Topsoil is the upper, outermost 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 subsoil.

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 100 years for one inch of topsoil to be deposited, if there is the correct ratio of organic material, inorganic material, and moisture. This can be improved by using the terra preta system. However, there are 25 billion tons of topsoil lost each year.

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