Topsoil is the uppermost layer of soil, typically with a thickness of 13 to 25 centimeters. It has the greatest concentration of organic matter and microorganisms, and it is where most of soil's biological activity occurs. It also usually has a high concentration of roots, because this is where plants obtain most of their nutrients.
Topsoil erosion, which is the blowing or washing away of topsoil, mainly as a result of conventional agriculture, is an environmental issue of extreme concern. Although topsoil is replenished naturally, it is estimated to take between five hundred and a thousand years to restore one inch, which is far slower than the speed of erosion. If current trends continue, the world's topsoil may become mostly depleted before the end of this century. Sustainable agricultural techniques attempt to slow erosion through the use of cover crops to increase the volume of organic matter in the soil.