Many factors can affect how long it takes for an inch of topsoil to form, but it normally takes at least 100 years and often over 500 years. The factors that affect soil formation are parent material, topography, climate, organisms and time.
Even though it can take over 500 years for an inch of topsoil to form naturally, it is possible to speed up the process. Adding extra earthworms to soil helps to promote topsoil formation; the earthworms transform organic material into food for plants. Adding dead plants, leaves, lawn clippings, kitchen scraps and other organic matter to soil increases the amount of plant food that earthworms can produce. This can make it possible for an inch of topsoil to form in five years.