From the Mailbag: Can soap be safely used as an insecticide?

July 11 - Cory R. asks:

"I had ants on my strawberries a couple of days ago, so I sprayed most of garden with soap and water. It looks like it did the trick, but now my radishes look terrible. I don’t think they liked the soap. Can all plants with stand water and soap spraying to get rid of ants, aphids etc? "

Soap can be a great insecticide, but you have to make sure you're using a plant-safe soap. There are insecticidal soaps available - Safer's Soap is one brand. You don't mention what soap you used... but there can be an issue with using "cleaning" soaps because they can damage plant foliage. It depends on the length of the fatty acid chains in the soap, although off the top of my head I don't remember if it's the short chains or the long chains that are the issue.

It's worth noting that there are several brands of organic herbicidal soaps that are intentionally designed to kill plant foliage. Also, as you found out, some plants are more succeptible than others - even insecticidal soaps can harm some very sensitive plants (fortunately, the problematic plants are listed in the soap's instructions).

Follow-up: A second email exchange indicated that the soap used was indeed standard liquid dish soap. Also, happily, the radishes did eventually recover.

Additionally, there is a fair bit of contradictory information on the web regarding whether it is short chains or long chains (of the fatty acid salts) that can cause plant damage. Stay tuned...