All soaps are long chain fatty acids, but not all soaps have inseciticidal properties. Insecticidal soaps are specifically formulated to have high insect-killing properties, while being safe for most plant species. Insecticidal Soap is a Contact Material Insecticidal soaps kill susceptible insects by washing away the protective coating on the surface of the insect and by disrupting normal membrane functions inside the insect. The insects must come into direct contact with the spray droplets for the material to be effective.
Good coverage is essential.
The soaps have no residual activity toward insects, but repeated applications may have damaging effects on some types of plants. Water Quality and Insecticidal Soap Effectiveness Water hardness reduces the effectiveness of insecticidal soaps. Calcium, magnesium and iron precipitate the fatty acids and render them useless against the insects. It is important to use the purest water possible. Mix the concentration of soap that you intend to use with water in a glass jar.