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I am curious of the lower boundary of soil. Soil is formed after going through complex processes and it has been through coevolution with living organisms. According to the USDA, for soil classification, the lower boundary of soil is arbitrarily set at 2 m. So, this is obviously not the actual depth. Yost and Hartemink (2020), in a review article mentioned soil depth/thickness:

A general term for the vertical dimension of the soil. Some authors consider it includes only the solum, some others include the solum plus the C and the G horizons (whether they represent or not the soil parent material), and some others consider it as equivalent to the root-restricting depth.

Considering C horizon while calculating the soil depth is also stated in a research by Richter and Markewitz(1995). They have mentioned varying depth of soils from 20 m to 100 m depending on spatial variations in climatic features. So, till which distance from our earth's surface is soil? Is there any standard depth of soil?

References:

  1. Yost, J.L., Hartemink, A.E. (2020). How deep is the soil studied – an analysis of four soil science journals. Plant Soil 452, 5–18. https://doi.org/10.1007/s11104-020-04550-z
  2. Richter, D., & Markewitz, D. (1995). How Deep Is Soil? BioScience, 45(9), 600-609. doi:10.2307/1312764
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    $\begingroup$ Soil has a specific meaning/composition that is not dependent on "depth". Soil depth can vary based on many factors. Here's a nice dataset that shows soil depth maps. daac.ornl.gov/SOILS/guides/Global_Soil_Regolith_Sediment.html $\endgroup$
    – f.thorpe
    Jun 10 at 20:48
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    $\begingroup$ So, there is no way we can say that soil has a standard depth. It will vary from region to region. $\endgroup$ Jun 12 at 4:38
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"Soil" is a loose amalgam of various earthen materials and organic detritus, thus conditions that create soil in various regions; soil "Thickness" depends on where soil ends and regolith (solid rock) begin

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