It seems my raised garden bed needs more soil added to it cause of removing weeds and such a few years back. It looks lower like it needs 7 inches more soil added to the exhisting soil and clay and rocks soil. I wonder what i should add to fill in. I want to do this on the cheap but i like to garden organically if possible even though it will just be flower beds in this area. the area i need to fill in is about 12 inches long by 4 inches wide and 7 inches deep. How much will i need? Im looking into see if my town has free compost to add. Would this work?