After a brief jaunt into the paleopedogenically-affected Chugwater Fm, we return now to the issue of soils and their stratigraphic occurrence. Before diving into the fun stuff (like correlation strategies, biotic and climatic inferences from paleosols, or paleogeomorphology), we have to lay down a terminological brickwork. First and foremost: WHAT is a soil, anyway?
“Soil” is actually a tough nomenclatural nut to crack. The Soil Science Glossary (published by the Soil Science Society of America) presents the following definition:
“soil - (i) The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. (ii) The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time. A product-soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics.”
Importantly, this definition rests heavily on the role of soil in cultivation and agriculture (a future post will deal with the implications of accepting or ignoring the necessity of land plant-activity on soil occurrences). Part (ii) offers a definition that implies a genetic connotation; however, this part also reads like a simple definition of weathered material. Is all clastic sediment a soil, then?
A different, though similar, definition is taken from the bible of Soils Sciences, the 2nd edition of Soil Taxonomy, written by the USDA’s Soil Survey Staff (and available in all its 800+ page glory here):
“Soil - Soil is a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons, or layers, that are distinguishable from the initial material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment.
The upper limit of soil is the boundary between soil and air, shallow water, live plants, or plant materials that have not begun to decompose. Areas are not considered to have soil if the surface is permanently covered by water too deep (typically more than 2.5 meters) for the growth of rooted plants.
The lower boundary that separates soil from the nonsoil underneath is most difficult to define. Soil consists of horizons near the earth's surface that, in contrast to the underlying parent material, have been altered by the interactions of climate, relief, and living organisms over time. Commonly, soil grades at its lower boundary to hard rock or to earthy materials virtually devoid of animals, roots, or other marks of biological activity. For purposes of classification, the lower boundary of soil is arbitrarily set at 200 cm.”
Aside from wordiness, these two definitions have a lot common, though there are important differences. The Soil Taxonomy definition singles out the presence of recognizable horizons within the soil as key. These horizons are the result of the differentiation of the parent material into separate components, driven by subtle differences in processes during soil formation. Unstated but implicit within this definition is the fact that soils occur in situ; they are not transported, but rather represent a geomorphic surface of exposure that reflects a period of non-deposition that allows for the parent material to undergo soil forming processes.
In the more geologically-centered literature, Retallack (2001) defines a soil more generally as “material forming the surface of a planet or similar body and altered in place from its parent material by physical, chemical and biological processes.”
The nature of how these processes proceed, interact, and change plays an important role in soil taxonomy, which will be the focus of a future post, as well. In the mean time, I’ll leave you with a picture of how these horizons manifest themselves in the field. Behold:
The picture above is of a soil pit from ‘round these parts, showing the interpretations of different horizons, based on LOTS of different parameters, including pH, root density, bioturbation evidence, and clay content. How these horizons are organized plays an important role BOTH in tagging a soil with a technical name as well as in interpreted the soil-forming processes, which are what we'll be talking about next time.
Just for book-keeping, I think the future posts will proceed roughly in the following order:
- Soil Forming Processes
- Soil Taxonomy
- Paleosol Nomenclature
- Paleosols and Stratigraphy
- Paleosols and Bioturbation
- Soils on Other Planets!
Retallack, G.J., 2001, Soils of the Past: An Introduction to Paleopedology, 2nd Ed, Oxford, Blackwell, 404 p.
Soil Science Survey, 1999, Soil Taxonomy, USDA, 871 p.