Watering plants with soapy water is not recommended. Dishwater contains harmful substances in concentrations too high for houseplants to tolerate. Phosphorous, a primary fertilizer ingredient contained in many soaps, must be in the correct proportion to be of benefit to plants. Cooking water can be safely used to water houseplants.
Horticultural soaps are formulations designed specifically for the purpose of killing certain plant pests and applied as a spray directly onto the insects. These can be made at home easily and inexpensively. They are preferable to soapy water because they are not toxic to beneficial insects and contain no ingredients that harm soils.