Information on soil starts with topsoil then subsoil and finally substratum.
Here is information on topsoil. On the topsoil layer there are many roots and living bugs and animals. Depending on the area you are in it contains many minerals and humus. Topsoil is the first layer (horizon) of soil.
Next is subsoil. Subsoil has very little humus but a lot of minerals. Also contains roots and animal burrows like moles, earthworms, and some beetles. It is the middle layer of soil.
Last is bedrock or substratum. It can be igneous, sedimentary, or metamorphic. Can be exposed by high erosion if that does happen that is called outcrop. Substratum is the bottom layer of soil.