Soap does not kill grass. In fact, soap, and more specifically, dishwashing liquid, is actually part of a process to help grass grow even more lush, notes Reader's Digest.
According to Reader’s Digest, special uses for dishwashing soap include a method that helps to turn a lawn more green. This method involves filling the reservoir of a 10- or 20-gallon hose sprayer with water and adding a special mixture of a 12-ounce can of cola (not diet), 1 cup of corn syrup and 1 cup of dishwashing liquid. The soap acts as an agent to spread the sugary mixture over the lawn and keep it on the blades of grass once the grass is sprayed with the garden hose. Reader's Digest recommends using this technique every three weeks to maintain the lawn’s color.