The Green

Topsoil = "the uppermost layer of soil, usually the top 2 to 8 inches. It has the highest concentration of organic matter and microorganisms, and is where most of the Earth's biological soil activity occurs. Plants generally concentrate their roots in, and obtain most of their nutrients from this layer. The actual depth of the topsoil layer can be measured as the depth from the surface to the first densely packed soil layer known as hardpan.

"As one of the layers of soil on the Earth's surface, topsoil is sometimes referred to as the A horizon. This layer is formed from the deposition of eroded material as well as decaying organic matter. A variety of soil mixtures are sold commercially as topsoil, usually for use in improving gardens and lawns, or for ideal growing conditions in container gardens, by using potting soil, for example.

"A major environmental concern known as topsoil erosion, occurs when the topsoil layer is blown or washed away. Without topsoil, little plant life is possible. It takes approximately 500 years for one inch of topsoil to be deposited, but there are 25 billion tons of topsoil lost each year."
(Wikipedia) [en.wikipedia.org]

In The Future is Green: Natural Habitats and Culture and Heritage, Colin Dunn, author of sundancechannel.com's Treehugger Blog, concludes that "as we encroach further into natural habitat, we cut down more trees (and trees absorb carbon dioxide as they grow), which leads to less water retention in the soil (which leads to less groundwater that we can access with wells) and less stable topsoil (which leads to more mudslides). What to do? Regenerate degraded environments and halt biodiversity loss, and protect or regenerate existing natural environments and the habitats they provide to fauna and flora."
More... [www.sundancechannel.com]