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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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Part of the Agriculture Commons, Physical and Environmental Geography Commons, Soil Science
Commons, and the Spatial Science Commons
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This Article is brought to you for free and open access by the Agronomy at Iowa State University Digital Repository. It has been accepted for inclusion in Agronomy Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected].
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
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
In press with Progress in Physical Geography, October 2019
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
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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
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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
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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
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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
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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
In press with Progress in Physical Geography, October 2019
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
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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
In press with Progress in Physical Geography, October 2019
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
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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
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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
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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
In press with Progress in Physical Geography, October 2019
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
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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
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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
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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
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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
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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
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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
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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
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(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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
In press with Progress in Physical Geography, October 2019
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
In press with Progress in Physical Geography, October 2019
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
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38
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