The area covered by a cubic yard of topsoil depends on the depth of the layer. For example, according to Keleny Top Soil's calculations, 1 cubic yard spread 1 inch deep covers 324 square feet whereas the same volume of dirt at 12 inches deep only covers 27 square feet.
Topsoil is the uppermost layer of soil that contains organic matter, which supplies plants with the nutrients needed for healthy roots. According to SF Gate's Home Guides, plants grow best in topsoil that is 36 inches or more in depth. Shallow or nutrient-deficient topsoil often is supplemented by additional topsoil or the addition of compost.