To make the neem oil root drench, JB places 1 T. neem oil and ½ t. Castile soap in a plastic measuring cup. He then adds a splash of water and stirred to ensure the concentrated mixture emulsified. He said to check after a few minutes to see if the mixture separates. (If it does, one can add a bit more Castile soap.) Then JB fills his 2-gallon watering can with water, pours the neem oil concentrate into it, and stirs.
The neem oil solution can be poured on the base/roots of plants in containers, or into a plastic bottle buried adjacent to plants in raised beds or the ground.