Hydrogeology, Chemical Weathering, and Soil Formation. Allen Hunt

order. For this reason, and given the earlier date of Zakharov’s formulation, Igor V. Florinski (2011, 2012) suggested it should be called the Zakharov–Jenny equation. However, Jenny “framed and expanded the existing theory and lifted it to a level that is the accepted to the present day,” which explains his influence (Hartemink, 2016, 88).

      Several researchers proposed modifications of Jenny’s original formula. Sergius A. Wilde (1946, 13) recast Dokuchaev’s formula so that time was expressed as a differential:

where s is soil, g is geological (parent) material, e is environmental influences, and b is biological activity. This model was further modified by Charles G. Stephens (1947) who split environmental influences into climate, c, relief, r, and water table, w, and changed the g and b factors into parent material, p, and organisms, o:

      which is almost identical to the CLORPT equation except time is seen to influence all factors and the factors can have dependent and independent status.

      Jack Major (1951) extended the CLORPT equation to embrace the entire ecosystem: soils, vegetation, and animal life. Jenny (1961a, 1980) offered his own extension that included ecosystems (entire sections of landscapes). He derived a general state‐factor equation of the form

      where l is ecosystem properties such as total carbon content, primary production, and respiration; v is vegetation properties such as biomass, species frequency, and sodium content; a is animal properties such as size, growth rate, and color; s is soil properties such as pH, texture, humus content; L₀ is the initial state of the system, that is, the assemblage of properties at time zero when development starts (the L stands for the ecosystem, or larger system as Jenny styled it, of which the soil is part); P_x are external flux potentials; and t is the age of the system.

      The state factors are groups of variables associated with L₀ and P_x. The initial state of the system is defined by parent material, p, and by the original topography and water table, r. The external flux potentials are environmental properties that lead to additions and subtractions of matter and energy to and from the system. They include environmental climate, cl, a biotic factor, o, comprising fauna and flora as a pool of species or genes, active or dormant, that happen to be in the ecosystem at time zero or that enter it later. The biotic factor is thus distinct from the vegetation that grows as the system develops; this appears as a system property on the left‐hand side of the equation. Other external fluxes would include dust storms, floods, and the additions of fertilizers; these could be given symbols and entered separately in the equation if so desired. In an extended form, the general state‐factor equation becomes

      That brings us back to the CLORPT equation, only this time it applies to ecosystems and not just soils. An even later version of the CLORPT equation considered the place of the human species in the state‐factor theory of ecosystems (Amundson & Jenny, 1991).

      Henry Lin (2011) pointed out that climate, organisms, relief, and parent material vary across geographical space and all change with time, which in turn means that soil environments change continuously through time, a point stressed by Dokuchaev and others (e.g. Johnson et al., 1990). To address this problem, Lin suggested an equation for the total change of a soil profile over a pedogenic time period t = t_n – t₀ (where t_n is the current time and t₀ is the beginning of pedogenesis) to capture the cumulative effects of all four time‐dependent spatial factors on pedogenesis:

      Grouping the spatial soil‐forming factors into flux factors (cl and o) and site factors (p and r), Lin writes this equation more succinctly as:

      which specifies the cumulative effects of climate and organisms on pedogenesis conditioned by parent material and topography.

      Jenny’s formulation has proved itself exceedingly valuable, both as a conceptual and a research tool. It has deepened understanding of soil variation across individual state factors (see below), although attempts to tackle the combined effects of all state factors using multivariate techniques have had little success (Yaalon, 1975; Amundson & Jenny, 1997). Nevertheless, the CLORPT equation is still cited and used, albeit in slightly modified forms, by pedologists and, to a lesser degree, ecologists. And it has acted as a springboard for later concepts regarding the soil as a system, a development instigated by Jenny himself.

1.3. SOIL AS A SYSTEM

Figure 1.1 depicts the soil system and its connections to the other terrestrial spheres. It serves to illustrate the various concepts about the soil system that have appeared since Hans Jenny first suggested that soil could be usefully conceived in that manner. Jenny’s earliest ideas seem to have been first set down in 1930 when he approached soils from a general theory of state, in which “soil properties, soil processes, and soil‐forming factors are united into a comprehensive system” (Jenny, 1930, 1053). Later, in his classic book Factors of Soil Formation (1941), he treated soil as a physical system to underline its status as a natural body. He also recognized that the soil system is an open system to which substances may be added or removed, a point taken up by later pedologists.

      1.3.1. Jenny’s Soil System

      To characterize a soil system, Jenny used the symbols s₁, s₂, s₃, and so forth to stand for soil properties (nitrogen content, acidity, clay content, or whatever). In modern parlance, these properties are state variables, a term adopted by Jenny in his 1980 book. The soil properties are interrelated, such that if one changes the others may also change:

      In Jenny’s original discussion of the soil as a system, he distinguished soil formers or soil‐forming factors (climate, relief, parent material, organisms, and time) and explored their relationships to the soil system itself. He later styled them state factors and stressed that they are “independent” variables that condition the state of the soil system (e.g. Jenny, 1980, 10). In Figure 1.1, these variables are shown as atmosphere and hydrosphere (climate), biosphere (organisms), toposphere (relief), and lithosphere (parent material); the entire system changes through time. Jonathan D. Phillips (1989) argued that the purpose and scope of Jenny’s state factors is sometimes misunderstood: they are not intended to describe pedogenetic processes or components of the soil itself; rather, they provide the context and the boundary conditions within which soil formation and development occurs. As Jenny (1961a, 385) put it, “The factors are not formers, or creators, or forces: they are the variables (state factors) that define the state of the soil system.” Jenny made it clear that his factorial approach is very much an ecological approach in which the soil component cannot be considered in isolation from its physical and biotic environment (Jenny, 1961a, 1980). However, it is true to say that Jenny’s work tended to consider the effect of one state factor on a single soil property, all other factors being held constant, and then expressing the relationship as a mathematical function of some kind, usually a linear