Answer: In a technical sense, soil can be divided into three general categories or horizons. The deepest is the parent material or rock; the next layer is subsoil such as clay or muck; the top layer or "top soil" can be several feet deep or just an inch or two, it is composed of decomposed rock plus rotted down plant and animal matter along with, in some areas, a top coating of humus. This means your topsoil depth, quality and composition will vary depending on where it is located, and on anything that may have occurred to disturb the native ground at that location.
If you purchase top soil, there is no specific definition for it, it is not a regulated product.
I hope this helps.
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