Topsoil is the dark upper layer that contains the nutrients plants need. If the undersoil is not compact, most landscapers recommend a four- to six-inch layer beneath lawns. On top of this base, many flower and vegetable gardeners add two more inches of fresh topsoil to their garden beds each year to counteract the effects of leeching and erosion.
Factors Requiring Additional Depth
Although it can be expensive, some gardeners take a "more is better" approach to topsoil, and certain situations always call for a thicker layer. Add extra topsoil to rocky fill dirt, hard clay soils and raised flower or vegetable beds. To give a boost to plants growing in rocky soil or hard clay, add at least an eight-inch layer of soil. Raised beds require a mix of approximately half compost, half topsoil and little sand for drainage.
For standard lawn and garden applications where soil is not too rocky or compact, four to six inches of topsoil is enough. However, for special soil types, you may need eight or more inches, and you may need to create a custom mix for raised garden beds.