Killing Aphids with Soap. HOMEMADE INSECTICIDAL SOAP CONCENTRATE 1/4 cup grated soap (I use Fels-Naptha, but Ivory or other real soap - see above - will work) 4 cups hot water Stir soap into water until dissolved. Store in a jar until ready to use. To dilute for plant use: Add 1 tsp. concentrate per 4 cups (32 oz.) of water. Pour solution into a spray bottle. This soap is a contact poison - spray directly onto the aphids. It will not harm your plants.