Agronomy Publications Agronomy
2019
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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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 Agriculture | Physical and Environmental Geography | Soil Science | Spatial Science
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 2 Bradley A. Miller1,2*, Eric C. Brevik3,4, Paulo Pereira5, Randall J. Schaetzl6
3 1 – Department of Agronomy, Iowa State University, Ames, IA, USA; [email protected] 4 2 – Leibniz Centre for Agricultural Landscape Research (ZALF) e.V., Institute of Soil Landscape Research, 5 Eberswalder Straße 84, 15374 Müncheberg, Germany 6 3 – Department of Natural Sciences, Dickinson State University, Dickinson, ND, USA; 7 [email protected] 8 4 – Department of Agriculture and Technical Studies, Dickinson State University, Dickinson, ND, USA 9 5 – Environmental Management Center, Mykolas Romeris University, Ateities g. 20, LT-08303 Vilnius, 10 Lithuania; [email protected] 11 6 – Department of Geography, Environment, and Spatial Sciences, Michigan State University, East 12 Lansing, MI, USA; soils@msu.edu 13 * - corresponding author
14 Abstract 15 The geography of soil is more important today than ever before. Models of environmental systems and
16 myriad direct field applications depend on accurate information about soil properties and their spatial
17 distribution. Many of these applications play a critical role in managing and preparing for issues of food
18 security, water supply, and climate change. The capability to deliver soil maps with the accuracy and
19 resolution needed by land use planning, precision agriculture, as well as hydrologic and meteorologic
20 models is, fortunately, imminent due to advances in the geospatial data related to soil. Digital soil
21 mapping, which utilizes spatial statistics and data provided by modern geospatial technologies, has now
22 become an established area of study; over 100 articles on digital soil mapping were published in 2018
23 alone. The first and second generations of soil mapping – discussed in this paper - thrived from
24 collaborations between Earth scientists and geographers. Now, as we enter the dawn of the third
25 generation of soil maps, those collaborations remain essential. To that end, we review the historical
26 connections between soil science and geography, examine the recent disconnect between those
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27 disciplines, and draw attention to opportunities for the reinvigoration of the longstanding field of soil
28 geography. Finally, we emphasize the importance of this reinvigoration to geographers.
29 1. Introduction 30 Geography and soil science have much in common. One of those commonalities is a connected origin in
31 natural resource inventory, which today make both disciplines essential to address key environmental
32 issues. The two disciplines also share a highly interdisciplinary nature (Shaw and Oldfield, 2007; Rodrigo-
33 Comino et al., 2018). Being naturally interdisciplinary is a strength in that it allows both soil science and
34 geography to bridge gaps between other, complimentary disciplines (Krupenikov et al., 1968; Fridland,
35 1976; Ostaszewska, 2008; Brevik and Hartemink, 2010). On the other hand, it can become a weakness
36 when allied fields seek to absorb portions of soil science or geography into their own academic spheres
37 (Nikiforoff, 1959; Fridland, 1976; Harvey, 1984; Brevik, 2009); this concern seems to have been shared
38 by both fields at various times in their histories.
39 Soil forms at the interface among the atmosphere, lithosphere, hydrosphere, and biosphere.
40 Traditionally, this interface has been called the pedosphere (Targulian et al., 2019). However, more
41 recently the concept of the pedosphere has been extended to include the top of the vegetative canopy,
42 forming the concept of Earth’s critical zone (Chorover et al., 2007; Brantley et al., 2017) (Figure 1).
43 Critical zone research has generated new excitement in interdisciplinary fields, as it focuses on the
44 processes occurring within this interface, from micro- to global scales (Brantley et al., 2007). Regardless
45 of the terminology or scientific approach, two concepts about soil are clear: 1) soil and the processes
46 occurring within it are essential to life on Earth, and 2) to understand soil, one must consider the
47 interacting processes in their respective spheres, including their positive and negative feedback loops.
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48
49 Figure 1. Conceptual diagram illustrating the Earth’s critical zone. Although difficult to illustrate in a 50 single diagram, soil is the subsurface environment shaped over time by geological, chemical, physical, 51 and biological processes. After Chorover et al. (2007). 52 53 The terminology used in the preceding description of soil should resonate with geographers, as the
54 concept of feedback loops is central in modern geography, including soil geography (Torrent and
55 Nettleton, 1978; Muhs, 1984; Phillips, 1993; Chadwick and Chorover, 2001). Similarly, a goal of physical
56 geography is to “explain the spatial characteristics of the various natural phenomena associated with
57 the Earth’s hydrosphere, biosphere, atmosphere, and lithosphere” (Pidwirny and Jones, 2017). Even
58 though soil science and geography have evolved as interdependent fields (Rodrigo-Comino et al., 2018),
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59 their overlap is evident. However, in various parts of the world, different academic structures and
60 funding sources have led to some academic disconnects, despite their apparent commonalities.
61 Traditionally, soil science and geography have intersected in subfields such as soil geomorphology
62 (Holliday et al., 2002), pedology (Schaetzl and Thompson, 2015), and soil geography (Arnold, 1994).
63 Although soil geomorphology research has been active in geography, geology, Earth science, and soil
64 science/agronomy departments, soil geography research has not been as active for a better part of the
65 20th century. If a reinvigoration in soil geography is occurring, it is time for a reconnection between the
66 two disciplines to be recognized and implemented, so as to build upon the strengths of both fields.
67 The concept of soil has received increased attention recently due to rising concerns about sustainability,
68 especially in the context of the ecosystem services that soil system provides (Adhikari and Hartemink,
69 2016; Baveye et al., 2016). One of the key services that soil provides in sustaining human life is mediated
70 through agriculture. Since the emergence of farming, human impacts on the soil system have become an
71 important aspect of soil dynamics (Grieve, 2001; McLauchlan, 2006; Veenstra and Burras, 2015). The
72 change from a nomadic to a sedentary lifestyle for early cultures changed the relationship between soil
73 and people, beginning an era of increasing impacts that people have on soil functions (Beach et al.,
74 2006; Sandor and Homburg, 2017; Goudie, 2018). The recent exponential growth of the human
75 population has intensified demand for food and resources, resulting in anthropogenic impacts on soil to
76 unprecedented levels (Ferreira et al., 2018). However, it should be noted that human manipulation of
77 soil has a long history and does not necessarily lead to negative impacts. For example, there is
78 archeological evidence that humans have changed and therefore influenced soil to increase
79 productivity, such as the terra preta de Indio (Fraser et al., 2011; McMichael et al., 2014). With the
80 expansion of human activities, humans therefore have become an important soil-forming factor (Bidwell
81 and Hole, 1965; Bockheim et al., 2014; Bajard et al., 2017). Concerns about environmental issues have
82 resulted in increasing interest in both soil science and geography (e.g., Jonsson and Daviosdottir, 2016;
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83 Pereira et al., 2018). For the same reasons that both disciplines were born in an era of investment in the
84 management of natural resources, they are once again poised to help answer related questions in
85 today’s world (Hartemink and McBratney, 2008).
86 Surges in these disciplines appear to occur at the convergence of new technologies and pressing issues
87 facing society. When these two ingredients come together, doors into areas of new exploration are
88 opened, new approaches are tested, and investment becomes a priority for government leaders. By the
89 19th century, surveying technology had reached a point that facilitated the unprecedented, quantitative
90 study of spatial relationships, at the same time that nations recognized the critical role that natural
91 resources played in the accumulation of wealth (Miller and Schaetzl, 2016). Today, as we begin the 21st
92 century, geospatial technologies such as global positioning systems (GPS), remote sensing, and
93 geographic information systems (GIS), coupled with vastly improved spatial datasets such as LiDAR
94 elevation and frequently updated aerial imagery archives, are revolutionizing the capabilities and
95 opportunities of both geography and soil science. The second ingredient - societal demand - is at our
96 doorstep. Concerns about environmental issues, including global climate change, ecosystem services,
97 water quality and quantity, soil degradation, loss of biodiversity, food security and quality, are all well
98 known to geographers and soil scientists. As recognition of these issues progresses around the world,
99 the need for geography and soil science expertise can only grow.
100 In this paper, we review (1) the historical connections between soil science and geography, and (2) how
101 recent technological advancements have provided an impetus to reinvigorate each of these two
102 respective disciplines’ interest in the other. Future papers in this three-part series will explore the
103 opportunities for future collaboration between geography and soil science in greater depth.
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104 2. Historical origins of soil science and geography 105 Soil science and geography have similar historical roots as academic disciplines. Although some aspects
106 of geography have been important to human societies for thousands of years (Harvey, 1984) and
107 geographical concepts have been taught in universities for centuries (Johnston, 2003), geography was
108 only established as a formal academic discipline in the latter part of the 19th century (Harvey, 1984;
109 Godlewska, 1989; Schelhaas and Hönsch, 2001; Sack, 2002; Johnston, 2003; Shaw and Oldfield, 2007;
110 Claval, 2014) and was not taught in some countries as a formal university subject until much later
111 (Barnes, 2007). Similarly, although soil and soil science concepts have been societally important for
112 thousands of years, soil science per se was only organized as an independent field of scientific study in
113 the late 19th century (Krupenikov, 1993; Brevik and Hartemink, 2010). In the late 19th and early 20th
114 centuries, both soil science and geography received strong, foundational contributions from scientists
115 trained as geologists; in fact, with a lack of formally trained geographers it was common for early
116 geography scholars to come from fields such as biology, geology, history, journalism, and mathematics
117 (Johnston, 2008). The beginnings of soil science were largely the same, with early concepts of soil being
118 driven by many of the same respective base disciplines. For example, in the 19th century, chemists
119 favored an emphasis on the humic content of soil, whereas geologists emphasized the mineral content
120 (Krupenikov, 1993). Regardless, the motivating purpose to study soil at the time was for agriculture,
121 leading to terms such as agrochemists and agrogeologists (Krupenikov, 1993).
122 Both soil science and geography benefited from the scientific advancements stemming from the Age of
123 Exploration, and from the associated motivations of national governments. Enough scientific
124 advancement occurred in the 15th century for the European empires to realize the benefit of, and to
125 invest in, the accurate mapping of national borders and resource inventories. In part, these
126 developments were based on improved survey methods, but that effort in turn gave rise to more
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127 scientific study of spatial patterns for the purpose of better spatial prediction and understanding of
128 processes (Miller and Schaetzl, 2014).
129 The confluence of these two disciplines and the rise of scientifically based "spatial thinking" is
130 exemplified by one of the founders of geography, Alexander von Humboldt (1769-1859) (Figure 2)
131 (Hartshorne, 1958; Bouma, 2017). Humboldt published a treatise on the basalt formations along the
132 Rhine River early in his career (Humboldt, 1790), but was mostly known for his botanical work while on
133 expeditions to explore the western hemisphere. What made Humboldt remarkable was his use of
134 quantitative methods, including careful recording of latitude and longitude, and attention to the
135 covariation of phenomena over space, later termed spatial association in geography. Prime examples of
136 this were Humboldt’s identification of relationships between vegetation and elevation (Humboldt,
137 1807/2009), and with global climate zones (Humboldt, 1817). His scientific achievements made von
138 Humboldt an academic superstar. For this reason, Russia repeatedly invited Humboldt to conduct
139 expeditions into Asia, an offer that was finally realized in 1829 (Wulf, 2015).
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140 141 Figure 2. Alexander von Humboldt (pictured here in 1814 at the age of 44), implemented quantitative 142 methods during the Age of Exploration to advance understanding of spatial patterns in the physical 143 environment. His work inspired a new generation of geographers and approaches to studying the Earth.
144
145 Like Humboldt, the naturalist Charles Robert Darwin (1809-1882) (Figure 3) became famous from his
146 studies during his explorations of the western hemisphere. Besides his well-known work on evolution,
147 Darwin also made contributions to understanding soil processes, particularly mixing by soil fauna
148 (bioturbation) (Darwin, 1869; 1881). Darwin’s work laid the foundation for an array of multidisciplinary
149 studies on pedogenic processes during the ensuing century, even though this approach to soil science
150 remained in the shadow cast by the Russian Vasily Dokuchaev’s more geographic approach to the study
151 of soil (Johnson and Schaetzl, 2014; see below). In 1975, Darwin’s ideas reappeared in Soil Taxonomy
152 (Soil Survey Staff, 1975), if only minimally, as a part of the then-emerging “biomantle” concept (Johnson
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153 et al., 2005). Recently, however, the bioturbation concepts first espoused by Darwin have gained
154 considerable traction, e.g., Humphreys et al. (1996), Balek (2002), and Fey (2010).
155 156 Figure 3. Charles Darwin (pictured here in 1855 at the age of 46), best known for his work as a naturalist, 157 contributed to soil science with his observations of the effect of bioturbation on the soil.
158 The founder of modern soil science was born into this academic environment. The Russian Vasily
159 Vasilyevich Dokuchaev (1846-1903) (Figure 4) was trained as a geologist and early in his career worked
160 on mapping the geology and soils of Russia (Dokuchaev, 1877; 1879). Expanding on Humboldt’s
161 approach of spatial association between organic life and environmental conditions, Dokuchaev
162 recognized that soil spatially co-varied with both biota and other environmental conditions (Brown,
163 2006). Specifically, Dokuchaev identified soil as resulting from the combined factors of climate,
164 vegetation, parent material, relief, and time (Dokuchaev, 1883/1967). Although other scientists from
165 around the world have made significant contributions to the development of modern soil science
166 (Jenny, 1961; Krupenikov, 1993; Brevik and Hartemink, 2010), Dokuchaev has become widely recognized
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167 as the father of modern soil science (Jenny, 1961; Krupenikov, 1993; McNeill and Winiwarter, 2004; Buol
168 et al., 2011; Landa and Brevik, 2015).
169 170 Figure 4. Vasily Vasilyevich Dokuchaev (pictured here in 1888 at the age of 42) had profound impacts on 171 the inception of soil geography as a science. Dokuchaev did not associate with geography, but his work 172 and the work of his students laid the groundwork for most of the soil maps in use today.
173
174 The interactions between geology, geography, and soil science in the late 19th to early 20th century were
175 numerous and complex, frequently making it difficult to place individuals into one of these disciplinary
176 categories. For example, the geologist Arthur E. Trueman (1894-1956), who had originally joined
177 University College, Swansea as the first head of the Department of Geology, expanded the department
178 to include geography, which later separated into two successful departments (Pugh, 1958). Another
179 example is Konstantin Dmitrievich Glinka (1867-1927), who made significant contributions to Russian
180 soil science and geography (Shaw and Oldfield, 2007). Glinka began his academic career as a
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181 mineralogist and his first academic position was as professor of mineralogy and geology at the
182 Agricultural Institute at Novo Alexandria, Russia (Ogg, 1928).
183 In the late 19th century, geologists at Harvard University in Cambridge, Massachusetts generated the
184 foundations of geography and soil science in the USA (Figure 5). Nathaniel S. Shaler (1841-1906)
185 authored a classic work on “The Origin and Nature of Soils” (1891), which, in part, extended Darwin’s
186 work on bioturbation. Shaler’s student and later colleague, William Morris Davis (1850-1935), is
187 recognized today as the father of American academic geography (Sack, 2002). Collier Cobb (1862-1934),
188 a student of both Shaler and Davis, later became the head of the Geology Department at the University
189 of North Carolina where he conducted research on human geography, as well as coastal and aeolian
190 processes. In addition, while at the University of North Carolina, Cobb established a Bachelor of Science
191 program in Soil Investigation, which supplied many of the mappers for the early soil survey program in
192 the USA (Brevik, 2010). Another of Davis’s students, Curtis Fletcher Marbut (1863-1935), served as the
193 director of the USA’s Soil Survey Division from 1913-1935, a time critical in the formation of the
194 procedures that produced soil maps used in the USA today. Davis was the first and fifth president, and
195 Marbut was the twentieth president, of the Association of American Geographers.
196 197 Figure 5. Some of the academic tree of key influencers in American soil geography. Many of the names
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198 in this chart will be recognized by geographers, geologists, and soil scientists as members of their own 199 discipline. (a – Graduate School of the USDA, c – University of Chicago, e – Earlham College, h – Harvard 200 University, n – North Carolina State University, p – Imperial University of St. Petersburg, w – University 201 of Wisconsin at Madison) (Modified from Tandarich et al., 1988) 202
203 In central Europe, Dokuchaev’s work laid the groundwork that would establish soil science as an
204 independent scientific field (Tandarich and Sprecher, 1994; Johnson and Schaetzl, 2014). Although
205 modern soil scientists celebrate Dokuchaev’s recognition of soil as an independent body of study, the
206 core of his work laid the foundations of soil geography, not pedology (Buol et al., 2011). Before World
207 War I, American geographers regularly studied and corresponded with German geographers (Martin,
208 2015). Among them was Marbut, who translated “The Great Soil Groups of the World and their
209 Development” from German to English. The author of that text was Glinka, the first director of the
210 Dokuchaev Soil Science Institute in Leningrad, Russia. Between Marbut’s study of the Russian
211 philosophies on soil science and the work of American geologist Eugene W. Hilgard (1833-1916), the
212 notion that soil was more than the product of only geologic processes had begun to be adopted in the
213 USA.
214 In 1927, the USA hosted the first meeting of the re-organized Congress of the International Association
215 of Soil Science. That conference was monumental in its gathering of influential leaders for soil mapping
216 at the time when soil survey programs were gaining major momentum (Figure 6). The list of congress
217 attendees was filled with notable scientists that geographers and soil scientists of the respective
218 countries will recognize, such as J.H. Ellis (Canada), E.J. Russell (England), A. Penck (Germany), H.
219 Stremme (Germany), L. Kreybig (Hungary), P. Treitz (Hungary), H. Jenny (Switzerland), M. Baldwin (USA),
220 T.M. Bushnell (USA), C.F. Marbut (USA), K.D. Glinka (USSR Russia), S. Neustruev (USSR Russia), and L.
221 Prasolov (USSR Russia). These connections and commonalities between the founders of modern
222 geography and soil science, with geology frequently a common link, illustrate the natural and historical
223 ties between these disciplines.
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224 225 Figure 6. Curtis Marbut (second from left) and Konstantin Glinka (middle) – two men who were the 226 leaders of the two most active soil survey programs in the world, during the most critical time of soil 227 mapping methods development. Marbut was a proponent of the concepts that Glinka had written 228 about. Photograph taken at the first re-organized Congress of the International Association of Soil 229 Science, hosted by the USA in 1927.
230
231 2.1 The connections
232 Soil geography is, in its most fundamental sense, the study of the spatial distribution of soil. Inherent in
233 that study are the patterns of soils, soil properties, and the processes that produced those patterns.
234 There is evidence of interest in soil geography per se, starting well before the time that soil science and
235 geography had become established as academic fields of study. Archeologists have found evidence that
236 farming practices were adapted according to soil fertility patterns dating back to 3000-2000 BCE.
237 Information on the spatial distribution of soil properties was recorded in China as early as 300 CE (Miller
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238 and Schaetzl, 2014), and maps of soil attributes were made in Europe by the early 1700s (Brevik and
239 Hartemink, 2010). In North America, long before European settlers had arrived, native peoples
240 recognized that soil in floodplain areas were fertile places for crop production (Brevik et al., 2016b). In
241 the American Southwest, farming had been concentrated in locations where soil water was
242 preferentially retained in the root zone due to restrictive layers such as shallow bedrock, petrocalcic, or
243 argillic horizons (Sandor et al., 1986; Homburg et al., 2005). Although these examples do not indicate the
244 existence of a formal academic field, they nonetheless show that the fundamental recognition of spatial
245 variation in soil properties has had a long history.
246 In Europe, early mapping of soil tended to be by boundaries of land ownership, due to the connection
247 with land valuation and taxation. Although largely a geology map, William Smith (1769-1839) mapped
248 the variation of soils in his landmark map of England, Wales, and Scotland (1815). In Germany, unique
249 soil classification systems were being proposed by the mid-1850s (Krupenikov, 1993). For example,
250 Friedrich Fallou (1794-1877) published books on the soil types of Saxony and Prussia (Fallou 1853, 1868,
251 1875). With advancements in understanding soil fertility, soil mapping endeavors in Germany shifted
252 from cadastral to the ability of soil to respond to different management practices. In general, the soils of
253 Europe were first mapped by geologists, as they were the most familiar with surveying and mapping
254 techniques. Approaching soil from this perspective, the scientists working in this area advocated for the
255 study of soil to be defined as agrogeology, a subdiscipline of geology (Berendt, 1877; Ehwald, 1964).
256 József Szabó (1822-1894) published soil maps of this style for Hungary in 1861, adding considerations of
257 groundwater (Szabó, 1861). In 1867, A. Orth’s map entitled “Geologic-Agronomic Mapping” won a
258 competition for “agricultural geognosy,” sponsored by the Agricultural Union of Potsdam
259 (Mückenhausen, 1997). German unification occurred in 1871 and coincidently the reputation of German
260 geography as well as German soil mapping rose in the world. Indicative of the influence of German soil
261 geography, M. Fresca was invited to map the soil of Japan from 1885 to 1887 (Krupenikov, 1993).
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262 The recognition and mapping of soil spatial properties were central to the establishment of soil science
263 as an independent field of study (Ogg, 1928; Cline, 1961; Krupenikov, 1993; Richter and Yaalon, 2012).
264 The foremost individual in this undertaking was Dokuchaev, who although trained as a geologist
265 specializing in mineralogy (Tandarich and Sprecher, 1994), became noted for his studies of soils and
266 their distributions. As with all major scientific advances, the contributions that elevated soil science
267 were made by a number of individuals, but because Dokuchaev’s contributions led to definite changes in
268 the way soil science was viewed and conducted, he is widely recognized as the father of soil science
269 (Jenny, 1961; Krupenikov, 1993; Landa and Brevik, 2015; Johnson and Schaetzl, 2014; Schaetzl and
270 Thompson, 2015). A major piece of that contribution was Dokuchaev’s publication on the Russian
271 Chernozem (1883/1967), which espoused the interdisciplinary view that soil was a product of more than
272 only geologic processes. Ironically, Dokuchaev expressly refused to associate himself with the field of
273 geography and did not feel that the fledgling science he was helping to create coincided with geography
274 (Shaw and Oldfield, 2007). Nonetheless, his student, Glinka, produced the first soil map of the world in
275 1908 (Hartemink et al., 2013). In 1909, several followers of Dokuchaev, all wrestling with how to better
276 map soil, attended the first International Agro-Geological Conference hosted by the Royal Hungarian
277 Geological Institute in Budapest. That conference marked a turning point for soil science in that it
278 addressed the “confusion [that is] for a time inevitable in a borderland subject like the present, that
279 joins up with geology, botany, and chemistry, and is closely connected with agriculture; indeed, even its
280 very name has not yet been settled, for we find the subject of the conference referred to as
281 agrogeology, agricultural geology, pedology, or simply ‘the science of soil’” (Russell, 1910, p. 157).
282 Similar to the experience in Europe, early soil mapping efforts in North America were generally
283 performed by trained geologists, largely because academic programs in soil science did not yet exist
284 (Coffey 1911; Lapham 1949; Brevik, 2010). The USA established the first nationally coordinated soil
285 survey effort in 1899 (Marbut, 1928). This undertaking was significant for soil geography in that it
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286 represented the first attempt to catalog the soil of a country using uniform standards and practices.
287 Under the direction of Milton Whitney (1860-1927) (Figure 7), the first generation of these maps were
288 produced using an agrogeology approach. In the 1930’s, Marbut’s integration of Dokuchaev’s multi-
289 factor approach and the wider availability of aerial photography came together to facilitate a second
290 generation of soil maps, using the concept of the soil-landscape paradigm. Essentially, this paradigm
291 established that soil map units should occur together in a regular, repeatable pattern, based on the
292 spatial patterns of the five soil-forming factors. Those areas with similar factors, especially topography,
293 were predicted to have similar soil properties (Hudson, 1992). These kinds of soil map units tended to be
294 based on the factors of soil formation identifiable in stereo-orthophotographs (Simonson, 1989). A set
295 of soil map units occurring together was called a soil association in the USA’s Soil Survey system; this
296 term was parallel to Milne’s catena concept established for soil mapping in Africa (Milne, 1935;
297 Bushnell, 1943).
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298 299 Figure 7. Milton Whitney (pictured here in the 1910s) was the first chief of the American Bureau of Soils, 300 which was charged with the first nationally coordinated soil survey, including uniform standards and 301 practices for the staff producing the soil maps.
302
303 Other examples of pioneering work done by geographers are easily found. The first soil fertility map of
304 Britain was prepared in the 1930s by the geographer Sir Dudley Stamp (1898-1966), with help from
305 other geographers such as E.C. Willatts (1908-2000) (Willatts, 1987). Stamp, whose academic training
306 was as a geologist but who made his career as a geographer, became one of the most influential
307 geographers in Britain (Johnston, 2008). Willatts was also widely known and became the organizing
308 secretary of the Land Utilisation Survey of Great Britain (Wise, 2000).
309 Hugh Hammond Bennett (1881–1960) (Figure 8), trained as a geologist by Cobb at the University of
310 North Carolina, began his career as a soil surveyor, which took him across the USA and other countries
311 conducting soil research. As a result of those experiences and an address given by Chamberlain, Bennett
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312 became concerned about the problem of soil erosion in the 1920s. In 1928, he co-authored “Soil
313 Erosion: A National Menace”, which would be influential in the development of the USA’s Soil
314 Conservation Service (SCS). Bennett became the director of the Soil Erosion Service when it was
315 established within the USA’s Department of Interior in 1933 and then became head of the SCS when it
316 was established within the Department of Agriculture in 1935. Bennett’s advocacy for protecting soil
317 resources was pioneering, strengthened by increased public awareness during the Dust Bowl which
318 occurred between 1934 and 1940 (Helms, 2010; Lee and Gill, 2015). Hugh Hammond Bennett served as
319 president of the Association of American Geographers from 1943-1944.
320 321 Figure 8. Hugh Hammond Bennett (center), calling attention to the severity and impacts of soil erosion. 322 Image courtesy of the USDA-NRCS. 323
324 Carl O. Sauer (1889-1975) was a highly influential American geographer (Kenzer, 1985) who served as
325 president of the Association of American Geographers from 1940-1941. He influenced soil geography
326 largely through his role on President Franklin D. Roosevelt’s (1882-1945) Presidential Science Advisory
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327 Board in the 1930s. Sauer suggested that the SCS should integrate pedology, geology, and climatology in
328 their research (Holliday et al., 2002). This recommendation was in line with Sauer’s own background, for
329 even though he made his reputation as a cultural geographer, he began his graduate studies in geology,
330 with a specialization in petrography (Kenzer, 1985). Sauer was an advocate for broad academic training
331 and for including individuals from related fields with geographic interests in the study of geography
332 (Sauer, 1956). The SCS accepted his advice and started a research program under the
333 geographer/climatologist Charles W. Thornthwaite (1899-1963), who studied under Sauer at the
334 University of California at Berkeley (Mather, 2005). This research began to form the foundation of soil
335 geomorphology, work that was unfortunately interrupted by World War II (Holliday et al., 2002).
336 Perhaps the academic crown jewel of geology-soil academic linkages was the soil geomorphology
337 program, established by the USA’s National Cooperative Soil Survey (NCSS) in the 1930s. The NCSS is a
338 special partnership between the American federal government, state and local governments, and
339 universities, to develop and improve soil maps. Using many of the ideals espoused by Sauer, the NCSS
340 led the development of the soil geomorphology program, which was to be focused on “surface and soil”,
341 and to have pedologists, geologists, as well as climatologists work together and focus on the interactions
342 and co-development of soil and landscapes (Effland and Effland, 1992). Under the leadership of Charles
343 Kellogg (1902-1980) and assisted by Guy D. Smith (1907-1981), the program had a stated research
344 mission to understand soil-landform relationships in support of soil mapping (Grossman, 2004). Smith
345 and the NCSS established several research locales where soil geomorphology was to be studied in detail,
346 including subhumid Iowa (led by Robert V. Ruhe (1918-1993), a geologist), a desert site in New Mexico
347 (led by Leland H. Gile (1920-2009), a soil scientist), a humid site in the Pacific Northwest of Oregon (led
348 by Robert B. Parsons, a soil scientist), and one in North Carolina (led by Raymond B. Daniels (1925-2009),
349 a soil scientist). Theories and data that poured out of these four sites spurred considerably more work of
350 this kind within the university community, had profound effects on theories of soil and landscape
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351 evolution, and greatly influenced the way soils were classified (Effland and Effland, 1992). Much of this
352 effort culminated in the first textbook devoted to soil geomorphology, written by geologist Peter W.
353 Birkeland (Birkeland, 1974).
354 Although this brief discussion is by no means exhaustive (there are many additional individuals and
355 advances that could be discussed), it serves to demonstrate that there are strong historical ties between
356 soil science and geography, and that significant advances were being made in both soil science and
357 geography in the late 19th and early 20th centuries (Figure 9). It also demonstrates that advances in soil
358 geography were driven by individuals trained in a number of fields, including chemistry (e.g., Whitney),
359 geography (e.g., Sauer, Thornthwaite), geology (e.g., Dokuchaev, Marbut), natural science (e.g., Darwin),
360 and others (Helms, 2002; Johnston, 2008; Johnson and Schaetzl, 2014; Landa and Brevik, 2015).
361 362 Figure 9. Some milestones in the development of map-making technologies, leading to important 363 advancements for both geography and soil science.
364
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365 2.2 The disconnect
366 Soil science and geography share some history; they are both highly interdisciplinary, and in fact,
367 overlap. They share many "founding fathers." And yet, despite the connections and commonalities, in
368 many ways it seems that there has been a disconnect between the fields for much of their recent
369 history. This is particularly true in the USA. Although geographers have made numerous contributions to
370 soil science in Europe, where an academic association between soil science and geography is more
371 common (Freeman, 1987; Willatts, 1987; Brandt, 1999), soil science research in the USA has largely been
372 conducted in colleges of agriculture at the land grant universities, particularly in departments of
373 agronomy, plant and soil science, soil science, etc. (Landa, 2004; Brevik et al, 2016a). Brevik (2009)
374 estimated that there were only about 50 soil specialists (as compared to approximately 2,500 total
375 geographers) employed in the geography departments of USA colleges and universities in 2005. Only
376 about one in five geography programs had a stated soil specialist. This contrasted with approximately
377 640 soil specialists employed in agricultural-based soil programs, and despite the fact that geography
378 programs were offered at >260 universities in the USA, whereas agricultural-based soil programs were
379 offered at only 76 universities (Brevik, 2009). Landa and Brevik (2015) found that 76% of the soil science
380 programs in the USA were offered by land grant universities, only 5% were offered by Earth science
381 departments, whereas the remaining 20% were offered by non-land grant universities that had
382 agriculture programs.
383 In addition to soil research and teaching being primarily within agriculture departments at American
384 universities, federal soil mapping and research programs have traditionally been housed within the
385 USA’s Department of Agriculture (USDA). This organization occurred despite early attempts by Eugene
386 W. Hilgard (1833-1916) and John Wesley Powell (1834-1902), two prominent American scientists, to
387 create a division of agricultural geology within the USA’s Geological Survey (Amundson and Yaalon,
388 1995). Within the USA, the largest professional soil science society, the Soil Science Society of America
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389 (SSSA), evolved from the American Society of Agronomy (ASA) and routinely holds their annual meetings
390 in association with ASA and the Crop Science Society of America (CSSA). Only one annual meeting of
391 SSSA to date has been in association with an Earth science society, that being a meeting with the
392 Geological Society of America in 2008, which was also held in conjunction with ASA and CSSA (Brevik,
393 2011). Because of its association with agriculture at both the academic and federal government levels,
394 soil science has typically not been viewed as a geoscience in the USA (Landa, 2004), likely weakening
395 potential ties between soil science and geography.
396 The evolution of geography as a discipline has likely had its own effect on the drift of geography away
397 from soil studies. At the beginning of the 20th century, what we would today call physical geography
398 dominated geography departments, and soil was often studied by geomorphologists. For example,
399 geographers may consider Davis the “father of American geography,” but geologists also consider him as
400 one of their own. At least in the USA, physical geography became a smaller component of geography as
401 the rise of human geography proceeded. Today, physical geographers sometimes wrestle with
402 distinguishing themselves from their colleagues in geology or ecology/botany departments, disciplines
403 that also share connections with soil science. In many ways, this is a natural situation for
404 interdisciplinary topics, but in general, soil research has been largely overlooked by non-agricultural
405 disciplines over the past century.
406 In recent decades, the field of soil science has moved away from the science of mapping. Soil science
407 was born out of the recognition of multiple factors affecting the processes and thus the spatial
408 distribution of different soil properties. However, by the early 20th century the scientific study of
409 mapping soil, sensu stricto, was fading. In 1929, Thomas Bushnell complained that the meetings of the
410 organization once called the “American Association of Soil Survey Workers” was no longer balanced
411 between the study of soil and the study of surveying or mapping. That organization evolved into the
412 SSSA, which is today comprised of 14 divisions of interest. Soil mapping is a subset of the pedology
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413 division, which also includes soil formation, classification, physical and chemical properties,
414 interpretation of soil behavior, human land use decisions, and ecosystem evolution. To be fair, there are
415 also separate divisions for soil chemistry, mineralogy, biology, physics, as well as for different
416 ecosystems. In addition, soil formation and interpretation of soil behavior are natural pairings with soil
417 mapping. Nonetheless, investment and research activity on the geographic nature of soil studies has
418 been decreasing. For example, other than a brief spurt of interest between 2009 and 2011,
419 presentations on soil mapping at national SSSA meetings have been sparse (Figure 10) and the USA’s
420 federal budget for soil mapping has declined from an all-time high in the late 1980s to a long-time low in
421 2013-2015 (Brevik et al., 2016a). Soil scientists today need to answer questions such as: Why should
422 more investment be made in soil mapping? Weren’t the strategies for mapping soil worked out by the
423 1940s? Why should areas that have already been mapped be revisited (e.g., >85% of the USA has
424 already been mapped (Indorante et al., 1996))? At the international level, some specialized conferences
425 reflect the reinvigorated interest in soil mapping. However, they also indicate some departure from
426 traditional disciplinary frameworks.
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160 140 120 100 80 60 40
Quantity Presentations of Quantity 20 0
Geographic (Horizontal) Pedogenesis (Vertical) Classification and Interpretation History and Eduction 427 428 Figure 10. Topical trends for presentations given in the pedology division of the Soil Science Society of 429 America (SSSA), for 2005-2018. After reaching a peak in 2011, which corresponds with a surge of 430 interest by Americans in digital soil mapping, the quantity of presentations on the spatial prediction or 431 geographic distribution of soil has been declining. *The 2008 meeting was held jointly with the 432 Geological Society of America. **The 2018 meeting was held in January of 2019, independently of the 433 usual association with the Crop Science Society of America and the American Society of Agronomy.
434
435 A symptom of the disconnect between soil science and geography is the lack of recognition of core
436 geographic concepts as the basis for the soil mapping paradigm. Geographical context is crucial to
437 understand soil formation and disturbances. Ask a soil scientist how they map soil, they will likely cite or
438 describe the soil-landscape paradigm (Hudson, 1992). If pressed to explain why that works as a means of
439 spatial prediction, they would likely describe the five factors of soil formation that broadly describe
440 processes influencing soil properties. What many do not think about is that this concept of spatial
441 covariation has a long history in the geographic study of many topics. When Bushnell (1929, p. 23) was
442 complaining about the lack of attention to mapping concepts by soil scientists, he made the observation
443 that “once we enter the map making game, there are rules to be obeyed and standards, which must be
444 met.” Because of the relatively small change in soil mapping methods over the past century, the old
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445 rules are largely adhered to, with minimal thought about methodological improvement. Meanwhile,
446 geography has evolved, and new datasets have emerged; there now exist new methods of analysis,
447 awareness of spatial complications (e.g., modifiable areal unit problem), and higher standards for map
448 production.
449 There are, of course, exceptions to these broad generalizations. Vladimir M. Fridland’s (1972/1976)
450 work on analyzing soil cover patterns is one example. In 1985, Francis D. Hole and James B. Campbell
451 wrote Soil Landscape Analysis, which uses the term “spatial association” to describe the traditional
452 method of soil mapping. In many ways, the analyses presented in that book echo the style of thinking
453 advanced by geographer William W. Bunge (1928-2013) in his seminal text for geography’s quantitative
454 revolution, Theoretical Geography (Bunge, 1962). It is also worth noting that Hole (1913-2002), who was
455 trained in geology and soil survey, held a joint appointment at the University of Wisconsin in both the
456 Departments of Soil Science and Geography (Devitt, 1988; Tandarich et al., 1988; Brevik, 2010).
457 3. Soil science’s renewed interest in soil mapping
458 3.1 The geospatial revolution’s effect on soil mapping
459 The tools made available by the geospatial revolution of the past 20 years have undoubtedly had major
460 impacts on many avenues of scientific investigation (Longley et al., 2015). This impact is especially true
461 for mapping applications in soil science. Similar to the stimulation that the first and second waves of soil
462 mapping provided to soil science, the current third wave is setting the stage for discovering spatial
463 patterns that will undoubtedly prompt further research into the processes producing those patterns.
464 Not long ago, the locations of representative soil samples were commonly described in writing as
465 distances and directions from an available landmark. Today, GPS units are commonly used for this
466 purpose, making it simpler and more accurate to record sample locations using geographic coordinates
467 (Figure 11). In addition to improving the ability to return to the same sampling sites, the association of
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468 the observed soil data with accurate geographic coordinates has opened a completely new realm of
469 spatial analysis and mapping. Sampling designs can now be planned in a GIS and the soil data collected
470 can be intersected with multiple layers of environmental covariates.
471 472 Figure 11. Global position systems (GPS) have changed soil sampling by improving pre-planning, re-use 473 of sample data, and the accuracy of relating soil properties with covariates. The researchers in this 474 photo created a sampling design using digital terrain analysis and are now locating those sample points 475 using a small, handheld GPS receiver, held by woman in the middle of the photo.
476
477 This same geospatial revolution produced a tremendous number of new base maps and related data, all
478 of which could contribute to the new mapping effort. Satellite images can span the electromagnetic
479 spectrum and cover large areal extents, providing indicators of vegetation, land use, natural hazards,
480 and other physical landscape properties (Xie et al., 2008; Joyce et al., 2009; Mulder et al., 2011). LiDAR-
481 based elevation data are highly accurate and detailed, and are becoming increasingly common (Hodgson
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482 and Bresnahan, 2004). Adding value to elevation alone, various types of digital terrain derivatives
483 provide important information related to surface topography and wetness (O’Loughlin, 1986; Moore et
484 al., 1993; Gessler et al., 2000). Although usually for smaller extents, proximal sensing systems such as
485 ground penetrating radar, electromagnetic induction, and electrical conductivity can be used evaluate
486 sediments in the subsurface (Huisman et al., 2003; Hedley et al., 2004; Rhoades and Corwin, 2008; Molin
487 and Faulin, 2013; Doolittle and Brevik, 2014). Some of these data products can be incorporated into the
488 manual process of delineating map units, but the quantity of data set layers quickly becomes more than
489 can be utilized by visual inspection. By quantifying the relationships between these covariates and soil
490 properties with tools such as machine learning, the mapping process can be made more automated and
491 more repeatable (McBratney et al., 2003; Scull et al., 2003).
492 In the pre-digital soil mapping paradigm, experience from surveying the soil landscape helped the
493 mapper develop a mental model for predicting the expected spatial distribution of soil types and their
494 associated soil properties (Hudson, 1992). Mappers would seek to establish relationships among soil
495 types and, in particular, landscape position and vegetation cover (e.g., Parsons et al., 1970; Barrett et al.,
496 1995). That mental model was then applied to the best available base map, often a stereopair of aerial
497 photographs, to delineate soil map units (Miller and Schaetzl, 2014). In the case of a stereopair, the soil
498 mapper would use cues from changes in topography and vegetation to hand-draw map unit boundaries
499 in the office, and field-check them later. Although difficult to directly overlay, the soil mapper would also
500 make use of any existing maps – such as those of surficial geology – to better predict where different soil
501 types occurred. GIS software changed that system by making it easier to overlay different base maps
502 and to edit map unit delineations (MacDougall, 1975; Chrisman, 1987). Of course, GIS has the power to
503 do much more, but for traditional soil mappers, GPS-logged field observations, access to better base
504 maps, and easier overlays best describe the first step into the geospatial revolution.
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505 A major asset of the traditional soil mapping approach was the human mind’s ability to synthesize years
506 of field experience and add a degree of intuitive knowledge to the field mapping effort. Unfortunately,
507 that mental model approach, however accurate it may be, has two major limitations: 1) it is based on
508 human judgment, making it largely not repeatable, and thus, 2) much of the knowledge is lost when the
509 soil mapper retires (Hudson, 1992). This latter problem has been a major issue for the current mapping
510 effort within the USA’s Soil Survey, where most mappers from the 1970’s and 1980’s have since retired;
511 the brain drain is real and there may be no solution. The number of soil scientists employed by the US
512 federal government declined by 39% from 1998 to 2017 (Vaughan et al., 2019). If time has run out for
513 documenting localized expert knowledge, the best approach for the next generation of soil maps may be
514 to “start over,” using the enhanced data sets and powerful mapping and modeling software that have
515 recently emerged. Again, GIS offers the tools to quantify the spatial relationships between soil
516 properties and covariates, making the spatial models more efficient and repeatable. Fortunately, the
517 original base soil maps and their underlying data remain, from which new and better mapping efforts
518 can be built. At the minimum, the previous generation of soil data provide the opportunity to study soil
519 change (e.g., Veenstra and Burras, 2015).
520 Early work connecting geospatial technologies with soil mapping began simply with storing and
521 representing soil information in a GIS (Webster and Burrough, 1972; Legros and Hensel, 1978;
522 Tomlinson, 1978; Webster et al., 1979). Although digital cartography is (was) an achievement in itself,
523 the visualization of soil maps using a computer did not utilize the potential to improve the maps with
524 the spatial analysis components of the geospatial revolution. Soil scientists simply took note of the
525 effects of spatial autocorrelation and instituted spatial sampling designs to avoid this type of bias in the
526 early 20th century (Mercer and Hall, 1911; Fisher, 1925; Youden and Mehlich, 1937). The availability of
527 general use computers reignited the application of computationally intensive statistics again later in that
528 century (e.g., Hole and Hironaka, 1960; Rayner, 1966; Webster and Burrough, 1972). With the addition
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529 of Matheron’s concepts for geostatistics (Matheron, 1965, 1969), these combined elements spurred
530 enthusiasm for spatial models to predict the distribution of soil properties (e.g., Burgess and Webster,
531 1980; McBratney and Webster, 1983; Vauclin et al., 1983). By 1994, the study of soil science with the
532 statistical and probability approaches afforded by computers, came to be known as pedometrics
533 (Webster, 1994). Early approaches to digital soil mapping emphasized geostatistics, but over time
534 methods for digital soil mapping relying on the covariation of soil properties with variables measured by
535 remote or proximal sensing have become more dominant (Figure 12).
120 Hybrid 100 Spatial Covariation 80 Spatial Autocorrelation 60
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Quantity Publications of Quantity 20
0 2005 2007 2009 2011 2013 2015 2017 536 537 Figure 12. Trends in three different types of spatial prediction methods used in digital soil mapping, 538 from 2005 to 2018. Geostatistics (spatial autocorrelation) was an early favorite for digital approaches to 539 soil mapping. However, more recently spatial regression (spatial covariation) approaches have gained in 540 popularity. Data obtained from a search of Scopus (2019) using the search term “digital soil mapping.”
541
542 3.2 The era of digital soil mapping
543 Two papers were coincidently published in 2003 that explored new trends in geospatial technologies
544 and their increasing utilization in soil mapping research. One was published in the journal Progress in
545 Physical Geography (Scull et al., 2003), the other in Geoderma (McBratney et al., 2003). Both papers
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546 were exemplary reviews of the state of the art for utilizing geospatial technologies to model the spatial
547 distribution of soil. The publication of these papers marked the widespread recognition that geospatial
548 technologies were providing new ways of thinking about soil mapping. The article by McBratney et al.
549 (2003) - targeted to soil scientists - had 2,238 citations as of 3 August 2019 (Google Scholar). Although
550 the article by Scull et al. (2003) – targeted to geographers – has not exactly been ignored, it nonetheless
551 had 449 citations on the same date. This disparity may be an important indicator of the respective
552 disciplines’ interest in soil mapping; soil scientists may be more interested in digital soil mapping.
553 Although this is a natural situation for interdisciplinary topics, it is not the same as directly interacting
554 with researchers that have the scientific study of spatial analysis and prediction as part of their academic
555 heritage. Our paper argues for greater collaboration between geographers and soil scientists, but there
556 is no doubt that great strides have already occurred in the realm of digital soil mapping. The annual
557 number of papers that Google Scholar has indexed using the words ‘digital soil mapping’ increased from
558 22 to 557 in the decade following the two landmark review papers in 2003 (Figure 13).
900 800 Digital Soil Mapping 700 600 Predictive Soil Mapping 500 400 300
200 Quanity Publications of Quanity 100 0 559 1980 1985 1990 1995 2000 2005 2010 2015 560 Figure 13. Trend in digital soil mapping activity as indicated by Google Scholar results for the terms 561 “digital soil mapping” and “predictive soil mapping”, from 1980 to 2018. Although the term “predictive 562 soil mapping” at times gains in popularity as an alternative term for the current revolution in soil 563 mapping methods, it should be noted that soil mapping is inherently an exercise in spatial prediction.
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564
565 In response to demands for a global data set to assist decision making related to issues of food security,
566 climate change, and environmental degradation, the Digital Soil Mapping Working Group of the
567 International Union of Soil Science established the Global Soil Map initiative in 2006. This project is
568 coordinating national soil mapping agencies to produce a standardized digital product of soil
569 information (Sanchez et al., 2009). Although several countries were already reinvigorating their soil
570 mapping programs with digital soil maps, the Global Soil Map project has created a target for map
571 quality and important soil properties to be included. For example, one of the major objectives of the
572 project is to bring to the fore issues and estimates of spatial uncertainty in digital soil mapping. The
573 ambitions of this project have attracted funding to assist areas with the greatest need for this
574 information, particularly Sub-Saharan Africa.
575 Beyond the obvious use of digital tools to represent the spatial distribution of soil, digital soil mapping
576 aims to incorporate other improvements to soil maps. When we discuss or envision digital soil mapping,
577 we must be careful to distinguish digitized soil maps from digital soil maps. The former does not
578 leverage the benefits of spatial analysis in a GIS. Although legacy soil maps still hold a lot of value, simply
579 digitizing them into a GIS fails to advance the science of soil mapping. Towards realizing the potential of
580 digital soil mapping, three goals have been identified as benchmarks: 1) addition of soil observations
581 using statistical sampling techniques, 2) production of soil maps by quantitative spatial models, and 3)
582 inclusion of uncertainty associated with predictions (Lagacherie and McBratney, 2006).
583 Researchers of digital soil mapping regularly debate the most appropriate data structures and
584 computational techniques to capture, represent, process, and analyze soil information. When
585 attempting to produce better digital soil maps, uncertainties that arise from overlaying multiple data
586 layers, identifying spatial patterns, and making predictions based on those patterns must be evaluated.
587 Further, questions on how to best communicate the resulting map and represent the associated
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588 uncertainty are omnipresent. Complications of scale, such as the modifiable areal unit problem and the
589 minimum map unit size (e.g., Hupy et al., 2004), continue to cause confusion on how to best analyze
590 patterns of soil and covariates. In short, soil is an excellent test subject for the systematic study of issues
591 of scale, accuracy, and spatial analysis.
592 In the earlier section describing the geospatial revolution’s effect on soil mapping, geospatial
593 technologies were described as tools for digital soil mapping. Soil is a quintessential geographic entity,
594 the product of complex interactions of phenomena occurring at different scales. This characteristic
595 makes soil and its spatial distribution a highly suitable subject for geographers and geographic
596 information scientists.
597
598 3.3. Modern issues creating new demands for soil maps
599 Soil is intimately intertwined with the topical alignments of both human and physical geography (Figure
600 14) (Kuby et al., 2013; Arbogast, 2017). Soil is a physical feature of the Earth. Soil is affected by human
601 activity and influences populations, as it is an essential natural resource.
602 The original motivation for soil mapping was resource inventory for the purpose of land valuation, then
603 later, for guiding landowners to optimize agricultural production (Miller and Schaetzl, 2014). Following
604 awareness of soil erosion as an issue, soil maps became important for identifying areas of erosion risk,
605 helping to better plan conservation practices. Subsequently, soil maps have been used for many
606 applications of land suitability including identifying limitations for water management, building
607 development, and wildlife habitat. In response to these recognized uses of soil maps, the attribute
608 tables for soil map units have been adapted and expanded. For the most part, this continues to be the
609 case for issues of newly increased concern, but some new issues will require new kinds of soil maps.
610 Interpreting legacy soil maps for the new demands has been challenging, but generally useful. However,
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611 the spatial accuracy and precision of those maps may not be sufficient for some of the new uses of soil
612 maps (Miller, 2012).
613
614 Figure 14. Some of the interdisciplinary overlaps between soil science and geography.
615
616 Environmental issues are one of the primary areas putting new demands on soil maps (Hartemink and
617 McBratney, 2008; Thompson et al., 2012). Because soil plays a key role in all of the spheres of the Earth,
618 most models of the environment benefit from spatially explicit soil data. Prediction of flood prone areas,
619 for example, depends on the interactions among rainfall, topography, and the spatially variable ability of
620 soil to absorb that water. Water quality models need to account for biological, physical, and chemical
621 interactions of water with the soil across watersheds. Many of the models for these kinds of issues, such
622 as the Water Erosion Prediction Project (WEPP) and Soil and Water Assessment Tool (SWAT), aggregate
623 soil information at the watershed or sub-watershed scale, which means that much of the spatial
624 information is discarded in the interest of model efficiency. With increasing computing power, however,
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625 there exists an opportunity to improve these models with better soil maps and better considerations of
626 spatial connectivity.
627 Soil and water connectivity is an emerging topic, and mapping soil and water flows is fundamental to
628 understand the impact of different land uses on overland flow and erosion. Under natural conditions,
629 connectivity depends on the parent material type, soil texture and structure, topography (e.g., slope,
630 aspect), climate patterns, and vegetation distribution (e.g., patchy or continuous). Connectivity can be
631 affected by natural phenomenon such as fire, or other human induced impacts (e.g., mining, grazing, or
632 agriculture). Normally, these disturbances increase connectivity, as compared to the natural condition.
633 The spatial distribution of connectivity can be complex, however, and mapping provides an important
634 contribution to a better understanding of where soil and water fluxes are high. Several indexes have
635 been developed to measure connectivity; these have been applied at different spatial and temporal
636 scales (Heckman et al. 2018). Soil and water connectivity maps have been produced in several
637 environments, such as mountain catchments (Cavalli et al. 2013; Zuecco et al. 2019), abandoned and
638 afforested mountainous areas (Lopez-Vicente et al. 2017), mountainous areas with heterogeneous land
639 use (Lopez-Vicente and Ben-Salem, 2019), agricultural lowland basins (Casamiglia et al., 2018), places
640 affected by landslides (Persichillo et al. 2018), and urban environments (Kalantari et al., 2016). Despite
641 the recent progress in this area, further work should focus on the validation of these models.
642 The soil-atmosphere interface is another aspect of how soil affects quality of life. The interactions of soil
643 with the atmosphere, such as evapotranspiration, are important processes to consider for improving the
644 accuracy of weather forecasting (Fennessy and Shukla, 1999; Koster et al., 2004). Similarly, soil stores a
645 large stock of carbon, which makes it a major potential carbon source or sink (Lal, 2004). Soil’s role in
646 the carbon cycle makes it an important factor in the positive and negative feedback loops of global
647 climate change (Stokstad, 2004; Zhuang et al., 2006).
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648 The early interest in soil maps to improve food production and protect soil as a resource has not gone
649 away. The global population is expected to reach 8.5 billion by 2030 (United Nations, 2019), during
650 which time 30-60 Mha of cropland is expected to be lost to infrastructure (e.g., housing, industry, roads,
651 etc.) (Döös, 2002). Soil degradation works against goals to increase agricultural crop productivity.
652 Although soil erosion has been recognized as a problem for a century and great effort has been made to
653 address the issue, large losses of valuable soil resources continue (Brevik et al., 2017).
654 Innumerable works have focused on mapping soil erosion at local, regional and global scales (e.g.,
655 Bahadur, 2009; Nachtergaele et al. 2010; Panagos et al., 2015; Gelder et al., 2017). The first attempt to
656 map soil degradation (including erosion) at the global level occurred with the Global Assessment of Land
657 Degradation and Improvement (GLASOD), but, surprisingly, this work did not use soil data (Pereira et al.,
658 2017). The accuracy of soil erosion maps increased appreciably with the availability of covariate maps
659 with more types of information and higher resolution, primarily developed in concert with the recent
660 revolution in geospatial technologies. Included among those covariate maps are continuous maps made
661 possible by spatial interpolation methods (Borrelli et al. 2018).
662 The majority of the soil erosion maps produced today focus on erosion risk (Ochoa-Cueva et al. 2013;
663 Farhan et al. 2015; Mancino et al. 2016; Haregeweyn et al. 2017) and the estimations are often carried
664 out using the Revised Universal Soil Loss Equation (RUSLE) (Pal and Al-Tabbaa, 2009; Boni et al., 2015).
665 RUSLE calculates soil loss rates (E) by rill and sheet erosion based on the rainfall (R), erodibility factor (K),
666 cover management factor (C), slope length and slope steepness factor (LS), and a support practices
667 factor (P) (Renard et al., 1997). Fewer studies have been carried out to map wind erosion, despite
668 widespread recognition that wind erosion increases soil degradation (Borrelli et al. 2016). Wind erosion
669 is much more complex to model than water erosion. Nevertheless, some attempts have been made at
670 the local (Sterk and Stein, 1997; Zobeck et al., 2000; Harper et al. 2010) and regional (Borrelli et al. 2014;
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671 2016) levels. Other works have mapped sediment sources and deposition areas (Petropoulos et al. 2015;
672 Cavalli et al. 2017).
673 The assessment of soil ecosystems and their services has increased rapidly in the last decade, and
674 mapping is crucial to identify the distributions of these services (regulating, provisioning, cultural, and
675 supporting ecosystem services). Maps can represent the synergies and trade-offs between ecosystem
676 services (ES), trends, costs and benefits, monetary value, and aid in estimating costs and benefits (Maes
677 et al. 2012; Burkhard et al. 2018). An extensive body of literature exists on mapping ES; this effort uses
678 soil variables to assess regulating and provisioning services (e.g., Burkhard et al. 2012; Syrbe and Walz,
679 2012). The relationship between the quality and quantity of ES with the services provided directly or
680 indirectly by soil has been demonstrated (Adhikari and Hartemink, 2016; Pereira et al. 2018; Brevik et al.
681 2019). Soil functions are crucial for assessments of ecosystem vitality; thus, they are normally integrated
682 into ES estimations in well-known ES assessment models such as InVEST (Sharp et al. 2018) and Aires
683 (e.g., Bagstad et al. 2014) (e.g., carbon storage, sediment delivery ratio).
684 There are several works that link soil functions with ES (e.g., Barrios, 2007; Pulleman et al. 2012; De
685 Vries et al. 2013; Lavelle et al. 2016) and which quantify soil ES (Dominati et al. 2010; Robinson et al.
686 2013). However, more effort should be made to map soil ES individually in order to understand the real
687 value of soil in ES assessment, and more research is needed to optimize the mapping of soil ES. In
688 addition, soil ESs are overlooked and not considered in some ES classifications such as ‘The Economics of
689 Ecosystems and Biodiversity’ (TEEB) (Pereira et al., 2018).
690 4. Conclusions 691 Clearly, a reinvigoration is occurring in soil geography. This renewed interest in improving soil maps is
692 stimulated by the confluence of improving capabilities for producing maps, and the increasing need for
693 spatial soils data. Innovations from the geospatial revolution, coupled with the increasing power of
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694 computing and machine learning, have added many new and useful opportunities available to the soil
695 mapper’s toolkit and theoretical base. To address modern issues facing society, including supporting a
696 growing population and other impacts on ecosystem services, more frequent improvements of soil
697 maps, of often manifested as "updates", will be required. Even though other disciplines continue to have
698 a strong vested interest in the study of soil, there is a clear need for the spatial sciences in the future of
699 this field. Mapping the environment, discovering processes, and understanding interactions between
700 these processes across space work cyclically with each other, which makes the new wave of soil
701 mapping exciting both for soil science and for geography and speaks to the need for these fields to work
702 collaboratively on issues of mutual interest.
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