You can pick up regular potting soil at your local hardware store, and fill your boxes with that. However, if you really want to make sure that they thrive, there are a few simple steps that you can take. First of all, you will want to have a loose, breathable layer of compost at the bottom of the box. For this, you can use anything from grass clipping to old leaves to line the bottom. This will allow for maximum drainage, and over time it will break down to create incredibly rich soil. You can then place your potting soil on top of the compost. For the top layer, you will want to place some type of fertilizer, or plant food. With that type of soil, there is no reason that your crops won’t grow wonderfully!