PROBLEMS AND SUGGESTIONS RELATED TO SOIL
CLASSIFICATION AS PRESENTED IN INTRODUCTION TO PHYSICAL
GEOLOGY TEXTBOOKS
Eric C. Brevik
Department of Physics, Astronomy, and Geosciences, Valdosta State University,
Valdosta, GA 31698-0055; [email protected]
ABSTRACT
Complex concepts and ideas are often presented to
introductory level students in a simplified form for
educational purposes. This simplification sometimes
requires that the “whole truth” not be presented.
However, the simplified material should be current and
factual to the extent that future lessons that build on the
introductory material will expand on the originally
presented material rather than contradicting it. The
presentation of soil classification in many current
introductory physical geology textbooks is over 35 years
out of date. In addition, the way soil classification
terminology is used would be questionable even if it
were still current. Individuals teaching introductory
geology courses that include sections on soils and the
authors of textbooks using incorrect classification
terminology should obtain current references on soil
classification and correct these deficiencies.
Keywords: geoscience teaching and curriculum; soils;
Soil Taxonomy
INTRODUCTION
Most textbooks intended for use in introductory physical
geology classes include a discussion of soils. However,
not all these discussions are accurate and up to date. In
fact, only one of the 11 introductory texts I have reviewed
actually presents the modern system of soil taxonomy
used in the United States, and that one does not contain
the full and complete list of soil orders used in modern
soil taxonomy. Although soil scientists have changed
taxonomies several times within the last century,
perhaps making it frustrating for those who do not
specialize in soils to keep up, the current U.S. system
(Soil Taxonomy) has been accepted for at least the last 37
years. It is a disservice to our students when many of our
current physical geology textbooks present a soil
classification system that is at best 37 years outdated,
particularly when this exposure to the study of soils may
be the only one many students receive. This paper is
meant to serve as constructive criticism and will,
hopefully, lead to improvement of the soils sections in
introductory textbooks where needed. The task of
effectively covering the wide variety of topics found in
such books is enormous; therefore this paper is not
meant to be overly critical of those who wrote the
introductory geology texts I reviewed.
Brevik - Soil Classification in Geology Textbooks
A BRIEF HISTORY OF SOIL CLASSIFICATION IN THE UNITED STATES
Part of the Problem - Part of the problem encountered
by non-soil scientists when writing about soil
classification is the frequency with which soil scientists
themselves change the system. The national soil survey
in the United States started in 1899 under the direction of
Milton Whitney (Fanning and Fanning, 2001). The first
U.S. soil classification system was based largely on the
underlying geology, and the soil maps generated were
little more than surficial geology maps (Simonson, 1989).
Some soils workers rapidly discovered this approach
was inadequate. George Coffey, a soil survey employee
who was in charge of soil classification and correlation
from 1905-1909 (Brevik, 1999), published his own ideas
on soil classification in 1912 (Coffey, 1912). The U.S. soils
community overlooked Coffey’s ideas. Actual change
didn’t come until Curtis Marbut took up the cause in the
1920s. Marbut published several drafts of his concepts of
soil classification between 1921 and 1935, and it is from
Marbut that we get the terms “Pedalfer” and “Pedocal”,
terms still used in many introductory geology texts. A
revised version of Marbut’s classification system was
published in 1938 (Baldwin et al., 1938) and was adopted
by the U.S. soil survey. At its highest level this system,
referred to by soil scientists simply as the 1938 system,
designated soils as Zonal soils, Intrazonal Soils, and
Azonal soils. Pedalfers and Pedocals were listed as
subdivisions of the Zonal soils, and Lateritic soils were
listed as one of three subdivisions of Pedalfers. Laterites
were one of five subdivisions of Lateritic soils. The 1938
system presented a number of problems to soil scientists,
and by 1950 the U.S. soils community decided the system
needed to be replaced with a completely new one
(Simonson, 1989).
The task of developing a new system of soil
classification fell largely to Guy D. Smith of the United
States Department of Agriculture’s Soil Conservation
Service (SCS). Six proposals (proposals three through six
were called approximations) were developed and
circulated for comment between 1950 and 1958
(Simonson, 1989). In 1960 the Seventh Approximation
was unveiled to the world at the Seventh International
Congress of Soil Science, held in Madison, WI, and in
1965 the SCS officially adopted a modified version of the
Seventh Approximation as the official system of soil
classification in the United States of America (Simonson,
1989). In 1975 the “final” version of the new U.S. Soil
Taxonomy system was published (Soil Survey Staff,
1975). This new system consisted of 10 soil orders at its
539
highest category with additional subdivisions. In the
years since 1975 two additional soil orders have been
added, giving a current total of 12 (Table 1). It is worth
noting here that the name of the SCS was changed to the
National Resource Conservation Service (NRCS) in 1994;
therefore the NRCS is the government agency currently
responsible for soil classification in the United States.
Soil
Classification
Typically
Presented
in
Introductory Physical Geology Texts - I reviewed 11
textbooks advertised as suitable for use in introduction
to physical geology classes. Of those eleven, one presents
the current system of soil taxonomy but gives 11 soil
orders instead of 12 (Gelisols were added to soil
taxonomy only about 4 years ago, so omitting them is
understandable). Two make note of the fact there is an
official USDA system but never present one (such an
approach is, obviously, at the author’s discretion). Eight
present a system of soil classification that is based on
three major soil groups: Pedalfers (soils formed in
humid-temperate climates), Pedocals (soils formed in
arid climates), and Laterites (soils formed in
humid-tropical climates). There are at least two major
problems with this Pedalfers-Pedocals- Laterites
presentation.
First, no system of soil classification that has ever
been used in the United States, official or unofficial, uses
Pedalfers, Pedocals, and Laterites as its major
subdivisions
of
soils.
The
United
Nations
FAO/UNESCO classification system does not use
Pedalfers, Pedocals, or Laterites either. The system that
came closest to using this subdivision of soils was that of
Marbut (1935), which used Pedalfers and Pedocals as the
subdivisions of its highest category and Laterite soils at a
lower level under Pedalfers, but this system was never
officially adopted by the SCS. The closest official system I
have been able to locate is that of Baldwin et al. (1938),
but in that system Pedalfers and Pedocals are
subdivisions of Zonal soils and Laterites are one of the
subdivisions of Lateritic soils, which in turn are
subdivisions of Pedalfers. Pedalfers, Pedocals, and
Laterites have never formed the three divisions of soils at
the highest level for any system of soil classification,
unlike the implication given in the soils presentations of
many physical geology textbooks. Laterites are in fact
Pedalfers in the 1938 system, but the presentations given
in many introductory physical geology texts imply
Laterites are a completely different category of soils. To
list Pedalfers and Laterites in the same discussion as two
different soils and not point out the direct relationship
between them is simply wrong.
The second problem is the system of Baldwin et al.
(1938), the official system that comes closest to matching
the one presented by so many of our introductory
physical geology texts, has not been the official system of
soil classification in the United States since 1965. The
eight
texts
I
reviewed
that
used
the
Pedalfers-Pedocals-Laterites
(PPL)
system
have
540
copyrights of 2002, 2002, 2001, 2001, 2001, 1997, 1995, and
1990, and each of these texts represent distinctly different
books by different authors. These introductory texts are
25 to 37 years out of date and present a system of soil
classification that never actually existed. Given that U.S.
soil taxonomy has experienced at least three major
changes in classification theory in the last 100 years, the
soils sections of these texts need to be updated.
Although I am not certain where the system of soil
classification given in many physical geology texts of
today originally came from, its roots seem to lie in some
distortion of the systems developed by Marbut (1935)
and Baldwin et al. (1938). It is easy to see several
attractive features of the PPL system that may account
for its frequent use in introductory physical geology
textbooks. The 1938 system was the official system of soil
classification in the United States until 1965. Therefore,
geologists who were in school in the 1960s (a group that
includes some of the authors of our current textbooks)
are probably familiar with terms from the 1938 system,
including Pedalfers, Pedocals, and Laterites. This
familiarity may lead them to hang onto these terms.
Another attraction is that modern Soil Taxonomy has 12
orders at its highest level as opposed to three in the PPL
system, and three is easier to remember. The PPL system
is also conceptually easy: Pedalfers form in humidtemperate climates, Pedocals form in arid climates, and
Laterites form in humid-tropical climates. Finally, some
authors are probably simply following the example set
by previous authors in using the PPL system. However,
these are not good reasons to present incorrect
information to our students. For those who like maps, the
PPL system is again simple; a map of the United States
can be presented with the eastern United States shown as
Pedalfers and the western United States as Pedocals with
a division roughly along the Mississippi River. While it is
more difficult to present a map of the United States
showing the distribution of soils according to modern
U.S. Soil Taxonomy it is not impossible, and the NRCS
has such maps available.
SOIL TAXONOMY FOR THE
INTRODUCTORY GEOLOGY STUDENT
The imagined complexity of soil terminology and
seemingly large number of soil orders are probably the
greatest impediments faced by geoscience educators
when deciding whether or not they will present U.S. Soil
Taxonomy to introductory level classes. However, stop
and ask yourself, is it really that hard? Compare it to
some other things you teach in your introductory level
physical geology courses. How many rocks and minerals
with their associated terms do you require your
introductory students to learn, is it more than 12? Are the
terms used to describe rocks and minerals foreign to
your students, and were they foreign to you when you
started your career as a college freshman? Are they any
more familiar with those rock and mineral terms upon
Journal of Geoscience Education, v. 50, n. 5, November, 2002, p. 539-543
Brevik - Soil Classification in Geology Textbooks
541
entering your course than they are with soils
terminology? Just as with rocks and minerals, U.S. Soil
Taxonomy can be simplified for introductory level
classes in a way that makes it easier but still educational
and still relevant if your students proceed on to
additional coursework involving soils or if they ever
need to work with soils information in the future.
The best way to accomplish this is to focus on the
links between soil classification and the five soil forming
factors. The five soil forming factors are 1) climate, 2)
parent material, 3) topography, 4) organisms (flora and
fauna, not just vegetation), and 5) time. Using these soil
forming factors as appropriate, point out how they lead
to the formation of a given soil in a given location. Take,
for example, Entisols, or soils that lack subsurface
horizons. They commonly form on floodplains, because
soil forming processes will start leading to formation of
an A horizon. Then a flood comes along and buries the
soil surface (or A horizon) under a layer of sediment, and
the whole process starts over again, meaning sequences
of A-C-A-C-A-C horizons (as one proceeds down) are
common on floodplains, no matter what the climate or
vegetation. B horizons often do not have an opportunity
to form because of continual burial of poorly developed
soils by flood deposits. Entisols are common on
floodplains whether they are located in the eastern
United States, the western United States, or the tropics
(you can’t rely on the PPL system). Entisols are also
common in mountainous and badlands areas where
erosion rates are approximately equal to the rate of soil
formation; again it doesn’t matter if those mountainous
areas are in the eastern or western United States. The
dominant factor in the formation of Entisols is the
amount of time that the parent material has been exposed
to soil forming processes.
Mollisols and Alfisols can make another good
example. In Iowa (and other states), there are places
where Mollisols and Alfisols occur side by side on the
landscape. Climate, parent material, topography, and
time are all the same, but the organisms differ. The
Mollisols formed in a prairie ecosystem, while the
Alfisols formed in a woodland ecosystem. The different
conditions created by the different organisms present in
the two ecosystems have lead to different soil properties.
In the southeastern United States soils formed in a
woodland ecosystem are often classified as Ultisols
instead of Alfisols. Ultisols are similar to Alfisols but are
more intensely leached. The southeastern United States
has a warmer, more humid climate than Iowa does and
the geomorphic surfaces are often older, therefore the
different climate and time factors have created soils that
are often more leached than those in Iowa. Vertisols
develop deep cracks during dry periods because they
contain high percentages of shrink-swell clays; thus
parent material has a strong control over the formation of
542
Vertisols regardless of the climate they formed in.
Vertisols are found in India, on the coastal plain of Texas,
and in the Red River Valley of North Dakota, locations
with a diverse range of climate, organism, and time
factors.
As can be seen from the discussion above, there is
much more to the classification of soils than soils of
humid-temperate climates, soils of arid climates, or soils
of humid-tropical climates. The simplified definitions
suggested for the 12 soil orders in Table 1 include
information that can be used to link the soils to the five
soil forming factors, allowing instructors to work these
relationships into their soil discussions. By showing
students how these soil forming factors combine to
create unique soils at any given location the study of soils
can be made interesting yet not be overwhelming
difficult. In addition, the material offered to students
during the class will maintain the accuracy and relevance
that all introductory level courses should strive for.
An additional point to be made in the teaching of
soils is that everyone lives near some. You may or may
not live in an area that has nice rock outcrops to visit,
glacial landforms to explore, a desert nearby, or a beach
to see. Depending on where you live, certain sections of a
physical geology course can be really abstract to the
average student because they simply have no
experiences to relate to what is being taught. But
virtually everyone lives near soil, so make soils a class
exercise or lab. Have the students find out what the
dominate soils in your area are, or go out and let them get
their fingers dirty in a soil pit, a ditch, or along a road cut.
Soils are something a student anywhere in the country
can really see and relate to, and it’s a lot more fun to find
out which of the 12 soil orders is most common in your
area than it is to look a map and notice that you share
Pedalfers or Pedocals with half the rest of the country!
WHERE TO OBTAIN NEEDED INFORMATION
There are a number of sources available to the authors of
geology texts or to those teaching physical geology
classes who want to update their soils knowledge. A
comprehensive discussion of the 12 soil orders can be
found in Buol et al. (1997). For those seeking a briefer
discussion of modern soil classification, Brady and Weil
(2002) provides a good overview, as do Miller and
Gardiner (2001) and Singer and Munns (2002). There are
also a number of recent introduction to physical
geography and geomorphology textbooks that use the
proper system of soil taxonomy, although you need to be
certain they include the recently added (post 1975)
Andisol and Gelisol soil orders. Brief descriptions of the
soil orders with accompanying photographs and maps
showing their geographic distribution across the United
Journal of Geoscience Education, v. 50, n. 5, November, 2002, p. 539-543
States can be found at the National Soil Survey Center Coffey, G.N., 1912, A Study of the Soils of the United
States, USDA Bureau of Soils Bulletin 85:
website at http://www.statlab.iastate.edu/soils/photoWashington, D.C., U.S. Government Print Office, 114
gal/orders/soiord.htm.
p.
Easterbrook,
D.J., 1999, Surface Processes and
CONCLUSION
nd
Landforms, 2 Edition: Upper Saddle River, NJ,
Prentice Hall, 546 p.
Most of the introductory geology textbooks I have
reviewed do not present the proper version of soil Fanning, D.S., and Fanning, M.C.B., 2001, Milton
Whitney: Soil Survey Pioneer: Soil Survey Horizons,
classification used in the United States today. I hope this
v. 42, p. 83-89.
article will help teachers of physical geology courses
improve their course and that it will lead to positive Marbut, C.F., 1935, Soils of the United States, In O.E.
Baker (ed.), Atlas of American Agriculture, Part III:
changes in the current introductory textbooks. When
Washington, D.C., U.S. Government Printing Office,
writing about or teaching soils one should obtain the
p. 12-15.
most recent references available. Soil science is still a
D.T., 2001, Soils in Our
young, dynamic field, and concepts, theories, ideas, and Miller, R.W., and Gardiner,
th
Environment, 9 Edition: Upper Saddle River, NJ,
classification changes rapidly. When it comes to soils you
Prentice Hall, 642 p.
can’t just go back to a 1960s publication and have any
confidence that what you are writing or teaching is up to Simonson, R.W., 1989, Historical Highlights of Soil
Survey and Soil Classification with Emphasis on the
date. I would like to leave with one parting thought:
United States: Wageningen, The Netherlands,
teaching modern U.S. Soil Taxonomy doesn’t have to be
International Soil Reference and Information Center,
hard, in fact, it can be a lot of fun to notice patterns once
83 p.
you spend a little time to figure out some of the basic
relationships between the soil forming factors and soil Singer, M.J., and Munns, D.N., 2002, Soils: An
Introduction, 5th Edition: Upper Saddle River, NJ,
taxonomy. So take some time to look U.S. Soil Taxonomy
Prentice
Hall, 448 p.
over, and then go out and have some fun with it.
Soil Survey Staff, 1975, Soil Taxonomy: A Basic System of
Soil Classification for Making and Interpreting Soil
ACKNOWLEDGMENTS
Surveys: Washington, D.C., U.S. Government
Printing Office, 754 p.
I thank Annabelle Foos (The University of Akron), Mary
Strahler,
A. and Strahler, A., 2000, Introducing Physical
Savina (Carleton College), and an anonymous reviewer
Geography, 2nd Edition: John Wiley & Sons, Inc.,
for the comments and suggestions they contributed
New York, NY, 575 p.
toward the improvement of this paper.
REFERENCES
About the Author
Baldwin, M., Kellogg, C.E., and Thorp, J., 1938, Soil
Classification, In Soils and Men, USDA Yearbook for
1938: Washington, D.C., U.S. Government Print
Office, p. 979-1195.
Brady, N.C., and Weil, R.R., 2002, The Nature and
th
Properties of Soils, 13 Edition: Upper Saddle River,
NJ, Prentice Hall, 988 p.
Brevik, E.C., 1999, George Nelson Coffey, Early
American Pedologist: Soil Science Society of
America Journal, v. 63, p. 1485-1493.
Buol, S.W., Hole, F.D., McCracken, R.J., and Southard,
R.J., 1997, Soil Genesis and Classification: Ames, IA,
Iowa State University Press, 527 p.
Eric C. Brevik earned his BS and MA degrees in Geology
from the University of North Dakota and his PhD in Soil
Morphology and Genesis at Iowa State University. He
currently teaches soil science, geomorphology, and
introduction to physical geology and geography courses
at Valdosta State University in Valdosta, GA. His
research interests include carbon sequestration by soil,
use of electromagnetic induction techniques in soil
research, soil science history, and the integration of
geological and soils information.
Brevik - Soil Classification in Geology Textbooks
543