Hi, I have so far bought some sleepers and had them cut down to 24 inch pieces, I have buried them 6 inches underground with 18 inches showing above ground side by side and screwed together, the wall of sleepers is 24 inches from the hedge. My next job is to fill the area before I buy the plants. The area in question if you could picture it is 18 inches high x 24 inches back to front and 12 feet long, What I would like to know is do I need to fill the whole area with top soil and if so would it be expensive to do or could I fill the area with earth and then some top soil, Any help appreciated thank you.