Alkaline Degradation of Pesticides
Some pesticides are chemically denatured by mixing with alkaline water, or in alkaline soil. As an example, the label of Azatin XL mentions that dilute solutions should be maintained at a pH between 3 and 7, and applied soon after preparation. It states that the diluted solution must not be stored for later use. In order to use this product effectively, you will have to correct the pH of your water before mixing. Azatin is not the only pesticide that degrades rapidly in alkaline water (pH greater than 7). The carbamates and organophosphates are generally more susceptible than chlorinated hydrocarbons or pyrethroids.
The first step in preventing alkaline hydrolysis is to determine the pH of the water used for measuring chemicals. Because of the seasonal variability, it is important to measure the pH several times over the course of the growing season. 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 determined 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 less expensive and more practical in the field. In general, the indicator is mixed with or dipped into the water and the resulting color is compared to a chart. It is often necessary to do a preliminary test with a wide range indicator included in the test kit to make a rough estimate. An indicator with a narrower range is then used for a more precise determination.
Buffering agents are available to add to the tank water if the spray chemical is subject to alkaline degradation. Chemical breakdown can take place quickly, before the tank is emptied. If you are using a tank mix, it is important to know that susceptible materials should not be mixed with anything that raises the pH of the solution, such as lime sulfur and liquid ammonia. Also, fixed copper fungicides such as Bordeaux mixture should not be acidified. Copper is more soluble under acid conditions, and so if acidified, more copper will dissolve and could be phytotoxic.
Excerpted from "Preventing Decomposition of Agricultural Chemicals by Alkaline Hydrolysis in the Spray Tank," by A. J. Seaman and H. Riedl, New York’s Food and Life Sciences Bulletin no. 118, Cornell University, Ithaca.
Information on Stability of Pesticides
Diazinon is most stable in pH 7 water, with a half life of 10 weeks; at pH 5, it is 2 weeks.
Chlorpyrifos (Dursban) at pH 8.0 has a half life of 1.5 days.
Malathion is stable at pH 5.0-7.0 but rapidly hydrolyzes in more acidic or alkaline conditions.
Carbaryl (Sevin) has a half-life of 24 days at pH 7.0, but only 1 day at pH 9.
Bendiocarb (Dycarb) can be less effective if alkaline spray water is used.
Azadirachtin (Azatin XL) should be maintained at pH 3-7, and applied soon after mixing.
Iprodione (Chipco 26019) rapidly hydrolyzes at pH above 8.0.
Mancozeb (Dithane) is most stable at pH 5.5-6.0.
Maneb may be sensitive to alkaline hydrolysis.
Author: Jana S. Lamboy, IPM Extension Educator, Cornell University, Geneva, NY