Subsoil is the layer of soil below the topsoil level of the earth. It is made up of mineral deposits, clay and fewer organic materials compared to the layers above it, and it is lighter in color.
Generally, soil is formed in layers known as horizons. The top layer of soil is referred to as the "A" horizon, and in the prairies and forests, this layer is covered with decayed remains of plants and animal matter known as humus. The "A" horizon topsoil merges with the subsoil or "B" horizon that contains necessary elements in a form that must undergo a change before plants can make use of it. Beneath these two layers is the "C" horizon or "parent material," which is made of glacial rock that has been weathered by the ages but remains unchanged.