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1 Soil as a System: A History Richard J. Huggett ABSTRACT The idea of soil as a system is not yet a hundred years old. Its origins lie in Dokuchaev’s view of soil as an independent object, an idea promoted so successfully and eloquently by Hans Jenny. It was Jenny who first thought of soil as a system. His CLORPT equation focused on state factors (external drivers) of the soil system and, later, ecosystems. His approach was largely statistical and empirical. Later, a few researchers investigated energy as a soil-system driver. A different line of investigation, spurred by Milne’s catena concept, saw soil as a spatial system. Research in this field began in earnest with Simonson’s concept of soil as an open system, which at first involved one-dimensional soil profiles but was later extended to catenas and three-dimensional soil land- scapes, all researched using a rich variety of statistical and deterministic models. Last came the recognition that soil is part of an interdependent system. This line of enquiry began with conceptual models of the ecosphere. Since the millennium it has made big advances from cross-disciplinary enquiries focusing on the Earth’s critical zone and interactions between soils and geomorphology, soils and hydrology, soils and life, and soils and humans. 1.1. INTRODUCTION system, soil as a spatial system, and soil as an interdependent system. The chapter will end with a brief look at Ideas about soil have a long and rich history. It is per- prospects for the systems approach in pedology. haps easy to dismiss older notions as outmoded, but the In developing ideas about soil and soil formation, foundations of soil science laid down by the creators of Jenny, his predecessors, and later researchers have put the subject still have currency, even though later thinkers forward various models that attempt to explain the struc- have refined them and added new elements. Expanding a ture and function of soil systems and their component metaphor, if Isaac Newton could see further by standing parts. Table 1.1 summarizes some of these models and on the shoulders of giants, then modern soil scientists can serves as a guide for the discussion that follows. see further by standing in the soil pits of their predecessors. This chapter will explore the view taken by Hans 1.2. SOIL AS AN INDEPENDENT BODY Jenny, a veritable giant among soil scientists, that soil may be regarded as a system. JennyCOPYRIGHTED mooted this idea in As aMATERIAL discipline in its own right, soil science emerged 1930, but soil concepts developed in the five decades and flowered during the second half of the nineteenth before that date provide an essential background and century when a few researchers proposed the idea of soil they will be discussed first, before considering soil as a as an independent entity (Brevik & Cerdà, 2016). This radical idea was presaged by Friedrich Albert Fallou School of Environment, Education, and Development, (1862), who argued that soil was distinct from the under- University of Manchester, Manchester, UK lying geology, and who also coined the term pedology. Hydrogeology, Chemical Weathering, and Soil Formation, Geophysical Monograph 257, First Edition. Edited by Allen Hunt, Markus Egli, and Boris Faybishenko. © 2021 American Geophysical Union. Published 2021 by John Wiley & Sons, Inc. DOI: 10.1002/9781119563952.ch1 c01.indd 3 11/24/2020 6:58:08 PM 4 HYDROGEOLOGY, CHEMICAL WEATHERING, AND SOIL FORMATION Table 1.1 Soil models with selected examples. Type of Model Qualitative Quantitative Subject of Model Conceptual Statistical and Empirical Deterministic Soil as an independent system System drivers Dokuchaev (1899) Climofunctions, biofunctions, Kline (1973) (state factors) Zakharov (1927) topofunctions, lithofunctions, Huggett’s (1991, 1995) BRASH Shaw (1930) chronofunctions (Jenny 1946, equation Jenny’s (1941) CLORPT 1958, 1980) Phillips (1993a, 1993b, 1998) equation Yaalon (1975) Wilde (1946) Birkeland (1999) Stephens (1947) Major (1951) Lin (2011) Energy as a system Runge (1973) Volobuyev (1963) Regan (1977) driver Lin (2011) Rasmussen et al. (2005) Quijano and Lin (2014) Rasmussen and Tabor (2007) Rasmussen et al. (2015) Shepard et al. (2017) Soil as a spatial system 1D soil profile Simonson (1959, 1968) Parton et al. (1987) Kline (1973) Runge (1973) Salvador-Blanes et al. (2007) Johnson and Watson-Stenger Finke and Hutson (2008) (1987) 2D hillslope Milne (1935a, 1935b) Huggett (1976) Heimsath et al. (1997) Ruhe and Walker (1968) Brown et al. (2004) Minsany and McBratney (1999) Conacher and Dalrymple Grealish and Fitzpatrick (2014) Yoo et al. (2007) (1977) Brillante et al. (2017) Wackett et al. (2018) 3D landscape Ruhe and Walker (1968) McBratney et al.’s (2003) Ahnert (1967) Huggett (1975) SCORPAN equation Huggett (1975) Shepard et al. (2017) Sommer (2006) Iticha and Takele (2018) Vanwalleghem et al. (2013) Soil as an interdependent system Ecosphere Vernadsky (1926, 1929, 1998) Huggett (1991, 1995, 1997) Cole (1958) Phillips (1993b) Critical zone National Research Council Banwart et al. (2017) Banwart et al. (2017) (2001) Chorover et al. (2007) Biopedology Darwin (1881) Peacock and Fant (2002) Saco and Moreno-de las Heras Johnson (1990) Johnson et al. (2005b) (2013) Johnson et al. (2005a) Johnson Gabet et al. (2014) and Schaetzl (2015) Geopedology and Zinck et al. (2016) Brillante et al. (2017) Temme and Vanwalleghem topopedology (2016) Willgoose (2018) Hydropedology Lin (2003) Pereira et al. (2018) Ma et al. (2017) Anthropopedology Yaalon and Yaron (1966) Barton et al. (2016) Leguédois Amundson and Jenny (1991) et al. (2016) Source. Partly inspired by discussion in Hoosbeek and Bryant (1992) and discussion and tables in Minasny et al. (2008). 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. c01.indd 4 11/24/2020 6:58:08 PM SOIL AS A SYSTEM: A HISTORY 5 Arguably, Eugene Woldemar Hilgard recognized the (1930). Shaw argued that soils are formed by the modifi- independent nature of soil in his Report on the Geology cation, and partial decomposition and disintegration, of and Agriculture of Mississippi published in 1860 (Jenny, parent material owing to the action of water, air, temper- 1961b). But it is undeniably the case that modern soil sci- ature change, and organic life. He expressed soil formation ence was born in the early 1880s when Dokuchaev pub- according to the formula lished his Russian Chernozem in 1883. T SMCV D, 1.2.1. Dokuchaev’s Formula for Soil Formation which states that soil, S, is formed from parent material, Vasilii V. Dokuchaev was a Russian geologist turned M, by the work of climatic factors, C, and vegetation, V, pedologist who surveyed large stretches of the cherno- over a time, T, but the process may be modified by erosion zems underlying the Russian steppes. This work led him of, or deposition upon, the soil surface, D. Shaw noted to express the view that soil is an independent object and that each of the factors in soil formation is important in not simply a geological formation; it is a surficial body of determining the character of soil, though under local mineral and organic substances produced by the conditions any one factor may exert a dominant influence. combined activity of animals and plants, parent material, climate, and relief (Dokuchaev, 1880, 1883). In taking 1.2.3. Jenny’s CLORPT Equation this view, he rejected the then-prevalent agrogeological definition of soil, the chemical approach to soil classi The most famous and lasting development of fication, and the agronomic view of soils (Krupenikov, Dokuchaev’s approach was the CLORPT equation given 1992). In their place, he put forward two seminal ideas: by Hans Jenny (1941). This equation expresses any soil or (1) that soil is an independent natural body worthy of soil property, s, as a function of soil-forming factors: study in its own right and (2) that five soil-forming factors determine the course of soil genesis. s fclo, , rp, , t, , Dokuchaev’s original formula for soil formation appeared in an 1899 publication (Florinsky, 2011, 2012; where cl is environmental climate; o is organisms (the see also Stockman et al., 2011) and read fauna and flora originally in the system and that entering later); r is topography, including hydrological features П = f (К, О, Г)В, such as the water table; p is parent material, defined as the initial state of soil when pedogenesis starts; t is the age of where is soil or soil properties, is climate, is organ- П К О the soil, or absolute period of soil formation; and the isms, is parent material, and is age of the soil. Г В dots are additional factors such as fire. In short, the state Topography was not included in the expression, most of the soil or specific soil properties is a function of the likely owing to a stenographer’s mistake in the original external environment. text, in which a discussion of the role of topography pre- It is clear that this CLORPT equation is the same as ceded the equation (Florinsky, 2012). Using English sym- Zakharov’s equation but with the soil-forming factors bols, Dokuchaev’s equation becomes appearing in a different order. For this reason, and given the earlier date of Zakharov’s formulation, Igor V. s fclo, , pt. Florinski (2011, 2012) suggested it should be called the This became known as the factorial or state-factor Zakharov–Jenny equation. However, Jenny “framed and approach to soil genesis (Table 1.1). expanded the existing theory and lifted it to a level that is In 1927, Sergey Zakharov, building on Dokuchaev’s the accepted to the present day,” which explains his work, presented a general soil formation equation in his influence (Hartemink, 2016, 88).

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