General Information on Use of Soap
Soft soaps made from potassium compounds are preferable because they can add useful minerals to the soil. Avoid using strong powder detergent soaps because they contain caustic soda which can scorch plant leaves and, if used frequently, will reduce soil fertility. Liquid soap and soft soap solutions can also burn leaves if their concentration is too strong or if applied too frequently.
Among pests controlled by soap solutions are: aphids, crickets, armyworm, caterpillars, leafminers, mites, small beetles, slugs and snails, thrips and whiteflies (Elwell and Maas, 1995).
In general, soaps will kill aphids and small caterpillars at concentrations of 5-8 g per litre of water. Concentrations of 8 g per litre will kill beetles and larger caterpillars, while concentrations of 10 g per litre can kill plants and are sometimes used as herbicides. Soap solutions are effective only when wet, once dry, they lose their insecticidal properties (Rappaport, 1992).