<P> Topsoil is the primary resource for plants to grow and crops to thrive and the main two parameters for this are Carbon and Nitrogen . The Carbon provides energy and Nitrogen is a tissue builder and plants require them in a range of ratios to enable suitable growth . An optimum figure for Topsoil in the UK is a ratio of less than 20: 1 . This ensures that the soil has a suitable energy reserve as well as tissue building material to enable the plants to thrive . A sawdust typically has a carbonaceous base and this a high C: N ratio (in the order of c. 400: 1) while an Alfalfa Hay has a low carbonaceous content and can typically have a C: N ratio in the order of 12: 1 . </P> <P> 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)." </P> <P> 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 between 500 and 1,000 years to form naturally . On current trends, the world has about 60 years of topsoil left . </P> <P> Because of its use in commercial application and due to the environmental concerns regarding erosion, it is important for consumers to accurately determine how much topsoil they need for a given project . Topsoil is mainly sold by the cubic yard in the United States . To calculate the amount of topsoil you will need, "simply take your length (in feet) multiplied by your width (in feet) multiplied by your depth (in feet so if you wanted 5 inches that would be 5 / 12) then divide the total by 27 . This will give you your cubic yards needed for the project . Ex: 10L x 10W x (5 / 12) = 41.66 / 27 = 1.54 cubic yards of topsoil needed .". </P>

When does top to bottom erosion take place
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