Topsoil is the top layer of soil, which contains most humus and animal organisms, and is therefore the most fertile. It varies in depth from place to place, but is usually about a foot deep. This soil should never be permanently removed as it is that which sustains healthy plant life.
Subsoil is the soil under the layer of topsoil. It can differ considerably from the top layer for geological reasons. It is mainly infertile as the soil organisms do not penetrate so far down — consequently there is little humus in it and therefore few soluble plant foods. However, subsoil can play an important part in the type of topsoil in a particular garden. A free-draining, gravelly subsoil will ensure that surface water drains quickly from the top layer, while a clay subsoil prevents water from draining freely from the topsoil. This can have the benefit of retaining moisture in the upper layers, but can also be disadvantageous if it is so solid or compacted that the surface water only drains very slowly, as it can cause flooding or sour, boggy conditions in the topsoil.
Subsoil should generally not be brought to the surface when digging, as it is of no benefit at all to plants. Neither should topsoil be buried, for example, by covering it with subsoil during building operations. However, if time and energy allow, it is a good practice to break up the subsoil when digging to improve drainage. Subsoil is easily distinguished from topsoil as it is usually lighter or different in colour and has no visible signs of humus (usually identifiable as a brown substance reminiscent of well-rotted vegetation) or organisms such as earthworms in it.