I found a recipe posted on the gardenweb forum: "Blend 6 ounces of castor oil with 2 tablespoons of dish soap. Mix that icky stuff with one gallon of water. This is your concentrate. Using a HOSE END SPRAYER, add 1 ounce of your concentrate per gallon of water and apply (as a drench) to the soil. You do not need to flood the area with this solution, nor should you merely mist it. Wet the top layer of soil and that will do the trick. If you're spraying over grass, you would need to apply a heavier dose."
I've attached a link to the thread so you can read the whole discussion if you wish.