By soap, I think she means dish soap. I've heard that pickling vinegar is even better, but I use regular white vinegar on my brick path to kill the weeds that come up through the bricks. I drop one tablespoon of dish soap (it helps the vinegar stick to the plant) into the gallon jug, shake it up, and then pour it carefully onto the path. I suppose if I diluted it by half it wouldn't take quite so many jugs to do the whole path.
It works great, though!
