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Pesticides and pH

Entering "water pH pesticides" into Google, yields 592,000 hits. Many of these are government or university web sites that include versions of the same table that lists the half-life (the time it takes for 50% of the pesticide to be inactivated) of pesticides at different pH's. For example, captan is listed as being most stable at pH 4 (acidic - 32.4 hr half-life), moderately stable at pH 7 (neutral - 8.3 hr half-life) and unstable at pH 10 (alkaline - 2-10 min half-life). However, none of the labels of captan products indicate the necessity to adjust pH. A search of the literature yielded a paper from 1976 that described alkaline hydrolysis of technical grade captan (pure, unformulated active ingredient) at pH10 in 10 minutes. However, formulated captan products (Maestro, Supra Captan, etc.) contain buffers, among the "inert" ingredients that make up the other part of the product that isn't active ingredient captan. Buffers work to maintain pH at a specific level, regardless of the pH of the water. To confirm this, a scientist from Arysta LifeScience recently ran an experiment. Maestro was put into solutions of pH 2-10.5 and samples were collected and analyzed for the concentration of active captan 0-48 hr after mixing. At all pH's, the concentration of captan remained 98-100% of what was originally added, even after 48 hr.

Granted, this information is only one pesticide from one company, but if pH adjustment is necessary to optimize activity of a pesticide, the manufacturers typically will include it on the label -- after all, it's in their own economic interest to assure that their products work properly. Also be aware of potential interactions when tank-mixing multiple products. If a particular combination isn't on the label, it is worth it to ask your dealer or company rep for more information.