Progress in soil geography I: Reinvigoration

Agronomy Publications Agronomy

2019

Progress in soil geography I: Reinvigoration Progress in soil geography I: Reinvigoration

Bradley A. Miller Iowa State University, [email protected]

Eric C. Brevik Dickinson State University

Paulo Pereira Mykolas Romeris University

Randall J. Schaetzl Michigan State University

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Progress in soil geography I: Reinvigoration Progress in soil geography I: Reinvigoration

Abstract Abstract The geography of soil is more important today than ever before. Models of environmental systems and myriad direct field applications depend on accurate information about soil properties and their spatial distribution. Many of these applications play a critical role in managing and preparing for issues of food security, water supply, and climate change. The capability to deliver soil maps with the accuracy and resolution needed by land use planning, precision agriculture, as well as hydrologic and meteorologic models is, fortunately, imminent due to advances in the geospatial data related to soil. Digital soil mapping, which utilizes spatial statistics and data provided by modern geospatial technologies, has now become an established area of study; over 100 articles on digital soil mapping were published in 2018 alone. The first and second generations of soil mapping – discussed in this paper - thrived from collaborations between Earth scientists and geographers. Now, as we enter the dawn of the third generation of soil maps, those collaborations remain essential. To that end, we review the historical connections between soil science and geography, examine the recent disconnect between those disciplines, and draw attention to opportunities for the reinvigoration of the longstanding field of soil geography. Finally, we emphasize the importance of this reinvigoration to geographers.

Disciplines Disciplines Agriculture | Physical and Environmental Geography | Soil Science | Spatial Science

Comments Comments This is a manuscript of an article published as Miller, Bradley A., Eric C. Brevik, Paulo Pereira, and Randall J. Schaetzl. "Progress in soil geography I: Reinvigoration." Progress in Physical Geography: Earth and Environment 43, no. 6 (2019): 827-854. doi: 10.1177/0309133319889048. Posted with permission.

This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/agron_pubs/613

https://doi.org/10.1177%2F0309133319889048


In press with Progress in Physical Geography, October 2019

1

Progress in Soil Geography I: Reinvigoration 1

Bradley A. Miller1,2*, Eric C. Brevik3,4, Paulo Pereira5, Randall J. Schaetzl6 2

1 – Department of Agronomy, Iowa State University, Ames, IA, USA; [email protected] 3

2 – Leibniz Centre for Agricultural Landscape Research (ZALF) e.V., Institute of Soil Landscape Research, 4 Eberswalder Straße 84, 15374 Müncheberg, Germany 5

3 – Department of Natural Sciences, Dickinson State University, Dickinson, ND, USA; 6 [email protected] 7

4 – Department of Agriculture and Technical Studies, Dickinson State University, Dickinson, ND, USA 8

5 – Environmental Management Center, Mykolas Romeris University, Ateities g. 20, LT-08303 Vilnius, 9 Lithuania; [email protected] 10

6 – Department of Geography, Environment, and Spatial Sciences, Michigan State University, East 11 Lansing, MI, USA; [email protected] 12

* - corresponding author 13

Abstract 14

The geography of soil is more important today than ever before. Models of environmental systems and 15

myriad direct field applications depend on accurate information about soil properties and their spatial 16

distribution. Many of these applications play a critical role in managing and preparing for issues of food 17

security, water supply, and climate change. The capability to deliver soil maps with the accuracy and 18

resolution needed by land use planning, precision agriculture, as well as hydrologic and meteorologic 19

models is, fortunately, imminent due to advances in the geospatial data related to soil. Digital soil 20

mapping, which utilizes spatial statistics and data provided by modern geospatial technologies, has now 21

become an established area of study; over 100 articles on digital soil mapping were published in 2018 22

alone. The first and second generations of soil mapping – discussed in this paper - thrived from 23

collaborations between Earth scientists and geographers. Now, as we enter the dawn of the third 24

generation of soil maps, those collaborations remain essential. To that end, we review the historical 25

connections between soil science and geography, examine the recent disconnect between those 26






2

disciplines, and draw attention to opportunities for the reinvigoration of the longstanding field of soil 27

geography. Finally, we emphasize the importance of this reinvigoration to geographers. 28

1. Introduction 29

Geography and soil science have much in common. One of those commonalities is a connected origin in 30

natural resource inventory, which today make both disciplines essential to address key environmental 31

issues. The two disciplines also share a highly interdisciplinary nature (Shaw and Oldfield, 2007; Rodrigo-32

Comino et al., 2018). Being naturally interdisciplinary is a strength in that it allows both soil science and 33

geography to bridge gaps between other, complimentary disciplines (Krupenikov et al., 1968; Fridland, 34

1976; Ostaszewska, 2008; Brevik and Hartemink, 2010). On the other hand, it can become a weakness 35

when allied fields seek to absorb portions of soil science or geography into their own academic spheres 36

(Nikiforoff, 1959; Fridland, 1976; Harvey, 1984; Brevik, 2009); this concern seems to have been shared 37

by both fields at various times in their histories. 38

Soil forms at the interface among the atmosphere, lithosphere, hydrosphere, and biosphere. 39

Traditionally, this interface has been called the pedosphere (Targulian et al., 2019). However, more 40

recently the concept of the pedosphere has been extended to include the top of the vegetative canopy, 41

forming the concept of Earth’s critical zone (Chorover et al., 2007; Brantley et al., 2017) (Figure 1). 42

Critical zone research has generated new excitement in interdisciplinary fields, as it focuses on the 43

processes occurring within this interface, from micro- to global scales (Brantley et al., 2007). Regardless 44

of the terminology or scientific approach, two concepts about soil are clear: 1) soil and the processes 45

occurring within it are essential to life on Earth, and 2) to understand soil, one must consider the 46

interacting processes in their respective spheres, including their positive and negative feedback loops. 47


3

48

Figure 1. Conceptual diagram illustrating the Earth’s critical zone. Although difficult to illustrate in a 49 single diagram, soil is the subsurface environment shaped over time by geological, chemical, physical, 50 and biological processes. After Chorover et al. (2007). 51

52

The terminology used in the preceding description of soil should resonate with geographers, as the 53

concept of feedback loops is central in modern geography, including soil geography (Torrent and 54

Nettleton, 1978; Muhs, 1984; Phillips, 1993; Chadwick and Chorover, 2001). Similarly, a goal of physical 55

geography is to “explain the spatial characteristics of the various natural phenomena associated with 56

the Earth’s hydrosphere, biosphere, atmosphere, and lithosphere” (Pidwirny and Jones, 2017). Even 57

though soil science and geography have evolved as interdependent fields (Rodrigo-Comino et al., 2018), 58


4

their overlap is evident. However, in various parts of the world, different academic structures and 59

funding sources have led to some academic disconnects, despite their apparent commonalities. 60

Traditionally, soil science and geography have intersected in subfields such as soil geomorphology 61

(Holliday et al., 2002), pedology (Schaetzl and Thompson, 2015), and soil geography (Arnold, 1994). 62

Although soil geomorphology research has been active in geography, geology, Earth science, and soil 63

science/agronomy departments, soil geography research has not been as active for a better part of the 64

20th century. If a reinvigoration in soil geography is occurring, it is time for a reconnection between the 65

two disciplines to be recognized and implemented, so as to build upon the strengths of both fields. 66

The concept of soil has received increased attention recently due to rising concerns about sustainability, 67

especially in the context of the ecosystem services that soil system provides (Adhikari and Hartemink, 68

2016; Baveye et al., 2016). One of the key services that soil provides in sustaining human life is mediated 69

through agriculture. Since the emergence of farming, human impacts on the soil system have become an 70

important aspect of soil dynamics (Grieve, 2001; McLauchlan, 2006; Veenstra and Burras, 2015). The 71

change from a nomadic to a sedentary lifestyle for early cultures changed the relationship between soil 72

and people, beginning an era of increasing impacts that people have on soil functions (Beach et al., 73

2006; Sandor and Homburg, 2017; Goudie, 2018). The recent exponential growth of the human 74

population has intensified demand for food and resources, resulting in anthropogenic impacts on soil to 75

unprecedented levels (Ferreira et al., 2018). However, it should be noted that human manipulation of 76

soil has a long history and does not necessarily lead to negative impacts. For example, there is 77

archeological evidence that humans have changed and therefore influenced soil to increase 78

productivity, such as the terra preta de Indio (Fraser et al., 2011; McMichael et al., 2014). With the 79

expansion of human activities, humans therefore have become an important soil-forming factor (Bidwell 80

and Hole, 1965; Bockheim et al., 2014; Bajard et al., 2017). Concerns about environmental issues have 81

resulted in increasing interest in both soil science and geography (e.g., Jonsson and Daviosdottir, 2016; 82


5

Pereira et al., 2018). For the same reasons that both disciplines were born in an era of investment in the 83

management of natural resources, they are once again poised to help answer related questions in 84

today’s world (Hartemink and McBratney, 2008). 85

Surges in these disciplines appear to occur at the convergence of new technologies and pressing issues 86

facing society. When these two ingredients come together, doors into areas of new exploration are 87

opened, new approaches are tested, and investment becomes a priority for government leaders. By the 88

19th century, surveying technology had reached a point that facilitated the unprecedented, quantitative 89

study of spatial relationships, at the same time that nations recognized the critical role that natural 90

resources played in the accumulation of wealth (Miller and Schaetzl, 2016). Today, as we begin the 21st 91

century, geospatial technologies such as global positioning systems (GPS), remote sensing, and 92

geographic information systems (GIS), coupled with vastly improved spatial datasets such as LiDAR 93

elevation and frequently updated aerial imagery archives, are revolutionizing the capabilities and 94

opportunities of both geography and soil science. The second ingredient - societal demand - is at our 95

doorstep. Concerns about environmental issues, including global climate change, ecosystem services, 96

water quality and quantity, soil degradation, loss of biodiversity, food security and quality, are all well 97

known to geographers and soil scientists. As recognition of these issues progresses around the world, 98

the need for geography and soil science expertise can only grow. 99

In this paper, we review (1) the historical connections between soil science and geography, and (2) how 100

recent technological advancements have provided an impetus to reinvigorate each of these two 101

respective disciplines’ interest in the other. Future papers in this three-part series will explore the 102

opportunities for future collaboration between geography and soil science in greater depth. 103


6

2. Historical origins of soil science and geography 104

Soil science and geography have similar historical roots as academic disciplines. Although some aspects 105

of geography have been important to human societies for thousands of years (Harvey, 1984) and 106

geographical concepts have been taught in universities for centuries (Johnston, 2003), geography was 107

only established as a formal academic discipline in the latter part of the 19th century (Harvey, 1984; 108

Godlewska, 1989; Schelhaas and Hönsch, 2001; Sack, 2002; Johnston, 2003; Shaw and Oldfield, 2007; 109

Claval, 2014) and was not taught in some countries as a formal university subject until much later 110

(Barnes, 2007). Similarly, although soil and soil science concepts have been societally important for 111

thousands of years, soil science per se was only organized as an independent field of scientific study in 112

the late 19th century (Krupenikov, 1993; Brevik and Hartemink, 2010). In the late 19th and early 20th 113

centuries, both soil science and geography received strong, foundational contributions from scientists 114

trained as geologists; in fact, with a lack of formally trained geographers it was common for early 115

geography scholars to come from fields such as biology, geology, history, journalism, and mathematics 116

(Johnston, 2008). The beginnings of soil science were largely the same, with early concepts of soil being 117

driven by many of the same respective base disciplines. For example, in the 19th century, chemists 118

favored an emphasis on the humic content of soil, whereas geologists emphasized the mineral content 119

(Krupenikov, 1993). Regardless, the motivating purpose to study soil at the time was for agriculture, 120

leading to terms such as agrochemists and agrogeologists (Krupenikov, 1993). 121

Both soil science and geography benefited from the scientific advancements stemming from the Age of 122

Exploration, and from the associated motivations of national governments. Enough scientific 123

advancement occurred in the 15th century for the European empires to realize the benefit of, and to 124

invest in, the accurate mapping of national borders and resource inventories. In part, these 125

developments were based on improved survey methods, but that effort in turn gave rise to more 126


7

scientific study of spatial patterns for the purpose of better spatial prediction and understanding of 127

processes (Miller and Schaetzl, 2014). 128

The confluence of these two disciplines and the rise of scientifically based "spatial thinking" is 129

exemplified by one of the founders of geography, Alexander von Humboldt (1769-1859) (Figure 2) 130

(Hartshorne, 1958; Bouma, 2017). Humboldt published a treatise on the basalt formations along the 131

Rhine River early in his career (Humboldt, 1790), but was mostly known for his botanical work while on 132

expeditions to explore the western hemisphere. What made Humboldt remarkable was his use of 133

quantitative methods, including careful recording of latitude and longitude, and attention to the 134

covariation of phenomena over space, later termed spatial association in geography. Prime examples of 135

this were Humboldt’s identification of relationships between vegetation and elevation (Humboldt, 136

1807/2009), and with global climate zones (Humboldt, 1817). His scientific achievements made von 137

Humboldt an academic superstar. For this reason, Russia repeatedly invited Humboldt to conduct 138

expeditions into Asia, an offer that was finally realized in 1829 (Wulf, 2015). 139


8

140 Figure 2. Alexander von Humboldt (pictured here in 1814 at the age of 44), implemented quantitative 141

methods during the Age of Exploration to advance understanding of spatial patterns in the physical 142

environment. His work inspired a new generation of geographers and approaches to studying the Earth. 143

144

Like Humboldt, the naturalist Charles Robert Darwin (1809-1882) (Figure 3) became famous from his 145

studies during his explorations of the western hemisphere. Besides his well-known work on evolution, 146

Darwin also made contributions to understanding soil processes, particularly mixing by soil fauna 147

(bioturbation) (Darwin, 1869; 1881). Darwin’s work laid the foundation for an array of multidisciplinary 148

studies on pedogenic processes during the ensuing century, even though this approach to soil science 149

remained in the shadow cast by the Russian Vasily Dokuchaev’s more geographic approach to the study 150

of soil (Johnson and Schaetzl, 2014; see below). In 1975, Darwin’s ideas reappeared in Soil Taxonomy 151

(Soil Survey Staff, 1975), if only minimally, as a part of the then-emerging “biomantle” concept (Johnson 152


9

et al., 2005). Recently, however, the bioturbation concepts first espoused by Darwin have gained 153

considerable traction, e.g., Humphreys et al. (1996), Balek (2002), and Fey (2010). 154

155 Figure 3. Charles Darwin (pictured here in 1855 at the age of 46), best known for his work as a naturalist, 156

contributed to soil science with his observations of the effect of bioturbation on the soil. 157

The founder of modern soil science was born into this academic environment. The Russian Vasily 158

Vasilyevich Dokuchaev (1846-1903) (Figure 4) was trained as a geologist and early in his career worked 159

on mapping the geology and soils of Russia (Dokuchaev, 1877; 1879). Expanding on Humboldt’s 160

approach of spatial association between organic life and environmental conditions, Dokuchaev 161

recognized that soil spatially co-varied with both biota and other environmental conditions (Brown, 162

2006). Specifically, Dokuchaev identified soil as resulting from the combined factors of climate, 163

vegetation, parent material, relief, and time (Dokuchaev, 1883/1967). Although other scientists from 164

around the world have made significant contributions to the development of modern soil science 165

(Jenny, 1961; Krupenikov, 1993; Brevik and Hartemink, 2010), Dokuchaev has become widely recognized 166


10

as the father of modern soil science (Jenny, 1961; Krupenikov, 1993; McNeill and Winiwarter, 2004; Buol 167

et al., 2011; Landa and Brevik, 2015). 168

169 Figure 4. Vasily Vasilyevich Dokuchaev (pictured here in 1888 at the age of 42) had profound impacts on 170

the inception of soil geography as a science. Dokuchaev did not associate with geography, but his work 171

and the work of his students laid the groundwork for most of the soil maps in use today. 172

173

The interactions between geology, geography, and soil science in the late 19th to early 20th century were 174

numerous and complex, frequently making it difficult to place individuals into one of these disciplinary 175

categories. For example, the geologist Arthur E. Trueman (1894-1956), who had originally joined 176

University College, Swansea as the first head of the Department of Geology, expanded the department 177

to include geography, which later separated into two successful departments (Pugh, 1958). Another 178

example is Konstantin Dmitrievich Glinka (1867-1927), who made significant contributions to Russian 179

soil science and geography (Shaw and Oldfield, 2007). Glinka began his academic career as a 180


11

mineralogist and his first academic position was as professor of mineralogy and geology at the 181

Agricultural Institute at Novo Alexandria, Russia (Ogg, 1928). 182

In the late 19th century, geologists at Harvard University in Cambridge, Massachusetts generated the 183

foundations of geography and soil science in the USA (Figure 5). Nathaniel S. Shaler (1841-1906) 184

authored a classic work on “The Origin and Nature of Soils” (1891), which, in part, extended Darwin’s 185

work on bioturbation. Shaler’s student and later colleague, William Morris Davis (1850-1935), is 186

recognized today as the father of American academic geography (Sack, 2002). Collier Cobb (1862-1934), 187

a student of both Shaler and Davis, later became the head of the Geology Department at the University 188

of North Carolina where he conducted research on human geography, as well as coastal and aeolian 189

processes. In addition, while at the University of North Carolina, Cobb established a Bachelor of Science 190

program in Soil Investigation, which supplied many of the mappers for the early soil survey program in 191

the USA (Brevik, 2010). Another of Davis’s students, Curtis Fletcher Marbut (1863-1935), served as the 192

director of the USA’s Soil Survey Division from 1913-1935, a time critical in the formation of the 193

procedures that produced soil maps used in the USA today. Davis was the first and fifth president, and 194

Marbut was the twentieth president, of the Association of American Geographers. 195

196 Figure 5. Some of the academic tree of key influencers in American soil geography. Many of the names 197


12

in this chart will be recognized by geographers, geologists, and soil scientists as members of their own 198 discipline. (a – Graduate School of the USDA, c – University of Chicago, e – Earlham College, h – Harvard 199 University, n – North Carolina State University, p – Imperial University of St. Petersburg, w – University 200 of Wisconsin at Madison) (Modified from Tandarich et al., 1988) 201 202

In central Europe, Dokuchaev’s work laid the groundwork that would establish soil science as an 203

independent scientific field (Tandarich and Sprecher, 1994; Johnson and Schaetzl, 2014). Although 204

modern soil scientists celebrate Dokuchaev’s recognition of soil as an independent body of study, the 205

core of his work laid the foundations of soil geography, not pedology (Buol et al., 2011). Before World 206

War I, American geographers regularly studied and corresponded with German geographers (Martin, 207

2015). Among them was Marbut, who translated “The Great Soil Groups of the World and their 208

Development” from German to English. The author of that text was Glinka, the first director of the 209

Dokuchaev Soil Science Institute in Leningrad, Russia. Between Marbut’s study of the Russian 210

philosophies on soil science and the work of American geologist Eugene W. Hilgard (1833-1916), the 211

notion that soil was more than the product of only geologic processes had begun to be adopted in the 212

USA. 213

In 1927, the USA hosted the first meeting of the re-organized Congress of the International Association 214

of Soil Science. That conference was monumental in its gathering of influential leaders for soil mapping 215

at the time when soil survey programs were gaining major momentum (Figure 6). The list of congress 216

attendees was filled with notable scientists that geographers and soil scientists of the respective 217

countries will recognize, such as J.H. Ellis (Canada), E.J. Russell (England), A. Penck (Germany), H. 218

Stremme (Germany), L. Kreybig (Hungary), P. Treitz (Hungary), H. Jenny (Switzerland), M. Baldwin (USA), 219

T.M. Bushnell (USA), C.F. Marbut (USA), K.D. Glinka (USSR Russia), S. Neustruev (USSR Russia), and L. 220

Prasolov (USSR Russia). These connections and commonalities between the founders of modern 221

geography and soil science, with geology frequently a common link, illustrate the natural and historical 222

ties between these disciplines. 223


13

224 Figure 6. Curtis Marbut (second from left) and Konstantin Glinka (middle) – two men who were the 225

leaders of the two most active soil survey programs in the world, during the most critical time of soil 226

mapping methods development. Marbut was a proponent of the concepts that Glinka had written 227

about. Photograph taken at the first re-organized Congress of the International Association of Soil 228

Science, hosted by the USA in 1927. 229

230

2.1 The connections 231

Soil geography is, in its most fundamental sense, the study of the spatial distribution of soil. Inherent in 232

that study are the patterns of soils, soil properties, and the processes that produced those patterns. 233

There is evidence of interest in soil geography per se, starting well before the time that soil science and 234

geography had become established as academic fields of study. Archeologists have found evidence that 235

farming practices were adapted according to soil fertility patterns dating back to 3000-2000 BCE. 236

Information on the spatial distribution of soil properties was recorded in China as early as 300 CE (Miller 237


14

and Schaetzl, 2014), and maps of soil attributes were made in Europe by the early 1700s (Brevik and 238

Hartemink, 2010). In North America, long before European settlers had arrived, native peoples 239

recognized that soil in floodplain areas were fertile places for crop production (Brevik et al., 2016b). In 240

the American Southwest, farming had been concentrated in locations where soil water was 241

preferentially retained in the root zone due to restrictive layers such as shallow bedrock, petrocalcic, or 242

argillic horizons (Sandor et al., 1986; Homburg et al., 2005). Although these examples do not indicate the 243

existence of a formal academic field, they nonetheless show that the fundamental recognition of spatial 244

variation in soil properties has had a long history. 245

In Europe, early mapping of soil tended to be by boundaries of land ownership, due to the connection 246

with land valuation and taxation. Although largely a geology map, William Smith (1769-1839) mapped 247

the variation of soils in his landmark map of England, Wales, and Scotland (1815). In Germany, unique 248

soil classification systems were being proposed by the mid-1850s (Krupenikov, 1993). For example, 249

Friedrich Fallou (1794-1877) published books on the soil types of Saxony and Prussia (Fallou 1853, 1868, 250

1875). With advancements in understanding soil fertility, soil mapping endeavors in Germany shifted 251

from cadastral to the ability of soil to respond to different management practices. In general, the soils of 252

Europe were first mapped by geologists, as they were the most familiar with surveying and mapping 253

techniques. Approaching soil from this perspective, the scientists working in this area advocated for the 254

study of soil to be defined as agrogeology, a subdiscipline of geology (Berendt, 1877; Ehwald, 1964). 255

József Szabó (1822-1894) published soil maps of this style for Hungary in 1861, adding considerations of 256

groundwater (Szabó, 1861). In 1867, A. Orth’s map entitled “Geologic-Agronomic Mapping” won a 257

competition for “agricultural geognosy,” sponsored by the Agricultural Union of Potsdam 258

(Mückenhausen, 1997). German unification occurred in 1871 and coincidently the reputation of German 259

geography as well as German soil mapping rose in the world. Indicative of the influence of German soil 260

geography, M. Fresca was invited to map the soil of Japan from 1885 to 1887 (Krupenikov, 1993). 261


15

The recognition and mapping of soil spatial properties were central to the establishment of soil science 262

as an independent field of study (Ogg, 1928; Cline, 1961; Krupenikov, 1993; Richter and Yaalon, 2012). 263

The foremost individual in this undertaking was Dokuchaev, who although trained as a geologist 264

specializing in mineralogy (Tandarich and Sprecher, 1994), became noted for his studies of soils and 265

their distributions. As with all major scientific advances, the contributions that elevated soil science 266

were made by a number of individuals, but because Dokuchaev’s contributions led to definite changes in 267

the way soil science was viewed and conducted, he is widely recognized as the father of soil science 268

(Jenny, 1961; Krupenikov, 1993; Landa and Brevik, 2015; Johnson and Schaetzl, 2014; Schaetzl and 269

Thompson, 2015). A major piece of that contribution was Dokuchaev’s publication on the Russian 270

Chernozem (1883/1967), which espoused the interdisciplinary view that soil was a product of more than 271

only geologic processes. Ironically, Dokuchaev expressly refused to associate himself with the field of 272

geography and did not feel that the fledgling science he was helping to create coincided with geography 273

(Shaw and Oldfield, 2007). Nonetheless, his student, Glinka, produced the first soil map of the world in 274

1908 (Hartemink et al., 2013). In 1909, several followers of Dokuchaev, all wrestling with how to better 275

map soil, attended the first International Agro-Geological Conference hosted by the Royal Hungarian 276

Geological Institute in Budapest. That conference marked a turning point for soil science in that it 277

addressed the “confusion [that is] for a time inevitable in a borderland subject like the present, that 278

joins up with geology, botany, and chemistry, and is closely connected with agriculture; indeed, even its 279

very name has not yet been settled, for we find the subject of the conference referred to as 280

agrogeology, agricultural geology, pedology, or simply ‘the science of soil’” (Russell, 1910, p. 157). 281

Similar to the experience in Europe, early soil mapping efforts in North America were generally 282

performed by trained geologists, largely because academic programs in soil science did not yet exist 283

(Coffey 1911; Lapham 1949; Brevik, 2010). The USA established the first nationally coordinated soil 284

survey effort in 1899 (Marbut, 1928). This undertaking was significant for soil geography in that it 285


16

represented the first attempt to catalog the soil of a country using uniform standards and practices. 286

Under the direction of Milton Whitney (1860-1927) (Figure 7), the first generation of these maps were 287

produced using an agrogeology approach. In the 1930’s, Marbut’s integration of Dokuchaev’s multi-288

factor approach and the wider availability of aerial photography came together to facilitate a second 289

generation of soil maps, using the concept of the soil-landscape paradigm. Essentially, this paradigm 290

established that soil map units should occur together in a regular, repeatable pattern, based on the 291

spatial patterns of the five soil-forming factors. Those areas with similar factors, especially topography, 292

were predicted to have similar soil properties (Hudson, 1992). These kinds of soil map units tended to be 293

based on the factors of soil formation identifiable in stereo-orthophotographs (Simonson, 1989). A set 294

of soil map units occurring together was called a soil association in the USA’s Soil Survey system; this 295

term was parallel to Milne’s catena concept established for soil mapping in Africa (Milne, 1935; 296

Bushnell, 1943). 297


17

298 Figure 7. Milton Whitney (pictured here in the 1910s) was the first chief of the American Bureau of Soils, 299

which was charged with the first nationally coordinated soil survey, including uniform standards and 300

practices for the staff producing the soil maps. 301

302

Other examples of pioneering work done by geographers are easily found. The first soil fertility map of 303

Britain was prepared in the 1930s by the geographer Sir Dudley Stamp (1898-1966), with help from 304

other geographers such as E.C. Willatts (1908-2000) (Willatts, 1987). Stamp, whose academic training 305

was as a geologist but who made his career as a geographer, became one of the most influential 306

geographers in Britain (Johnston, 2008). Willatts was also widely known and became the organizing 307

secretary of the Land Utilisation Survey of Great Britain (Wise, 2000). 308

Hugh Hammond Bennett (1881–1960) (Figure 8), trained as a geologist by Cobb at the University of 309

North Carolina, began his career as a soil surveyor, which took him across the USA and other countries 310

conducting soil research. As a result of those experiences and an address given by Chamberlain, Bennett 311


18

became concerned about the problem of soil erosion in the 1920s. In 1928, he co-authored “Soil 312

Erosion: A National Menace”, which would be influential in the development of the USA’s Soil 313

Conservation Service (SCS). Bennett became the director of the Soil Erosion Service when it was 314

established within the USA’s Department of Interior in 1933 and then became head of the SCS when it 315

was established within the Department of Agriculture in 1935. Bennett’s advocacy for protecting soil 316

resources was pioneering, strengthened by increased public awareness during the Dust Bowl which 317

occurred between 1934 and 1940 (Helms, 2010; Lee and Gill, 2015). Hugh Hammond Bennett served as 318

president of the Association of American Geographers from 1943-1944. 319

320 Figure 8. Hugh Hammond Bennett (center), calling attention to the severity and impacts of soil erosion. 321

Image courtesy of the USDA-NRCS. 322

323

Carl O. Sauer (1889-1975) was a highly influential American geographer (Kenzer, 1985) who served as 324

president of the Association of American Geographers from 1940-1941. He influenced soil geography 325

largely through his role on President Franklin D. Roosevelt’s (1882-1945) Presidential Science Advisory 326


19

Board in the 1930s. Sauer suggested that the SCS should integrate pedology, geology, and climatology in 327

their research (Holliday et al., 2002). This recommendation was in line with Sauer’s own background, for 328

even though he made his reputation as a cultural geographer, he began his graduate studies in geology, 329

with a specialization in petrography (Kenzer, 1985). Sauer was an advocate for broad academic training 330

and for including individuals from related fields with geographic interests in the study of geography 331

(Sauer, 1956). The SCS accepted his advice and started a research program under the 332

geographer/climatologist Charles W. Thornthwaite (1899-1963), who studied under Sauer at the 333

University of California at Berkeley (Mather, 2005). This research began to form the foundation of soil 334

geomorphology, work that was unfortunately interrupted by World War II (Holliday et al., 2002). 335

Perhaps the academic crown jewel of geology-soil academic linkages was the soil geomorphology 336

program, established by the USA’s National Cooperative Soil Survey (NCSS) in the 1930s. The NCSS is a 337

special partnership between the American federal government, state and local governments, and 338

universities, to develop and improve soil maps. Using many of the ideals espoused by Sauer, the NCSS 339

led the development of the soil geomorphology program, which was to be focused on “surface and soil”, 340

and to have pedologists, geologists, as well as climatologists work together and focus on the interactions 341

and co-development of soil and landscapes (Effland and Effland, 1992). Under the leadership of Charles 342

Kellogg (1902-1980) and assisted by Guy D. Smith (1907-1981), the program had a stated research 343

mission to understand soil-landform relationships in support of soil mapping (Grossman, 2004). Smith 344

and the NCSS established several research locales where soil geomorphology was to be studied in detail, 345

including subhumid Iowa (led by Robert V. Ruhe (1918-1993), a geologist), a desert site in New Mexico 346

(led by Leland H. Gile (1920-2009), a soil scientist), a humid site in the Pacific Northwest of Oregon (led 347

by Robert B. Parsons, a soil scientist), and one in North Carolina (led by Raymond B. Daniels (1925-2009), 348

a soil scientist). Theories and data that poured out of these four sites spurred considerably more work of 349

this kind within the university community, had profound effects on theories of soil and landscape 350


20

evolution, and greatly influenced the way soils were classified (Effland and Effland, 1992). Much of this 351

effort culminated in the first textbook devoted to soil geomorphology, written by geologist Peter W. 352

Birkeland (Birkeland, 1974). 353

Although this brief discussion is by no means exhaustive (there are many additional individuals and 354

advances that could be discussed), it serves to demonstrate that there are strong historical ties between 355

soil science and geography, and that significant advances were being made in both soil science and 356

geography in the late 19th and early 20th centuries (Figure 9). It also demonstrates that advances in soil 357

geography were driven by individuals trained in a number of fields, including chemistry (e.g., Whitney), 358

geography (e.g., Sauer, Thornthwaite), geology (e.g., Dokuchaev, Marbut), natural science (e.g., Darwin), 359

and others (Helms, 2002; Johnston, 2008; Johnson and Schaetzl, 2014; Landa and Brevik, 2015). 360

361 Figure 9. Some milestones in the development of map-making technologies, leading to important 362

advancements for both geography and soil science. 363

364


21

2.2 The disconnect 365

Soil science and geography share some history; they are both highly interdisciplinary, and in fact, 366

overlap. They share many "founding fathers." And yet, despite the connections and commonalities, in 367

many ways it seems that there has been a disconnect between the fields for much of their recent 368

history. This is particularly true in the USA. Although geographers have made numerous contributions to 369

soil science in Europe, where an academic association between soil science and geography is more 370

common (Freeman, 1987; Willatts, 1987; Brandt, 1999), soil science research in the USA has largely been 371

conducted in colleges of agriculture at the land grant universities, particularly in departments of 372

agronomy, plant and soil science, soil science, etc. (Landa, 2004; Brevik et al, 2016a). Brevik (2009) 373

estimated that there were only about 50 soil specialists (as compared to approximately 2,500 total 374

geographers) employed in the geography departments of USA colleges and universities in 2005. Only 375

about one in five geography programs had a stated soil specialist. This contrasted with approximately 376

640 soil specialists employed in agricultural-based soil programs, and despite the fact that geography 377

programs were offered at >260 universities in the USA, whereas agricultural-based soil programs were 378

offered at only 76 universities (Brevik, 2009). Landa and Brevik (2015) found that 76% of the soil science 379

programs in the USA were offered by land grant universities, only 5% were offered by Earth science 380

departments, whereas the remaining 20% were offered by non-land grant universities that had 381

agriculture programs. 382

In addition to soil research and teaching being primarily within agriculture departments at American 383

universities, federal soil mapping and research programs have traditionally been housed within the 384

USA’s Department of Agriculture (USDA). This organization occurred despite early attempts by Eugene 385

W. Hilgard (1833-1916) and John Wesley Powell (1834-1902), two prominent American scientists, to 386

create a division of agricultural geology within the USA’s Geological Survey (Amundson and Yaalon, 387

1995). Within the USA, the largest professional soil science society, the Soil Science Society of America 388


22

(SSSA), evolved from the American Society of Agronomy (ASA) and routinely holds their annual meetings 389

in association with ASA and the Crop Science Society of America (CSSA). Only one annual meeting of 390

SSSA to date has been in association with an Earth science society, that being a meeting with the 391

Geological Society of America in 2008, which was also held in conjunction with ASA and CSSA (Brevik, 392

2011). Because of its association with agriculture at both the academic and federal government levels, 393

soil science has typically not been viewed as a geoscience in the USA (Landa, 2004), likely weakening 394

potential ties between soil science and geography. 395

The evolution of geography as a discipline has likely had its own effect on the drift of geography away 396

from soil studies. At the beginning of the 20th century, what we would today call physical geography 397

dominated geography departments, and soil was often studied by geomorphologists. For example, 398

geographers may consider Davis the “father of American geography,” but geologists also consider him as 399

one of their own. At least in the USA, physical geography became a smaller component of geography as 400

the rise of human geography proceeded. Today, physical geographers sometimes wrestle with 401

distinguishing themselves from their colleagues in geology or ecology/botany departments, disciplines 402

that also share connections with soil science. In many ways, this is a natural situation for 403

interdisciplinary topics, but in general, soil research has been largely overlooked by non-agricultural 404

disciplines over the past century. 405

In recent decades, the field of soil science has moved away from the science of mapping. Soil science 406

was born out of the recognition of multiple factors affecting the processes and thus the spatial 407

distribution of different soil properties. However, by the early 20th century the scientific study of 408

mapping soil, sensu stricto, was fading. In 1929, Thomas Bushnell complained that the meetings of the 409

organization once called the “American Association of Soil Survey Workers” was no longer balanced 410

between the study of soil and the study of surveying or mapping. That organization evolved into the 411

SSSA, which is today comprised of 14 divisions of interest. Soil mapping is a subset of the pedology 412


23

division, which also includes soil formation, classification, physical and chemical properties, 413

interpretation of soil behavior, human land use decisions, and ecosystem evolution. To be fair, there are 414

also separate divisions for soil chemistry, mineralogy, biology, physics, as well as for different 415

ecosystems. In addition, soil formation and interpretation of soil behavior are natural pairings with soil 416

mapping. Nonetheless, investment and research activity on the geographic nature of soil studies has 417

been decreasing. For example, other than a brief spurt of interest between 2009 and 2011, 418

presentations on soil mapping at national SSSA meetings have been sparse (Figure 10) and the USA’s 419

federal budget for soil mapping has declined from an all-time high in the late 1980s to a long-time low in 420

2013-2015 (Brevik et al., 2016a). Soil scientists today need to answer questions such as: Why should 421

more investment be made in soil mapping? Weren’t the strategies for mapping soil worked out by the 422

1940s? Why should areas that have already been mapped be revisited (e.g., >85% of the USA has 423

already been mapped (Indorante et al., 1996))? At the international level, some specialized conferences 424

reflect the reinvigorated interest in soil mapping. However, they also indicate some departure from 425

traditional disciplinary frameworks. 426


24

427 Figure 10. Topical trends for presentations given in the pedology division of the Soil Science Society of 428

America (SSSA), for 2005-2018. After reaching a peak in 2011, which corresponds with a surge of 429

interest by Americans in digital soil mapping, the quantity of presentations on the spatial prediction or 430

geographic distribution of soil has been declining. *The 2008 meeting was held jointly with the 431

Geological Society of America. **The 2018 meeting was held in January of 2019, independently of the 432

usual association with the Crop Science Society of America and the American Society of Agronomy. 433

434

A symptom of the disconnect between soil science and geography is the lack of recognition of core 435

geographic concepts as the basis for the soil mapping paradigm. Geographical context is crucial to 436

understand soil formation and disturbances. Ask a soil scientist how they map soil, they will likely cite or 437

describe the soil-landscape paradigm (Hudson, 1992). If pressed to explain why that works as a means of 438

spatial prediction, they would likely describe the five factors of soil formation that broadly describe 439

processes influencing soil properties. What many do not think about is that this concept of spatial 440

covariation has a long history in the geographic study of many topics. When Bushnell (1929, p. 23) was 441

complaining about the lack of attention to mapping concepts by soil scientists, he made the observation 442

that “once we enter the map making game, there are rules to be obeyed and standards, which must be 443

met.” Because of the relatively small change in soil mapping methods over the past century, the old 444

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rules are largely adhered to, with minimal thought about methodological improvement. Meanwhile, 445

geography has evolved, and new datasets have emerged; there now exist new methods of analysis, 446

awareness of spatial complications (e.g., modifiable areal unit problem), and higher standards for map 447

production. 448

There are, of course, exceptions to these broad generalizations. Vladimir M. Fridland’s (1972/1976) 449

work on analyzing soil cover patterns is one example. In 1985, Francis D. Hole and James B. Campbell 450

wrote Soil Landscape Analysis, which uses the term “spatial association” to describe the traditional 451

method of soil mapping. In many ways, the analyses presented in that book echo the style of thinking 452

advanced by geographer William W. Bunge (1928-2013) in his seminal text for geography’s quantitative 453

revolution, Theoretical Geography (Bunge, 1962). It is also worth noting that Hole (1913-2002), who was 454

trained in geology and soil survey, held a joint appointment at the University of Wisconsin in both the 455

Departments of Soil Science and Geography (Devitt, 1988; Tandarich et al., 1988; Brevik, 2010). 456

3. Soil science’s renewed interest in soil mapping 457

3.1 The geospatial revolution’s effect on soil mapping 458

The tools made available by the geospatial revolution of the past 20 years have undoubtedly had major 459

impacts on many avenues of scientific investigation (Longley et al., 2015). This impact is especially true 460

for mapping applications in soil science. Similar to the stimulation that the first and second waves of soil 461

mapping provided to soil science, the current third wave is setting the stage for discovering spatial 462

patterns that will undoubtedly prompt further research into the processes producing those patterns. 463

Not long ago, the locations of representative soil samples were commonly described in writing as 464

distances and directions from an available landmark. Today, GPS units are commonly used for this 465

purpose, making it simpler and more accurate to record sample locations using geographic coordinates 466

(Figure 11). In addition to improving the ability to return to the same sampling sites, the association of 467


26

the observed soil data with accurate geographic coordinates has opened a completely new realm of 468

spatial analysis and mapping. Sampling designs can now be planned in a GIS and the soil data collected 469

can be intersected with multiple layers of environmental covariates. 470

471 Figure 11. Global position systems (GPS) have changed soil sampling by improving pre-planning, re-use 472

of sample data, and the accuracy of relating soil properties with covariates. The researchers in this 473

photo created a sampling design using digital terrain analysis and are now locating those sample points 474

using a small, handheld GPS receiver, held by woman in the middle of the photo. 475

476

This same geospatial revolution produced a tremendous number of new base maps and related data, all 477

of which could contribute to the new mapping effort. Satellite images can span the electromagnetic 478

spectrum and cover large areal extents, providing indicators of vegetation, land use, natural hazards, 479

and other physical landscape properties (Xie et al., 2008; Joyce et al., 2009; Mulder et al., 2011). LiDAR-480

based elevation data are highly accurate and detailed, and are becoming increasingly common (Hodgson 481


27

and Bresnahan, 2004). Adding value to elevation alone, various types of digital terrain derivatives 482

provide important information related to surface topography and wetness (O’Loughlin, 1986; Moore et 483

al., 1993; Gessler et al., 2000). Although usually for smaller extents, proximal sensing systems such as 484

ground penetrating radar, electromagnetic induction, and electrical conductivity can be used evaluate 485

sediments in the subsurface (Huisman et al., 2003; Hedley et al., 2004; Rhoades and Corwin, 2008; Molin 486

and Faulin, 2013; Doolittle and Brevik, 2014). Some of these data products can be incorporated into the 487

manual process of delineating map units, but the quantity of data set layers quickly becomes more than 488

can be utilized by visual inspection. By quantifying the relationships between these covariates and soil 489

properties with tools such as machine learning, the mapping process can be made more automated and 490

more repeatable (McBratney et al., 2003; Scull et al., 2003). 491

In the pre-digital soil mapping paradigm, experience from surveying the soil landscape helped the 492

mapper develop a mental model for predicting the expected spatial distribution of soil types and their 493

associated soil properties (Hudson, 1992). Mappers would seek to establish relationships among soil 494

types and, in particular, landscape position and vegetation cover (e.g., Parsons et al., 1970; Barrett et al., 495

1995). That mental model was then applied to the best available base map, often a stereopair of aerial 496

photographs, to delineate soil map units (Miller and Schaetzl, 2014). In the case of a stereopair, the soil 497

mapper would use cues from changes in topography and vegetation to hand-draw map unit boundaries 498

in the office, and field-check them later. Although difficult to directly overlay, the soil mapper would also 499

make use of any existing maps – such as those of surficial geology – to better predict where different soil 500

types occurred. GIS software changed that system by making it easier to overlay different base maps 501

and to edit map unit delineations (MacDougall, 1975; Chrisman, 1987). Of course, GIS has the power to 502

do much more, but for traditional soil mappers, GPS-logged field observations, access to better base 503

maps, and easier overlays best describe the first step into the geospatial revolution. 504


28

A major asset of the traditional soil mapping approach was the human mind’s ability to synthesize years 505

of field experience and add a degree of intuitive knowledge to the field mapping effort. Unfortunately, 506

that mental model approach, however accurate it may be, has two major limitations: 1) it is based on 507

human judgment, making it largely not repeatable, and thus, 2) much of the knowledge is lost when the 508

soil mapper retires (Hudson, 1992). This latter problem has been a major issue for the current mapping 509

effort within the USA’s Soil Survey, where most mappers from the 1970’s and 1980’s have since retired; 510

the brain drain is real and there may be no solution. The number of soil scientists employed by the US 511

federal government declined by 39% from 1998 to 2017 (Vaughan et al., 2019). If time has run out for 512

documenting localized expert knowledge, the best approach for the next generation of soil maps may be 513

to “start over,” using the enhanced data sets and powerful mapping and modeling software that have 514

recently emerged. Again, GIS offers the tools to quantify the spatial relationships between soil 515

properties and covariates, making the spatial models more efficient and repeatable. Fortunately, the 516

original base soil maps and their underlying data remain, from which new and better mapping efforts 517

can be built. At the minimum, the previous generation of soil data provide the opportunity to study soil 518

change (e.g., Veenstra and Burras, 2015). 519

Early work connecting geospatial technologies with soil mapping began simply with storing and 520

representing soil information in a GIS (Webster and Burrough, 1972; Legros and Hensel, 1978; 521

Tomlinson, 1978; Webster et al., 1979). Although digital cartography is (was) an achievement in itself, 522

the visualization of soil maps using a computer did not utilize the potential to improve the maps with 523

the spatial analysis components of the geospatial revolution. Soil scientists simply took note of the 524

effects of spatial autocorrelation and instituted spatial sampling designs to avoid this type of bias in the 525

early 20th century (Mercer and Hall, 1911; Fisher, 1925; Youden and Mehlich, 1937). The availability of 526

general use computers reignited the application of computationally intensive statistics again later in that 527

century (e.g., Hole and Hironaka, 1960; Rayner, 1966; Webster and Burrough, 1972). With the addition 528


29

of Matheron’s concepts for geostatistics (Matheron, 1965, 1969), these combined elements spurred 529

enthusiasm for spatial models to predict the distribution of soil properties (e.g., Burgess and Webster, 530

1980; McBratney and Webster, 1983; Vauclin et al., 1983). By 1994, the study of soil science with the 531

statistical and probability approaches afforded by computers, came to be known as pedometrics 532

(Webster, 1994). Early approaches to digital soil mapping emphasized geostatistics, but over time 533

methods for digital soil mapping relying on the covariation of soil properties with variables measured by 534

remote or proximal sensing have become more dominant (Figure 12). 535

536 Figure 12. Trends in three different types of spatial prediction methods used in digital soil mapping, 537

from 2005 to 2018. Geostatistics (spatial autocorrelation) was an early favorite for digital approaches to 538

soil mapping. However, more recently spatial regression (spatial covariation) approaches have gained in 539

popularity. Data obtained from a search of Scopus (2019) using the search term “digital soil mapping.” 540

541

3.2 The era of digital soil mapping 542

Two papers were coincidently published in 2003 that explored new trends in geospatial technologies 543

and their increasing utilization in soil mapping research. One was published in the journal Progress in 544

Physical Geography (Scull et al., 2003), the other in Geoderma (McBratney et al., 2003). Both papers 545

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were exemplary reviews of the state of the art for utilizing geospatial technologies to model the spatial 546

distribution of soil. The publication of these papers marked the widespread recognition that geospatial 547

technologies were providing new ways of thinking about soil mapping. The article by McBratney et al. 548

(2003) - targeted to soil scientists - had 2,238 citations as of 3 August 2019 (Google Scholar). Although 549

the article by Scull et al. (2003) – targeted to geographers – has not exactly been ignored, it nonetheless 550

had 449 citations on the same date. This disparity may be an important indicator of the respective 551

disciplines’ interest in soil mapping; soil scientists may be more interested in digital soil mapping. 552

Although this is a natural situation for interdisciplinary topics, it is not the same as directly interacting 553

with researchers that have the scientific study of spatial analysis and prediction as part of their academic 554

heritage. Our paper argues for greater collaboration between geographers and soil scientists, but there 555

is no doubt that great strides have already occurred in the realm of digital soil mapping. The annual 556

number of papers that Google Scholar has indexed using the words ‘digital soil mapping’ increased from 557

22 to 557 in the decade following the two landmark review papers in 2003 (Figure 13). 558

559 Figure 13. Trend in digital soil mapping activity as indicated by Google Scholar results for the terms 560

“digital soil mapping” and “predictive soil mapping”, from 1980 to 2018. Although the term “predictive 561

soil mapping” at times gains in popularity as an alternative term for the current revolution in soil 562

mapping methods, it should be noted that soil mapping is inherently an exercise in spatial prediction. 563

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564

In response to demands for a global data set to assist decision making related to issues of food security, 565

climate change, and environmental degradation, the Digital Soil Mapping Working Group of the 566

International Union of Soil Science established the Global Soil Map initiative in 2006. This project is 567

coordinating national soil mapping agencies to produce a standardized digital product of soil 568

information (Sanchez et al., 2009). Although several countries were already reinvigorating their soil 569

mapping programs with digital soil maps, the Global Soil Map project has created a target for map 570

quality and important soil properties to be included. For example, one of the major objectives of the 571

project is to bring to the fore issues and estimates of spatial uncertainty in digital soil mapping. The 572

ambitions of this project have attracted funding to assist areas with the greatest need for this 573

information, particularly Sub-Saharan Africa. 574

Beyond the obvious use of digital tools to represent the spatial distribution of soil, digital soil mapping 575

aims to incorporate other improvements to soil maps. When we discuss or envision digital soil mapping, 576

we must be careful to distinguish digitized soil maps from digital soil maps. The former does not 577

leverage the benefits of spatial analysis in a GIS. Although legacy soil maps still hold a lot of value, simply 578

digitizing them into a GIS fails to advance the science of soil mapping. Towards realizing the potential of 579

digital soil mapping, three goals have been identified as benchmarks: 1) addition of soil observations 580

using statistical sampling techniques, 2) production of soil maps by quantitative spatial models, and 3) 581

inclusion of uncertainty associated with predictions (Lagacherie and McBratney, 2006). 582

Researchers of digital soil mapping regularly debate the most appropriate data structures and 583

computational techniques to capture, represent, process, and analyze soil information. When 584

attempting to produce better digital soil maps, uncertainties that arise from overlaying multiple data 585

layers, identifying spatial patterns, and making predictions based on those patterns must be evaluated. 586

Further, questions on how to best communicate the resulting map and represent the associated 587


32

uncertainty are omnipresent. Complications of scale, such as the modifiable areal unit problem and the 588

minimum map unit size (e.g., Hupy et al., 2004), continue to cause confusion on how to best analyze 589

patterns of soil and covariates. In short, soil is an excellent test subject for the systematic study of issues 590

of scale, accuracy, and spatial analysis. 591

In the earlier section describing the geospatial revolution’s effect on soil mapping, geospatial 592

technologies were described as tools for digital soil mapping. Soil is a quintessential geographic entity, 593

the product of complex interactions of phenomena occurring at different scales. This characteristic 594

makes soil and its spatial distribution a highly suitable subject for geographers and geographic 595

information scientists. 596

597

3.3. Modern issues creating new demands for soil maps 598

Soil is intimately intertwined with the topical alignments of both human and physical geography (Figure 599

14) (Kuby et al., 2013; Arbogast, 2017). Soil is a physical feature of the Earth. Soil is affected by human 600

activity and influences populations, as it is an essential natural resource. 601

The original motivation for soil mapping was resource inventory for the purpose of land valuation, then 602

later, for guiding landowners to optimize agricultural production (Miller and Schaetzl, 2014). Following 603

awareness of soil erosion as an issue, soil maps became important for identifying areas of erosion risk, 604

helping to better plan conservation practices. Subsequently, soil maps have been used for many 605

applications of land suitability including identifying limitations for water management, building 606

development, and wildlife habitat. In response to these recognized uses of soil maps, the attribute 607

tables for soil map units have been adapted and expanded. For the most part, this continues to be the 608

case for issues of newly increased concern, but some new issues will require new kinds of soil maps. 609

Interpreting legacy soil maps for the new demands has been challenging, but generally useful. However, 610


33

the spatial accuracy and precision of those maps may not be sufficient for some of the new uses of soil 611

maps (Miller, 2012). 612

613

Figure 14. Some of the interdisciplinary overlaps between soil science and geography. 614

615

Environmental issues are one of the primary areas putting new demands on soil maps (Hartemink and 616

McBratney, 2008; Thompson et al., 2012). Because soil plays a key role in all of the spheres of the Earth, 617

most models of the environment benefit from spatially explicit soil data. Prediction of flood prone areas, 618

for example, depends on the interactions among rainfall, topography, and the spatially variable ability of 619

soil to absorb that water. Water quality models need to account for biological, physical, and chemical 620

interactions of water with the soil across watersheds. Many of the models for these kinds of issues, such 621

as the Water Erosion Prediction Project (WEPP) and Soil and Water Assessment Tool (SWAT), aggregate 622

soil information at the watershed or sub-watershed scale, which means that much of the spatial 623

information is discarded in the interest of model efficiency. With increasing computing power, however, 624


34

there exists an opportunity to improve these models with better soil maps and better considerations of 625

spatial connectivity. 626

Soil and water connectivity is an emerging topic, and mapping soil and water flows is fundamental to 627

understand the impact of different land uses on overland flow and erosion. Under natural conditions, 628

connectivity depends on the parent material type, soil texture and structure, topography (e.g., slope, 629

aspect), climate patterns, and vegetation distribution (e.g., patchy or continuous). Connectivity can be 630

affected by natural phenomenon such as fire, or other human induced impacts (e.g., mining, grazing, or 631

agriculture). Normally, these disturbances increase connectivity, as compared to the natural condition. 632

The spatial distribution of connectivity can be complex, however, and mapping provides an important 633

contribution to a better understanding of where soil and water fluxes are high. Several indexes have 634

been developed to measure connectivity; these have been applied at different spatial and temporal 635

scales (Heckman et al. 2018). Soil and water connectivity maps have been produced in several 636

environments, such as mountain catchments (Cavalli et al. 2013; Zuecco et al. 2019), abandoned and 637

afforested mountainous areas (Lopez-Vicente et al. 2017), mountainous areas with heterogeneous land 638

use (Lopez-Vicente and Ben-Salem, 2019), agricultural lowland basins (Casamiglia et al., 2018), places 639

affected by landslides (Persichillo et al. 2018), and urban environments (Kalantari et al., 2016). Despite 640

the recent progress in this area, further work should focus on the validation of these models. 641

The soil-atmosphere interface is another aspect of how soil affects quality of life. The interactions of soil 642

with the atmosphere, such as evapotranspiration, are important processes to consider for improving the 643

accuracy of weather forecasting (Fennessy and Shukla, 1999; Koster et al., 2004). Similarly, soil stores a 644

large stock of carbon, which makes it a major potential carbon source or sink (Lal, 2004). Soil’s role in 645

the carbon cycle makes it an important factor in the positive and negative feedback loops of global 646

climate change (Stokstad, 2004; Zhuang et al., 2006). 647


35

The early interest in soil maps to improve food production and protect soil as a resource has not gone 648

away. The global population is expected to reach 8.5 billion by 2030 (United Nations, 2019), during 649

which time 30-60 Mha of cropland is expected to be lost to infrastructure (e.g., housing, industry, roads, 650

etc.) (Döös, 2002). Soil degradation works against goals to increase agricultural crop productivity. 651

Although soil erosion has been recognized as a problem for a century and great effort has been made to 652

address the issue, large losses of valuable soil resources continue (Brevik et al., 2017). 653

Innumerable works have focused on mapping soil erosion at local, regional and global scales (e.g., 654

Bahadur, 2009; Nachtergaele et al. 2010; Panagos et al., 2015; Gelder et al., 2017). The first attempt to 655

map soil degradation (including erosion) at the global level occurred with the Global Assessment of Land 656

Degradation and Improvement (GLASOD), but, surprisingly, this work did not use soil data (Pereira et al., 657

2017). The accuracy of soil erosion maps increased appreciably with the availability of covariate maps 658

with more types of information and higher resolution, primarily developed in concert with the recent 659

revolution in geospatial technologies. Included among those covariate maps are continuous maps made 660

possible by spatial interpolation methods (Borrelli et al. 2018). 661

The majority of the soil erosion maps produced today focus on erosion risk (Ochoa-Cueva et al. 2013; 662

Farhan et al. 2015; Mancino et al. 2016; Haregeweyn et al. 2017) and the estimations are often carried 663

out using the Revised Universal Soil Loss Equation (RUSLE) (Pal and Al-Tabbaa, 2009; Boni et al., 2015). 664

RUSLE calculates soil loss rates (E) by rill and sheet erosion based on the rainfall (R), erodibility factor (K), 665

cover management factor (C), slope length and slope steepness factor (LS), and a support practices 666

factor (P) (Renard et al., 1997). Fewer studies have been carried out to map wind erosion, despite 667

widespread recognition that wind erosion increases soil degradation (Borrelli et al. 2016). Wind erosion 668

is much more complex to model than water erosion. Nevertheless, some attempts have been made at 669

the local (Sterk and Stein, 1997; Zobeck et al., 2000; Harper et al. 2010) and regional (Borrelli et al. 2014; 670


36

2016) levels. Other works have mapped sediment sources and deposition areas (Petropoulos et al. 2015; 671

Cavalli et al. 2017). 672

The assessment of soil ecosystems and their services has increased rapidly in the last decade, and 673

mapping is crucial to identify the distributions of these services (regulating, provisioning, cultural, and 674

supporting ecosystem services). Maps can represent the synergies and trade-offs between ecosystem 675

services (ES), trends, costs and benefits, monetary value, and aid in estimating costs and benefits (Maes 676

et al. 2012; Burkhard et al. 2018). An extensive body of literature exists on mapping ES; this effort uses 677

soil variables to assess regulating and provisioning services (e.g., Burkhard et al. 2012; Syrbe and Walz, 678

2012). The relationship between the quality and quantity of ES with the services provided directly or 679

indirectly by soil has been demonstrated (Adhikari and Hartemink, 2016; Pereira et al. 2018; Brevik et al. 680

2019). Soil functions are crucial for assessments of ecosystem vitality; thus, they are normally integrated 681

into ES estimations in well-known ES assessment models such as InVEST (Sharp et al. 2018) and Aires 682

(e.g., Bagstad et al. 2014) (e.g., carbon storage, sediment delivery ratio). 683

There are several works that link soil functions with ES (e.g., Barrios, 2007; Pulleman et al. 2012; De 684

Vries et al. 2013; Lavelle et al. 2016) and which quantify soil ES (Dominati et al. 2010; Robinson et al. 685

2013). However, more effort should be made to map soil ES individually in order to understand the real 686

value of soil in ES assessment, and more research is needed to optimize the mapping of soil ES. In 687

addition, soil ESs are overlooked and not considered in some ES classifications such as ‘The Economics of 688

Ecosystems and Biodiversity’ (TEEB) (Pereira et al., 2018). 689

4. Conclusions 690

Clearly, a reinvigoration is occurring in soil geography. This renewed interest in improving soil maps is 691

stimulated by the confluence of improving capabilities for producing maps, and the increasing need for 692

spatial soils data. Innovations from the geospatial revolution, coupled with the increasing power of 693


37

computing and machine learning, have added many new and useful opportunities available to the soil 694

mapper’s toolkit and theoretical base. To address modern issues facing society, including supporting a 695

growing population and other impacts on ecosystem services, more frequent improvements of soil 696

maps, of often manifested as "updates", will be required. Even though other disciplines continue to have 697

a strong vested interest in the study of soil, there is a clear need for the spatial sciences in the future of 698

this field. Mapping the environment, discovering processes, and understanding interactions between 699

these processes across space work cyclically with each other, which makes the new wave of soil 700

mapping exciting both for soil science and for geography and speaks to the need for these fields to work 701

collaboratively on issues of mutual interest. 702


38

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