Soil Survey & Mapping
Purpose of a Soil Survey
• Determine characteristics of the soils
• Classify the soils according to a standard system of
classification,
• Plot boundaries of the soils on a map, and
• Make predictions about the behavior and best management
of the soils (Interpretations).
Natural vs. Single Purpose Soil Survey
• “soil map”-- shows the distribution of different kinds of soil as
an integration of all properties rather than the distribution of
a single soil property
• From this map, maps showing the distribution of soil
properties, expected response to management, and
suitability for various uses can be constructed
– These maps can also be easily revised without additional field work
• Single-purpose soil inventories can be made at a lower cost
– Can only be used for the single-purpose
– Cannot be used for new use or management system without
additional field work
– Examples: forest productivity, septic tank suitability, agricultural use
Taxonomic Units and Cartographic Units
• A taxonomic unit is a category in an arbitrary system for
classifying individuals (“soil series”)
– An individual must meet a strict set of criteria
• Failure of any one of these criteria excludes it from the taxonomic unit.
• Taxonomic units are strictly arbitrary and do not really exist in nature.
• A cartographic unit is a naturally occurring body that can be
delineated on a map of some form (“soil phase”)
– Cartographic units, in most cases, contain individuals from several
taxonomic units
– One or more soils may be dominant in the landscape unit delineated
on the map, but there are always other soils present as inclusions
– “Phase” mapping unit specifies dominant series, surface texture, and
slope gradient (“Cecil sandy loam, 2-6% slopes”)
SERIES CONCEPT: defined by NRCS based on soil/horizon properties as
specified in “Official Series Description” (OSD)
For “CECIL” series:
RANGE IN CHARACTERISTICS: The Bt horizon is at least 24 to 50 inches thick and extends to 40 inches or more. Depth
to bedrock ranges from 6 to 10 feet or more. The soil ranges from very strongly acid to moderately acid in the A horizons
and is strongly acid or very strongly acid in the B and C horizons. Limed soils are typically moderately acid or slightly acid
in the upper part. Content of coarse fragments range from 0 to 35 percent by volume in the A horizon and 0 to 10 percent
by volume in the Bt horizon. Fragments are dominantly gravel or cobble in size. Most pedons have few to common flakes of
mica in the Bt horizon and few to many flakes of mica in the BC and C horizons.
The A or Ap horizon has hue of 2.5YR to 10YR, value of 3 to 5, and chroma of 2 to 8. A horizons with value of 3 are less
than 6 inches thick. The texture is sandy loam, fine sandy loam, or loam in the fine earth fraction. Eroded phases are sandy
clay loam, or clay loam in the fine earth fraction.
The E horizon, where present, has hue of 7.5YR or 10YR, value of 4 to 6, and chroma of 3 to 8. It is sandy loam, fine sandy
loam, or loam in the fine-earth fraction.
Etc…
COMPETING SERIES: These are the Appling, Bethlehem, Georgeville, Herndon, Madison, Nanford, Nankin, Pacolet, Saw,
Tarrus, and Wedowee series in the same family. Those in closely related families are the Cataula, Chestatee, Cullen,
Hulett, Lloyd, Mayodan, and Mecklenburg series. Appling soils have dominant hue of 7.5YR or yellower or where hue is
5YR it has evident patterns of mottling in a subhorizon of the Bt or BC horizon. Bethlehem soils have soft bedrock at depths
of 20 to 40 inches. Cataula soils have a perched water table at 2 to 4 feet, Chestatee soils contain more than 15 percent,
by volume, coarse fragments throughout. Cullen soils have more clay in the Bt horizon. Mayodan and Mecklenburg soils
have mixed mineralogy and in addition, Mayodan soils formed in Triassic age sediments and Mecklenburg soils formed
from basic diabase parent material. Georgeville, Herndon, Nanford, and Tarrus soils formed in Carolina slate and contain
more than 30 percent silt. Hulett, Nankin, and Wedowee soils have a Bt horizon with hue of 5YR or yellower. In addition,
Nankin soils formed from marine sediments. Lloyd soils have rhodic colors to depths of 40 inches or more. Madison,
Pacolet, and Wedowee soils have thinner argillic horizons. Saw soils have hard bedrock at depths of 20 to 40 inches.
Inclusions
• Soil map units are named for one, two, or more kinds of soil
that are most abundant in the area
– “Madison-Pacolet soils, 25-40% slopes”
• There are almost always inclusions of soils with different
properties than the named soil(s)
– Similar soils - behavior and response to management similar to the
named soils.
– Dissimilar soils - behave and respond to management much
differently than the named soil
• Goal of soil survey is to delineate the soils in a manner that
limits dissimilar soils in map units.
Inclusions
• Inclusions occur in map units for a number of reasons.
– Similar inclusions may be the result of
• incomplete understanding of subtle differences in landscape properties or
• soils may be a continuum over short distances
– Similar inclusions may occur as a choice rather than by chance.
• Too many delineations can be as bad as too few.
• Dissimilar soils occur in map units because of
– poor understanding of soil landscape relationships or,
– map scale
• If the minimum size area that can be shown on the map is 4 ha, an area
of a contrasting soil that is 2 ha cannot be shown
Inclusions
Map Scale
Minimum Size Delineation*
ha
1:1,200
1:5,000
1:12,000
0.005
0.10
0.41
1:24,000
1:62,500
1:250,000
2.3
15.8
252
1:1,000,000
1:5,000,000
4,000
101,000
Soil Survey as a Paradigm Based Science
• A "paradigm" is a broad explanatory structure that provides a
foundation for an entire field of scientific inquiry.
– This enables focus on a narrow range of problems.
– When the paradigm is accepted, there is no need to explain the
meaning of each concept.
• In soil survey, the paradigm is the soil-landscape model.
• The origin of the paradigm is in the state factor concept:
• S = C, PM, R, O, T
• The equation implies that, by looking for changes in one or
more of these factors, boundaries between different bodies
of soil can be accurately located.
Soil Survey as a Paradigm Based Science
• This paradigm requires that one discard the common
concept that soils are a “continuum”.
– There are frequent, often abrupt, discontinuities that can be
discerned by trained observers.
– However: soils often change over a gradient that includes transitional
properties between “end-members” at extremes of the gradient.
– Distinct “soil types” in this case are largely a human construct.
Summary of the Soil-Landscape Paradigm
• Within a soil-landscape unit, the five factors interact in a distinctive
manner which leads to development of the same kind of soil
• The more difference between two adjoining soil-landscape units, the
more abrupt and striking the discontinuity between them
• The more similar two soil-landscape units, the more similar their
associated soils tend to be
• Adjacent areas of different soil-landscape units have a predictable spatial
relationship.
• Once the relationships among soils and landscape units have been
determined, the soil can be inferred by identifying the characteristic soillandscape unit.
• This paradigm makes soil mapping possible because of observable
discontinuities between adjoining soil-landscape units.
Soil-Landscape Model
• Largest weakness is an extreme reliance on learning the
concepts through experience rather than the classroom.
• To the untrained eye, the landscape is nothing but a
backdrop for objects of more interest
• To use the soil-landscape model to recognize and delineate
soil bodies, one must learn to see the landscape as an entity
unto itself.
• Soil scientist must then learn to group these units into a
smaller number of similarity groups called map units.
– This grouping is not made based on all properties of the soillandscape units
• Analogous to recognizing people’s faces
Soil Survey Model
• It is impossible to observe and make measurements on the
entire population of soils in a region.
• Observe soils at selected points in the landscape
• Relate soil properties to landscape features
• Integrate observations into conceptual models used to
predict properties of soils in new areas with similar
vegetation, geology, climate, and landscape characteristics.
• Observations to confirm the model predictions
– Refine the model if the observation reveals a soil different from that
expected.
– Iterative process
Observations
• Numbers and locations depend on the complexity of the
area, experience of the soil surveyor, and validity of the soil
surveyor's model
– Observations are not random - Soils are not distributed randomly
– Observations made at locations expected to best represent a
particular segment of the landscape
• The soil-landscape model is rarely written in textbooks or
technical journals.
– Concepts reside in the minds of experienced soil surveyors
• Few express the concepts verbally
• They can show you, but may not be able to tell you
– "We all see the world through colored glasses. The challenge is to
recognize the color of the glasses we are wearing."
Soil Landscape Model
• Lack of written expression is one reason that soil mapping is
often considered an art rather than a science
– What is called scientific knowledge is first and foremost expressed in
language rather than pictures
• The result of studying and mapping soils is a map rather than written
words.
• Unless you understand the soil-landscape relationships in the area, the
map is nothing but an abstract representation of a spatial distribution.
• Maps are ineffective for conveying the scientific concepts and complex
relationships shown on the imagery.
• "A sensible philosophy controlled by relevant set of concepts
can nearly act as a substitute for genius“ -- LTW