Pedogenesis

Pedogenesis (from the Greek pedo-, or pedon, meaning 'soil, earth,' and genesis, meaning 'origin, birth') is the science and study of the processes that lead to the formation of soil (soil evolution)^[1] and first explored by the Russian geologist Vasily Dokuchaev (1846 – 1903), the so-called grandfather of soil science, who determined that soil formed over time as a consequence of climatic, mineral and biological processes, which he demonstrated using the soil forming equation:

Soil = f(<var>C</var>, <var>PM</var>, <var>O</var>) × time (where <var>C</var> = climate, <var>PM</var> = parent material, <var>O</var> = biological processes)

In 1941, the Swiss scientist Hans Jenny expanded Vasily Dokuchaev equation by adding relief/topology as a factor and separating the biological processes into the fauna and flora coming up with the equation:

Soil = f(<var>C</var>, <var>PM</var>, <var>R</var>, <var>O</var>, <var>V</var>) × time (where <var>C</var> = climate, <var>PM</var> = parent material, <var>R</var> = relief/topology, <var>O</var> = fauna, <var>V</var> = flora)

Pedogenesis is though more a parent than a branch of pedology, whose other aspects include the soil morphology, classification (taxonomy) of soils, and their distribution in nature, present and past (soil geography and paleopedology).

Climate

Climate regulates soil formation. Soils are more developed in areas with higher rainfall and more warmth. The rate of chemical weathering increases by 2-3 times when the temperature increases by 10 degrees Celsius. Climate also affects which organisms are present, affecting the soil chemically and physically (movement of roots).

Organisms

The organisms living in and on the soil form distinct soil types. Coniferous forests have acidic leaf litter and form what are known as inceptisols. Mixed or deciduous forests leave a larger layer of humus, changing the elements leeched and accumulated in the soil, forming alfisols. Prairies have very high humus accumulation, creating a dark, thick A horizon characteristic of mollisols.

For example three species of land snails in the genus Euchondrus in the Negev desert are noted for eating lichens growing under the surface limestone rocks and slabs (endolithic lichens).^[2] They disrupt and eat the limestone.^[2] Their grazing resulting in the weathering of the stones, and the subsequent formation of soil.^[2] They have a significant effect on the region: the total population of snails is estimated to process between 0.7 and 1.1 metric ton per hectare per year of limestone in the Negev desert.^[2]

Parent material

The rock from which soil is formed is called parent material. The main types are: aeolian, glacial till, glacial outwash, alluvium, lacustrine parent material and residual parent material, or bedrock.

Pedologists use the following generic functional relationship for understanding soil formation:^[3]

<var>S</var> = f(<var>CL</var>, <var>O</var>, <var>R</var>, <var>P</var>, <var>T</var>, <var>…</var>)

(where <var>S</var> = soil properties, <var>CL</var> = regional climate, <var>O</var> = potential biota, <var>R</var> = topography, <var>P</var> = parent material, <var>T</var> = time, <var>…</var> = additional variables)

Examples

A variety of mechanisms contribute to soil formation, including siltation, erosion, overpressure and lakebed succession. A specific example of the evolution of soils in prehistoric lake beds is in the Makgadikgadi Pans of the Kalahari Desert, where change in an ancient river course led to millennia of salinity buildup and formation of calcretes and silcretes.^[4]

See also

• CLORPT
• Parent material
• Soil horizon
• Soil production function
• Topography

Notes

References

• Stanley W. Buol, F.D. Hole and R.W. McCracken. 1997. Soil Genesis and Classification, 4th ed. Iowa State Univ. Press, Ames ISBN 0-8138-2873-2
• C. Michael Hogan. 2008. Makgadikgadi, The Megalithic Portal, ed. A. Burnham
• Francis D. Hole and J.B. Campbell. 1985. Soil landscape analysis. Totowa Rowman & Allanheld, 214 p. ISBN 0-86598-140-X
• Hans Jenny. 1994. Factors of Soil Formation. A System of Quantitative Pedology. New York: Dover Press. (Reprint, with Foreword by R. Amundson, of the 1941 McGraw-Hill publication). pdf file format.
• Ben van der Pluijm et al. 2005. Soils, Weathering, and Nutrients from the Global Change 1 Lectures. University of Michigan. Url last accessed on 2007-03-31

v t e Geologic principles and processses Stratigraphic principles Principle of original horizontality Law of superposition Principle of lateral continuity Principle of cross-cutting relationships Principle of faunal succession Principle of inclusions and components Walther's law Petrologic principles Intrusive Extrusive Exfoliation Weathering Pedogenesis Diagenesis Compaction Metamorphism Geomorphologic processes Plate tectonics Salt tectonics Tectonic uplift Subsidence Marine transgression Marine regression Sediment transport Fluvial processes Aeolian processes Glacial processes Mass wasting processes