Chemical Compatibilty

Maggie Scarbrough

In order to save time, money, and fuel, applicators often mix more than one pesticide for application. Occasionally manufacturers will pre-mix them. It is legal to mix them yourself, unless labels specifically instruct you not to mix a particular chemical. The chemicals being mixed must be compatible to prevent undesired effects like loss of effectiveness or chemical reactions that will damage surfaces or equipment. Charts that tell the compatibility of chemicals are available in pesticide trade publications, through the Cooperative Extension service, and industry recommendations.


If chemicals are not compatible, they can form lumps, gels or even separate. Any of these circumstances negatively affects your ability to apply the pesticide. Other effects are those you cannot see but will result in increased toxicity, loss of effectiveness against the pest or injury to a surface.


If you are unable to find whether two chemicals are compatible, mix a small amount first to use as a test. Before you proceed, be sure to put on enough personal protective equipment, including gloves, eyewear and apron. Use a clean, clear glass container and be sure to use the same diluents and amounts as will be used later. Unless there are other directions from the label, use the W-A-L-E plan.


  • First add some diluent.
  • Next add Wettable and other powders or Water-dispersible granules.
  • Thoroughly Agitate and add any more diluent.
  • Add the Liquid products.
  • Finally add Emulsifiable concentrates.


Shake the jar after adding all the ingredients and feel the sides to see if a chemical reaction is causing it to produce heat. If so, they probably should not be combined. Check the mixture 15 minutes later for heat.


If the mixture forms clumps or develops surface scum or solids settle out, these chemicals should probably not be mixed. If there are no signs of problems, test the mixture on the target area.