Detergent poured onto the soil may bring soil dwelling insects to the surface and may act as a surfactant to help water penetration in fine, silty soils. Killing weeds? Doubtful. Apple cider vinegar used full strength is a natural weed killer.
Regular household vinegar might give weeds a pause, but it won't kill them either; however, you can buy vinegar for horticultural use. It's very strong & will burn your skin if you don't apply it using safety precautions.