Neither fill dirt nor topsoil is composed according to standards, but any new homeowner who hasn't demanded topsoil can tell you they differ. The first late-summer hot stretch will give a good indication as to what lies beneath your lush, green lawn, and no amount of fertilizer can remedy an insufficient layer of topsoil.
All soil descends from the rock formed as the primordial stuff of Earth cooled. Wind, heat, cold, rain and moving water -- all involved in the evolution of cooling rock and conversion to a nitrogen-based atmosphere -- broke down the surface into rocks, gravel, pebbles and, finally, grains of sand. As life began, plants and animals -- carbon-based lifeforms -- lived and died upon the surface, adding organic material to the mix. Lacking gardeners or developers to mix the organics uniformly, however, local organic matter rested only in the top inches of local sand, creating a mosaic of combinations of minerals and essential elements atop the rocky subsoil. This narrow band became topsoil.
The material called fill dirt, or only fill, typically contains topsoil, but it also contains rocky subsoil and lots of other material in a mixture without a standard composition. When farmland, forests or old roadbeds are cleared, the materials, organic or not, all go into the same pile. Organic contents, as anyone who has ever cut down a tree and left the stump and roots intact knows, sinks as it decomposes. So fill provides the substance to fill depressions and contour berms. In addition to cleared land and leftovers from previous construction projects, fill contains the tops of steep hills and hollows of retention ponds, including their topsoil. Fill raises grades and contours the rolling hills of subdivisions, but it provides little fertility for plants.
The topsoil in the first 4 to 12 inches of Earth’s covering supports the roots of a variety of plant roots -- the nutrient gatherers. Successful topsoil contains nitrogen, carbon and other nutrients. It provides easy drainage yet retains moisture and allows some penetration of light and air. Topsoil is important; a study by the University of California-Davis and California Department of Transportation found that dry-weight production of growing plants along roadwork laid over topsoil exceeded that planted on fill by about 250 percent when the same seed and fertilizer were used. The study also found that plantings on fill tended to die out after three to five years.
Every lawn might do better when planted on topsoil, but all topsoils do not make superb garden soil. A garden typically requires proportions and amounts of nutrients different from those that occur in the few inches on which lawn grass can succeed; corn and squash are demanding guests that pay big dividends. Because no standard exists for topsoil, your topsoil may be thin or deep and clay, sand or loam. Amending topsoil with organic matter in the form of compost increases its fertility and depth. Soil tests can determine what help your part of the soil mosaic might need.