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Although the ingredient label for both products says “potassium salts of fatty acids,” they aren't exactly the same thing. All soaps are made by mixing fats or oils with an alkali such as lye or a similar substance. By chemical reaction, the mineral in the alkali forms a salt with the fat. This process is called saponification and results in the soaps that we are familiar with.
Insecticidal soap is made using unsaturated long-chain fatty acids (such as oleic acid) from animal fats. These long-chain fatty acids dissolve insect skin or cuticle while preserving the protective cuticle of most plants. Herbicidal soap is made from shorter-chain fatty acids from vegetable sources—mostly coconut and palm oils—and these plant-based fats are extremely effective at disrupting plant cells.
