Pedology: The Science of Soil Development

1. 2
Pedology: The Science
of Soil Development
WHA T PEDOLOGISTS DO
The term pedology is derived from the Russian pedologiya, which
itself comes from the Greek words
1f'f06p (soil) and M'Yo~ (speech).
Thus, at its inception, pedology was a discourse
on soil. In modern
terminology, pedology
is the branch of soil science that integrates and
quantifies the formation, morphology, and classification
of soils as
natural landscape bodies. Pedology
is foremost an integrative discipline
that seeks to elucidate developmental processes governing the observed
properties and distribution patterns of soils worldwide (collectively
termed the
pedosphere) in
relation to landform evolu­
tion, biogeochemical envi­
ronments, and organism
habitats. Pedologists study
both the mechanisms and the
magnitudes
of spatial and
temporal variability
in soil
features ranging across scales
from microscopic to global
and from seasonal to epoch­
FACTORS OF SOIL FORMATION
al. They look for the systematic and random sources of soil variability
as a basis for broad generalizations about how their observations may
be interrelated at every level of resolution. Pedologists examine soil as
a natural porous material at the surface of the earth, without primary
focus
on its many practical uses, either for engineering purposes or as
a medium in which to grow plants. The basic understanding gained
from their investigations, however,
is applied widely by others to solve
problems
of societal concern in commerce, agriculture, and natural
resource management.
Soil at the landscape scale develops its unique properties under
mutually interacting biotic, climatic, topographic, geologic, and
temporal influences.
By contrast, at the microscopic scale (micrometers
9

2. 10 OPPORTUNITIES IN SOIL SCIENCE
to centimeters), the soil "architecture" is governed by relationships
among porosity, the zonation
of inorganic and organic constituents, and
the structures
of aggregates (heterogeneous masses of solid particles
bound together). Water and chemical movement or retention, mineral
synthesis or weathering, plant root environments, and microorganism
habitats all are influenced strongly by microscale soil architecture.
Pedologists are foremost among the basic soil scientists who help to
develop integrated-system models to scale knowledge from small
samples up to the global pedosphere. Through their knowledge
of the
sources and magnitudes
of soil variability, they serve a pivotal role in
broadening the terrestrial information base via an integrative, hierarchi­
cal approach. Basic pedological research
is directed also at reconstruct­
ing past earth surface environments, integrating natural resource
databases, and even extending soil development concepts to extraterres­
trial systems.
SOILS IN
THE EARTH SYSTEM
We have noted that soils are part
of a landscape continuum that
varies in space and time. Some
of this variability is not easily ex­
plained, but many
of the gradational features are understood as
consequences of interdependent variations in the geologic, climatic,
topographic, and biotic factors that affect soil formation through time.
A major challenge
is to structure basic research so that findings for a
soil sample at one location can be extrapolated to
an appropriate popula­
tion
of soils within the continuum as a whole. This kind of understand­
ing
of soils as a component of earth systems science is made easier if
we think in terms of organizational hierarchies. The objects of interest
at anyone level
of organization provide the environment for objects at
the next lower level, and are themselves a component
of the objects at
the next higher level. This approach can be used to systematize
landscapes, to understand the interactions
of their components with the
oceans or the atmosphere, and to monitor the impacts
of human activi­
ties, such
as agriculture or manufacturing.
The highest level
of organization, the pedosphere, results from
large-scale variations of climate and vegetation. The next level
is
regional physiography, with specific attributes of topography, water
distribution, climate, and geology that have corresponding regional
influences
on soil-forming processes. Within a physiographic region,
local, topographically related assemblages
of soils, called topo-

3. CHAPTER 2: PEDOLOGY
sequences, are the
components at the next
lower level
of organi­
zation. They form in
response to local
changes in soil drain­
age, vegetation, parent
material ,microclimate,
and time to yield a
characteristic hillslope
relationship. Soils with
similar properties are
comprised in a
soil
series. They are identi­
fied on the basis
of the
characteristics
of their
pedons, which are
three-dimens ional
bodies of soil large
enough to encompass
soil horizons and their
microscopic variabili-
*~
Pedosphere
Regional
physiography
Toposequence
I
~ Mineral structure
~~ (molecular)
Pedon
11
ty. The soil profile is an arrangement of soil horizons that represents
the results
of soil-forming processes with time. Horizons are the
components
of the soil profile. They, in turn, are composed of
aggregates that provide the variety in the size, shape, and arrangement
of pores that determines the aeration, water retention, and drainage
of
soils. At the next lower level of organization, we can identify complexes
of minerals, organic matter, and microbial cells that make up soil
aggregates. Below this scale, we find the level
of molecular structure.
The methods used to study these hierarchical levels of organization
vary with the levels themselves. Objects at small scales may be studied
with spectroscopy and microscopy to infer their form and internal
spatial arrangement (for more about this, see Chapters 4 and 5). Objects
...... .---------Hierarchical levels -----------:l.~
Remote sensing Visual
... .. ..
Microscopy
Crystal
Structure
Spectroscopy
.....

4. 12 OPPORTUNITIES IN SOIL SCIENCE
at the mesoscale (kilometers) can be viewed directly at a single location
or
be seen more comprehensively by using satellite-based observation.
Data collated from satellites in space allow pedologists to view a large
part
of our planet, but the image is actually a composite of data
representative (at a minimum)
of only a few square meters of land
surface (more
on this in Chapter 5).
Everyone recognizes that our planet constitutes one large system
of
interconnected components, and that no single part -oceans, atmo­
sphere, organisms, or soils -can be understood completely by treating
it in isolation. Pedologists have special skills to enhance the integration
of our knowledge of terrestrial processes and to help predict, alter, or
alleviate future environmental changes. As they become more involved
in both global and regional change issues, they help to generalize site­
specific information (e.g
., from a small sample, where plant roots, soil,
and microorganisms interact biochemically) to global scales, where
"greenhouse gases" that may have originated from interactions in soil
can adversely affect the climate
of our planet (see Chapter 3). By
examining a wider range of environmental variability than is usually
considered
in land management decisions, we can obtain a better
understanding of how soil-landscape systems function and interact.
Management decisions then can be made with greater sensitivity to
environmental quality, exploration
of land-use options, and public
participation. (See page
13.)
These contributions
of pedologists can be brought into earth systems
science in an especially powerful and exciting way through geographic
information systems (GIS) analysis. This computer-based technology
offers an approach to modeling soil-forming processes, soil properties,
and landscapes from geographic databases that simulate the five soil­
forming factors. The components
of GIS include data collection, storage
and retrieval, analysis, and display. Data collection entails using line
digitizers, scanners,
and other computer
tools to capture and
convert information
from maps, sensors,
and direct field
observations into a
digital form that
is
compatible with
.GIS. Once data
have been so cap­
tured and
stored,

5. CHAPTER1: PEDOLOGY
PEDOTECHNOlOGY:
PEDOLOGY APPLIED TO
SOCIETAL NEEDS .
Pedologists usually
study natural soils that are influenced
relatively little
by human beingsexcept fonraditionaLa~ricultur ·
al or silviculturalpractices. Their studies have led to broad
concepts
of ~oil ·forming processes and factors of soH formation
to explain hoiN soils have come into existence and dev~loped
through time. But today, pedologists are fully aware that
humans have· major effects on the genesis of soils and are
becoming more
important in controlling processes that make
soils change .. SoCiety
at large is concerned that human effects
on soils· be environmentally acceptable and aesthetically
pleasing. Since pedologists
know how soil formation occurs,
they should be able to predict the influence of human actions on
soil. Indeed, pedologists have an obligation
to make such
predictions
to assure a continued abundance of the soil resource
in the future. .
. Pedology has coined a
nar.ne for human impact on the
making or modifying of soil which is analogous to the familiar
termbiotechn%gy, defined· a~ follows: ..
Pedotechn%gy. Deliberate or planned human involve­
ment in the genesis of soils, by assessing and selecting
the pool.of materials from which soils are to be created,
or by choosing soil manipulation procedures or amend­
ments
to promote the formation of soil for specifi~
purposes.
A major
goal of pedotechnology is to avoid undesirable
environmental consequences
from an ignorance Of basic
pedology when the manipulation of earth surface materials
occurs in. agriculture·, .
waste disposal, mining .. operations,
construction of buildings and highways, dredging. operations,
etc. Whenever there is large-scale land manipulation,
new
landscapes are· needed as well as new soils. For example, with
current societal interest in the preservation of wetlands, there
is much discussion
of how to create new wetlands to improve
water quality, to enhance wildlife habitat, or simply to replace
wetlands
that have been destroyed because of construction or
(Continued on next page)
',' ...... .
. .. ... . . .
13

6. 14 OPPORTUNITIES IN SOIL SCIENCE
(continued from previous page)
agricultural practices. Pedologists, enQineers, and ecologists
working together can learn how to make new wetlands.There
a're excellent opportunities for basic pedological' research, with
the results' then applied to improving designs for wetlands;
waste-disposal areas, or biological habitats' accOrding
to
pedological. principles.
How To Make A Wetland?
Interdisciplinary collaboration is essential to the success of
pedotechnology. Only when field-, laboratory-, and modeling­
oriented scientists cooperate
to identify appropriate avenues of
research will the right types of data be gathered andrele.vant
. mathematical models be generated. Some
of the projects that
tEl"ams of scientists may work on are:
• field studies in
whichthe applicability. of basic pedology'
and ecology
to land manipulation is tested during the
construction ·
of new landscapes;
• examination
of the taxonomy ~base .d · transect.studies
currently performed
by some soil survey parties in an
effort to characterize the map units employed' in the
National Cooperative Soil Survey
(a ~once -over" inven­
to~y of soils in the United States);
• . develOpment
of new s;:lmpling techniques and computer
software for data collection, statistical analysis, and
"
efficient selection of field sampling strategies.

7. CHAPTER 2: PEDOLOGY
GIS software is
used to study the
relationships be­
tween data layers.
Soil data and digital
elevation models
can be combined in
GIS with hydrologic
and landscape
mod­
els to generate
either visual dis­
plays or statistical
information con­
15
cerning the complex spatial relationships among a variety of soil
variables. For example, soil-landscape relationships can
be modeled
and landscape processes simulated using digital geographic databases,
with the results displayed
on color video monitors showing three­
dimensional landscape images. Satellite-based Global Positioning
Systems can facilitate the rapid geodetic referencing
of field sampling
sites and remote-sensing products, thus paving the way for the
generation
of detailed digital elevation models in relation to GIS maps.
In agriculture, this innovation will allow more efficient placement
of
fertilizers based on precise soil management maps derived from a field­
scale GIS. Other examples include large-scale (Le., the size
of a state)
maps that show areas that are vulnerable to erosion, susceptible to the
leaching
of chemicals to groundwater supplies, or suitable for the
production
of specific new crops. (See page 16.)
The application
of GIS to many kinds of land-use questions will be
facilitated
in the next decade because soil maps of all farmlands and
most forest and grasslands
in the United States will be completed.
Armed with this new information, GIS will provide
us with basic
knowledge about environmental processes at the field, watershed, and
global scales. Building
on this knowledge, we can combine interactive
"expert systems" that create friendly, easily understood means
of
presenting soil information to technical personnel and the public with
predictive models and GIS software.
RECONSTRUCTING PAST EARTH ENVIRONMENTS
Paleopedology is the study of soils, (termed paleosols), that have
formed
on landscapes of the past. These soils are valuable to the study

8. 16 OPPORTUNITIES IN SOIL SCIENCE
.AGRICUl TURE ON SOilS; NOT FiElDS·
Modern agriculture
must optimize among c6riflictingissues
of environmental protection, sustainabilitY,and profitability. No
longer can we indulge past management practicf;ls emphasizing
maximum yield
with little consideration for I~rger ~cosystem
effects. Conventional agriculture .. manages ... croplandCls a
spatially homogeneous soil body, using,
for example, one
fertilizer or one herbicide application rate across an entire field.
Soils,
however, exhibit spatial variability .as a result of differing
parent material, topography, native vegetation, microclimate,
age, management practices, and cultural history.
As a result,
very significant crop yield variability
(e.g., from about 60 to 160
bushels per acre for corn) can occur in a typical field under quite
uniform fertilizer and pesticide application. Uniform-field
management
thus creates inefficiencies by overamen'ding some
areas and underamending others, thereby unnecessarily
increasing
total material inputs and energy consumption,
. reducing farm profitability, and possibly
contributing to the
pollution of our water supplies.
New technologies, such as GIS, remote sensing techniques;
simulationmodeli.ng, and precise application of cheniic?ls, offer
farmers new opportunities for managing soils according to their
inherent potential as media for pl~nt growth. The conGElpt novv
emerging is
to optimize field management, on a real-time basis,
200
o
UNIFORM
MANAGEMENT
MAKES
CORN YIELDS
VARIABLE
by varying the agricultural operation as farm equipment travers­
.
es a field. With a microcomputer, a digitized map of soil
conditions, and a guidance system, a chemical spreader can
alter fertilizer or herbicide applications as a
tractor moves across
fCOfltinued on next page)·

9. CHAPTER 2: PEDOLOGY
(continued from previouS page)
., ." . . .
a field. Recently this approach has been used in the preplant
application
of anhydrous ammonia (NH 3) fertilizer, with a·laptop
microcomputer controlling the rate of NH3 addition in response
to soil conditions, as portrayed by a digital map created from'
soil surveys, aerial photographs, grid-design soil
s~ 'rnpling and
geostatistical analysis, historic crop yields, and
other 'relevant
data. A recent
study has compared soil-specific and:co.nvention­
al agricultural management in southwestern Minnesota. Thene!..
profit from the soil-specific mana' gement was significantly
. higher than the net return from conventional0hole-field
management. .
Farming
by cOmputer-assisted, soil-specific management is'
not limited to spreading preplant fertilizers and chemicals. This
approach can also
be adapted to other farm operations, such as
sidedres$ application of fertilizers or pesticides, seed variety and
population control, tillage, and irrigation. The
concept of
agriculture on soils, not fields offers a significant breakthrough
in farm management. Although the
technology to realize such
precision agriculture is already available, there is still the need
for improved basic understanding of soil-landscape relationships
to create better databases for the soil mapping unit. When this
information is applied to soil-landscape units as they occur in
nature, some
of the current concerns in the agricultural commu­
nity about profitability and environmental protection can be
more easily reconciled
with societal pressures for less pollution.
17
of biological and physicochemical evolution on Earth. Like modem
soils, paleosols are the products
of the conditions under which they
formed and, depending on their degree
of preservation, they can serve
as sensitive indicators
of former soil environments. Paleosols are
abundant
in the geological record and types as old as 3100 million years
have been identified. The study
of paleosols that formed prior to 1.5
million years ago is in its infancy, but it has already offered some
fascinating avenues
of investigation that address major issues in the
history
of our planet: the possible origin of life in soil instead of in an
aquatic environment, with clay minerals serving as essential templates
for the synthesis
of biological molecules; the evolution of the atmo­
sphere as revealed in paleosols formed from about 3000 to 1000 million
years ago, with particular focus on the chemistry and mineralogy of

10. 18 OPPORTUNITIES IN SOIL SCIENCE
iron (Fe) for assessing primordial levels of oxygen (0); and the long­
term control and dynamics
of the global environment, either by living
organisms defining the material conditions
of survival or by the
geochemistry
of those conditions operating independently of organismal
control.
The antiquity
of large plants and animals on land, their subsequent
evolution, and the corresponding diversification
of soils also present
challenging and fruitful areas for paleopedology research. Significant
contributions have been made to the interpretation
of geologic fossil
records by the provision
of detailed stratigraphic control, discrimination
between
in situ and transported fossils, and independent reconstructions
that have broader utility than can be developed from examining fossils
alone, which often are spatially and temporally constrained. Paleoped­
ology also can provide a link between geological and ecological time
scales that describe both increases and losses
of biological diversifica­
tion.
Paleopedology has provided unique insights into soil-landscape
} modern soil
} poleosol
in loess
deposits
dynamics under climatic
change. The transition
from humid-subhumid
conditions with a well­
established vegetative
cover to drier conditions
with a sparse vegetative
cover
is marked typically
by an increase in land­
scape instability, mass
movement
of unconsoli­
dated material, and
considerable physical
disruption of soil. Recent
studies in regions not
} poleosol in Coostol influenced by glaciation
Ploins deposits
during the past 1.5
million years (e. g., the
arid southwestern portion
of the United States) show that these regions
have experienced periods
of higher precipitation in the past than occur
today. The past wet periods produced increased mineral weathering,
biomass production, and soil development relative to modem condi­
tions, in which the paucity
of vegetation and periodic high-intensity
rainstorms contribute to landscape instability and considerable redistri­
butioll
of materials on the landscape.

11. CHAPTER 2: PEDOLOGY 19
The impact of both prehistoric and historic cultures on soil and
landscape
is well-documented and, in some instances, has been
explained in detail through paleopedological studies. The rise and fall
of civilizations (e.g., the Sumerian and Phoenician) often has been
driven by their utilization and ultimate degradation
of soil and water
resources. Soil acidification during prehistoric times in western Europe
may be the result
of deforestation instead of climatic change. These
kinds
of global issues can be clarified through the detailed examination
of paleosols beneath dated archaeological materials. They raise
challenging questions concerning the relatively slow but deleterious
processes by which certain present-day land-use practices may adversely
affect soil and water resources over large areas. Indeed, many
of our
contemporary environmental problems have ominous parallels
in the
past.
The use
of fossil fuels ultimately may produce global changes in
climate, which, in tum, may lead to a rising sea level that will impact
coastal land areas. Paleosols can yield valuable information about
prehistoric climates and their relationship to global changes in sea level.
The present-day high position
of sea level worldwide is unprecedented,
except for during the Sangamon period, approximately 122,000 yr ago,
when it was 8 m (26
ft) higher than at present. Detailed studies of
Dating Metbod
Radioisotopic
Microscopicl Cbemical! Geomorphic Correlation
Spectroscopic Mineralogical
Carbon Fission Track Soil Analyses Soil Development Lithostratigrapbic
Potassiwn Luminescence
Rock Varnish Landscape Volcanic Ash
Uranium Elecuon Spin Obsidian Evolution Fossils
Resonance
Hydration Weathering Rinds Artifacts
Sedimentation Stable Isotopes
Rates
Type
01 Soil Age
Numerical Age ---------......................... .
Calibrated Age
................................. -----------........................... .
Relative Age
............................... ---------....................... .
Correlated Age
.............................. --------

12. 20 OPPORTUNITIES IN SOIL SCIENCE
paleosols give us clear evidence of climatic change, and we expect that
the study
of coastal soils developed during and since the Sangamon
period will help
us to understand and predict future changes in global
climate.
Advances in paleopedology will rely on the development
of new
techniques that better defme, resolve, and measure paleosol attributes
(e.g., stable isotope methods). More precise estimates
of the geological
age and period
of formation of paleosols are essential for improving the
value
of these soils as "chronometers." Models of soil formation rates
are now developing broad utility. But a more reliable distinction
between features developed during soil formation and those acquired
afterward
is essential for accurate model interpretations. Analytical
techniques that combine microscopy with
in situ chemical analysis offer
great potential for resolving this dilemma, which generally becomes
more serious with increasing age
of a paleosol.
ISOTOPIC SIGNATURES IN SOIL DEVELOPMENT
Isotopes are atoms whose electronic structure (and, therefore,
chemical reactivity)
is the same, but whose nuclei differ in the number
of neutrons (stable neutral particles) present. Stable isotopes are
isotopes that do not undergo radioactive decay processes. They exist for
a number
of chemical elements commonly found in soil and water (e.g.,
hydrogen [H], carbon
[C], nitrogen [N], 0, and sulfur [S]), and the
natural rarity of some
of them makes their use as environmental
markers or "signatures"
of pedological processes of keen interest. Both
equilibrium and kinetic factors influence the distribution
of stable
isotopes among the phases
of matter and among different species of
living organisms, which means that measurements of the ratios of the
concentrations of a pair
of stable isotopes for one or more chemical
elements can lead to unique information about soil phenomena. This
type
of measurement has become possible with the development of
sophisticated mass spectrometry and attendant sample-preparation
methods. The application
of stable-isotope studies to pedological
research has expanded greatly
as awareness of their utility has increased
among basic soil scientists.
Two approaches utilizing stable isotopes have been used in basic
pedological research. The first involves the measurement
of naturally
occurring isotopes
in soil to infer its mode of origin or past environ­
ment
of formation. The second involves the experimental use of
substances which have been enriched or depleted in a particular isotope

13. CHAPTER2: PEDOLOGY 21
(e.g., the nitrogen isotope, ISN). When introduced to soil, these
substances serve
as tracers to identify transformations or transport of
components during the course of a soil process (e.g., N cycling).
As an example
of the first approach, we can tum to the stable C
and 0 isotopes in the mineral calcium carbonate
(CaC03). Calcium (Ca)
and magnesium (Mg) carbonates
in soil are sensitive indicators of both
water movement and soil formation, because they readily undergo
dissolution and precipitation reactions. The isotopic composition
of
CaC03 formed by soil development (pedogenic carbonate) is known to
differ from that
of CaC03 formed in geologic materials. Thus we can
use isotope measurements to distinguish these two kinds
of CaC03, a
task that would be difficult, if not impossible, using chemical or
microscopic techniques. Carbon and 0 isotope studies
of CaC03 have
added greatly to our
CARBON ISOTOPE RATIOS ARE SIGNATURES OF understanding of the
VEGETATION,SOIL,AND GEOLOGIC ENVIRONMENTS rate and extent of
C3PLANTS gpf1=';fri'l!
C.PLANTS
SOIL HUMUS
PEDOGENIC CARBONATE
E!\;%fr;;m;iiiif1
ANCIENT MARINE CARBONATE
I
-40
I
-30
I
-20
I
-10
I
o
S 13C(%0 relative to PDB)
I
+10
mineral weathering in
soils formed on calcar­
eous (CaC03-rich)
parent materials. Re­
cent research has
shown that a large
proportion of CaC03 in
soils formed
on lime­
stone and related par-
ent materials is not
inherited, but actually
is a weathering product of the original rock.
Stable C and 0 isotopes are also used to detect anthropogenic weather­
ing
of soil minerals by irrigation waters. We have learned from this
approach that a significant portion (up to
40%) of the total CaC03 in
irrigated soils can dissolve and reprecipitate during < 50 yr of water
application. Similar to
CaC03, during the weathering of silicate (silicon
[Si]-and O-containing) minerals through their reaction with percolating
natural waters, 0 isotopic differences can be detected between the
parent and newly precipitated minerals. These isotopic differences help
us study both the mechanism and the degree of silicate weathering
under either field or laboratory conditions. Oxygen isotope ratios
in
silicate minerals are useful also for tracing their geologic sources.
Carbon and 0 isotope ratios
in soil are strongly influenced by
environmental conditions, and many studies have been done to correlate
modem temperature to 0 isotope ratios
in pedogenic carbonates. Other
recent work has shown that there
is a relationship between the density

14. 22 OPPORTUNITIES IN SOIL SCIENCE
and photosynthetic pathway (C 3 vs. C 4) of plants and the C isotope
ratios
of pedogenic carbonates. These results, in tum, have led to basic
research in arid and semiarid regions
of the world to establish the
relationship among climate, vegetation, and the C and 0 isotope ratios
of pedogenic carbonate. We have now learned that C isotope ratios are
influenced both by the photosynthetic pathway and by the density and
biological activity
of the vegetation, and that 0 isotope ratios of
carbonate are related to that of rainfall, which itself depends on regional
temperature. The door
is then wide open for using stable isotopes in
paleosols to understand aspects
of past climates. A fine example of this
kind
of application is a recent study of paleosols in the famous Olduvai
Gorge in Tanzania. This study demonstrated a trend
of increasing
aridity and higher temperature during the past million years, based
simply on changes in C and 0 isotope ratios in soil carbonate.
Stable-isotope studies can be used to add to the information already
provided by other approaches in research on C and N cycling in soils
(see Chapter 3 for more about C and N cycling). One
of the most
important repositories
of these two chemical elements is in humus, the
non-biomass soil organic matter. Changing patterns
of land use in the
United States necessitate a clearer understanding
of the role of humus
within soil profiles, particularly in subsurface horizons. We need
consider only the problem
of land disposal of hazardous wastes to see
the relevance
of basic knowledge about the movement and stability of
such toxic materials and their incorporation into soil humus. Radioac­
tive isotopes
of C often have been used to investigate the degradation
and synthesis
of humus and to shed light on its formation and retention
in surface soils. Radiocarbon dating techniques have allowed
us to
estimate the residence time
of organic C in soil profiles and the age of
humus in paleosols. We have learned that part of soil humus "turns
over" quite rapidly,
on the order of years to decades, while more
resistant humus fractions have residence times on the order
of hundreds
to thousands
of years. Because distinct stable C isotope ratios have now
been recognized
in the C 3 vs. C 4 groups of plants, it is also possible to
measure these ratios
in soil humus to determine the rate at which the
decomposition products
of plant biomass are incorporated into the soil
humus pool. Also, differences in stable N isotope discrimination among
biological and chemical transformations
in the N cycle make it possible
for us to determine, for example, the source
of N in nitrate-polluted
groundwater (whether from applied fertilizer or degrading soil humus),
or the amount
of soil N that has been introduced via fixation by
legumes.

15. CHAPTER2: PEDOLOGY 23
EXTRATERRESTRIAL PEDOLOGY
We humans (at least the more adventuresome among us!) often have
dreamed about visiting and even living on the Moon, Mars, and other
planetary bodies. Now we are entering an era where such dreams can
become reality. The National Aeronautics and Space Administration
(NASA)
is considering several planetary missions as a part of the
human exploration
of the inner solar system. These missions include
expeditions to establish the first human presence on another planet
(Mars), establishment
of lunar outposts to conduct extraterrestrial
science, and evolutionary expansion to establish a self-sufficient human
presence beyond our planet. Evolutionary expansion will be a step-by­
step program. The first step probably will be the establishment
of a
lunar outpost that will lead to a self-sufficient human colony. The lunar
outpost then will serve to prepare
us for the human exploration of
Mars. Prior to human exploration, a number of robotic missions will
occur
on Mars to conduct basic science (e. g., the Mars Rover/Sample
Return [MRSR] Mission).
These extraterrestrial missions will probably take place in the first
two decades
ofthe 21 st century, so planning for them has to start now.
A vast amount
of science and technology is necessary to ensure their
safety and success, and soil scientists will play an important role. Their
research will include the development
of extraterrestrial "soils" for
plant growth, the study
of soil formation on planetary surfaces, and the
study of past aqueous weathering
on planetary bodies.

16. 24 OPPORTUNITIES IN SOIL SCIENCE
Self-sufficient bases (Le., bases that do not require continuous
resupply from Earth) require the use
of on-site planetary materials. One
scenario proposed
is to use planetary materials as a plant-growth
medium to produce food for the base crew. This possibility opens a new
research area for pedologists. Some
of these materials have never been
exposed to water, organic matter, or an
02 atmosphere (e.g., those on
the lunar surface) and it will be necessary to understand their solubility
reactions in a terrestrial environment before they can be used to sustain
plant growth. Because
of our extensive knowledge about the lunar
surface from past missions, we can design experiments to be conducted
in terrestrial laboratories to examine the interaction of simulated lunar
materials and water. We shall also need to understand the factors that
influence the activity, ecology, and population dynamics
of microbes in
extraterrestrial materials. Not only will microbes play an important role
in converting these materials into productive soils, but they will also be
instrumental in the degradation
of waste products as part of a regenera­
tive life-support system on a planetary base.
Aqueous weathering has not played a role in the formation
of the
lunar surface rock. However, a number
of other planetary surfaces have
undergone transformations from aqueous weathering (e.g., Mars,
asteroids, and comets). Space missions are being planned to study some
of these surfaces in detail, including the MRSR and the Comet
Rendezvous Asteroid Fly-by (CRAF) Mission, both
of which may be
conducted
by the end of the decade. The basic science expertise
- -.... - .

17. CHAPTER2: PEDOLOGY 25
required to interpret data from these missions should provide a number
of interdisciplinary research opportunities for pedologists when samples
are returned from space. The planetary-science community will look to
pedologists for help
in defining the soil-forming conditions (e.g., past
climates) that have affected surface development. Doing pedology on
planetary surfaces other than our own also allows
us the opportunity to
examine the soil-forming factors from an extraterrestrial perspective.
For example, because Mars appears to lack biological activity, soil
formation may have to be examined without the influence
of a
biological factor. We
may also have to describe minerals or other solid
phases that are not found on Earth. These types
of research can be very
rewarding because they should not only improve our fundamental
understanding
of soil, but also aid adventuresome people in the
colonization
of extraterrestrial bodies.