be certain it is correct. Add more buffer if necessary and recheck the pH.
  • Once the pH is correct, add the pesticides to the spray tank.

Buffers will enhance the effectiveness of your pesticides and growth regulators and give you a better residual.  This will, in the long term, reduce the number of sprays you make, reduce pesticide resistance and harm to the environment, and save money while helping you to produce a good crop. Simple, yet effective.

References:


Extension Greenhouse Crops and Floriculture Program
University of Massachusetts, Amherst
2004, updated 2010, 2012
Topics: 
Commercial Horticulture
Commercial Horticulture topics: 
Greenhouse Management
Integrated Pest Management
Weeds