What is Fill Dirt: Fill dirt is earthy material which is used to fill in a depression or hole in the ground or create mounds or otherwise artificially change the grade or elevation of real property. Fill dirt is usually subsoil (soil from beneath the top soil) and underlying soil parent material which has little soil organic matter or biological activity. Fill dirt is taken from a location where soil is being removed as a part of leveling an area for construction; it may also contain sand, rocks, and stones, as well as earth. Fill dirt should be as free of organic matter as possible since organic matter will decompose creating pockets of empty space within the fill which could result in settling. Settling of the fill can and will possibly result in damage to any structures built on the fill.