What is topsoil and why does losing it matter?

The answer

Topsoil is the upper layer of the soil profile, richer in organic matter, biological activity and nutrients than the layers beneath. It is where most roots feed and where most rain is absorbed, so it is the most productive layer and the first one erosion removes. When it is lost, plants have a harder time, the remaining subsoil is denser and less fertile, and the exposed ground erodes more easily. Pale patches on slopes are a typical sign that it has gone.