Every time you are Foliar Feeding & when ever the pH changes out of acceptable range on the reservoir (Usually not more then a half a point. ex: 6.0-6.5 if it went to 6.7 you would want to lower it a little). One should keep in mind to check and adjust ph AFTER adding the Nutrients, Additives, and/or Wetting Agents as most nutrients and additives will change pH. You are wasting your time adjusting pH before hand. You should Adjust pH AFTER mixing up Foliar Spray and BEFORE spraying plants.