Water pH and its Effect on Pesticides

The effectiveness of a pesticide can be influenced by many factors: e.g. errors in mixing and application, equipment problems, timing of application, resistance, choice of product, incorrect identification of a pest or disease. Another possibility that needs to be considered is the water being mixed with the pesticide product. Most of the pesticides we use are applied with water; however, its quality can have a large impact on the performance of the pesticide.

In particular, the pH of the water is critical. At higher pH levels (>7), some pesticides undergo a chemical reaction known as alkaline hydrolysis, a process that degrades the active ingredient, reducing its effectiveness. Pesticides that are particularly affected include insecticides in the organophosphorus and carbamate families. The rate of hydrolysis is measured in terms of its half-life; i.e. the time it takes for the active ingredient in a pesticide to degrade to half of its original level. The shorter the half life, the more rapidly the pesticide degrades.

There is no one authoritative source where information on this subject can be obtained for all pesticides. However, a quick search of the internet will find a number of articles on the subject that are in general agreement on the pH effects on different pesticides. Be aware that there are some discrepancies in actual half-life values between different sources, although the general trends are consistent.

To give you some idea of the impact, the half life of various greenhouse-registered pesticides at different pHs includes:

• Captan - 8 hours at pH 10, reducing to 10 minutes at pH 8
  
• Trumpet - 48 days at pH 5 ? 3 days at pH 7 ? 45 minutes at pH 9
  
• Malathion - 8 days at pH 6 ? 3 days at pH 7 ? 19 hours at pH 8
  
• Kelthane - >20 days at pH 5 ? 5 days at pH 7 ? 15 minutes at pH 10

To make life more difficult, there are some pesticides, especially the metal-based fungicides, such as Aliette or copper compounds, where you need to stay away from a low pH. Under these conditions, the solubility of the metal is greater, increasing the chances of toxicity to the plants.

So what is the take home message on this? Well, you could conduct an extensive internet search each time you use a new product, adjusting your spray tank solution to the appropriate pH level. However, for many pesticides, this information can be hard to find and it would certainly complicate your decisions around pesticide use. Probably the most efficient approach is always to make sure that your spray tank water is adjusted to a pH of about 6 and you should be in good shape for most pesticides you need to apply.

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