Pedogenesis

Pedogenesis

Soil Profile

Pedogenesis is the process of soil formation. Soil has many biological, chemical, and physical factors that are variable, and constantly subject to change. Its nature, profile build-up and specific properties are the direct result of several pedogenic processes. In the late 18th century (1880s), Russian scientist Vasily Dokuchaev perceived soil as an “independent natural body” regarded as a function of local maternal rock variety, age of the land (time since it became surface land), climate, and vegetation. His ideas were expanded in the next century by scientist Hans Jenny, who in the 1940s, established that the development of soil is influenced by five interrelated factors: climate, organisms, relief (topography), parent material, and time. In 1941, Jenny detailed his ideas and observations in his book, “Factors of Soil Formation: A System of Quantitative Pedology”, where he coined the term pedogenesis, and outlined an equation that would account for soil formation. This equation also known as the State Factor Model is summarized as: s = f(cl, o, r, p, t, ...).

Origin (Founders)

The origin of pedogenesis as a form of study dates back to Vasily Dokuchaev. He believed that pedogenesis was principally

controlled by vegetation and climate. This was based on observations that alike soils developed in spatially separate areas when their vegetation and climate were similar. Konstantin Glinka, a student of Dokuchaev, went on to express soil as “not only a natural body with definite properties, but also its geographical position and surroundings, I.e. climate, vegetation, and animal life”. Hans Jenny’s interpretation, which was based on Dokuchaev and those that came before him, provided a detailed definition of both soil and the "larger system," as well as a method to quantitatively and numerically link soil and larger system properties to state factors. This “larger system” would be “composed of the upper part of the lithosphere, the lower part of the atmosphere, and a considerable part of the biosphere.”

Pedogenesis Factors

Hans Jenny formulated the pedogenesis concept into the “fundamental equation of soil-forming factors”:

s = f (cl, o, r, p, t, …).

This equation states that soil formation (s) is a function (f) of climate (cl), organisms (o), relief (r), parent material (p), and time (t). CLORPT, for short, is an acronym for these pedogenesis factors. He also left an ellipses (…) in the equation, for other possible considerations that he did not ponder at the time.

Pedogegenesis Factors

Climate (cl):

The climate factor of soil genesis is the climate (I.e. temperature, precipitation, and humidity) of the ecosystem. Temperature and moisture are two climatic components that are very influential in pedogenesis. The production of mineral particles caused by weathering is directly influenced by temperature. Bedrock weathering rates typically increase with higher temperatures. Moisture levels in soils are mainly regulated by water additions through precipitation minus the losses due to evapotranspiration. Moisture availability also has impacts on the decomposition of organic matter and the soil pH.

Organisms (o):

The organism factor deals with the potential biota of the system. It is the microbial, plant, and animal gene flux that cycles through the system from its surroundings. Organic components dealing with pedogeneis include organic matter accumulation, profile mixing, and biogeochemical nutrient cycling. Litter and decomposing processes adds to the top layer of soil and increases nutrients influencing fertility and structure. Vegetation also helps in binding soil and protecting the surface from erosion from water and wind.

Relief (r):

The relief or topography deals with the subvariables that correspond to the physical make up of an ecosystem at the beginning of its development, or the start of an observation period. The site’s position on a hillslope, the range of its slope, the proximity of the site to the water table are examples of the subvariables. Relief typically moderates the formation of soil on a regional or local scale, and pedogenesis is directly related to microclimate and drainage influenced by the topography. The development of soil horizons are caused by illuviation and eluviation drainage processes. Microclimate would be influenced by which side of a hill is warmer than a side facing another direction. This results in soils of different areas having differences in terms of texture, depth, biota, and profile development.

Parent Material (p):

The parent material factor is defined as the initial state of the rock, sediment, or minerals of an ecosystem from which the soils develop. Weathering of bedrock and transported sediments from erosive means can make up this factor. For a habitat that has been clear-cut or burned down, the parent material would be the present soil at which the new fauna begins to grow over. Parent material influences are generally related to soil chemistry and texture, and nutrient cycling.

Time (t):

Time is the period since the ecosystem began forming, or since the assemblage of state factors changed. It is the temporal consequences of all the summed up state factors. Time at 0 (t=0), the starting point could be after a depositional event, or a major disturbance or configuration of all the pedogenesis factors. Through time, soils go receives positive and negative feedback in an attempt to reach equilibrium. Steady state is reached over time when a soil reaches maturity.

Other Possible Factors (...):

There could be additional state factors dealing with soil formation. One could say that Humans could be an element to the equation. Based on our extreme influence and control over ecosystems, soil formation could be swayed by human direct or indirect involvement.

References

Veldkamp, Antonie. “PEDOGENESIS AND SOIL FORMING FACTORS .” LAND USE, LAND COVER AND SOIL SCIENCES – Vol. VI, [1]

“CHAPTER 10: Introduction to the Lithosphere (u). Soil Pedogenesis.” Physical Geography, [2].

Amundson, Ronald, and Hans Jenny. “On a State Factor Model of Ecosystems.” BioScience, 1 Sept. 1997, [3].

“Plate Tectonics.” Soils, Weathering, and Nutrients, 16 Sept. 2013, [4].

Historical Overview of Soils and the Fitnes of the Soil Environment.” Fundamentals of Soil Ecology, by David C Coleman, 2nd ed., 2004.