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Topsoil is the upper, outermost layer of soil, usually the top 2 inches (5.1 cm) to 8 inches (20 cm). 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.
The two common types of Topsoil are Bulk and Bagged Topsoil. The following table illustrates major differences between the two. 
When starting a gardening project, it is very crucial to check whether or not the Soil is satisfactory. Following are the desired levels of Topsoil nutrients. 
|pH Level||5.8 to 6.2|
|Phosphorus (P-I)||Index of 50|
|Potassium (K-I)||Index of 50|
|Calcium (Ca%)||40-60% of Cation Exchange Capacity (CEC)|
|Magnesium (Mg%)||8-10% of CEC|
|Base saturation (BS%)||60-80% of CEC|
|Manganese (Mn-I)||Index > 25|
|Zinc (Zn-I)||Index > 25|
|Copper (Cu-I)||Index > 25|
A variety of soil mixtures are sold commercially as topsoil, usually for use in improving gardens and lawns, e.g. container gardens, potting soil and peat. Another important yet not commonly known use for topsoil is for proper surface grading near residential buildings such as homes. "The ground around the home should slope down six inches for the first ten feet away from the home. This can often be done by adding topsoil (not sand or gravel)."
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. The estimated annual costs of public and environmental health losses related to soil erosion exceed $45 billion. Conventional agriculture encourages the depletion of topsoil because the soil must be plowed and replanted each year. Sustainable techniques attempt to slow erosion through the use of cover crops in order to build organic matter in the soil. The United States alone loses almost 3 tons of topsoil per acre per year. This is of great ecological concern as one inch of topsoil can take 500 years to form naturally. However, farmer and engineer P. A. Yeomans developed a technique known as Keyline design, which has been proven to convert subsoil into topsoil much faster; Yeomans published a book, called Water For Every Farm, in 1954 about the technique.