The depth of a soil is measured by how far below the surface plant roots can extend before being stopped by barriers, such as rock, sand, gravel, heavy clay, compacted dirt, or cement. Below the topsoil (usually only a few inches deep) lies another layer called subsoil. Usually more dense than topsoil and containing less organic matter, this 24- to 36-inch-deep band serves as both a water and nutrient reservoir for plants.
Even the largest trees can grow successfully in 36 to 60 inches of topsoil and subsoil. Conversely, shallow soils less than 10 inches deep are suitable only for shallow-rooted plants: lawns, annuals, and some alpines and perennials.
How to determine soil depth
How deep is your soil? To check, simply dig a hole. If you encounter an impenetrable layer in the first two feet, the soil may not be deep enough to support all the plants you are planning to grow.