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# Soils, Science, Society, and the Environment

## Abstract and Keywords

Soils are the complex, dynamic, spatially diverse, living, and environmentally sensitive foundations of terrestrial ecosystems as well as human civilizations. The modern, environmental study of soil is a truly young scientific discipline that emerged only in the late 19th century from foundations in agricultural chemistry, land resource mapping, and geology. Today, little more than a century later, soil science is a rigorously interdisciplinary field with a wide range of exciting applications in agronomy, ecology, environmental policy, geology, public health, and many other environmentally relevant disciplines. Soils form slowly, in response to five inter-related factors: climate, organisms, topography, parent material, and time. Consequently, many soils are chemically, biologically, and/or geologically unique. The profound importance of soil, combined with the threats of erosion, urban development, pollution, climate change, and other factors, are now prompting soil scientists to consider the application of endangered species concepts to rare or threatened soil around the world.

# Introduction

## Soils and the Environment

Soils are multi-layered, living, breathing, three-dimensional environmental systems formed by the complex interactions of Earth’s biosphere, lithosphere, hydrosphere, atmosphere, and (to an ever increasing extent) human activity. Outside of gardening, agriculture, civil engineering, and the natural sciences, it can be argued that few humans peer below the surface of the soil, let alone consider that the world contained within any one soil profile might be as important, complex, or even as beautifully alien and fantastic as the enigmatic environments of the deep sea. Nonetheless, the far-reaching importance of soil (or in many cases, the lack of soil) is increasingly irrefutable, and it is important to emphasize that this incalculable value extends far beyond agriculture and food security.

Soil is, simply put, one of our most precious natural resources and life support systems (Wilding & Lin, 2006; Sandor, WinklerPrins, Barrera-Bassols, & Zinck, 2006). As the physical and biogeochemical foundations of most terrestrial ecosystems, soils constitute some of the most dynamic media of study within the Earth and environmental sciences. For example, today, a comprehensive, scientific understanding of soil is requisite for agricultural sustainability and resilience, climate change modeling (e.g., carbon flux and sequestration) and predictions of landscape responses to environmental change, hydrology and water resource management, pollution transport and remediation (with applications to both water and air quality), habitat and rare species conservation, civil engineering and sustainable urban design, the pursuit of new antibiotics, the management of environmental and agricultural pathogens, paleoenvironmental reconstruction, forensic science, contextual interpretation of archaeological data, and many other endeavors.

## Soil Definitions

Soil science can be broadly defined as the application of the scientific method to the study of the dynamic, layered environment immediately underfoot, but there is no one, universally accepted definition of soil itself. Instead, definitions have varied through time, and they currently vary depending upon the ecosystem service demanded of soil, or even with the disciplinary perspective of the person interacting with the soil. For instance, soil scientists, geomorphologists, hydrologists, and ecologists are all different kinds of environmental scientists; each may tolerate distinct magnitudes of biological, chemical, geomorphic, or hydrological variability, whether natural or anthropogenically influenced, in delineating soil boundaries across the same landscape.

Thus, soil is perhaps best approached as a concept with a degree of flexibility. Pedologists are scientists who study soil formation and evolution (i.e., “pedogenesis”). Pedologists define soil as: (1) a layered, three-dimensional medium formed at and roughly parallel to the Earth’s surface, that is (2) generally but not always capable of supporting growth by vascular plants and/or microbes, in which (3) each layer (horizon) has a suite of chemical and physical properties different from those of the parent material (i.e., rock, sediment, or anthropogenic fill) in which the soil has formed (Bockheim et al., 2005; Buol, Southard, Graham, & McDaniel, 2011; Soil Survey Staff, 2014; IUSS Working Group WRB, 2015). Incorporating these characteristics into a genetic definition, five broad factors form soils and determine their properties. Specifically, soils vary across landscapes as a function of: (1) local (micro) climate, (2) topography, (3) vegetation and biological activity, (4) parent material (geology), and (5) time (i.e., the age of the landform or geomorphic surface in which the soil has formed).

Soils are also physically defined by pedologists. Every soil is composed, in variable proportions, of: (1) minerals, (2) living organisms and decaying organic matter, (3) solutions, and (4) gases. As a consequence of the incessant reactions between the five genetic factors and these four physical components, soils are almost never static; their horizons, textures, minerals, nutrients, colors, and other properties evolve over timescales ranging from seconds (though more typically decades) to millions of years as a direct consequence of physicochemical losses, gains, translocations, and transformations (Simonson, 1959; Brady & Weil, 2008). Such reactions induce change vertically within any soil profile and also laterally across landscapes (Soil Survey Staff, 2014). Vertically, changes are expressed in the differentiation of discrete “master” horizons (Soil Survey Staff, 2014). Horizons are formally classified by process and character, for instance: “O” horizons are composed almost entirely of organic matter; “A” horizons are chemically leached mineral horizons darkened by organic matter, generally at or near the soil surface, “B” horizons form below the surface from the accumulation (as well as leaching and transformation) of ions or minerals, “C” horizons at the base of the profile represent the top of unweathered to only slightly weathered parent material, and “R” designates hard bedrock (Tandarich et al., 2002; Soil Survey Staff, 2014). Additional master horizons are described for forest, desert, and wetland or permafrost soils.

Soil can also be more ecologically or functionally defined. Any natural, “healthy” soil has evolved with and in response to past and present ecosystems to provide services including: habitat for flora and fauna, chemical and physical filtration of water, nutrient (re)cycling, the exchange of gases with the atmosphere, and, finally, an engineering medium in, on, and from which humans might construct homes or infrastructure (Brady & Weil, 2008; Bouma, 2010).

By contrast, a civil or environmental engineer’s soil concept tends to more generally encompass any of the unconsolidated, porous, and permeable geologic material, weathered or not, that extends from the Earth’s surface to a depth of perhaps tens or even hundreds of meters. This engineering concept merges soils with unaltered, unweathered sediments because, together, soils and sediments represent a continuous set of physical Earth materials with which human infrastructure, and the environmental footprint of society, must closely interact. In some cases, especially from a materials perspective, it can prove difficult to definitively point to the depth at which “soil” ends and “sediment” or “regolith” (i.e., weathered geological materials that can include bedrock altered deep within the Earth’s crust) can be considered unaltered from its original state. For example, alluvial fan sediments derive from the erosion of mountainside or hill slope soil profiles, and may be deposited with inherited mineralogical or geochemical traits suggestive of the older, eroded, precursor soil. Unfortunately, when used in its strictest sense, the classical engineering definition permits soil to be considered as lifeless, static material. By contrast, genetic or ecological approaches posit soil as an ever dynamic, both living and nonliving reflection of the terrestrial biosphere (Lin, 2014). In fact, the interaction of physical and biological soil processes may be one of the most important aspects of soil (Young & Crawford, 2004).

Currently, many soil scientists, geologists, ecologists, and other researchers are applying the term “Critical Zone” as a more broadly encompassing framework with which to consider soils, and with which to emphasize the need for interdisciplinary, systems-based, and place-based approaches in the Earth and environmental sciences (Wilding & Lin, 2006; Amundson, Richter, Humphreys, Jobbágy, & Gaillardet, 2007; Brantley, Goldhaber, & Ragnarsdottir, 2007; Giardino & Houser, 2015). Recognizing that soils are open systems in which exchanges of matter and energy, biologically mediated or otherwise, strongly link the lithosphere, biosphere, atmosphere, and hydrosphere, the Critical Zone approach integrates analysis of landscape components including soils, sediments, bedrock, vegetation, microorganisms, groundwater, and the atmosphere so that no one component is studied as if in isolation from the others, nor ignored as the exclusive purview of another scientific subdiscipline (Chorover, Kretzschmar, Garcia-Pichel, & Sparks, 2007; Richter & Yaalon, 2012). This is not entirely distinct from the pedogenic perspective. The Critical Zone encompasses the outermost skin of the Earth down to the deepest reach of groundwater (Brantley et al., 2007); it therefore includes but is not synonymous with soil. Aptly, Lin (2011) calls soil the “critical component of the Earth’s Critical Zone” (p. 2049).

Given the range of soil definitions in use today, of which only a few are offered here, improving international soil data accessibility and standardization remains a priority for many scientists (Hartemink & McBratney, 2008; Brevik & Hartemink, 2010; Adewopo et al., 2014). As in any field, standardization is intended (and necessary) to improve the quality, clarity, and efficiency of scientific data communication. In the United States as in other countries, a variety of freely available, technical resources now exists to help different stakeholders (e.g., farmers, land managers, ecologists, environmental engineers, geologists, and others) standardize the description, mapping, sample collection, and analysis of soils for various purposes (Burt, 2004; Schoeneberger, Wysocki, Benham, Broderson, & Soil Survey Staff, 2013; Soil Survey Staff, 2014; IUSS Working Group WRB, 2015; and others). However, no single approach can ever be perfect or universally applicable, and several soil classification systems are currently in use internationally, sometimes in direct competition with one another (Brevik & Hartemink, 2010). Furthermore, new metrics and techniques must be and are periodically added to accommodate new technologies, the rise of new disciplines, the discovery of new contexts or circumstances in which existing field or analytical methods prove inadequate, changes in environmental conditions or even the formation of new environments entirely, and the rise of new scientific priorities (Buol et al., 2011; Adewopo et al., 2014). The evolution of soil science illustrates this process quite well.

Soil science as an independent, formal discipline within the natural sciences is very young (Brevik & Hartemink, 2010). Most soil textbooks justifiably point to the work of Vasily Dokuchaev (1846–1903) in the late 19th century as a formally definable starting point for modern, genetic soil science, but the reality is more nuanced. Hundreds of naturalists, thinkers, and scientists shaped the development of soil science into a separate scientific discipline, especially from the mid-19th century onwards. Krupenikov (1992) uses the divisions of Strzemski (1947, as cited by Krupenikov, 1992) and others to propose a chronology for the development of soil knowledge and science from the Neolithic to the present (see Table 1). More recently, expert summaries of the nearly 11 millennia of soils knowledge and science history have been presented by Brevik and Hartemink (2010) and others (see Brevik, 2013).

Table 1: A generalized chronology of soil science developments

Time Range

Main Developments

Selected Writers or Thinkers

Ancient World & Middle-Ages

1

Neolithic - Early Bronze Age (~10 ka to 5 ka)

Accumulation of knowledge about soil properties, fertility, and early land management practices

2

~ 5 ka to 2.5 ka (Egypt, Mesopotamia, India, China, Meso-America)

Irrigated agriculture; attempts at soil salinization control; early soil/land use surveys (cadastral maps)

3

~ 400 BCE–400 CE (Greece, Rome, Mediterranean basin)

Attempts at soil classification; more systematic approaches to soil fertility, integration of soils into philosophical or religious concepts

Herodotus (ca. 484–425 BC); Theophrastus (371–c. 287 BCE; Cato (234 BC–149 BC); Varro (116 BC–27 BC); Strabo (ca. 64 BCE–CE 24); Columella (4–70 AD); Pliny the Elder (23–79 CE); and Palladius (ca. 4th–5th century CE)

4

6th–14th centuries (China, Byzantine empire, Germany, Europe, Russia)

Soil quality description for valuation or other landholding purposes

Abu Ibn Sina ("Avicenna") (980–1037); Ibn al-‘Awwām (ca. 11th century); Albert the Great (Albertus Magnus, ca. 1200–1280); and Peter Crescenzi (ca. 1230/35–1320)

Soil Science Foundations

5

15th–17th century

Further ideas on soil development, hypotheses regarding the role of "salts", water, and/or organic matter in plant nutrition

Leonardo da Vinci (1452–1519); Bernard Palissy (1510–1589); Francis Bacon (1561–1626)

6

17th–18th century

Rise of modern perspecitves on fertility, soil-plant relationships, and soil-rock relationships

Robert Boyle (1627–1691); Jethro Tull (1674–1741); J. G. Wallerius (1709–1785); M. V. Lomonosov (1711–1765); Francis Home (1719–1813); and others

7

End of 18th to mid-19th century

Agrogeology and mapping; intensive soil research and theoretical generalizations; the role of humus in nutrient dynamics

A. Thaer (1752–1828); F. A. Fallou (1794–1877); Charles Darwin (1809–1882); A. Orth (1835–1915); and P. E. Muller (1840−1926); and others

Modern Soil Science

8

late 19th century–early 20th century

Establishment of genetic perspectives in soil science (Dokuchaev's), birth of soil microbiology, expansion of soil physics

V. V. Dokuchaev (1846–1903); N. Sibirtsev (1860–1900); E. W. Hilgard (1833–1916); K. Glinka (1867–1927); S. N. Winogradskii (1856–1953); Curtis Marbut (1863–1935); and others

9

20th century

Dokuchaev school gains widespread traction; new soil classifications; emergence of subdisciplines; world soil maps; world soil congresses; soil textbooks; and more.

Too many to cite

10

Present Day

Disciplinary and interdisciplinary diversification of soils research; soils as a sensitive environmental system; statistics, quantification, modeling, and pedometrics; global land resource monitoring; food security considerations; climate change research; the Anthropocene perspective, and more.

Too many to cite

Table contents modified from a table by Krupenikov (1992) and text in Warkentin (2006), Brevik and Hartemink (2010), and others.

# History and Evolution of Soil Science

## Overview of Ancient World Soil Concepts and Land Use

The roots of human contact and experimentation with soil extend to the start of the Holocene epoch, with the beginning of cultivated agriculture. Farming developed independently at slightly different times between about 10000 and 8000 bce (perhaps as late as 3000 bce in North America), in as many as seven different cultural hearths, primarily in the Near East (Lal, Reicosky, & Hanson, 2007; McNeill & Winiwarter, 2004; Sandor, 2006; Dotterweich, 2013). Even before formal writing systems, any attempt to grow crops likely prompted observation and sharing of at least rudimentary knowledge about soil and climate (Krupenikov, 1992). Yet for millennia soil was not seen as the product of climate, organisms, relief, parent material, and time favored by scientific definitions today. Instead, soil was more generally and more simply considered, and with a bird’s-eye perspective (Jenny, 1968), as fertile land for plant growth.

In the ancient world as holds true in many regions even now, the primary concerns about soil were its fertility and its ability to support and sustain food production (Bockheim, Gennadiyev, Hammer, & Tandarich, 2005; Winiwarter, 2006). Thus, ancient forays into what is now soil science focused almost entirely on fertility, plant nutrition, and erosion control; soil biogeochemistry and geomorphology did not come into focus until the 19th to 20th centuries. Years of agricultural practice do not by default yield understanding or wisdom (Evtuhov, 2006); nonetheless, knowledge about soil, or at least agriculture and soil management more broadly, was indeed accrued and shared, steadily as well as in fits and starts.

All natural sciences require a classification system for their subject matter, ideally a formal and rigorously defined taxonomy (see Joel, 1926; Bockheim et al., 2005; Buol, Southard, Graham, & McDaniel, 2011; Soil Survey Staff, 2014; IUSS Working Group WRB, 2015). Even early civilizations around the world grouped soil types using properties such as color and texture (Krupenikov, 1992; Winiwarter, 2006; Brady & Weil, 2008). Furthermore, while most ancient soil descriptions would be considered subjective, generalized, or even vague by modern standards, they were still quite useful. Color is a coarse proxy for soil mineralogy, oxidation state, and organic matter content, while texture, a highly tactile trait describing a soil’s composition by sand, silt, and clay-sized particles, exerts a profound influence on infiltration, moisture retention, nutrient-holding capacity, structure, cohesion, and many other properties. In fact, soil texture and color are such fundamental and quickly assessable properties that both are still used as part of modern soil description and even taxonomy (Brady & Weil, 2008; Schoeneberger, Wysocki, Benham, Broderson, & Soil Survey Staff, 2013; Soil Survey Staff, 2014; IUSS Working Group WRB, 2015).

Ancient peoples obviously valued soil as a resource. The development of soil classification schemes and the drafting of at least rudimentary cadastral soil or land-use maps were closely linked to economic valuation—specifically, taxation—in ancient China, Egypt, and pre-Columbian Central America (Krupenikov, 1992; Gong, Zhang, Chen, & Zhang, 2003; Brevik & Hartemink, 2010; Winiwarter, 2006). Comparable land surveys would be conducted on and off throughout history, to be truly formalized with modern, objective, methodological, multi-scale protocols only in the mid- to late 19th century (Krupenikov, 1992; Gong et al., 2003; Brevik & Hartemink, 2010).

Records from ancient China, Egypt, Mesopotamia, Greece, Rome, and elsewhere also illustrate that soils knowledge, scientific and otherwise, understandably did not develop at the same rate in different regions or countries, nor with completely successive precursor information (Krupenikov, 1992). Oral traditions, the lack of formal writing systems, the rarity of successful, natural preservation of texts and other archaeological materials, and the loss or deliberate obliteration of written records during times of war, social upheaval, and environmental change or catastrophe mean that much remains uncertain and fragmentary about the history and true extent of ancient soil knowledge (Brevik & Hartemink, 2010). Even today, centuries to millennia of indigenous or other local soil knowledge is still being either lost to global economic development or even perhaps discounted or muted by modern science (see Barrera-Bassols & Zinck, 2003; Sandor, WinklerPrins, Barrera-Bassols, & Zinck, 2006). Nonetheless, given the dependence of societal stability, especially urbanized societies, on steady food production, most agriculturally based cultures developed a lexicon to describe soils in regard to fertility, color, texture, moisture, and suitability for certain types of crops or other vegetation (Sandor, 2006; Williams, 2006; Brevik & Hartemink, 2010). Again, it is almost certain that practical knowledge about crops and soil management practices, if not information about soils themselves, would have been traded alongside agricultural goods (Krupenikov, 1992).

Unfortunately, misconceptions about soil as a “limitless” resource, misconceptions about the nature of soil fertility, and beliefs about the supremacy of humans over the Earth also facilitated severe soil and ecosystem degradation in the ancient world as well as today (Hillel, 1991; Diamond, 2005; Showers, 2006). A devastating intersection of one or more factors including timber harvests or other land clearance practices, nutrient depletion caused by intensive, year-after-year cultivation with the expansion of human populations, time periods of variable precipitation or climate change, poor irrigation practices, and grazing led—immediately or eventually—to severe soil erosion, soil salinization, or both, in many regions. Around the globe, these slow disasters occurred sometimes more than once, over many years, dating as far back as the Uruk period (i.e., Mesopotamia ca. 4000 to 3100 bc) and up to and including the present day (Hillel, 1991; Lowdermilk, 1999; Gong et al., 2003; McNeill & Winiwarter, 2004; Diamond, 2005; Evtuhov, 2006; Montgomery, 2007; Casana, 2008; Janzen et al., 2011; Dotterweich, 2013). Concerted soil, water, or soil and water conservation efforts including strict irrigation laws, successful or not, were made by ancient world civilizations (Krupenikov, 1992; McNeill & Winiwarter, 2004; Dotterweich, 2013; and others). Nevertheless, the loss of topsoil to the world’s oceans and other depositional basins via water or wind is one of the most prevalent, globally occurring environmental impacts of agricultural settlement (Montgomery, 2007; Dotterweich, 2013). The timing and signature of anthropogenic soil erosion cannot always be clearly separated from natural erosion events, and a bias toward the proof of anthropogenic causes is argued to exist in some cases (Showers, 2006). However, after the extinction of large animal species through overhunting, the onset of intensive agriculture could be considered a herald, if not a starting point, of the Anthropocene.

## Neolithic to Renaissance

Early scientific studies of soil can of course be claimed with controlled, hypothesis-driven experimentation, however, few records of deliberate hypothesis testing have been recognized before the 16th to 18th centuries. Alternatively, the development of scientific approaches to soils can be sought in attempts to classify them. The oldest soil classification system is thought to have been established in ancient China approximately 2000 bce (Xia Dynasty, ca. 2070–1600 bce) (Gong et al., 2003; Brevik & Hartemink, 2010). The written record of this system is younger, at approximately 500 bce, but it documents a variety of soil types that can be at least generally translated into modern, scientific equivalents (Gong et al., 2003). The ancient classification incorporated color, texture, moisture, and vegetation, and each soil type would have had a correlative taxation class based on its fertility (Gong et al., 2003; Brevik & Hartemink, 2010). Agriculture was practiced in China by 9500 bce or more certainly by 7500 bce (Sandor, 2006). Officially sanctioned, public lessons on best practices in cultivation are reported from roughly 2200 bce (Gong et al., 2003) and definitively by the 2nd century bce (Krupenikov, 1992).

Farming itself is another type of applied soil science when practiced through systematic trial-and-error and adjusted according to accumulated knowledge (Winiwarter, 2006). The distribution of Neolithic farming settlements in many regions correlates with (then) naturally fertile soils, specifically those with at least 200 mm mean annual precipitation and also possessing naturally occurring, native strains of (eventual) crop plants (Krupenikov, 1992). Correlatively, lands that would not have been arable due to thin, rocky soil, aridity, or low fertility were either left completely unsettled (Krupenikov, 1992), or were used only temporarily or nomadically for hunting and gathering. With the establishment of agriculture in Mesopotamia, China, Egypt, India, and other centers of the ancient world, population size and political power tracked agricultural know-how, successes, and failures. Especially in the ancient Near East, political transitions can be linked to the successive, related processes of deforestation, soil erosion, sediment (silt) build-up in irrigation canals, water-logging, and major, irrigation-driven soil salinity crises (Hillel; 1991; Krupenikov, 1992; Brevik & Hartemink, 2010) accompanied by population declines (Lowdermilk, 1999).

Despite these failures, many ancient peoples knew their soils well, especially in Egypt (Krupenikov, 1992). Knowledge of soil types and climate was used to cultivate crops and to prescribe the timing of sowing and harvest, to evaluate land holdings, and to develop and strictly regulate irrigation infrastructure (see, for instance, the Code of Hammurabi) (Hillel, 1991; Krupenikov, 1992; Lowdermilk, 1999). The oldest irrigation system is known from the Samarra culture in northern Mesopotamia (6500/6300 to 6000 bce), consisting of extensive channels at Choga Mami, and evidenced by linseed found at Samarran sites (Roaf, 2004). Irrigation then developed in southern Mesopotamia (a non-dry farming area) during the Ubaid Period (5500–4000 bce) (Roaf, 2004).

Soil genesis was certainly pondered as well. Soils—fertile lands—were associated with rivers especially, and with volcanic ash. The regular floods of the Euphrates, Indus, and Nile Rivers were considered imperative for fertility and soil genesis, and were either partially managed or at least carefully monitored (Krupenikov, 1992). As further evidence of soil processes knowledge, aggressive manuring to offset fertility declines, terracing to counter soil erosion, the development of plows to suit a variety of soil types, and other practices were in wide use around the world (Krupenikov, 1992; Sandor, 2006; McNeill & Winiwarter, 2004). However, there exists scant evidence to claim that soils themselves were the subject of focused study, nor that they were ever truly untangled from fertility, moisture, or climate in the minds of ancient world peoples. Even observations of soil organisms were perhaps more equitable with soil mythology than with soil biology (Berthelin, Babel, & Toutain, 2006).

Soil science had taken firmer root by the first millennium bce in Europe and the Mediterranean basin. The ancient Greeks and their contemporaries did not experiment with soil, but their observations of the natural world fostered a significant knowledge base nonetheless (Krupenikov, 1992; Brevik & Hartemink, 2010). The philosophers, geographers, and other thinkers of Ancient Greece and the Mediterranean were arguably the first to describe in detail the layered nature of soil profiles below the surface (though not the origin of these layers nor the genetic relationships between them), and they considered the soil linked to deeper geological materials (Krupenikov, 1992). They also recognized that soils were spatially variable, that soils changed over time, and that they supplied nutrition to plants. The ancient Greeks tailored their farming practices to conserve soil moisture, and documented differences not only between soil types but also within latitudinal climate gradients or biomes (Krupenikov, 1992; Brevik & Hartemink, 2010).

Greek writers or thinkers rarely dedicated long portions of their work to soil nor specifically focused on it, instead tending to remark about soils only within broader works on landscapes, agriculture, or other topics (Krupenikov, 1992). Theophrastus from Ares (ca. 371–287 bce) is considered one of the most impactful agricultural experts of the era, and his keen consideration of soil and its classification was far ahead of his time (Krupenikov, 1992). Informative remarks about soils and landscapes were also provided by many other writers and thinkers including Herodotus (484–425 bce), Aristotle (384–322 bce), and Eratosthenes of Cyrene (ca. 276–194 bce) (Krupenikov, 1992; Buol, Southard, Graham, & McDaniel, 2011). A 28-book agricultural volume written in Punic by Mago from Carthage (predating the fall of Carthage in 146 bce, true authorship uncertain) is known to have been at least partially translated, cited, and updated by Roman and other writers, but it does not survive in its original text, nor in complete form (see Winiwarter, 2006).

Later, agricultural knowledge and prescribed soil/land management practices in the Roman Empire and neighboring regions developed in no small part from the Greek foundation (McNeill & Winiwarter, 2004; Winiwarter, 2006; Brevik & Hartemink, 2010). Winiwarter (2006) presents an excellent chronology and discussion of Roman-era writings on agriculture and economic ecology, noting variable emphases on soil itself. Texts on agriculture between 250 bce and 550 ce are especially well-known from Cato the Elder (M. Porcius Cato, 234–149 bce), Varro (M. Terentius Varro, 116 bc–27 bc), Strabo (ca. 64 bce–24 ce), Columella (Lucius Iunius Moderatus Columella, 4–70 ce), Pliny the Elder (23–79 ce) and Palladius (Rutilius Taurus Aemilianus Palladius, ca. 450–550 ce). Of their works, De Re Rustica by Columella ranks as one of the most originally impactful, scientific, and long recirculated Roman works on agriculture (Krupenikov, 1992; Winiwarter, 2006). The oft-cited compendium Naturalis Historia by Pliny the Elder is a comparably informative and invaluable source of past and contemporary knowledge (Krupenikov, 1992; Brevik & Hartemink, 2010). Varro’s emphasis on the need for crop rotation to slow the decline of soil fertility is also remarkable and oft-cited (Krupenikov, 1992; Winiwarter, 2006). Overall, these Roman works contain a range of insights on environmental concerns including erosion, fertility decline, and possibly even the role of slavery in facilitating soil degradation through poorly incentivized land stewardship (Columella, De Re Rustica). Roman writers even described reasonable field tests of soil fertility and soil cohesion (Winiwarter, 2006).

Ancient soil-agricultural knowledge is also known from the pre-Columbian Americas. New World cultivation was under way by 3000 to 1800 bce (Smith & Yarnell, 2009), with irrigated agriculture especially developed in the 1st millennium bce (Krupenikov, 1992) perhaps as a consequence of drought. Soil-agricultural knowledge and land management, though less well attested than in the Old World, has been documented most notably in Central America, especially for Aztec culture (see Williams & Ortiz-Solorio, 1981; McNeill & Winiwarter, 2004; Williams, 2006).

Despite the extent of generalized soil knowledge and groupings demonstrated by writers and thinkers of most cradles of the ancient world (many are not explored here), focused, detailed, methodologically consistent, scientific study of soil is difficult to claim. None of the writers or civilizations mentioned previously provide what can be considered scientific metrics of soil itself (Krupenikov, 1992; Warkentin, 2006; Brevik & Hartemink, 2010). Rather, the oldest known, detailed soil profile descriptions date only from the 19th century (see Feller, Blanchart, &Yaalon, 2006; Hartemink, 2009).

Manuring, land reclamation, terracing, and other land management practices, as well as land valuation surveys, certainly did continue to develop across Europe, the Middle East, and Asia (especially China and Japan) during the Middle Ages, and especially after the 11th century, but the works of thinkers or writers who discussed soils (within agricultural contexts) during this period remain relatively little known to modern scientists (Krupenikov, 1992; Winiwarter, 2006). This may be partly due to lack of translation, among other reasons (Winiwarter, 2006). As prominent exceptions, Abu Ibn Sīnā (“Avicenna”; 980–1037), Ibn al-‘Awwām (ca. 11th century), Albert the Great (Albertus Magnus, ca. 1200–1280), and Peter Crescenzi (ca. 1230/35–1320) are recognized as prominent writers and soil-agricultural information gatherers of the Middle Ages (Olson & Eddy, 1943a; Krupenikov, 1992). Ibn al-‘Awwām’s Kitāb al-filāḥa, written in Seville, Spain, in the 11th century, was advanced and comprehensive for the era (Banqueri, 1802; Olson & Eddy, 1943a).

## The 16th to 19th Centuries

By at least the 16th century, focused observations and experiments on plant nutrition were under way, nudging perspectives on chemistry and soil fertility a bit closer to the reality of soil biogeochemical nutrient cycles. These experiments laid the foundation for truly scientific chemistry (rather than alchemy) and soil research. So focused on the cause and nature of fertility were these early efforts that they were eventually labeled “agricultural chemistry”; soil science did not yet exist as a separate discipline (Krupenikov, 1992; Hartemink, 2009). Hypothesized origins of fertility in this period ranged from organic matter, to water, to soil particles.

Among those to contribute in this area of study was Leonardo da Vinci (1452–1519), oft-quoted for noting that “we know more about the celestial bodies than the soil underfoot” (e.g., Brevik & Hartemink, 2010; Lin, 2014). In addition to his work on soil conservation (Olson & Eddy, 1943b), Da Vinci’s observations of potted grasses were ahead of their time in recognizing that decaying organic matter returns nutrients to the soil. Humus, the term now used to describe this vital, decomposed soil organic matter, would not be coined until the work of Johan Gottschalk Wallerius (1706–1764) and then Albrecht Thaer (1752–1828) more than two centuries later (Krupenikov, 1992; Feller et al., 2006). Similarly, Bernard Palissy (1510–1589), who was the first scientist to describe the use of an auger specifically for the study of soil profiles (Feller et al., 2006), postulated that manuring returned nutrients removed by plant growth to the soil (Krupenikov, 1992). Francis Bacon (1561–1626) was mistaken in his conclusion that water supplied the nutrients needed by plants, suggesting that soil served merely as a physical support structure for growth, but he was one of the first to scientifically investigate soil’s chemical (i.e., “salt” or cation) adsorption capacity (Krupenikov, 1992). It was not until the work of Johann R. Glauber (1604–1670), Francis Home (1719–1813), and others that the role of specific chemical compounds in soil fertility would be tested at first in isolation (e.g., Glauber, using KNO3) and then increasingly in combination (e.g., Home, using soluble nitrogen compounds and MgSO4) (Krupenikov, 1992). By contrast, Jethro Tull (1674–1741) suggested, like others before him, that tiny soil particles in water provided plant nutrition. Unfortunately, these ideas were often presented as competing, “either-or” truths rather than partial glimpses of a more complex whole that would emerge in the 19th and 20th centuries.

## Soil Science in the 19th–20th Century

During the 19th century, descriptions of soil profiles and horizons (though not so termed) would accelerate the development of the discipline. Soils gained recognition as something more than just the cultivatable surface of the Earth, and more complex than mere, unconsolidated geological material (Krupenikov, 1992). This perspective was influenced in no small part by the 18th-century foundations established by Mikhail Lomonosov (1711–1765), who perceived soil as an evolving medium, not static, among many other contributions (Krupenikov, 1992; Buol et al., 2011). The eventual development of soil science was also linked to mapping (Hartemink, Krasilnikov, & Bockheim, 2013). Land assessments and then soil surveys were conducted at increasingly finer, more localized, single plot scales, especially from the middle of the 19th century onwards (Evtuhov, 2006). Through this effort, horizons gained recognition as laterally traceable layers with distinct properties that tended to be similar in depth and thickness across a given soil landform (Feller et al., 2006). Between 1563 and 1883, the careful observation, description, and labeled depiction of soil profiles and their horizons increased (Feller et al., 2006; Hartemink, 2009). This trend can be traced among the works of Bernard Palissy (1510–1590), Georges-Louis Leclerc de Buffon (1707–1788), Albrecht Thaer (1752–1828), Friedrich A. Fallou (1794–1877), Charles Darwin (1809–1882), Albert Orth (1835-1915), Peter E. Muller (1840–1926), and many other influential natural scientists (Feller et al., 2006; Hartemink, 2009; Brevik & Hartemink, 2010).

Horizons were initially used for description only. The terms horizon and profile, let alone the use of pedon to describe the smallest possible three-dimensional expression of a more or less uniform column of a given soil would not appear until the 20th century (Tandarich, Darmody, Follmer, & Johnson, 2002). With the work in Russia of Dokuchaev and Nikolai Sibirtsev (1860–1900), who used an A-B-C profile designation similar to today’s system, and then Konstantin Glinka (1867–1927), and due to the efforts of Curtis Marbut (1863–1935) in the United States, horizons were eventually identified as genetically significant (Krupenikov, 1992; Tandarich et al., 2002; Bockheim et al., 2005). In the 20th century, each horizon was recognized as having unique morphological, chemical, and physical properties that revealed developmental history and could be used for soil classification, land management, or environmental applications (Krupenikov, 1992; Bockheim et al., 2005). Interestingly, Jenny (1968) and Hartemink (2009) explored the visual representation of soils and soil profiles over time, offering the insight that artistic appreciation may have prompted scientists to observe and study soils more closely.

The intensification of soil mapping in Europe and North America during the mid-to-late 19th century was driven by the linked factors of population increase and the need for agricultural expansion and also yield intensification (Krupenikov, 1992; Hartemink et al., 2013). To assess soil resources, exhaustive surveys were mandated to be completed district by district, to precisely quantify soil quality in every land parcel (Evtuhov, 2006). These 19th-century maps and their accompanying reports shaped and inspired the thinking of authors and readers alike, and the distribution of soils was increasingly understood to reflect natural processes or laws (Hartemink et al., 2013). The maps were novel in that they included careful documentation not only of bedrock lithology (rock type and mineralogy) and soil texture, but also native vegetation communities, geomorphology (landforms), and soil physical properties including profile thickness or depth (Krupenikov, 1992; Evtuhov, 2006). Among the many scientists involved, Albert Orth (1835–1915) indexed horizons with successive letters and described profiles down to parent material (Tandarich et al., 2002; Brevik & Hartemink, 2010). Orth’s work particularly influenced Peter E. Muller (1840–1926), a forerunner of pedology (Feller et al., 2006). Meanwhile, the first soil map in the United States was part of an 1848 Massachusetts state geological survey (Hartemink et al., 2013). In 1860, Eugene W. Hilgard (1833–1916), the founder of soil science in the United States and a founder of soil science overall, completed an influential soil map and report for Mississippi (Krupenikov, 1992; Amundson, 2006; Evtuhov, 2006).

With mapping, soil science in the 19th century began to emerge from agricultural chemistry as a new discipline; however, for much of the 19th century it was considered “chemical geology” or “agricultural geology” (Krupenikov, 1992; Brevik & Hartemink, 2010; Buol et al., 2011). This school of thought viewed soil as a physically weathered (albeit organic-rich), chemically leached rock layer that varied due to lithology. However, scientists like F. A. Fallou (1794–1877), a land tax-assessor interested in mineralogy who studied soil as a hobby, began to advocate for soil science as a separate discipline from geology (Brevik & Hartemink, 2010). A shift toward the new field of soil science had started, and would be accelerated by Dokuchaev’s genetic concepts in 1883. Evolving within geology and in parallel to biology, chemistry, and physics (Warkentin, 2006; Brevik & Hartemink, 2010), four correlative subdisciplines of soil science eventually resulted. Together, soil chemistry, pedology (the broadly “soil geology” subdiscipline), soil biology, and soil physics reflect the origins and historical evolution of soil science, and they are still represented in the core curricula of academic programs in soil and environmental science today.

Soil biology can be traced back to early studies of organic matter (humus) in plant nutrition; however, with 1881’s The Formation of Vegetable Mould through the Action of Worms, with Observations on Their Habits, Charles Darwin (1809–1882) published the first study of soil organisms specifically (Tandarich et al., 2002; Feller, Brown, Blanchart, Deleporte, & Chernyanskii, 2003; Brevik & Hartemink, 2010). Darwin identified bioturbation (though not so termed), describing the role of biological and physical mixing of soil in the formation of differentiated horizons, and he graphically depicted soil profiles with sequentially lettered horizons (Krupenikov, 1992; Feller et al., 2003; Berthelin et al., 2006). This book reflected years of observation; Darwin had presented early work, including a labeled soil profile illustration, in 1837 (Feller et al., 2006). In 1878, P. E. Müller (1840–1926) studied compositional components of soil humus and identified the role of biological factors in forest soil genesis (Feller et al., 2006; Buol et al., 2011). Thus, Darwin and Müller are variably credited not only as founders of soil biology but also as key contributors to soil science more generally (Feller et al., 2003). Microbiology became a part of soil science with the work of Pavel Kostychev (1845–1895), Martin Wöllny (1846–1901), Martinus Beijerinck (1851–1931), and Sergei Winogradsky (1856–1953), among many others (see Krupenikov, 1992; Berthelin et al., 2006).

Meanwhile, soil physics (agricultural physics) emerged as the study of soil structure, particle size, aggregation, and hydrology, again due to major contributions of M. Wöllny and many others (Krupenikov, 1992). In soil chemistry (e.g., “agricultural chemistry”), nutrient cycles and biogeochemistry were coming to the fore. For instance, the concept of limiting nutrients was established in 1838 by P. Carl Sprengel (1787–1859) (Sparks, 2006). Also particularly influential in the 19th century were the efforts of Justus von Liebig (1803–1873), considered the founder of soil chemistry, and then John Lawes (1814–1900) to support nitrogen, phosphorous, and potassium as requisites for plant growth (McNeil & Winiwarter, 2004; Sparks, 2006). The discovery of microbial nitrogen fixation in the 1880s led to a better understanding of the nitrogen cycle (McNeil & Winiwarter, 2004). The need for fertilizers, a strategy to cope with saline soils, and later, soil pollutants, all continued to stimulate interest in soil chemistry as well as soil physics through the 19th and into the 20th century (Hillel, 1991; Krupenikov, 1992). Work by K. K. Gedroiz (1872–1932) and others on the adsorption capacity of colloids established the now crucial principles of cation exchange capacity in 1922 (Krupenikov, 1992; Sparks, 2006).

The inception of modern soil science is typically credited to the late-19th-century ideas of Dokuchaev (The Russian Chernozem, 1883). Dokuchaev viewed soils as a natural body produced by the interacting factors of climate, organisms (especially vegetation), topography, and parent material acting over time. Like the many other naturalists who had observed the zoning of climate and organisms for millennia before him, Dokuchaev observed that regional and latitudinal gradients created correlative zones of soil properties, especially humus content associated with the extensive, agriculturally critical, and economically invaluable Russian chernozems (or Mollisols in U.S. Soil Taxonomy). Besides adding a genetic component to the soil concept, he recognized that soils are composed of unique, roughly surface-parallel horizons, and that they consist of both organic and inorganic materials. Further, Dokuchaev recognized that fine-scale local factors, especially substrate lithology and topography, were a secondary, important control on soil genesis. Dokuchaev’s conceptual model remains central to pedogenic theory in the 21st century, and it is the main reason for his standing as the primary founder of soil science. Dokuchaev, however, did not develop these ideas in a vacuum but, rather, worked with and was in constant intellectual exchange with many other prominent, influential scientists interested in soils and related research (Krupenikov, 1992; Evtuhov, 2006).

Meanwhile, beginning with the landmark Report on the Geology and Agriculture of the State of Mississippi (1860), the contributions of E. W. Hilgard influenced the development of modern soil science in the United States (Evtuhov, 2006; Amundson, 2006). Hilgard’s soil chemical research was a groundbreaking model for the period, and he explored the links between soils and climate with forays into arid land pedogenesis (Amundson, 2006). For these reasons, Hilgard is often closely ranked with Dokuchaev as a founder of genetic soil science.

At the time, the natural factor perspective on soil genesis reflected a truly profound and radical departure from both the long-standing definition of soils as a medium for plant growth, and also from the agrogeologic view of soils as (merely) loose, weathered rock (Bockheim et al., 2005). The new soil genesis model was not widely accepted until the 20th century, and the genetic approach would not be taught at U.S. institutions until the 1920s or even the 1930s (Krupenikov, 1992; Brevik & Hartemink, 2010). Curtis Marbut at the Bureau of Soils (now the USDA NRCS) was particularly key in aligning U.S. soil science with the Dokuchaev school in the early 20th century, and in establishing the first U.S. Soil Taxonomy (Krupenikov, 1992; Buol et al., 2011). Moreover, the 19th century’s proliferation of scientific societies (Evtuhov, 2006) culminated in a series of influential conferences on soil science in the early 20th century, facilitating a synergistic, international exchange of ideas (Krupenikov, 1992; Hartemink, 2009). Especially post–World War II, genetic soil science had been firmly established, and soil science was its own discipline (Hartemink, 2009).

A key 20th-century contribution to soil genesis was made by Hans Jenny (1899–1992), who reorganized Dokuchaev’s ideas into the modern, state-factor model of soil genesis (Jenny, 1994; Amundson & Jenny, 1991). Jenny postulated pedogenesis as a mathematical function, albeit quantitatively unsolvable, of climate, organisms, topography, parent material, and time, i.e.:

$Display mathematics$

The ellipsis leaves this state-factor model open for additional factors (Amundson & Jenny, 1991; Buol et al., 2011; Richter & Yaalon, 2012). Such flexibility was especially visionary from an environmental perspective given that, today, humans are cited as a sixth factor profoundly influencing soil change (Amundson & Jenny, 1991; Richter, 2007; Richter & Yaalon, 2012). Another important refinement was offered by Simonson (1959), who noted that genetic processes operate simultaneously as well as sequentially and who articulated the processes of losses, gains, transformations, and translocations in soil profile development (Buol et al., 2011).

Along with these developments in pedogenic theory, the subdiscipline of soil geomorphology gained a foothold after the 1950s. Accompanied by the space race and the profound, scientific revolution of plate tectonic theory in geology, the gradual extension of soil science into new environments, assisted by geomorphology, helped meet the scientific needs of geomorphologists, sedimentologists, archaeologists, and other scientists engaged in surface processes research (Birkeland, 1999; Tandarich et al., 2002). In turn, these disciplines enriched pedology with new analytical methods and scientific perspectives (Richter & Yaalon, 2012). Gradually, soil science expanded its scope from the agricultural fields of stable plains, valley floors, and temperate biomes to investigate the pedogenic processes of hill slopes, mountains, wetlands, and the important, spatially extensive environments of arid lands and the tundra. Coupled with soil conservation efforts and agricultural intensification, the soil science expansion into new biomes helped change the perception of non-arable regions as “wastelands” to, instead, unique, environmentally sensitive ecosystems in their own right.

## Modern Environmental Soil Science

During and after the 1970s, soils gained greater foothold not only as an agricultural resource meriting scientific study and stewardship, but also as an integral ecological and environmental component linked with all other Earth systems and processes (Hillel, 1991; Hartemink, 2009; Brevik & Hartemink, 2010). Decades of soil survey and geomorphic mapping were then and continue now to be accompanied by initially vast, and then ever-increasing technological improvements in analytical sensitivity, computational power, and remote sensing (satellite) capabilities, along with a better understanding of isotope geochemistry, microbiology, and biogeochemical cycles. Soil science has become more comprehensive, analytical, and quantitative (Wilding & Lin, 2006). Especially due to computer technology, quantitative models of complex soil system interactions are being used to try to predict soil change across spatial and temporal scales. This effort is a vital direction for soil science and its sustained contributions to societal needs going forward (Lin, 2011). While traditional soil science emphasized slow, gradual, uniformitarian rates and processes of change, modern soil science recognizes that many changes can in fact be dynamic from decade to decade (Richter, 2007).

With unprecedented demands now made on soil by still-increasing human populations (Richter & Yaalon, 2012), in an epoch increasingly referred to as the Anthropocene (Crutzen, 2002), the application of soil science to environmental problems has been one of the fastest growing areas of soil science overall (Hartemink, 2002). A profound new perspective in modern soil science as an environmental discipline lies in the recognized, global impact of humans on soil landscapes and all other linked Earth systems (Richter & Yaalon, 2012), for instance, through dry deposition of atmospheric aerosols in even very remote regions. Humans are a sixth factor driving soil change, having for millennia attempted to domesticate soils for their needs (Amundson & Jenny, 1991; Richter, 2007). While the scientific frontier in Dokuchaev’s era was the natural formation of uncultivated soils, the existence of entirely natural, virgin soils anywhere on the planet is now in question, and a major focus of modern soil science lies in the scientifically informed management of human-affected soils, even the most minimally human-affected among them (Richter, 2007; Lin, 2011). Tugel and colleagues (2005) suggested that soil surveys should now include data on the causes of soil change at human timescales, and address the implications of soil change for ecosystem services or land use. In addition, soils composed almost entirely of anthropogenically disturbed materials (Anthrosols) or manufactured materials (Technosols) are now recognized for their significance as well (IUSS Working Group WRB, 2015).

Recently, a decline in the number of soil science departments in U.S. universities, changes in research funding trends, and a de-emphasis on Earth science in primary and secondary schools, have all been interpreted as indicators of a decline in soil science overall or even as a threat to the survival of the discipline (Adewopo et al., 2014; Hartemink & McBratney, 2008). However, an opposing view suggests that current environmental challenges have opened the door for the application of soil science to such a broad range of other scientific disciplines that the apparent trend might more aptly be termed a “golden era” (Wilding & Lin, 2006) or a “soil science Renaissance” (Hartemink & McBratney, 2008).

# Environmental Soil Science Topics and Applications

Many environmentally related subdisciplines now fall within or overlap with soil science. Researchers working in these fields may more closely identify as chemists, biologists, geologists, environmental scientists, or physicists rather than soil scientists; however, the distinctions between disciplines are increasingly becoming blurred. Science is working to comprehensively understand the role of soil processes in mitigating some of the Earth’s most pressing environmental issues, including climate change, public health, agricultural sustainability, water resource quantity and quality, and widespread extinctions (Janzen et al., 2011; Adewopo et al., 2014). This section briefly introduces a few areas of research that at least generally illustrate the remarkable diversity of soil science applications across the Earth and environmental sciences.

## Conservation and Rare, Threatened, or Endangered Soils

The loss or degradation in quality of soil due to erosion, (sub)urban development, nutrient depletion, pollution, and other anthropogenic causes is of obvious concern for agricultural, ecological, and societal sustainability. For instance, Franklin Roosevelt famously stated that “the nation that destroys its soil destroys itself” (Roosevelt, 1937). Soil conservation efforts trace back millennia, and key thinkers from Varro and Pliny the Elder to Thomas Jefferson (see Bennet, 1944) noted with chagrin the loss of topsoil associated with cultivation. Poor farming practices in the United States, combined with drought, culminated in the devastating Dust Bowl of the 1930s and prompted the establishment of the Soil Conservation Service (now the USDA Natural Resources Conservation Service) (Roosevelt, 1937). Today, cover cropping, alley cropping, agroecology, conservation tillage, no-till cultivation, riparian buffers, and many other practices (see Matson, Parton, Power, & Swift, 1997; Brady & Weil, 2008) are each considered viable components of sustainable agriculture that seek to arrest soil erosion by maintaining soil cohesion and a protective vegetative cover, among other benefits. Combating soil erosion and maintaining or restoring the ecosystem functions of impacted soils continue to be globally important aims of applied soil science and land management. However, soil conservation involves more than just combating erosion, in the same way that protecting a rare species requires a comprehensive, integrated management of ecosystems and human activity.

Modern science is beginning to formally recognize the truly unique geological and environmental conditions that have shaped many soils (Amundson, Guo, & Gong, 2003). Moreover, some of the rarest, most spatially restricted soils are seen as potentially supporting comparably unique and rare flora and fauna, or even unique extremes of microbial diversity (Fitter et al., 2005; Tennesen, 2014). Extremely old soils are also relatively few in number. Without having the chance to fully study or understand a given soil, we cannot know what has been lost when it vanishes (Janzen et al., 2011). For example, Certini and colleagues (2002) describe the importance of nascent, “embryonic” soils formed in cavities left by the differential weathering of inclusions in an igneous rock substrate, noting that the cavities contain unique, miniature ecosystems supported only by the tiny bodies of soil slowly but steadily forming within them. Thus, even tiny, young soils formed in one outcrop may be significant for biodiversity. Similarly, the loss of pedodiversity caused by the extinction of even one soil taxon should pose no less ecological significance than the extinction of a biological species (Drohan & Farnham, 2006). Moreover, besides these biologically important roles, certain soils may have great cultural significance to different peoples (e.g., the Chimayo soil, Santuario de Chimayo, New Mexico). For these reasons, the extinction of soil taxa is now a recognized topic of serious scientific consideration (Amundson et al., 2003; Drohan & Farnham, 2006; Tennesen, 2014), an important and captivating environmental application of modern soil science. Furthermore, with the extension of soil science concepts into extremes of climate, time, and parent material, there is another important scientific and environmental implication of pedodiversity: soils are not everywhere the same and therefore cannot be managed nor even studied the same way in all environments either.

## Soils through Time and Space: Paleosols, Relict Soils, Early Life, and Other Planets

Paleopedology is an extension of soil science back into the geologic record. Paleosols are fossilized soils preserved through burial, most commonly in the alluvial (floodplain) strata of depositional basins (Kraus, 1997; Retallack, 1998; Birkeland, 1999; Retallack, 2001). Paleosols represent years to millennia of general sedimentary and geomorphic stability, that is, periods of time without either major erosion or deposition. When paleosols have a consistent stratigraphic position that can be laterally traced over long distances they may be formally described as “geosols” according to stratigraphic nomenclature (North American Commission on Stratigraphic Nomenclature, 2005). As with modern soils, the nature and properties of paleosols reflect their paleoclimate, paleobiology, paleogeomorphology, parent material, and duration of pedogenesis. Thus, paleosol morphology, mineralogy, and geochemistry can be cautiously used alongside any fossil plant and animal traces to reconstruct past environmental conditions (Retallack, 2001). In the aggregate, sequences of paleosols and sedimentary strata can provide a record of geologic basin evolution in response to tectonic events, climate change, or sea-level change (Kraus & Bown, 1993; Kraus, 1999).

Unfortunately, paleosols are at best only imperfect and incomplete records. With the conversion of soil to paleosol over geologic time, properties are almost always altered physically and chemically by burial, compaction, heating, mineral reactions with deep, slow-moving groundwater, and other processes. For instance, organic matter decomposes, phyllosilicates (clays) may recrystallize into new minerals, and isotopic ratios can be partially or completely reset as ions diffuse into or out of the soil. However, with a full understanding of these challenges, cautious interpretations made after carefully assessing the degree of post-burial profile alteration can nonetheless provide useful glimpses into the geologic past (e.g., Driese, Mora, Stiles, Joeckel, & Nordt, 2000). With careful, clearly stated assumptions, past climate regimes can be approximated from paleosol geochemistry, using chemical indices or clumped isotope ratios that are thought to serve as general precipitation or paleotemperature proxies (Sheldon & Tabor, 2009; Quade, Eiler, Daëron, & Achyuthan, 2013).

Today, some of the most exciting applications of soil science lie in the search for chemical or micromorphological traces of microbial life in Precambrian soils or sediments on Earth (e.g., Czaja, 2010), as well as in the comparable but even more (logistically) challenging search for life, past or present, on Mars. Over billions of years, Earth’s soils, atmosphere, and sedimentary environments have all profoundly changed in response to the evolution of life, the evolution of photosynthesis, the colonization of terrestrial ecosystems by bacteria, fungi, and then vascular plants, the evolution of conifer forests (before which there could exist no Spodosols or Podzols), and the evolution of angiosperms (especially grasses, before which there could exist no Mollisols or Chernozems) and other developments (see Mack, James, & Monger, 1993). Cyanobacteria or other crust-forming cryptogams being studied in modern, arid soil landscapes (e.g., Williams, Buck, & Beyene, 2012; Belnap et al., 2001) may have been among the first life forms to colonize and stabilize terrestrial (subaerial) substrates.

Paleopedology has also informed the study of modern landscapes. For example, paleosols are differentiated from relict profiles, which are extant, unburied soils formed under a previous biome, climate, or other environmental condition that is distinct from the present (Birkeland, 1999). Relict soils are sometimes considered “out of equilibrium” with the current environment; however, equilibrium in pedogenesis is rarely if ever reached because soil change may lag behind or respond only slowly to certain environmental changes. Soils that have experienced more than one climate, for instance a soil that has experienced both a glacial and an interglacial climate, and containing mineralogical, chemical, or physical properties reflective of both climate regimes, are considered “polygenetic.” With this perspective, Lin (2014) argues that most living soils are at least in part a kind of polygenetic paleosol, with some components or properties that more closely reflect the past, rather than the present environmental conditions. Thus, Dokuchaev’s soil genesis paradigm is today being adjusted from a deterministic model with roots in geologic uniformitarianism to a dynamic model that places soils firmly within the broader context of Earth surface and near-surface processes (e.g., the Critical Zone) (Lin, 2014) as well as biological evolution.

## Soil-Environmental Responses to Climate Change

By striving to understand the responses of soils, landscapes, and ecosystems to environmental change in the geologic past, soil scientists can also seek analogs that might help predict the response of soils and ecosystems to global temperature increases and climate change caused by anthropogenic greenhouse gas emissions in the present. In particular, soil science is one component of a systems-based approach to identify pedogenic, geomorphic, sedimentological, hydrological, biological, and atmospheric thresholds and feedback loops in landscape evolution.

Biogeochemical cycles are a key focus of this effort. For instance, as reservoirs of carbon, and as a vital component of terrestrial ecosystems, soils play an important role in the global carbon cycle. Soil carbon cycling is complex and microbially mediated, with carbon exchanged back and forth at dynamic rates between living biomass, readily decomposable organic litter, and recalcitrant pools of organic matter (Brady & Weil, 2008; Janzen et al., 2011). At one extreme, the total organic carbon content in desert soils might be less than 5% by mass of the surface horizon, but the accumulation of inorganic carbon as calcite (CaCO3) in calcic and petrocalcic horizons has been investigated as a reservoir or sink for atmospheric CO2. At another more critical extreme in the high latitudes, alarm is growing over the threat of catastrophic methane release from thawing gelisols—that is, tundra, or permafrost, soils—due to both present warming trends as well as those predicted for the next century (Stokstad, 2004). Estimates for the amount of carbon stored in boreal and alpine permafrost range from 350 to 450 gigatons, and neither the rate nor the magnitude of soil and environmental change that would result from release of this carbon is yet understood (Stokstad, 2004).

Thus, additional work is needed to quantitatively model the effects of increasing atmospheric CO2, changes in moisture availability, and changes in temperature on each soil biome around the globe. Sedimentological and geomorphic responses to climate change must also be constrained. Quantifying soil-environmental responses to climate change through the integration of soil chemistry, microbiology, ecology, land use, hydrology, and geomorphology is an urgent research priority.

## Nutrient Management

Although soil science now encompasses much more than fertility, sustainable nutrient management remains an important area of research in many ways. For instance, the environmental impacts of soil fertility management include energy consumption and carbon dioxide emissions associated with the manufacture of synthetic fertilizers, the mining of phosphates and nitrate from geologic deposits, cultural eutrophication of agricultural watersheds caused by nutrient runoff, the pollution of aquifers and other drinking water sources with nitrate (leading to methemoglobinemia in some agricultural communities) and other harmful compounds, and the loss of ecosystems and ecosystem services with agricultural land clearance and soil nutrient depletion (Brady & Weil, 2008; Janzen et al., 2011).

At least 14 elements essential for plant growth are derived from the soil (Brevik & Burgess, 2013). Thus, successful crop yields are also accompanied by exports of N, P, and other elements to consumers and, ultimately for some nutrients, wastewater treatment systems, watersheds, and ocean basins (Janzen et al., 2011). Synthetic fertilizers, in addition to their environmental costs, also have clear monetary costs. Yet through over-application, poorly timed applications, or other problems, many fertilizers are never taken up by crops, but instead are lost to agricultural runoff or soil microbes. Research continues to explore more sustainable, integrated strategies to manage soil fertility, crop yield, and ecosystem services in agricultural systems, and to recover nutrients, for instance phosphorus, from wastewater systems. Thus, research to establish locally tailored, sustainable nutrient management is needed to ensure food security and ecosystem health, especially within the context of continued population growth and climate change.

Nutrient management also extends to non-agricultural systems. In the United States and Europe, emissions of sulfur and nitrogen compounds from coal-burning power plants and other sources have been greatly reduced by legislation following the recognition of acid rain and its consequences. However, decades of soil leaching by acidic precipitation have left some forest soils depleted of important cations (nutrients) even today. Compounded by climate change, the long-lasting effects of the acid rain legacy for temperate forests are not yet clear. In urban settings, the heat island effect, soil sealing, and watershed reconfigurations can impact the quantity and quality of organic inputs into soils, as well as increase the risk of contamination by metals, trace organic contaminants, and pathogens. All of these changes can adversely impact nutrient cycling in soils, with risks of pollution posed to urban farms and gardens (Adewopo et al., 2014). Soil science research, therefore, is still sorely needed to ensure efficient, environmentally sustainable nutrient management in the 21st century.

## Water Quality, Urban Soils, and Environmental Remediation

Soils are the meeting place of the atmosphere, lithosphere, biosphere, hydrosphere, and human activity. As a consequence, agriculture, urban development, mining activities, forestry, and many other activities profoundly impact environmental quality through soil, and soil scientists are increasingly being called upon to study the role of soil in mitigating air and water quality issues in addition to continuing research on the remediation of soil pollutants.

Only a fraction of Earth’s fresh water is available to humans, and much of the actively circulating water available to us passes through the soil (Janzen et al., 2011). As fresh water resources grow scarce with increasing demands by cities and agriculture, especially with climate change, and as pollution of groundwater, lakes, and streams becomes ever more widespread, soil science is needed to help model and improve the intensive management of water resources and quality. In urban and suburban environments, green roofs, community gardens, greenways, bioswales, graywater systems, conversion of industrial sites into riverside parks, and other examples of environmental engineering all require soil science knowledge to ensure healthy ecosystem function, to ensure effective nutrient management, to ensure that runoff is not polluted, and to grapple with accumulation of heavy metals or other urban pollutants in soils used for gardens or recreational parks. Soil science also plays a role in current pollution remediation techniques including soil vapor extraction, air sparging, and more. As knowledge of soil microbial diversity increases, the use of bacteria to isolate heavy metals, or to degrade problematic hydrocarbons or other compounds is a continuing area of research. Similarly, research on soil-plant interactions can inform the use of hyperaccumulators in environmental remediation as well.

## Salinization and Desertification

Arid regions constitute more than a third of Earth’s landscapes and sustain nearly two billion people (United Nations, 1997). Unfortunately, the problems of salinization and desertification caused by irrigated agriculture and land clearance in arid and semi-arid landscapes is not a problem constrained to human history. Worldwide, salt-affected soils remain a threat to agricultural sustainability, water resources, and human health.

Soil salinization occurs whenever water repeatedly evaporates within the soil profile, leaving dissolved ions behind as precipitated salts. Sodium, in particular, along with other monovalent cations, can prevent soil colloids from flocculating and can cause aggregates to collapse and disperse. Dispersion can lead to pore-plugging and the formation of a hardpan or surface crust that impedes infiltration. This leads to more near-surface evaporation and salt build-up. Alternatively, conversion of natural ecosystems to cropland can raise water tables to such an extent that the capillary fringe reaches the soil profile, also promoting salt crystallization with evaporation. Efflorescent crusts formed by the evaporation of water brought to the soil surface by capillary rise can crystallize around phyllosilicates and other minerals, or may incorporate toxic metals such as arsenic that can occur naturally in alkaline groundwaters. When salt crusts formed by evaporating soil moisture are subjected to wind, the transport of harmful minerals and elements in that dust becomes an important concern for arid land peoples. Desertification may accompany soil salinization, as soils become too salty to sustain even native shrubs or grasses, leaving little surviving vegetation to anchor soil against severe erosion by wind.

Because small-magnitude changes in precipitation and potential evapotranspiration can cause disproportionately large effects on arid and semi-arid ecosystems and sediment thresholds, soil salinization and desertification remain significant concerns for agriculture in marginalized landscapes of the Sahel, the Mojave, and elsewhere. As the climate warms and populations increase, further demand may be placed on groundwater supplies used for irrigation, and the risk of salinity and sodicity problems in agricultural soils may increase. Thus, soil science research in arid lands will remain an important area of environmental science in the 21st century.

## Soil Microbiology

Soil microbiology is the study of soil organisms that are, at least as individual cells, invisible to the naked eye. Included among this scope are archaea, bacteria, as well as eukaryotes such as fungi and Protista, along with a number of nematodes and other organisms that fall between micro- and macro-scale divisions. Soil microbiology also studies the extensive coats, mats, or crusts formed by microbes on mineral and land surfaces, and another key area of research lies in determining the complex, multi-directional, multi-species relationships among distinct soil microbes and communities of vascular plants (Brady & Weil, 2008). Research in soil microbiology has many clear applications in the environmental sciences, with particular relevance to issues of pedodiversity, nutrient management, environmental (pollution) remediation, and public health, among others.

Soil microbes perform an amazing array of services in soils. They are among the main drivers of biogeochemical cycles in terrestrial ecosystems, they help to weather rock substrates into secondary (soil) minerals, they regulate the oxidation state of important elements, they facilitate mineral precipitation from soil solutions, and they help bind, or aggregate, soil particles together (Brady & Weil, 2008; Viles, 2012). Individual cells, network structures (for instance, cyanobacterial filaments or fungal hyphae), and secretions of sticky extracellular polymeric substances not only bind soil particles together but can also chemically alter mineral surfaces (Brady & Weil, 2008; Viles, 2012). Biofilms, biological crusts, and mycorrhizal communities can cover very large areas; thus, awareness is growing that microbial communities significantly influence soil geomorphology over long distances and, through biochemical weathering, over long, geologic timescales (Viles, 2012).

Soils are one of the last frontiers of biodiversity research because, even now, most soil bacteria and archaea remain unidentified (Fitter et al., 2005; Janzen et al., 2011). Estimates of the quantity and diversity of microbes in the hypothetical, “average” soil are both tantalizing and uncertain. One gram of soil may contain 109 to 1010 individual bacterial cells or archaea, and stretched end-to-end, 10 to 103 meters of fungi (Brady & Weil, 2008). The intensive study of one soil in Scotland yielded over 100 species of bacteria, 350 protozoa, and 24 species of arbuscular mycorrhizal fungi (Fitter et al., 2005).The total living biomass in soil (including larger soil fauna) is crudely estimated between 1 and 5% of the total organic matter content (Brady & Weil, 2008). More recent work estimates that microbes, more broadly, may even account for as much as 50% of global biomass (Viles, 2012).

The development of new techniques with which to study microbes in soil, water, and rock has shed new light on the staggering diversity of life in soil (Brady & Weil, 2008; Viles, 2012). These include microscopic and chemical analyses, along with culture-dependent methods of species identification. Most excitingly, culture-independent genetic profiling, molecular-level isotopic measurements, and other recent techniques (Viles, 2012) have dramatically increased the number of soil organisms identified, facilitated a clearer understanding of what some of these organisms do in soil, and aided the development of new antibiotics (Viles, 2012).

The diversity of species now beginning to be cataloged is at least in part indicative of the extreme, dynamic range of microhabitats that may exist in any given soil. Fractal surface areas, intricate microporosity, and diverse ranges of particle size and composition, coupled with chemical distinctions between soil horizons, mean that there exist myriad microenvironments within any given soil. Each of these microenvironments may have widely different temperature ranges, moisture contents, O2 concentrations, pH ranges, and other geochemical conditions (Brady & Weil, 2008). There is also remarkable functional redundancy among microbes in natural, healthy soils, meaning that some soils may prove to be resilient, at least for a time, to environmental change (Brady & Weil, 2008; Janzen et al., 2011). Sterilized soils, moreover, typically do not support plant growth as well as natural or inoculated soils, underscoring the role of soil microbial communities and symbionts in making nutrients available to plants for growth (Brady & Weil, 2008). It is now recognized that even in extremely cold, hot, arid, acidic, or sodic environments, weathering and soil genesis are facilitated or accompanied by a diverse community of microbial life (Viles, 2012)

Thus, soil microbiology remains a vibrant, critical topic in soil science. Continued areas of research include further investigation of functional redundancy in nutrient cycling and nutrient conservation, the development of new antibiotics to counter increasingly drug-resistant strains of bacteria, the impact of pharmaceuticals and other anthropogenic influences on soil biodiversity, the use of microbes for remediation of chemical pollutants, the management of pests and pathogens for crops, livestock, and humans, and many other topics.

## Soil Science, Public Health, and Medical Geology

Soil science also plays an important role in public health. More broadly encompassed within Medical Geology, this area of research is a young, growing, interdisciplinary science that explores the relationships of Earth processes, geological materials, and/or physical environments with public health or the health of animals. Research on soils and public health considers both natural factors as well as the impact of human interactions with the environment (see Brevik & Burgess, 2013).

Soils are linked to human health because of our dependence on the supply by soils of nutrients to plants and livestock (Deckers & Steinnes, 2004; Brevik, 2013). Soil pH and other properties influence ion solubility; thus, changes in soil solution chemistry can leach ions out of the soil profile or render them unavailable for plant uptake. Nutrient deficiencies in agriculturally important soils can cause nutrient levels in crops to decline also, and a variety of ailments can result in humans whose diet stems primarily from nutrient-poor soil (Brevik, 2013). Soils can also harm human health by introducing toxic substances or pathogens into the food chain, and by introducing toxins or pathogens via (1) physical contact of skin with soil, (2) ingestion of soil particles with food, or (3) inhalation of dust derived from wind erosion of soil (Deckers & Steinnes, 2004; Brevik, 2013). Soil science is also critically responsible for the development of antibiotics and other medicines. Most antibiotics today have been derived from soil actinomycetes; thus, the isolation of antibiotics from soil (micro)organisms is one of the most significant medical applications of soil science (Brevik, 2013).

Many soil-borne pathogens are now well-known. Besides a number of bacteria and well-known parasitic roundworms (see Brevik & Burgess, 2013 and others), even soil fungi can cause epidemics in certain regions. As one example, the fungus Coccidioides immitis is endemic to many soils of the southwestern United States and northern Mexico. The disease it causes, coccidioidomycosis, may resemble a severe flu or pneumonia and is commonly referred to as “Valley Fever” after California’s Central Valley, where, as in the Sonoran and Mojave Deserts, incidences of the disease are well-documented (Fisher, Bultman, & Pappagianis, 2000). Humans become infected after disturbing and inhaling soil containing the spores of C. immitis, which therefore especially affect earth scientists, farm workers, construction workers, and environmental engineers. However, broader populations are exposed when spores from endemic areas are blown as far as several hundred kilometers away, given windy, dusty conditions. These spores may lie dormant for years (Fisher et al., 2000). It is not entirely certain what impact strong El Niño years or projected climate change will have on this pathogen. Moreover, increasing modern dust levels may provide greater protection from ultraviolet radiation than in the past, permitting greater diversity of viable viruses, bacteria, or fungi to even cross ocean basins on stratospheric dust (Brevik, 2013). Thus, research on soil-borne pathogens is an important environmental application of soil science.

While airborne soil dust can carry pathogens, harmful gases, organic chemicals, heavy metals, and radioactive materials into the lungs and then the bloodstream, causing a variety of ailments, the main direct health effect of dust inhalation is respiratory irritation or disease (Brevik, 2013). Recently, Buck and colleagues (2013) identified a link between mesothelioma rates in southern Nevada and the prevalence of asbestiform minerals in local rock outcrops and soils. Thus, further research is urgently needed around the world to determine the nature and magnitude of health risks posed by natural and anthropogenically produced soil dust to growing human populations.

Additional priorities in the field of soils and public health include better integration of geochemical and environmental quality data with public health data, especially via spatial (i.e., Geographic Information System) data analyses (Filippelli, Morrison, & Cicchella, 2012). In light of climate change, population trends, urbanization, and changes in lifestyle or consumption patterns, a major research priority lies in better determining the intersections of soil and public health overall (Adewopo et al., 2014).

## Soil and Forensic Science

One final application of soil science worth mentioning even if not strictly environmental is forensic soil science (see Ritz et al., 2009; Fitzpatrick and Raven, 2012). Forensic soil science overlaps with forensic geology and forensic geomorphology, and it includes in its scope of work the use of mineralogy, geochemistry, and micromorphology to compare the “fingerprints” of soil or sediment samples from crime scenes and alibi localities, burial or murder implements, the clothing of suspects, stolen or smuggled fossils, and other evidence collected as part of any legal investigation (Fitzpatrick & Raven, 2012). Forensic soil science can even in some instances help constrain time of death based on decomposition and local soil chemical or environmental conditions (see Ritz et al., 2009). These types of analyses are not entirely different from those used to study modern soils, nor to reconstruct paleoclimate or paleoecology, described previously, nor even from the application of soil science and paleopedology to archaeological contexts. However, the documentation and sample tracking procedures that must be followed by soil scientists when handling and analyzing samples for court cases are far more detailed than general research practices, and the need for laboratory accreditation is also critical (Ritz et al., 2009; Makarushka, 2012). Moreover, the use of new analytical methods, rather than decades-old analytical practices that while well-established and cited may not be as accurate or appropriate for a given sample or question, can potentially be portrayed as unreliable to a jury. Forensic soil science also has bearing in environmental cases relating to the assessment of earth materials used in civil engineering or industry, and of course relating to soil pollution, its effects, provenance, and mitigation. The use of Earth science in criminal cases dates as far back as 1906 (e.g., the murder of Eva Disch in Germany) (Murray & Tedrow, 1975) and perhaps even to a Prussian railroad heist in 1856 (Makarushka, 2012). Thus, the continuing evolution of forensic soil science illustrates the capacity of soils, even in fragmentary, partial, or trace amounts, to provide scientific, environmental narratives and explanations relevant to modern society.

# Conclusions

Soils are the living foundation of terrestrial ecosystems, and they are increasingly appreciated as complex, three-dimensional, environmental systems that illustrate the myriad, intricate linkages between Earth’s biosphere, lithosphere, hydrosphere, atmosphere, and human populations. Human society has risen and fallen over at least 11 millennia in proportion to the developing understanding and successful or unsuccessful management of soil as an environmentally sensitive, ecological, and economic resource.

The study of soil as a separate scientific discipline is quite young, emerging only in the 19th century from foundations in agriculture, chemistry, and geology before adopting a genetic, evolutionary landscape perspective applicable to many scientific interests. In the 21st century especially, modern soil science has diversified in response to environmentally critical research needs associated with both population growth and climate change. The growing impact of human activity on the natural world is reflected in the changing philosophy of modern soil science, which now views humans as a sixth factor of soil and environmental change alongside climate, organisms, topography, parent material, and time. Recently, exciting new developments have been made in the fields of paleopedology, geochemistry, microbiology, and many other areas.

# Acknowledgments

R. Hazlett encouraged and facilitated my involvement in the Oxford Research Encyclopedia of Environmental Science, and both L. Oglesby and J. Moore-Kucera provided helpful early suggestions and discussion regarding the direction and scope of this article. Any shortcomings, however, are entirely my own. Some ideas in this article have origins in past conversations with mentors and collaborators over the years, including B. Buck, P. Drohan, J. Noller, R. Rogers, T. Spell, and countless others. I am indebted to C. Sauvage for suggestions and early comments on ancient world irrigation practices. I thank the W. M. Keck Science Department of Claremont McKenna, Pitzer, and Scripps Colleges, and the staff of the Honnold-Mudd Library of the Claremont University Consortium for supporting the research for this article.

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