The effective depth of a soil for plant growth is the vertical distance into the soil from the surface to a layer that essentially stops the downward growth of plant roots. The barrier layer may be rock, sand, gravel, heavy clay, or a cemented layer (e.g. caliche).

Terms that are used to express effective depth of soil are:

  • Very Shallow - surface is less than 10 inches from a layer that retards root development.
  • Shallow - Soil surface is 10 to 20 inches from a layer that retards root development.
  • Moderately deep - Soil surface is 20 to 36 inches from a layer that retards root development.
  • Deep - Soil surface is 36 to 60 inches from a layer that retards root development.
  • Very deep - Soil surface is 60 inches or more from a layer that retards root development.

Soils that are deep, well-drained, and have desirable texture and structure are suitable for the production of most garden or landscape plants. Deep soils can hold more plant nutrients and water than can shallow soils with similar textures. Depth of soil and its capacity for nutrients and water frequently determine the yield from a crop, particularly annual crops that are grown with little or no irrigation. Plants growing on shallow soils also have less mechanical support than those growing in deep soils. Trees growing in shallow soils are more easily blown over by wind than are those growing in deep soils.

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Source: University of Arizona Cooperative Extension. 1998. Arizona Master Gardener Manual: Chapter 2, Soils and Fertilizers. Tucson, AZ. Available at http://ag.arizona.edu/pubs/garden/mg/soils/index.html#index (verified 19 August 2004).