Top soil comes from the first several inches of the ground. Quality top soil contains loam with some clay and sand, and a high percentage of both decomposing and living organic material. There is no actual definition of "garden soil." Retail outlets may sell bags labeled as garden soil but it is more of a marketing idea than reality. A retailer may use garden soil as a term synonymous with top soil, but since there is no standard of quality for top soil, garden soil becomes a meaningless phrase.
Nature takes dozens of years to add an inch of top soil to the ground. The repeated growth and death cycle of plants and animals, weather of the soil and the work of earthworms, sow bugs, ants and billions of micro-organisms create top soil. When all the conditions are right, the soil is richly fertile, dark brown or black and crumbly to the touch. Land that has been undisturbed for thousands of years may have top soil 36 or more inches deep. Most farms consider a top soil of 4 inches to be sufficient.
Top Soil Standards
Official government standards don't exist for top soil; it is a case of "buyer beware." The American Society of Landscape Architects created its own standard for the percentage of clay, sand, organic material, pH and debris bagged top soil may contain. You can request topsoil that meets the ASLA specifications if you've hired a professional contractor for your landscape project.
If your home is in a subdivision it is likely the original top soil was removed some time ago by the contractors and never replaced. Depending upon the region, the soil can be mostly clay, sand or with luck, loam. If the homeowner rakes cut grass and fallen leaves, it is unlikely there is the proper abundance of organic material in the soil. If the homeowner buys bagged soil labeled "garden soil" there is even less official definition as to what that means and they could be buying soil no better than what is already in the backyard.
Creating Top Soil In the Garden
Good topsoil for a garden can be bought or created. For a spring garden, lay out the perimeter during the fall. Dig the existing soil out of the garden to a depth of 8 inches and lay to the side. Fill the hole with 1 inch of green organic material such as kitchen debris, grass clippings and coffee grounds. Add 1 inch of soil back into the hole with the grass side facing downward. Fill with 1 inch of brown material such as straw, hay, leaves or even shredded newspapers or cardboard. Add another 1-inch layer of soil. Continue layering until the soil has been added back to the garden spot. It will be considerably higher than when it was first dug out but will eventually settle back to the normal height. Cover with wet burlap and weigh it down with rocks along the edges. By spring planting the area will be 8 inches or so of good, black topsoil.