Hydrogeology, Chemical Weathering, and Soil Formation. Allen Hunt
drivers (state factors)
Note. The boundaries between the three types of model are often blurred, with some actual models having characteristics of more than one type; the terms geopedology and so on are explained in Figure 1.2.
1.2.1. Dokuchaev’s Formula for Soil Formation
Vasilii V. Dokuchaev was a Russian geologist turned pedologist who surveyed large stretches of the chernozems underlying the Russian steppes. This work led him to express the view that soil is an independent object and not simply a geological formation; it is a surficial body of mineral and organic substances produced by the combined activity of animals and plants, parent material, climate, and relief (Dokuchaev, 1880, 1883). In taking this view, he rejected the then‐prevalent agrogeological definition of soil, the chemical approach to soil classification, and the agronomic view of soils (Krupenikov, 1992). In their place, he put forward two seminal ideas: (1) that soil is an independent natural body worthy of study in its own right and (2) that five soil‐forming factors determine the course of soil genesis.
Dokuchaev’s original formula for soil formation appeared in an 1899 publication (Florinsky, 2011, 2012; see also Stockman et al., 2011) and read
where П is soil or soil properties, К is climate, О is organisms, Г is parent material, and В is age of the soil. Topography was not included in the expression, most likely owing to a stenographer’s mistake in the original text, in which a discussion of the role of topography preceded the equation (Florinsky, 2012). Using English symbols, Dokuchaev’s equation becomes
This became known as the factorial or state‐factor approach to soil genesis (Table 1.1).
In 1927, Sergey Zakharov, building on Dokuchaev’s work, presented a general soil formation equation in his textbook on soils. In English, the equation reads
soil = f (parent rock material, plant and animal organisms, climate, age of the terrain, topography)
or using symbols:
1.2.2. Shaw’s Elaboration
Another major elaboration of the state‐factor approach initiated by Dokuchaev was due to Charles F. Shaw (1930). Shaw argued that soils are formed by the modification, and partial decomposition and disintegration, of parent material owing to the action of water, air, temperature change, and organic life. He expressed soil formation according to the formula
which states that soil, S, is formed from parent material, M, by the work of climatic factors, C, and vegetation, V, over a time, T, but the process may be modified by erosion of, or deposition upon, the soil surface, D. Shaw noted that each of the factors in soil formation is important in determining the character of soil, though under local conditions any one factor may exert a dominant influence.
1.2.3. Jenny’s CLORPT Equation
The most famous and lasting development of Dokuchaev’s approach was the CLORPT equation given by Hans Jenny (1941). This equation expresses any soil or soil property, s, as a function of soil‐forming factors:
where cl is environmental climate; o is organisms (the fauna and flora originally in the system and that entering later); r is topography, including hydrological features such as the water table; p is parent material, defined as the initial state of soil when pedogenesis starts; t is the age of the soil, or absolute period of soil formation; and the dots are additional factors such as fire. In short, the state of the soil or specific soil properties is a function of the external environment.
It is clear that this CLORPT equation is the same as Zakharov’s equation