1
Soil taxonomy,teaching, and geography
2Kenneth L. White
ABSTRACT
Soil Taxonomy
adoptedby the Soil Conservation
Service andusedby soil scientists has not been
readily acceptedin manyother disciplines. This
lack of acceptance
is particularly evidentin many
introductorycoursesin physical scienceat college level. Frequently,professorsandtextbooks
refer to the 7th Approximation
if they refer to the
newsystemat all. A generalized presentation
whichcorrelates soil orders, vegetationbiomes,
andKoppen
climates at a regional scale is pro.
posed.Spatial coincidenceof the three environ.
mental subsystems
provides an increased appre.
ciation of Soil Taxonomy’s
nomenclature
to detail
environmental
settings, diagnostichorizons, and
quantifiedprofile differences.Thesoil ordersare
displayedin a flow chart organizedaccordingto
increasingsoil development
anddecreasingwater
supplyingcapacity.
Additional index words:Soil orders, Regional
soils.
THhlS
paper describes one way to reduce some of the
orror that is felt when a person is first confronted with the Comprehensive Soil Classification
Scheme presented in Soil Taxonomy, popularly known
as the 7th Approximation (1). Over the years, my experiences with university students enrolled in Introductory Physical Geographyindicate that they have a minimal knowledgeof such disciplines as climatology, biology, botany, chemistry, and the other elements that combine to make the study of soils so complex. Students
tend to lack understanding of relationships between
soils, vegetation, and climates in either a spatial or a
temporal setting. This lack does not seem to decrease
significantly with advance in class level. While most
students come from Liberal Arts or the Social Sciences,
there are still an appreciable number from Engineering
and Physical Sciences. Andalthough these deficiencies
create difficulties, they can be overcome; Soil taxonomy
assist in doingso.
How does one present this complex and seemingly
formidable system of soil classification,
particularly
when it is accompaniedby the strange-sounding vocabu’ A versionof a paperpresentedat the annualmeeting
of the Association of American
Geographers
in NewOrleans,La., April 1978.
2 Assistantprofessorof Geography,
TexasA&M
Univ.,CollegeStation, TX77843.
11
lary of Soil Taxonomy? Howdoes one make the system
intelligible,
even simple and understandable, to the
average undergraduate student?
A major complaint about Soil Taxonomy has been
that the system’s language is complex and esoteric. To
overcome this stumbling block, I have reduced the system to a regional approach, addressing the questions
"What", "Where", and "Why." When the terminology of Soil Taxonomyis introduced in this context and
the system makes "geographic sense", then the terminology can be presented at the level of detail needed for
the task at hand.
This approach is a simplification and is subject to the
old and familiar accusation of superficiality. But what is
superficial to the pedologist or physical geographer may
be very complex to the bedeviled and confused freshman. The challenge then, is to find that "Golden
Mean"between superficiality and scientific rigidity. I
do not believe this "Golden Mean" can really be attained, but an acceptable compromisemust be sought.
Wemust begin with the concept of soil classification.
Whyand how are soils classified? Soils are complex
bodies covering muchof the earth’s land surface. They
result from the actions and interactions of climate,
living organisms, parent material, relief, and time. Soils
are a response to the physical and biological environment. Any soil classification scheme is designed to reduce complexity to a level of organization which permits
an understanding of the relationships between soils and
soil environments. This allows the student to develop
predictions about either individual soils or soil groups.
Howsoils are classified is a more difficult concept.
The first major step in the developmentof soil classification occurred during the 1870’s in Russia under the
leadership of V. V. Dokuchayev,whorecognized soil as
a naturally occurring body with a distinct spatial arrangement (2). Curtis Marbut introduced Dokuchayev’s
ideas in the United States in the late 1920’s, leading to
development and publication of the classification
scheme knownas the 1938 System (3). This system emphasized the connection between soils and natural regions, i.e., vegetation and climate. Geographers quickly
adopted the system and, in many cases, became infatuated with it because of the pleasing aesthetic qualities of
maps developed using the scheme. However, since the
introduction of the 7th Approximation in 1960 and the
formal publication of Soil Taxonomyin 1975, a major
schism has appeared among physical geographers (4).
Which system is to be taught? The older, more familiar
system with a clear, easy-to-read map or a newer but
complex tongue-twister with a visually confusing map?
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JOURNAL OF AGRONOMIC EDUCATION
ENTISOL
INCREASING
DEVELOPMENT
(TIME)
ALLUVIAL
AZONAL
INCEPTISOL
TUNDRA
AZONAL
1
VERTISOL
...........
I
!
HISTOSOL
ALFISOL
BOGSOl LS
BOGS & SWAMPS
30% ORGANIC CARBON
GRAY BROWN PODZOLICS
NON-CALClC BROWNS
DECIDUOUS
FOREST/SAVANNA
ENRICHEDIN Fe & AI
CLAY RICH B-HORIZON
(D/C)
SPODOSOL
(D)
PODZOLS
ACID LITTER
TAIGA (CONIFERS)
WHITE A-HORIZON
RED B-HORIZON
(B&C)
(WET-DRY
SEASONS)
GRUMUSOLS
DARK COLORED
WIDE DEEP CRACKS
SHRINK-SWELL CLAYS
I
MOLLISOL
I
(BS/C)
ARIDISOL
BRUNIZEMS,
CHERNOZEMS, CHESTNUTS
STEPPE-PRAIRIE
THICK BLACK A-HORIZON
Ca ENRICHED B-HORIZON
(BW)
SIEROZEMS
SOLONCHALKS
XEROPHYTIC (DESERT)
LOW ORGANIC CARBON
:
LITHOLOGY
ULTISOL (A/C)
REDDISH BROWNLATOSOLS, RED-YELLOW PODZOLICS
TROPICAL DECIDUOUS, TEMPERATE EVERGREEN
RED/YELLOW COLORS
OXlSOL (A)
DECREASING
WATERSUPPLYING
CAPACITY
=
LATOSOLS
TROPICAL FOREST
RED COLORS
1 KOPPENCLIMATECLASSIFICATION
Fig. 1. A highly generalized conceptualization o! soil order development.
It is not just a matter of a new terminology. The 1938
system was marked by several problems. Even with the
1949 revisions, this system has strong tendencies to:
a. overemphasizevirgin soil conditions,
b. develop ambiguous categories, especially confusing at the family level,
c. be a non-bifurcated system,
d. have all categories not mutually exclusive,
e. have the nomenclature display a strong genetic
bias and not be hierarchical.
On the other hand, Soil Taxonomyis:
a. based on definable soil properties, most of which
can be defined in the field.
b. characterized by classification
keys that are
quantified and dichotomous--there are no "maybes."
Terminologyof Soil Taxonomyis critical in the method presented here. If we, as teachers of Introductory
Physical Science charged with the task of understanding
the relationships of manand the land, cannot communicate with other disciplines, our work runs the risk of
failure. Whensoil is discussed in the American educational scheme, we must use the "lingua franca" of the
American pedologist. For example, the Koppen climatic
classification
is understood and generally used as a
pedagogical technique in Introductory Physical Geography courses (5). Anyof the vegetation classification
schemes can be used in the same manner.
If we accept the notion of a "lingua franca" based on
a climatic classification system (I prefer Koppen, but
others will do) and vegetation regions, then perhaps we
can bring the three subsystems of soils, vegetation, and
cli=nate together. The interactions of the three can be
seen in soil development since the course and degree of
development are a major influence in grouping soils at
the highest level of classification, the order. The interactions of climate, vegetation, and soils can be expressed in terms of ten orders of soil taxonomyin a flow
chart (Fig. 1). Increasing development is portrayed from
top to bottom in the figure and reflects pedologic age
but indeterminate chronologic age. Moisture supplying
capacity decreases qualitatively from left to right on the
horizontal axis. Within this frameworkof soil development and moisture are displayed the soil orders, Koppen
climates, and vegetation types. Climate and |ife are the
active factors of pedogenesis and dominate in a large regional content (Fig. 2). Thus, like the temporal setting,
the spatial setting is more qualitative than quantitative.
In instruction, Figs. 1 and 2 may be used to help stu-
WHITE:
SOIL TAXONOMY,TEACHING,
AND GEOGRAPHY
13
Nostrong associations with either climate or vegetation
have been noted. Thus the Vertisols are entered in the
graph to the side of the main flow with a dashed line.
U.S. Regional Soils, Vegetation, and Climate
Fig. 2. Regionalsoil order distributions. Note1--Oxi.
sols are foundin hot-humidtropical regions. They
havenot beenidentified in the USAoutsideof Hawaii
andPuertoRico. Note 2--Climatic classification is
according to the Koppensystem as modified by
GeigerandPohl(5).
dents answer the question: "If you wanted to see a
prime example of (
), where, in the United
States, would you go and why?"
Myexperiences indicate that this graphic presentation
provides the student with a basic understanding of both
the nature of Soil Taxonomy and the distributions of
physical environmental factors.
Youngand ImmatureSoils and Vertisols
The method assumes a time base of zero with mixed
mineralogical material exposed to the pedogenic factors
of climate, life, parent material, relief, and time. Soon
an Entisol develops, a soil of recent origin with minimal
horizon development. The geographer must be careful
in applying numbers to the age of Entisols. Some, due
to restrictive environmental conditions precluding soil
development, are quite old chronologically yet very
young pedologically.
AnInceptisol is a soil with just the inception of horizon development. Inceptisols are still immature soils.
Since both of these soil orders are immature or
"azonal", they are not tied to a particular "natural region." Entisols are most commonin depositionally active areas such as flood plains. Inceptisols are more
widely scattered but commonin the tundra region.
The Vertisols are a difficult order to place in this
framework. In general terms, they have a high content
of swelling clay which occurs in areas of wet and dry
seasons and which displays noticeable cracking during
the dry season. They are primarily related to parent material of shallow marine depositional environments.
They must have at least one dry period per year for the
characteristic
cracks to develop. However, the dry
period may not be extensive enough to be displayed in
the KoppenSystem, as in the case of the Vertisols of the
Houston, Texas, area. Under natural conditions most
were covered by grasslands, but important areas of forest, woodland, and occasional desert shrub also existed.
The next step is to introduce regional patterns of
vegetation and climate for the United States. Vegetation, climate, and soil are similar in that all three
gradually change along environmental gradients. The
intent here is to associate all three of these elements
using moisture supplying capacity as the environmental
gradient. The horizontal positioning within Fig. 1 reflects higher levels of moisture supplying capacity on the
left, and low levels on the right. Moisture supplying
capacity includes precipitation regime, soil water-holding capacity, and temperature controls of plant-available moisture.
The first order, Histosol, has the greatest amountof
moisture available for plants (Organic soils are classified in the Histosol order.) They exhibit a poor sense of
regionality.
Their primary development control is a
waterlogged condition due to impaired drainage. Their
distribution is spotty, but associated with either glacially
deranged drainage patterns or with extremely flat
coastal areas. Histosols do not possess an associated climate. Specific local examples are found in southern
Louisiana, the California delta, the glaciated upper
Midwest, and NewEngland bogs.
The next order, Alfisol, also has a disjunct distribution but with a higher degree of spatial organization
than the previous orders. Within the USA, Alfisols are
found north and west of Ultisols, east of Mollisols and
south of Spodosols in the area of Cf climate (mesothermal, humid) (Fig. 2). Vegetatively, this area
bordered by the temperate evergreen forest on the
south, the grasslands on the west, and the coniferous
forest on the north. In other words, Alfisols are intermediate to the idealized pedogenic processes of
podzolization, calcification, and laterization which have
been used to reflect the interactions of climate, parent
material, and vegetation to form the soils in natural regions as used in the 1938 system. Alfisols tend to be
fertile soils with B horizons enriched in iron and aluminumand illuviated clay, although the concentrations are
still lower than in either Ultisols or Oxisols. Alfisols
have a secondary spatial concentration in the areas of
Cs climates (Mediterranean). Here they occupy an intermediate position between the Spodosols of the mountain slopes and the Spodosols of the sandy coastal environment. In some areas, such as southern California,
Alfisols are relics of past wetter climates.
Spodosols appear to the right of the Alfisols on the
chart. These soils also follow the "natural region" concept espoused in the 1938 system. The podzols of the
1938 scheme are the Spodosols of Soil Taxonomy.
Regionally they relate to D climates (micro-thermal or
snowclimates) and the coniferous forest (Taiga) of
northern hemisphere. Spodosol areas may or may not
14
JOURNAL OF AGRONOMIC EDUCATION
he wetter than Alfisols in terms of annual precipitation,
hut low winter temperatures and generally coarser textures reduce the moisture supplying capacity for plant
growth. The brief growing season also reduces bacterial
activity necessary for litter decomposition,resulting in a
minimumof humus, a maximumof litter,
and an acid
soil moisture environment. In other words, Spodosols
occupy the central position of the generalized process of
podzolization which produces the leached white albic A
horizon over the red-to-orange spodic B horizon containing illuviated humus, iron, and aluminum.
Based on decreasing moisture supplying capacity and
increasing regionality, soils of the Mollisol order also
occupy a "natural region." They relate well to the
brunizems, chernozems, and chestnuts of the 1938
system. They also fit the BS/C Koppen climatic boundary (semi-arid/sub-humid)
and the grasslands
(steppe/tall grass prairie) of vegetation classification.
These soils are the "bread baskets of the world." They
are high in natural fertility and have a thick, friable,
black A horizon. The life cycle of grasses continually replenishes the soil reservoir with organic material and nutrients. The decrease in moisture is due to generally
lower precipitation as opposed to the low moisture supplying capacity of the Spodosols.
At the extreme right of the horizontal arrangement
are the Aridisols, or desert soils. Areas of least moisture
availability are patently represented in the xerophytic
vegetation type, i.e., ephemeral grasses and forbs or
cacti. The Aridisols are strongly correlated to the Koppen BW(true desert) and the dry side of the BS (steppe).
Horizons maybe well developed; but if this is the case,
it is almost always the result of paleoclimatic regimes.
With present-day climates, precipitation tends to run
off rather than be absorbed into the soil. Dependingon
local topographic controls, the local process may be
salinization. The identification criterion, whenbordering Mollisols, is the low level of organic matter.
Next in line on the chart are the Ultisols which are
intermediate in both process and climate. The climatic
location is the polar edge of the A (tropical) climates
and the equatorial edge of the C (meso-thermal) climates. Apparently, warm temperatures and ample precipitation are required for development of Ultisols. In
terms of process, they are beyond a pure process of
podzolization and not yet to a pure laterization. This is
reflected in manyUltisols being previously classified as
Red Yellow Podzols. More clay movementand leaching
of bases also differentiate Ultisols from Alfisols. These
soils dominate the southeastern United States. Vegetation includes a mix of temperate evergreen forests to deciduous tropical forests which are adapted to low fertility levels. In the acidic and relatively infertile Ultisols, a
high percentage of nutrients is maintained in the
biomass by an effective root network of the trees which
capture and recycle the bases as they are released from
either the soil or the vegetative litter. The primary factor
that distinguishes the Ultisols from their spatially sur-
rounding orders is the low base saturation of the Ultisol
order.
In manycases in the southeastern United States the
break betweenAlfisols and Ultisols is related to lithology. The coarser materials develop Ultisols while the
finer grained lithology tends toward Alfisols. Figure 1
reflects this distinction in the broken line betweenAlfisols and Ultisols.
Following the sense of "natural regions", the Oxisols
fit the Af (tropical, wet) climate and the tropical rain
forest areas. As a group, these are the oldest soils in
terms of weathering. Age, coupled with the high levels
of chemical and biological activity, has produced an essentially featureless soil without distinct horizons primarily containing iron and aluminum oxides as opposed
to silicates in the other orders. Fertility levels are very
low; an agricultural use demandshigh fertilizer inputs
using advanced mechanization. Commonto these soils
is plinthite or laterite in the strictest definition. This is
an iron-rich clay and quartz mixture capable of irreversible hardening upon exposure to the atmosphere.
Soils belonging to this order have not been found in the
continental U.S.
The Student and the Presentation
Student response to the above described presentation
has been favorable. Difficulties often encountered in
teaching the nomenclature of Soil Taxonomy are reduced. In fact, experience with this type of presentation
has produced very few of the caustic commentsdemeaning to Soil Taxonomy,or its forerunner the 7th Approximation, which can be found in the literature or heard in
conversations with non-pedologists around the country.
At the introductory level, this scheme blends traditional ideas of soils and "natural regions" with the
quantitative methodology of Soil Taxonomy. The quantitative aspects are generalized in a set of qualitative
statements
combining
Soil Taxonomy, Koppen
climates, and vegetation types without destroying the essence of any component parts. For the student, the result is a single-page flow chart of environmental relationships at a level of detail selected by the instructor
(Fig. 1).
The presentation of the flow chart and map is concluded with the complete taxonomic classification of a
local soil. At Texas A&M
University, I use the Lufkin
series, which is classified as a "fine, montmorillonitic,
thermic Vertic Albaqualf" at the family placement
(Lufkin, established series description, 1979, SCSstaff).
The student is then led through this classification.
The term "fine" designates clayey texture (35 to 590/o
<2 microns) in the control section, 25 cm to 100 cm
depth. The dominant clay mineral is montmorillonite
(> 50°70). The mean annual soil temperature is between
15 and 22 C (59 to 72 F), or thermic. A Vertic Albaqualf
is an Alfisol with an aquic moisture regime and usually
requires artificial drainage. It also has a light colored
MILFORD AND MCBEE: PROFESSIONAL GRADUATE STUDIES
(albic) surface horizon. The Vertic subgroup adjective
indicates the clay-rich subsurface horizon has appreciable shrink-swell characteristics producing soil movement but does not fit the definition for a Vertisol.
By explaining that the formative elements and their
definitions are provided in most introductory texts, I try
to convey the idea that much can be learned about a soil
and its environment from its taxonomic classification
even though the name looks and sounds formidable.
The approach described in this paper has been presented at several different institutions. When students
who have been exposed to this system appear in upperdivision classes in other disciplines, instructors have
noted improvement in the students' ability to deal with
the interactions of soil, vegetation, climate, and geology. The system also displays a spatial arrangement
which is lost in the details of many regional or continental soil maps.
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