"If the planter box is just sitting on top of the ground, you can use top soil, essentially creating a raised bed. Most top soil benefits from amendment with compost or other organic matter. You can also use a commercial potting soil to fill the bed, but that would be more expensive.
The amount of soil you'll need depends on how deep you make the bed. At a minimum, the root zone should be 6 to 12 inches deep. If you make it a foot deep, you would need 30 cubic feet of soil."