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 or curvilinear regression equation. In Figure 1.1, this means that the arrows from the external or driving variables to the soil system are investigated one at a time, and that no interaction between the driving variables or feedback from the soil state variables to the driving variables is considered. Given five state factors, Jenny proposed five broad groups of functions or sequences: climofunctions or climosequences, biofunctions or biosequences, topofunctions or toposequences, lithofunctions or lithosequences, and chronofunctions or chronosequences. He also included dotfunctions and dotsequences to allow for the effects of other factors such as fire. Subsequent work has established a host of such functions and sequences (see Yaalon, 1975; Huggett, 1998; Birkeland, 1999).
Figure 1.1 The soil system.
1.3.2. Simonson’s Soil System
Roy W. Simonson’s (1959) generalized theory of soil genesis took a decidedly systems view of soils and elaborated upon Jenny’s view of the soil as an open system. He described four groups of physical, chemical, and biological processes common to all soils: additions of organic and mineral matter as solids, liquids, and gases; their removal; their transfer or translocation; and their transformation:
Simonson argued that the changing balances between these processes differentiate one soil from another. For example, mineralization and humification of plant litter engage more or less the same processes of transformation in all environments, but different process rates may lead to different end products.
Stanley Buol et al. (1980) categorized processes of soil formation using Simonson’s general scheme: enrichment, deposition on the soil surface, and littering are additions; leaching and surface erosion are removes; eluviation, lessivage, and pedoturbation are examples of transfers; and humification, mineralization, and weathering involve transformations. In Figure 1.1, Simonson’s model would engage the internal soil processes and forms with energy and material inputs from, and outputs to, the atmosphere, hydrosphere, biosphere, and anthroposphere. For the soil system to persist, incoming material must at least replace outgoing material. This fact was recognized by Constantin C. Nikiforoff (1959), who likened the situation to a section of an aggraded stream between two bends: water enters from upstream, water leaves downstream, but between the two bends nothing is lost and work is done. The corresponding “stream” in the soil system is the collection of surficial materials which constitute the soil. In the words of Buol et al. (1980, 11), “A soil is an evolving entity maintained in the midst of a stream of geologic, biologic, hydrologic, and meteorologic material.”
1.3.3. Soil Energy System
Coexisting with the material soil system is an energy exchange and storage system (Lin, 2011). Thermal energy (heat) is stored in the soil. The soil system gains heat from incoming solar radiation, terrestrial radiation emitted by the atmosphere, possibly from incoming soil materials and from exothermic reactions; it loses thermal energy in emitting radiation, by conduction out of the system, in outgoing soil materials and in endothermic reactions. Potential energy of a chemical or elevational nature is also stored, imported, and exported. Energy transfers in the soil are brought about by heat conduction, by convection associated with water and air movements, and by translocation of materials. Energy transformations in the system occur in chemical alterations, biological activity, wetting and drying, freezing and thawing, and evaporation and condensation in the soil atmosphere.
The soil energy system is not as well studied as the soil material system, but there are some interesting investigations (Table 1.1). A few researchers have developed models that considered the energy involved in weathering and soil formation, and in doing so quantify the climatic factor of soil formation though such measures as organic matter production and the amount of water available for leaching (Runge, 1973); the energy expended in soil formation (Volobuyev, 1963; Regan, 1977); a measure of the effective energy and mass transfer to the subsurface that accounts for local variations in topography, water and energy balances, and primary production (Rasmussen et al., 2005; Rasmussen & Tabor, 2007; Rasmussen et al., 2015); a probabilistic approach for quantifying soil property variability through integrating energy and mass inputs over time (Shepard et al., 2017); and a review of energy and entropy in near‐surface Earth systems (Quijano & Lin, 2014).
1.4. SOIL AS A SPATIAL SYSTEM
Soil may be viewed as a one‐dimensional, two‐dimensional, or three‐dimensional object. The soil profile concept, introduced to western soil scientists by Curtis F. Marbut in 1921 (see Tandarich et al., 2002; Brevik et al., 2016), is one dimensional, defining soil as the vertical cross‐section from the surface downward through all the soil horizons and into the parent material. It was adopted as the basic unit for soil survey. During the 1920s and first half of the 1930s, soil surveyors mapped the spatial distribution of soils and in doing so were assuredly aware that soils were part of landscapes, but they did not consider soil as a three‐dimensional functional landscape unit. Concepts of that nature appeared first with the two‐dimensional soil catena and later with the three‐dimensional soil landscape, both of which, along with the soil profile, were to lend themselves to a systems approach.
1.4.1. Soil Profiles
As explained above, Simonson (1959) suggested the concept of the soil profile as an open system. Quantitative models of soil profile development followed. An example was the CENTURY model used to simulate the formation of soil organic matter (Parton et al., 1987). It established statistical relationships between production and decomposition rates of soil organic matter and four driving variables (annual precipitation, temperature controls on decomposition, soil texture, and plant lignin content) and then predicted soil carbon and nitrogen levels for 24 grassland locations in the Great Plains of the USA. Donald Sasscer and his colleagues (1971) built a deterministic model to study stable and titrated water through an old‐field system in Argonne, Illinois; the system consisted of a vegetation compartment and 49 soil compartments, each 1 cm thick. Jerry Kline (1973) used the same basic model to investigate soil–plant relationships and soil genesis.
Donald Lee Johnson (1985) devised an evolutionary model of profile pedogenesis that considered soil thickness changes:
where T is soil thickness, D is profile deepening, U is soil upbuilding, and R is soil removal. An important feature of this model, and what makes it “evolutionary,” is that soil thickness is dynamic and may ebb or flow over time. This model formed the basis of Johnson and Watson‐Stegner’s (1987) comprehensive soil evolution model, which suggested that phenomena such as erosion, deposition, and pedoturbation can influence the soil formation processes. The model posited that soils may evolve along either progressive or regressive pathways:
