How deep is soil?

Question

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

Answer 1

It depends on your definition of soil, as also the type of soil in question. As some definitions include organic material. But taking this definition from Merriam Webster: "the superficial unconsolidated and usually weathered part of the mantle of a planet and especially of the earth" is a definition I'm more familiar with as an engineering geologist.

So what would be the maximum depth of soil? I would put it at the depth at which soil becomes rock, e.g. when sand becomes sandstone (under the aforementioned definition, and the definition engineers consider, sand is indeed a soil). Therefore the boundary where soil becomes rock would be when the soil has undergone the process of diagenesis.

Diagenesis is the process in which soil undergoes different physical/chemical/biological changes to become rock. Typically diagenesis occurs at around 1km of depth.

Deep boreholes have been drilled in the Ebro basin showing conglomerates at depths of up to 1km, but does that fit your definition of soil? If it does, roughly 1km.

Therefore there is no "standard" depth at which you define soil/rock but we can estimate that the maximum amount of soil that can be accumulated in an area is going to be roughly 1km.

Answer 2

"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

Answer 3

There was never a defined "bottom" to soil when I studied "Soil Science" (a co-production between geology and agriculture departments - with mineralogy, chemistry and various parts of the zoology departments collaborating too). Granted, in our region we were scraped clean by glaciers barely 10ka ago, so our local soils are only a few metres deep. But one of the SoilSci professors cut his intellectual teeth on bauxitic soils in the tropics where weathering profiles are hundreds of metres deep, and engineering the sides of the bauxite pits to be stable is a significant task.

Your question does not have a meaningful answer, as stated. You need to narrow conditions considerably.