Measurement of the acidity or alkalinity of a solution, commonly referred to as pH, is performed on a scale of 0 to14. A pH of 7 is considered neutral, whereas a high pH is in the 7 to14 range (alkaline), and a low pH is in the 0 to7 range (acidic).

Some pesticides can decompose quite rapidly in water with a high pH (greater than 7). If a spray mixture stands for several hours or overnight before use, much of the active ingredient may be neutralized by a reaction called Alkaline Hydrolysis, in which the pesticide molecule is split by the water and converted to an inactive form. Decomposition rates are determined by the individual chemical; each compound's rate is different. Usually the more alkaline the solution, the faster the rate of decomposition is. (See Table 1)

To prevent Alkaline Hydrolysis, you should:

1. Determine the pH of your spray solution. This should be done more than once over the growing season because of seasonal variation. Measuring your spray water pH before mixing can be misleading because the pesticides you add may lower or increase the pH of the overall spray solution. Always READ THE LABEL for each chemical to see if there are any warnings about water pH. If no specific instructions are found and you suspect a problem, run small tests with your most used chemicals after they have been mixed with your spray water. The most accurate way to measure pH would be through the use of an electronic pH meter. If you do not have one available, samples may be taken to the Provincial Soil and Feed Lab for analyses.

2. It is always a good practice to spray right after your chemical has been mixed. If a delay occurs, you may want to add a buffering solution to your tank to lower the pH and slow down the rate of alkaline hydrolysis. Contact your local Agricultural Chemical Representative for recommendations on which buffering solution to use and the rates required.

NOTE: There are a number of pesticide materials that should not be acidified under any circumstances because they become highly phytotoxic at low pH's. These include the fixed copper fungicides, lime and lime sulfur. However, if the label tells you to avoid alkaline materials, chances are that adjusting the pH to 6.0 or lower will be of benefit.

TABLE 1: Optimum pH's and half-lives of various Insecticides

References:

• 1998 Ohio Vegetable Production Guide, Bulletin 672.
• Michigan State University Extension, Fruit CAT Alerts - 05079602.
• PEI Dept of Environment.
• Wilbur-Ellis - Ideas to grow with; Factsheet.