Chapter

4

Soil Mapping Concepts

By Soil Science Division Staff. Revised by Kenneth Scheffe and 

Shawn McVey, USDA-NRCS.

Soil Mapping Process

Soil mapping is the process of delineating natural bodies of 

soils, classifying and grouping the delineated soils into map 

units, and capturing soil property information for interpreting 

and depicting soil spatial distribution on a map. 

The soils and miscellaneous areas (e.g., Rock outcrop) in a survey 

area are in an orderly pattern that is related to the geology, landforms, 

topography, climate, and natural vegetation. Each kind of soil and 

miscellaneous area is associated with a particular kind of landform or 

with a segment of the landform. Soil scientists delineate these repeating 

patterns of landform segments, or natural bodies, on a map. By observing 

the soils and miscellaneous areas in the survey area and relating their 

position to specific segments of the landform, a soil scientist develops 

a concept, or model, of how they formed. Thus, during mapping, these 

models enable the soil scientist to predict with considerable accuracy the 

kind of soil or miscellaneous area on the landscape (Hudson, 1992). 

The repetitive patterns imprinted in soils by the soil-forming factors 

can be observed at scales ranging from continental to microscopic. These 

patterns are the basis for soil identification and mapping at different scales. 

A system of terminology, definitions, and operations can be ascribed to 

the various scales. Hierarchical systems of classes and subclasses are 

established to produce groupings at the different scales.

Commonly, individual soils on the landscape merge into one another 

as their characteristics gradually change. To construct an accurate soil 

map, however, soil scientists must determine the boundaries between the 

soils. Some boundaries are sharp, where soils change over a few meters, 

while others are more gradual. Soil scientists can observe only a limited 

number of pedons. Nevertheless, these observations, supplemented 
236 Chapter 4

by an understanding of the soil-vegetation-landscape relationship, are 

sufficient to verify predictions of the kinds of soil and to determine their 

boundaries.

Soil scientists record the characteristics of the pedons, associated 

plant communities, geology, landforms, and other features that they 

study. They describe the kind and arrangement of soil horizons and their 

color, texture, size and shape of soil aggregates, kind and amount of 

rock fragments, distribution of plant roots, reaction, and other features 

that enable them to classify and identify soils (see chapters 2 and 3 

for details). They describe plant species present (their combinations, 

productivity, and condition) to classify plant communities, correlate 

them to the soils with which they are typically associated, and predict 

their response to management and change. After the soil scientists 

identify and describe the properties of landscape components, or 

natural bodies of soils, the components are correlated to an appropriate 

taxonomic class, which is used for naming map units. Correlation, or 

comparison of individual soils with similar soils in the same taxonomic 

class in other areas, confirms data and helps the staff determine the 

need to assemble additional data. Taxonomic classes are concepts. Each 

taxonomic class has a set of soil characteristics with precisely defined 

limits. The classes are used as a basis for comparison to classify soils 

systematically. Soil Taxonomy, the system of taxonomic classification 

used in the United States, is based mainly on the kind and character of 

soil properties and the arrangement of horizons within the profile (Soil 

Survey Staff, 1999). 

While a soil survey is in progress, samples of some of the soils in 

the area are collected for laboratory analyses. Soil scientists interpret 

the data from these analyses and tests as well as the field observed 

characteristics of the soil properties to determine the range of values for 

key soil properties for each soil. They also use these data to determine 

the expected behavior of the soils under different uses. Soil property data 

is organized and stored in a database, where it is used to generate soil 

interpretations for use and management. Interpretations for all of the 

soils are field tested through observation of the soils in different uses 

and under different levels of management. Special studies to document 

dynamic soil properties that are affected by use and management may 

be conducted (see chapter 9). Data are assembled from other sources as 

well, such as research information and field experience of specialists. 

After survey staff locate and identify the significant natural bodies 

of soil in the survey area, they draw the boundaries of these bodies on a 

map and identify each as a specific map unit by name. Imagery showing 

trees, buildings, roads, and rivers is commonly used as a base map to help 
Soil Survey Manual 237

in locating boundaries accurately. Tonal shades and patterns on aerial 

photographs or digital images are used to indicate potential changes 

in vegetation, drainage conditions, parent materials, and other factors 

affecting surface reflectance. As digital mapping techniques are being 

increasingly integrated into mapping (see chapter 5), additional sources 

of information, such as multispectral bands, digital elevation models, 

and other data layers (such as geology), along with global positioning 

systems (GPS) are used to accurately locate map unit boundaries. 

Although the processes used in digital mapping techniques are different 

from nondigital conventional methods, the principles are the same. 

In the United States, soil surveys vary in scale and in intensity of 

observations. The components of map units are designated by taxa in 

Soil Taxonomy (Soil Survey Staff, 1999) or as miscellaneous areas (i.e., 

nonsoil areas). In naming a map unit, soil taxa names (commonly a soil 

series name) are modified with phase terms (indicating surface texture, 

slope, flooding, stoniness, etc.) to convey information that is either 

more specific than the wider range of properties defined for the series 

(e.g., surface texture) or that represents a property outside of the soil 

itself (e.g., flooding). The phase commonly is a portion of the range of 

properties exhibited by the taxon. For example, a certain soil series may 

have slopes ranging from 3 to more than 60 percent but the map units are 

shown with narrower ranges (e.g., 3 to 8, 8 to 15, 15 to 25) to provide 

information that is useful in managing the soils in the area.

Historically, soil surveys have classified entire polypedons and 

grouped their properties for interpretative output as vector maps (polygons). Some contemporary surveys classify only certain soil properties, 

such as surface rock fragment cover, and output the information as raster 

maps, in which each pixel represents a specific value of the property. 

With these maps, commonly called “soil property maps,” the user can 

decide how to group or aggregate the information for their needs. Strictly 

speaking, maps of individual soil properties are not synonymous with 

soil surveys, which by definition delineate natural soil bodies. 

Soil Mapping and the Scientific Method

Soil mapping uses the scientific method, in which the scientist must: 

(1) develop questions, (2) generate hypotheses that answer those questions, (3) test the hypotheses, and (4) confirm or reject the hypotheses. 

After a tentative delineation of a soil body is drawn on an aerial photo or 

digital image, the soil mapper (step 1) questions what type of soil exists 

within that delineation. Typically, the delineation follows a landscape 

feature, such as a large flood plain or a ridge summit. Based on previous 
238 Chapter 4

knowledge about the soils of the region, the mapper (step 2) develops 

hypotheses, such as the Alpha and/or Beta series occurs within the delineation. The mapper (step 3) tests those hypotheses by augering, backhoe 

trenching, or observing natural exposures and (step 4) confirms or rejects 

each hypothesis. After documenting the results, the mapper returns to 

step 1 (develops questions) and repeats the process for a neighboring 

area. This process allows the soil scientist to map soils efficiently. Rather 

than making a large number of observations on a regular grid pattern 

to discover the kind of soil present, the mapper selects a limited number of strategically located points in the landscape to make observations. 

The observations confirm or reject the previously developed model. The 

mapper essentially is predicting the soil beforehand and only making an 

observation to confirm the prediction, rather than discovering the soil 

only after each observation is made. As long as the model is accurate, 

relatively few observations are required to make an accurate map (Hudson, 1992).

The scientific method is also used when investigating soil genesis. 

Although soil mapping and soil taxonomic classes are based on 

quantifiable properties rather than soil genesis (Smith, 1963), it is 

nevertheless useful for the soil mapper to develop conceptual models 

about soil genesis throughout the mapping process (Arnold, 1965). The 

most useful is the “multiple working hypotheses” method, which is 

based on the premise that when a scientist creates multiple hypotheses 

for an observed feature rather than one hypothesis, they are less likely to 

develop a parental attachment to “their” hypothesis (Chamberlin, 1897). 

Instead, the scientist becomes engaged in finding evidence that disproves 

each of the competing hypotheses. The “working hypothesis” is the one 

that survives. This method of testing multiple hypotheses simultaneously 

not only enhances the quality of conceptual models but also lessens 

antagonistic debates between scientific colleagues (Platt, 1964). 

Soil Maps

Historical Approach

Aerial photographs were used as the mapping base in most soil 

survey areas in the United States during the 20th century. Conventional 

panchromatic (black and white) photography, color photography, and 

infrared photography were used for remote sensing and as base maps for 

the soil survey. Information on the applicability of each type of base map 

and how the older map products were used is covered in the 1993 Soil 

Survey Manual (Soil Survey Division Staff, 1993).
Soil Survey Manual 239

Aerial Photographs

Even in the current digital age, the use of aerial photographs remains 

an effective means of mapping soils in areas where suitable digital 

imagery and data layers or the required skills, resources, or support 

for digital mapping techniques are not available. Chapter 5 covers the 

integration of digital soil mapping techniques in conducting soil surveys. 

Aerial photographs are still a viable mapping base in soil survey. They 

provide important clues about kinds of soil from the shape and color of 

the surface and the vegetation. The relationships between patterns of soil 

and patterns of images on photographs for an area can be determined. 

These relationships can be used to predict the location of soil boundaries 

and the kinds of soil within them.

Aerial photographs using spectral bands not visible to the eye, such 

as color infrared, enable subtle differences in plant communities to be 

observed. Other spectral bands in the infrared are useful in distinguishing 

differences in mineralogy and moisture on the soil surface and also have 

better cloud penetration. These data must be interpreted by relating 

the visual pattern on the photographs to soil characteristics found by 

inspection on the ground. 

Features, such as roads, railroads, buildings, lakes, rivers, and field 

boundaries, and many kinds of vegetation can be recognized on aerial 

photographs and serve as location aids. Cultural features commonly 

are the easiest features to recognize on aerial photos, but they generally 

do not coincide precisely with differences in soils, except in areas with 

significant anthropogenic alteration. Chapter 11 provides guidance on 

mapping human-altered landscapes and human-transported soil materials.

Relief can be perceived by stereoscopic study. Relief features are 

helpful in locating many soil boundaries on the map. Topographic maps 

also provide insight to relief, slope, and aspect. Relief also identifies 

many kinds of landforms commonly related to kinds of soil.

Many landforms (e.g., terraces, flood plains, sand dunes, kames, and 

eskers) can be identified and delineated reliably according to their shapes, 

relative heights, and slopes. Their relationship to streams and other 

landforms provides additional clues. The soil scientist must understand 

geomorphology (discussed in chapter 2) to take full advantage of photo 

interpretation.

Accurate soil maps cannot be produced solely by interpretation of 

aerial photographs. Time and place influence the clues visible on the 

photographs. Human activities have changed patterns of vegetation 

and confounded their relationships to soil patterns. The clues must be 

correlated with soil attributes and verified in the field. 
240 Chapter 4

Contemporary Approach

Digital imagery has replaced photographs as the mapping base 

in 21st century soil survey. The ability to overlay multiple imagery 

resources for comparisons, the ability to quickly adjust scale, and the 

use of raster-based soil maps have increased the speed of delivering soil 

survey products as well as the variety of products available. Customized 

soil survey products are enhanced by the choice of background imagery 

(e.g., color imagery and topographic imagery) used to display soil survey 

information. Methods for digital soil mapping and the products derived 

from digital imagery are discussed in chapter 5. 

Sources of Apparent Error on Existing Soil Maps

Soil surveys in the United States. meet the technical standards and 

design requirements in place at the time they were completed. However, 

standards in use varied from State to State or regionally. In addition, 

standards evolved with increased knowledge about soils and changes in 

user needs. One should not assume that the soil survey data and maps 

completed many years ago, which did not have the benefit of recent 

evaluation and update, meet the standards and expectations of users today.

Soil survey mapping scale and map unit design considerations likely 

cause the most apparent errors on soil maps. Projects were designed to 

collect and document soil distribution and properties based upon user 

needs and were not more detailed than necessary. For low-intensity uses 

of the soil (e.g., grazing on native rangeland, native forests, watershed, 

and wildlife habitat), soil mapping was performed on a small-scale photo 

base of 1:48,000 to 1:63,560, or smaller. Areas of soil that were markedly 

different in use and management but too small to be delineated at the 

scale of mapping were described as inclusions in map units or denoted 

by a spot symbol on the map. When the mapping is presented at a larger 

scale, these areas may appear to be errors in the map.

Soil surveys in the U.S. were initiated with “memoranda of understanding” between National Cooperative Soil Survey (NCSS) partners 

and other local partners. These documents included agreed-to scheduled 

progress targets and completion date. The schedule dictated the scale of 

mapping and the mapping intensity or order. Map units were designed 

to meet specific user needs, and fieldwork was conducted to create soil 

maps that met those needs. If the user needs change due to changes in 

land use, the map unit design may not adequately meet the new needs. 

Soils with markedly different potentials or risks based upon use may not 

be adequately separated in mapping units. 

Standards used in the soil correlation process set minimum extent 

requirements for both a map unit and a soil series included in the soil 
Soil Survey Manual 241

survey legend. Setting these limits was done prior to computerization 

to ensure that data and information would remain manageable. At the 

end of a project, map units and series that did not meet the minimum 

extent requirements were combined with the most similar map unit or 

component in the legend and the concepts were expanded to include 

these soils and areas. 

Boundaries of the soil mapping legends generally coincided with 

county or State lines. Small areas of soils having a different bedrock 

geology, physiography, or major land resource area (MLRA) that crossed 

the political soil survey boundary (map legend boundary) were too small 

in extent to appear in the legend alone, so they were combined with the 

most similar map unit. When two adjacent survey areas are viewed at the 

join, fault lines may appear between the two surveys. Under the MLRA 

approach currently used to update soil survey in the U.S., these faults are 

being corrected.

Significant changes in the soil resource itself may have occurred since 

the time of soil mapping. Extensive anthropogenic activities, including 

mining, excavation, land leveling, and construction, remove or bury native 

soils. Because of natural processes, such as changes in stream courses, 

landslides, and volcanic eruptions, soil materials at the surface may differ 

from those identified on soil maps completed earlier. These changes 

are generally dramatic and easily recognizable on the landscape. Some, 

however, may be subtle (e.g., filling of wet areas, alterations to hydrology, 

and mechanical alterations such as deep ripping and deep mixing).

Other actual errors may be discovered on soil maps, including 

labeling errors done in the field and map compilation and publication 

errors. They should be documented and corrected.

Field Operation and Equipment

The efficient operation of a soil survey requires the use of certain 

kinds of equipment. The three major equipment needs are: (1) tools 

to examine the soil profile; (2) soil testing, measuring, and recording 

devices for mapping; and (3) transportation vehicles. While some of the 

equipment used in soil survey reflects new technology, such as tools for 

proximal sensing of soil properties (discussed in chapter 6), many of the 

basic tools for observing soils in the field have changed little in recent 

years. The 1993 Soil Survey Manual contains a detailed discussion and 

description of many of these items. Short descriptions of commonly 

used field tools are also provided in the Field Book for Describing and 

Sampling Soils (Schoeneberger et. al., 2012, pp. 8-5 and 8-6).
242 Chapter 4

Tools for Examining and Testing the Soil

A soil scientist examines the soil often in the course of mapping. 

Examination of both horizontal and vertical variations is essential. The 

most commonly used tools are spades and soil augers. Augers are used 

in most areas for routine mapping. In some areas, however, a spade is 

used to examine the soil. In soils with no rock fragments, samples can be 

collected quickly and relatively easily using truck-mounted (fig. 4-1) or 

Figure 4-1

A truck-mounted hydraulic probe used to quickly obtain soil profiles. The Giddings 

probe (shown) has the ability to collect a large- or small-diameter core sample, and 

extensions can be added to it for deep coring. Driver’s side-mounted bull probes are 

preferred in some areas but are limited to collection of smaller diameter core samples. 

(Photo courtesy of Casey Latta)
Soil Survey Manual 243

hand-operated probes. Backhoes and shovels are used to expose larger 

soil sections for examination, sampling, and photography. Where a probe 

or auger is regularly used for examining the soil, a large pit exposed 

by a backhoe (see fig. 4-2, left image) can be used to ensure map unit 

concepts are as predicted and have not strayed from the conceptual 

model developed. 

Figure 4-2

Left image—A backhoe excavation providing a large view of the soil profile and 

improving access for description and sampling. Right image—Shoring, exit ramps, and 

other safety measures must be used to protect staff in deep trenches. (Photos courtesy of 

Wayne Gabriel)

Backhoes, however, have limitations. Cost as well as time to 

perform needed maintenance must be considered. Where available 

locally, renting a backhoe and operator only when needed may be an 

option. Some property owners do not want large equipment on their 

property. Operators must be trained to use the equipment efficiently, 

and safety standards must be met (see fig. 4-2, right image). Overhead 

and underground power lines, which pose electrical hazards, and other 

utilities must be located and avoided. Slopes may be too steep for safe 

operation of machinery. It is important to recognize soil properties that 

make trench walls prone to collapsing and thus dangerous for anyone 

in a pit. Designing backhoe trenches with benching, shoring, and exit 

ramps can improve safety. 

Equipment for unique environments must be considered. Power 

equipment is commonly used to save time and effort. Vibracore samplers 

are used to obtain subaqueous soil samples several meters below the 

water surface. Devices such as core catchers are used to prevent the 

sandiest materials from falling out of the sample tube. Various small 
244 Chapter 4

instruments can also be used to examine the soil, such as small handheld 

digital meters that determine salinity, soil reaction, sodicity, and soil 

nutrients. Proximal sensing tools, such as XRF meters, electromagnetic 

induction, and ground-penetrating radar, can also be used. See chapters 

6, 10, and 11 for more information on tools suited to proximal sensing 

and unique environments. 

Measuring and Recording Devices for Mapping

A handheld geographic positioning system (GPS) unit can assist in 

navigation and capture the location of soil descriptions. It can be indispensable as a navigation aide in remote or roadless areas. GPS provides 

both horizontal position in geographic coordinates and elevation. Most 

units can store and recall waypoints and so help workers identify and return to specific locations. Some provide background maps of geographic 

and cultural features to aid navigation.

A small digital camera is useful in capturing quality images of soil 

profiles and features, landscape settings, and vegetation and documenting 

land use and management. Smartphones, tablets, and some laptops have 

built-in cameras that can be used for capturing and storing images. If 

digital images are used as the mapping base, laptop computers or tablet 

PCs (provided they are sufficiently ruggedized and suited to outdoor 

viewing) can be used to display and annotate maps.

Waterproof data loggers can be installed at some study sites to 

automatically collect measurements of air and soil temperatures, water 

potential, and more. These data can be collected over 1 or more years as 

needed and summarized to characterize site conditions for classification 

and interpretation.

Transportation

Field operations in soil survey require transportation of workers, 

equipment, supplies, and soil samples. Vehicles are provided to the soil 

survey team for their daily operations. The time spent by soil scientists 

traveling to and from the field can be lengthy and mainly unproductive. 

Enough vehicles are provided to keep travel time as short as possible.

Additional equipment used for special purposes or for short periods 

is typically rented or supplied as needed. A passenger van, for example, 

may be furnished by one of the soil survey project’s partners during a 

field review. Some vehicles must carry power equipment or pull trailers. 

All vehicles should be suited to the needs of the survey area, whether it 

be for use off road or in paved areas; to carry workers efficiently, comfortably, and safely; to hold the regularly used equipment; to accommo-
Soil Survey Manual 245

date an extra load; or to protect workers and equipment from adverse 

weather.

Specialized vehicles are needed in some areas. Aircraft, particularly 

helicopters, are used in some soil surveys to transport workers and 

equipment and to provide broad views of landscapes and vegetation. 

Aircraft are useful for photographing landscapes, soil patterns, and land 

use. Availability, cost, and lack of conventional landing sites are the main 

limitations to using aircraft. Snowmobiles provide winter access where 

travel is impossible or impractical in other seasons. Tracked vehicles, 

trail bikes, and all-terrain vehicles (ATVs) may be needed in areas that 

otherwise can be reached only by walking. Pack horses may be the only 

viable means of transporting people and equipment in wilderness areas. 

Marsh buggies with large buoyant tires and airboats are used in swamps 

and marshes. Canoes and small boats may be used to navigate waterways 

or to access areas consisting of numerous islands. Shallow draft boats are 

useful in conducting soil surveys in areas consisting of subaqueous soils 

(see chapter 10). Specialized vehicles must be reliable in remote areas. 

Costs of buying or renting equipment, maintaining the equipment, 

and training operators can be high. Time is needed for transport, 

maintenance, and training. Some equipment is hazardous to operate. In 

addition, sensitive ecosystems may be damaged by the equipment.

Soil Identification and Classification

In soil surveys, the individual parts that make up the soil continuum 

are classified. The classes are defined for bodies of soil of significant 

kinds and extent. The taxonomic classes are conceptual. Their definitions 

are based on the knowledge of soils as they occur in nature and the 

understanding of the genetic processes responsible for their formation. 

The taxonomic classes themselves are not real soils, but they relate to 

their representatives in nature—the pedon and the polypedon. 

Pedon

A pedon is the smallest body of one kind of soil that is large enough 

to represent the nature and arrangement of horizons and the variability 

in the other properties. It lacks boundaries with neighboring pedons 

(Soil Survey Staff, 1999). It is a unit of observation, sampling, and 

classification. 

A pedon extends down to the lower limit of a soil, through all genetic 

horizons and, if the genetic horizons are thin, into the upper part of the 
246 Chapter 4

underlying material. It includes the rooting zone of most native perennial 

plants. For purposes of most soil surveys, a practical lower limit of the 

pedon is bedrock or a depth of about 2 m, whichever is shallower. A 

depth of 2 m allows a good sample of major soil horizons, even in thick 

soil. It includes much of the volume of soil penetrated by plant roots, and 

it permits reliable observations of soil properties.

The surface of a pedon is roughly polygonal and ranges from 1 to 

10 m2

 in area, depending on the nature of the soil’s variability. Where 

the cycle of variations is less than 2 m and all horizons are continuous 

and nearly uniform in thickness, the pedon has an area of approximately 

1 m2

. Where horizons or other properties are intermittent or cyclic over 

an interval of 2 to 7 m, the pedon includes one-half of the cycle (1 to 3.5 

m). If horizons are cyclic over an interval greater than 7 m, each cycle is 

considered to contain more than one soil. The range in size, 1 to 10 m2

, 

permits consistent classification by different observers where important 

horizons are cyclic or repeatedly interrupted over short distances.

Polypedon

A pedon by itself is too small to be the unit of soil mapping because 

it cannot account for features such as slope and surface stoniness. In 

addition, it is too small to embody the full range of variability occurring 

within a soil series. Instead, the polypedon is used to define a soil series 

and is the unit of soil mapping. It is the three-dimensional soil body or soil 

individual that is homogeneous at the soil series level of classification. 

It is big enough to exhibit all the soil characteristics considered in the 

description, classification, and mapping of soils (fig. 4-3). 

The concept of the polypedon is, from a practical standpoint, more or 

less equivalent to the component in soil mapping, but with one technical 

difference. Since the polypedon is defined as being homogenous at the 

series level of classification, each pedon making up the polypedon must 

fall within the class limits for all the properties (texture, color, reaction, 

thickness, etc.) of that series. When the limits of taxa are superimposed 

on the pattern of soil in nature, areas of taxonomic classes rarely, if 

ever, coincide precisely with mappable areas. In contrast, the map unit 

component represents a miscellaneous area or a natural soil body that 

includes all of the pedons making up the polypedon, as well as other very 

similar pedons within the mapped area that are just slightly outside the 

property ranges assigned for the series. A polypedon and similar or noncontrasting soils (discussed later in this chapter) occur within the concept 

and boundaries of the map unit component. Soil map units may consist of 

one or more components. 
Soil Survey Manual 247

Figure 4-3

Polypedons vary in size. This figure illustrates a small polypedon formed in a pipe 

through a petrocalcic horizon (Gile et al., 2003). It also illustrates the concept that 

pedons have no lateral boundaries with neighboring pedons, unlike polypedons, which 

do have boundaries with neighboring polypedons. 

The polypedon represents the minimum unit of interpretation and 

soil management. If the boundaries of an individual polypedon are 

gradual and diffuse, the polypedon is virtually impossible to delineate 

because its properties are confined by the taxonomic class it represents 

(series) and these properties vary in a sinuous or continuous manner in 

either vertical or horizontal dimensions. Boundaries between map unit 

components, however, are commonly evident where the differences 

produce contrasting native plant communities or changes in properties 
248 Chapter 4

that impact soil use and management. Boundaries of both map unit 

components and polypedons may be easily observed at discontinuities, 

such as erosional facies and geologic contacts, and following human 

alteration.

Polypedons link the real bodies of soil in nature to the mental 

concepts of taxonomic classes and are the basis of soil components used 

in mapping, interpreting, and managing soils. 

Soil Map Units

Soil map units are designed to efficiently deliver soil information to 

meet user needs for management and land use decisions. Map units can 

appear as individual areas (i.e., polygons), points, or lines on a map. A 

map unit is a collection of areas defined and named the same in terms 

of their soil components, miscellaneous areas, or both (components and 

miscellaneous areas are described below). Each map unit differs in some 

respect from all others in a survey area and is uniquely identified on 

a soil map. A map unit description is a written characterization of the 

component within a map unit and the relationship of one map unit to 

another. Appendix 2 provides an example of a map unit description. 

Soil map units consist of one or more components (defined below). 

A delineation of a map unit generally contains the major (dominant) 

components included in the map unit name, but it may not always contain 

a representative of each kind of minor component. In older soil surveys, 

minor components were neither described nor interpreted in detail and 

were referred to as inclusions within a map unit. A dominant or major 

component is represented in a delineation by a part of a polypedon, a 

complete polypedon, or several polypedons. A part of a polypedon 

is represented when the phase criteria, such as slope, require that a 

polypedon be divided. A complete polypedon occurs if there are no phase 

criteria that require the subdivision of the polypedon or the features 

exhibited by the individual polypedon do not cross the limits of the 

phase. Several polypedons of a component may be represented if the map 

unit consists of two or more dominant components and the pattern is such 

that at least one component is not continuous but occurs as an isolated 

body or polypedon. Similarly, each minor component in a delineation is 

represented by a part of a polypedon, a complete polypedon, or several 

polypedons. Their extent, however, is small relative to the extent of the 

major component(s). Because soil boundaries can seldom be shown with 

complete accuracy on soil maps, parts and pieces of adjacent polypedons 

are inadvertently included or excluded from delineations.
Soil Survey Manual 249

Some map unit delineations may not contain any of the dominant 

components named in the map unit description but contain what are 

termed “similar soils.” In most survey areas, there are soils that occur 

as mappable bodies but have very limited total extent within a specific 

survey area. They are typically included with other map units if, for all 

practical purposes, their soil interpretations are the same. The allowance 

of similar soils in map units is by design—it permits the number of 

map units and named components to be reduced without reducing the 

interpretative value of the soil map. 

The kinds of map units used in a survey depend primarily on the 

purposes of the survey and the pattern of the soils and miscellaneous 

areas in the landscape. The pattern in nature is fixed, and it is not exactly 

the same in each delineation of a given map unit. In soil surveys, these 

patterns must be recognized and map units designed to meet the major 

objectives of the survey based upon known or projected user needs. It is 

important to remember that soil interpretations are made for areas of land 

and the most useful map units are those that group soils based upon their 

similarities.

Component

Within the context of a map unit, a component is an entity that 

can be delineated at some scale. It is commonly a soil but may be a 

miscellaneous area. Components consisting of soil are named for a soil 

series or a higher taxonomic class. Those that are miscellaneous areas are 

given an appropriate name, such as “Rock outcrop” or “Urban land.” In 

either case, each component that makes up a map unit can be identified 

on the ground and delineated separately at a sufficiently large scale. Map 

unit components describe the properties of natural bodies of soils, or 

miscellaneous areas of nonsoil, in a particular landscape. Components 

can be major or minor in extent, depending upon the kind of map unit and 

percent composition. Designation of components as major or minor in 

soil databases is helpful for interpretive groupings. Typically, only major 

components are used in a map unit name. 

Table 4-1 lists the kinds of map unit components used in soil survey. 

Soil components typically represent less than the full range of some 

properties allowed in a taxonomic class, which is defined by limits of 

key diagnostic properties. They may also differ from taxonomic classes 

and be slightly outside the class limits of some properties that define 

the taxa. Soils having properties that are slightly outside the defined 

taxonomic limits but that do not adversely impact major land uses are 

called similar soils. Map unit components are commonly a subset of the 
250 Chapter 4

dominant taxonomic class or series in the delineation and similar soils. 

By identifying and naming components in map units, a soil scientist can 

quickly communicate interpretive information about a map unit and still 

indicate its complexity.

Table 4-1

Kinds of Map Unit Components Used in Soil Survey

Component kind Description 

Soil series Most common component. It is the lowest 

categorical level of Soil Taxonomy.

Taxonomic 

categories above 

the series

Components given a taxonomic reference term 

that implies no specific range of properties 

beyond what is given in the map unit 

description.

Taxadjuncts Components that are named for a soil series they 

resemble but have one or more differentiating 

characteristics that are outside the taxonomic 

class limits of that series. Their use and 

management is similar to that of the named 

soil series.

Miscellaneous areas Components that are not soil as defined in 

Soil Taxonomy (such as Rock outcrop) or 

are bodies of soil that are no longer capable 

of supporting plants, such as soils heavily 

contaminated by toxic substances. Examples 

are given in table 4-2.

Phases of 

components

Components that are assigned a descriptive 

term to help distinguish between multiple 

components of the same taxonomic or 

miscellaneous area occurring within the same 

map unit legend or geographically associated 

map units.

Soil Series

The series represents a three-dimensional soil body having a unique 

combination of properties that distinguish it from neighboring series. 

The soil series concept was developed more than 100 years ago and 

somewhat followed the logic of the series as used to describe sediments 
Soil Survey Manual 251

in the geologic cross-section. Like the geologic formation, the soil series 

has served as the fundamental mapping concept. In geology, strata 

closely related in terms of their properties and qualities were members 

of a series in the sedimentary record. Initially, the soil series did not 

conform to a specific taxonomic class nor property class limits but rather 

to the predominant properties and qualities of the soil landscape, climate, 

and setting in which the soil occurred.

Today, the soil series category is the lowest level and the most 

homogeneous category in the U.S. system of taxonomy. As a class, a 

series is a group of soils or polypedons that have horizons similar in 

arrangement and in differentiating characteristics. The soils of a series 

have a relatively narrow range in sets of properties. Although part of Soil 

Taxonomy, soil series are not recorded in it. In the United States, they 

are in the Official Soil Series Descriptions database (Soil Survey Staff, 

2016a). Appendix 1 provides an example of a soil series. 

The soil series is not the object mapped in soil survey. Natural soil 

bodies are mapped and then described and classified. Each map unit 

soil component is correlated to a soil series or other taxonomic class. 

Soil series serve as a bridge between real soil bodies and conceptual 

taxonomic classes. They are an important tool for naming, remembering, 

and communicating information about soils. They also serve as a tool for 

transferring knowledge about soil genesis, properties, and interpretations 

from place to place, wherever a given soil series was correlated to a map 

unit component.

Soil series are differentiated on all the diagnostic features of the 

higher categories in Soil Taxonomy plus those additional and significant 

characteristics in the series control section (Soil Survey Staff, 1999). 

Some of the characteristics commonly used to differentiate series are 

the kind, thickness, and arrangement of horizons and their structure, 

color, redoximorphic features, texture, reaction, consistence, content of 

carbonates and other salts, content of humus, content of rock fragments, 

temperature, kinds and thickness of human-altered materials, and 

mineralogical composition. A significant difference in any one of these 

can be the basis for recognizing a different series. Very rarely, however, 

do two soil series differ in just one of these characteristics. Most 

characteristics are related, and generally several change together.

Some soils are outside the limits of any recognized soil series and 

have unique sets of properties. These are potential new series. When such 

a soil is first recognized, it is described and identified as a taxon of the 

lowest category in which it can be classified. A phase of that taxon can 

be used to identify a map unit. In some surveys, including virtually all 

detailed surveys, definitions need to be further refined. For these, the 
252 Chapter 4

soil is proposed as a new series. The new series remains tentative until 

its properties can be described in detail, its extent determined, and any 

conflicts with established series resolved. If the tentative series remains 

through the correlation process, it is established as a new series at the time 

of final correlation. A taxonomic unit description includes the ranges in 

soil properties exhibited within the mapped areas for that taxonomic unit. 

The limits of these ranges are set for the taxonomic class, but generally 

the full range allowed by the taxonomic class is not exhibited in a survey 

area.

Taxa Above the Soil Series

The first level above the series is the family. Components mapped 

to the family level match the classification of a series, but not the series 

criteria. The name of a representative series belonging to the component 

taxonomic classification is used as the component name (e.g., Ezbin 

family). The component name represents the range in characteristics 

for many series within the family classification. Use of family-level 

components is generally limited to soil survey orders 3 and 4. 

Components mapped to levels higher than the family use the 

classification as a reference term and may include the range in 

characteristics for many families within the referenced classification. An 

example is coarse-loamy Typic Cryaquolls. In this example, the higher 

taxa is the subgroup Typic Cryaquolls and it is modified by the familylevel particle-size class term “coarse-loamy” to provide additional 

information. These components are used especially on small-scale maps 

in soil survey orders 3 through 5.

During a survey, the taxonomic system is tested and retested many 

times. The results of these tests are reported at field reviews and the field 

correlation. Problems in mapping or identifying soils and inconsistencies 

between the system and observed properties of the soils are recorded in 

field review reports and correlation memoranda. After appraising these 

reports, supervisory soil scientists bring any inadequacies to the attention 

of the office responsible for keeping the system up to date.

Taxadjuncts

Taxadjuncts are polypedons that have properties outside the range in 

characteristics of any recognized series and are outside higher category 

class limits by one or more differentiating characteristics of the series. A 

taxadjunct is given the name of an established series that is most similar 

in characteristics. These components classify differently taxonomically 

but have the same interpretations for use and management as the named 
Soil Survey Manual 253

series. Because the differences in properties between the named series 

and its taxadjunct are small and do not affect major interpretations, a 

new series is not established. The taxadjunct is treated as if it were a 

member of the named series, and its interpretations are similar to those 

for comparable phases of the series for which it is named. Differences 

from the established series are described. For example, a fine-silty map 

unit component differs slightly from an established fine-loamy series in 

only particle size, and no current soil series exist to accommodate the 

fine-silty classification. The fine-silty soil is correlated as a taxadjunct to 

the established fine-loamy series and a new series is not proposed. 

Miscellaneous Areas

Miscellaneous areas are land that has little or no identifiable soil and 

thus supports little or no vegetation without major reclamation. Examples 

of miscellaneous areas are shown in table 4-2. The areas can be a result of 

active erosion, washing by water, unfavorable soil conditions, or human 

activities. Some miscellaneous areas can be made productive but only 

after major reclamation efforts. Map units are designed to accommodate 

miscellaneous areas, and most map units named for miscellaneous areas 

include areas of soil. If the amount of soil exceeds the standards for minor 

components defined for the survey, the map unit is named as a complex 

or association of miscellaneous area and soil. One must be careful in 

determining that an area is a miscellaneous area rather than a soil. For 

example, not all areas that are mined should be named “Mined land.” If 

they are able to support vegetation and thus meet the definition of soil, they 

should be classified as soil. This is particularly important if it is possible to 

populate at least some major soil property data in the soil database and so 

provide meaningful interpretive information. The National Cooperative 

Soil Survey maintains an official list of miscellaneous areas and their 

definitions for use in the U.S. soil survey (USDA-NRCS, 2016). 

Phases

Phase terms added to map unit component names convey important 

information about a map unit and differentiate it from other map units on 

the map unit legend. A property of a taxon that has too wide a range for 

the interpretations needed or some feature outside the soil itself that is 

significant for use are a basis for phasing map units. Phases commonly 

include only part of the range of features exhibited by a taxon within a 

soil map unit. Soil phases can be based on attributes, such as frost hazard, 

character of the deeper substratum, or physiographic position, that are 

not characteristics used to identify taxa but nevertheless affect use and 
254 Chapter 4

Table 4-2

Miscellaneous Areas Used as Map Unit Components

Area Description

Badland Moderately steep to very steep barren land dissected by 

many intermittent drainage channels in soft geologic 

material. Ordinarily, it is not stony and occurs in 

semiarid and arid areas.

Beaches Sandy, gravelly, or cobbly shores washed and rewashed 

by waves. The areas may be partly covered with water 

during high tides or storms.

Chutes Elongated areas on steep mountain slopes that lack 

vegetation and have exposed bedrock, rock fragments, 

and woody debris. Avalanche or mass movement 

activity is evident. 

Cinder land Loose cinders and other scoriaceous magmatic 

ejecta. The water-holding capacity is very low, and 

trafficability is poor.

Dams Artificial structures that are oriented across a 

watercourse or natural drainage area for the purpose 

of impounding or diverting water.

Dumps Areas of smoothed or uneven accumulations or piles of 

waste rock and general refuse. “Dumps, mine” is an 

area of waste rock from mines, quarries, and smelters.

Dune land Unstable sand in ridges and intervening troughs that 

shifts with the wind.

Glaciers Large masses of ice formed by the compaction and 

recrystallization of snow. The ice front may be 

advancing or retreating. Areas may include incidental 

amounts of soil or rock. 

Gullied land Areas where erosion has cut a network of V-shaped or 

U-shaped channels deep enough to inhibit or prevent 

crossing.

Lava flows Areas covered with lava. Most flows have sharp, jagged 

surfaces, crevices, and angular blocks characteristic 

of lava. Others are relatively smooth and have a ropy, 

glazed surface. A small amount of earthy material 

may occur in a few cracks and sheltered pockets.

Mined land Areas that are significantly altered by mining activities. 

Soil material and rock have been moved into, out 

of, or within the areas designated. Because access 

to mined land may be limited by permissions or 

hazardous materials, identification of soil components 

can be difficult or impossible.
Soil Survey Manual 255

Table 4-2.—continued

Area Description

Oil-waste 

land

Areas where liquid oily wastes, principally saltwater 

and oil, have accumulated. They include slush pits 

and adjacent areas affected by the liquid wastes. The 

land is barren, although some of it can be reclaimed at 

high cost.

Pits Open excavations from which soil and commonly 

underlying material have been removed, exposing 

either rock or other material. Examples are “Pits, 

mine,” “Pits, gravel,” and “Pits, quarry.”

Playas Barren flats in closed basins in arid regions. Many 

areas are subject to wind erosion and many are saline, 

sodic, or both. The water table may be near the 

surface at times.

Riverwash Unstabilized sandy, silty, clayey, or gravelly sediment 

that is flooded, washed, and reworked frequently by 

rivers.

Rock 

outcrop

Exposures of bare bedrock other than lava flows and 

rock-lined pits. If needed, map units can be named 

according to the kind of rock, e.g., “Rock outcrop, 

chalk,” “Rock outcrop, limestone,” and “Rock 

outcrop, gypsum.” If small, they can be identified by 

spot symbols on maps.

Rubbleland Areas of cobbles, stones, and boulders commonly at 

the base of mountains or left on mountainsides by 

glaciation or periglacial processes.

Slickens Accumulations of fine textured material from placermine and ore-mill operations. They may have 

undergone chemical extractions. They are typically 

confined in constructed basins.

Urban land Land mostly covered by streets, parking lots, buildings, 

and other structures of urban areas.

Water Streams, lakes, ponds, and estuaries that are covered 

with water, deep enough or moving, that growth 

of rooted vegetation is precluded. Many areas are 

covered with water throughout the year.

management. Common phases are slope, surface texture, flooding and 

ponding, surface fragments, degree of erosion, and climate (see table 4-3 

in the “Naming Map Units” section). Overlying water depth is used as a 

phase term for some subaqueous soils. Phases such as “filled,” “graded,” 
256 Chapter 4

or “landscaped” are used for some map units consisting of soils that 

formed in human-altered or human-transported material. 

Phase terms are devised and used as needed to differentiate map 

units. The usefulness of each phase must be repeatedly tested and verified 

during a survey. Separate phases of a taxon must differ significantly in 

behavior. If no useful purpose is served by separating them in mapping, 

similar phases of different taxa may be combined and the combination 

described. The interpretations prepared during the course of a survey 

provide evidence of similarities and differences among map units.

The justification for most phases rests on the behavior of the soils 

under various uses. At least one statement about soil behavior must be 

unique to each phase of a taxon, and the differences of soil properties 

must exceed normal errors of observation. The use of phase terms is 

described in greater detail in the section “Naming Map Units” below. 

Classes

Classes of soil properties are not necessarily used directly as 

phases. Defined class limits of properties are designed for a convenient 

description of soil, but they can also be used to define phases of soil 

map units in some cases. For example, a map unit may be named as 

a moderately saline phase to distinguish it from another map unit with 

the same name but whose soils have no significant salinity. However, 

property class terms are not useful for all soils. Distinctions significant 

for one kind of soil are not significant for every other kind. Any 

single property is significant only through its interactions with other 

properties. 

Kinds of Map Units

Soils differ in size and shape of their areas, in degree of contrast 

with adjacent soils, and in geographic relationships. Four kinds of map 

units are used in soil surveys: consociations, complexes, associations, 

and undifferentiated groups.

In most map units, areas of soil occur that do not meet all of the 

taxonomic criteria of the soil (series or higher taxa) used to name the 

map unit. However, because these soils have properties similar to those 

of the named soils and interpret similarly for the dominant land uses, 

they are included as part of the named component. They are referred to 

as similar or non-contrasting soils. Conversely, minor components and 

unnamed soils which interpret differently for major uses, whether they 
Soil Survey Manual 257

are well suited (less limiting) or poorly suited (more limiting), are called 

dissimilar or contrasting soils. 

The total amount of dissimilar minor components in a map unit 

generally does not exceed about 15 percent if they are limiting and 25 

percent if they are nonlimiting. A single dissimilar limiting component 

generally does not exceed 10 percent if it is very contrasting. 

In most cases, soil map units can be delineated as polygons. However, 

in some cases a polygon cannot be drawn to conform to cartographic 

standards due to size or shape constraints. In these cases, lines or points 

may be used to designate map units. If this is necessary, the narrow width 

or small size is included in the map unit description to indicate the nature 

of the soils on the landscape. 

Consociations

In a consociation, delineated areas are dominated by a single soil 

component (or miscellaneous area). Commonly, at least one-half of the 

pedons in each delineation are of the same soil taxa as the named soil. 

The remainder of the delineation mostly consists of soil so similar to 

the named soil that major interpretations are not significantly affected. 

The map unit component thus consists of soil meeting the criteria for 

the taxonomic class (series or higher taxa) used to name the map unit 

plus similar soils. The soil in a consociation may be identified at any 

taxonomic level. 

A consociation that is named for a miscellaneous area (such as 

Rock outcrop) dominantly consists of that kind of area, and any minor 

components present do not significantly affect the use of the map unit. 

Generally, less than about 15 percent of any delineation is soil or less 

than about 25 percent is other kinds of miscellaneous areas. Percentages 

may vary, depending upon the kind of miscellaneous area and the kind, 

size, and pattern of the minor components.

Complexes

Complexes consist of two or more dissimilar major components that 

occur in a regularly repeating pattern or in an unpredictable pattern. The 

major components of a complex cannot be mapped separately at a scale 

of about 1:24,000 (fig. 4-4). Typically, each major component occurs 

in each delineation, although the proportions may vary appreciably 

from one delineation to another. The major components are sufficiently 

different from each other in morphology or behavior that the map unit 

cannot be a consociation. 
258 Chapter 4

Figure 4-4

An area that meets the definition of a soil complex—the major components cannot be 

mapped separately at a scale of about 1:24,000. The bottom photo shows the profile of 

the Bluepoint series (Typic Torripsamments) that has formed as a coppice dune. The 

middle photo shows the distribution of the coppice dunes on the eroded phase of the 

Rotura series (Typic Petroargids). The top photo shows a landscape of the BluepointRotura complex in southern New Mexico.
Soil Survey Manual 259

Associations

Associations consist of two or more dissimilar major components 

occurring in a regular and repeating pattern on the landscape. The 

major components of an association can be separated at a scale of 

about 1:24,000, but due to land use or user needs, the map unit design 

integrates the predictable and repeating pattern of soil occurrence. Many 

general soil maps use soil associations because they are at scales much 

smaller than 1:24,000 and can depict only the characteristic landscapes of 

associated soils, not the individual soils (fig. 4-5). The major components 

are sufficiently different in morphology or behavior that the map unit 

cannot be a consociation. 

Figure 4-5

Block diagram depicting the relationship of the soils in the Monona-Ida-Judson 

association in the general soil map (published scale of 1:125,000) of Woodbury County, 

Iowa (USDA-NRCS, 2006). Because a general soil map cannot show the location of 

each soil making up an association, accompanying diagrams such as this are commonly 

used. Monona soils are classified as Typic Hapludolls, Ida soils as Typic Udorthents, 

Judson soils as Cumulic Hapludolls, and Rawles soils as Oxyaquic Udifluvents.

Undifferentiated Groups

An undifferentiated group is a map unit of dissimilar soils that are 

not consistently associated geographically and, therefore, do not always 
260 Chapter 4

occur together in the same map unit delineation. These components are 

included in the same named map unit because use and management are 

the same or very similar for common uses. Generally, some common 

feature outside of the soil itself, such as steepness, stoniness, or flooding, 

determines use and management. If two or more very steep soils that are 

geographically separated are so similar in their potentials for use and 

management that defining two or more additional map units would serve 

no useful purpose, they may be placed in the same unit. Every delineation 

has at least one of the major components and some may have all of them. 

Minor Components Within Map Units

In all soil surveys, virtually every delineation of a map unit 

includes areas of soil components or miscellaneous areas that are not 

identified in the name of the map unit. Many areas of these components 

are too small to be delineated separately. Examples are small areas 

of steeper slopes or small areas of wet soils in an upland map unit. 

The location of some components cannot be identified by practical 

field methods. Some minor components are deliberately included in 

delineations identified as another map unit to avoid excessive detail on 

the map or the legend. These soils of very limited extent were referred 

to simply as “inclusions” in mapping before the mid-1990s, but are 

now identified as minor components and correlated to the lowest level 

of classification, as appropriate. Minor components are not indicated in 

the map unit name, but they are observed and documented in the map 

unit description. 

Minor components reduce the homogeneity of map units and may 

affect interpretations. The objective is to define map units that contain 

as few as possible minor components that behave differently from 

the named components. Map units must be defined, recognized, and 

delineated consistently in the field.

The number of minor components reflects the taxonomic purity of 

map units. This number and the degree that contrasting minor components 

differ from the major components can be used to estimate the interpretative 

purity of map units. The actual amount of minor components is estimated 

from observations made during the survey. Adjustments in mapping and 

map unit design may be needed.

In the definition of map units, a scientist must judge the effects of 

minor components on management against the amount of work required 

to minimize the number of minor components. Determining the impact
Soil Survey Manual 261

of the differences between the major and minor components is useful. 

If differences are small, the components are compared as similar. If 

differences are large, the components are contrasted as dissimilar.

Similar components are alike in most properties and share diagnostic 

properties and limits. Interpretations for most common uses are alike or 

reasonably similar and the interpretative value of the map unit is not 

affected. In contrast, dissimilar components differ appreciably in one or 

more properties, and the differences are great enough to affect major 

interpretations. Some dissimilar components are limiting and others are 

nonlimiting relative to the interpretations being considered.

If a minor component does not restrict the use of entire areas or 

impose limitations on the feasibility of management practices, its 

impact on predictions for the map unit is small. Minor components that 

have restrictions on use less severe than those of the dominant soil do 

not adversely affect predictions about the unit as a whole. They may 

even be beneficial. Such minor components are nonlimiting, and the 

interpretative purity of a map unit for most interpretations is not altered. 

For example, including small areas having slopes of 4 to 8 percent in 

an area having slopes mainly of 15 to 25 percent has no adverse effect 

on use of the area for most purposes. However, if the minor component 

has significantly lower potential for use than the dominant component in 

the map unit or affects the feasibility of meeting management needs, a 

small amount in a map unit can greatly affect predictions. These are the 

most critical minor components because they decrease the interpretative 

purity of map units. Even a small area having slopes of 15 to 25 percent 

in a map unit dominated by slopes of 4 to 8 percent can seriously affect 

the use of the area for many purposes. Even small minor components of 

wetter soils (such as Typic Epiaqualfs) in areas of upland soils (such as 

Aquic Hapludalfs) may control and limit the uses of the dominant soil 

component.

Soils that cannot be used feasibly for the same purposes as the 

surrounding soils are especially important to identify. They are delineated 

separately if the map scale is small enough and if delineating them will 

improve the usefulness of the map for the major anticipated uses. Areas 

too small to delineate may be identified and located on the map by special 

symbols.

Limiting Dissimilar Soils

Standards of purity are adjusted according to the precision required 

by the survey objectives. Generally, all delineations contain soils other 

than those identified in the map unit name. These minor components 
262 Chapter 4

reduce purity. Different kinds of minor components, however, have 

different effects on the value of the map for use. The minor components 

that most detract from purity are those that are distinctly more limiting 

for use than the named soil. These are called limiting dissimilar soils. A 

soil may be a limiting dissimilar soil for some uses but not for others. The 

survey objectives must be considered when assigning limiting dissimilar 

soils. Not only the amount of limiting soils but also the size of their 

individual areas is important. Soil survey standards for both are set at 

levels that do not seriously detract from the validity of interpretations 

based on the named soil.

Standards of purity are attained by adjusting the field operations. For 

example, if the standards require that areas of limiting dissimilar soils 

as small as 0.1 ha be delineated, the area must be traversed at intervals 

close enough to locate areas that small and the soil must be examined at 

enough places along each traverse to detect them.

Designing and Documenting Map Units

Designing Map Units

Well designed map units are based on accurate soil-landscape 

models. They can be consistently associated with features observable on 

the surface (e.g., vegetation and geomorphic position) and consistently 

delineated by the survey team. Initial investigations identify the pattern 

of occurrence for each component making up the map unit. In most cases, 

well designed map units require a relatively few number of observations 

to delineate accurately. 

Knowing the Parameters

Guidance documents (such as a memorandum of understanding 

or other project plan) outline considerations for the order of mapping, 

scale of mapping, minimum size delineation, base imagery (if used), 

documentation requirements, and specific interpretive needs of the user. 

The survey team is responsible for collecting complete and accurate soil 

data, assessing the complexity of the soil landscape, and designing map 

units that support land use decisions and meet the objectives of the survey 

project. If the information is too broad or too complex, the objectives of 

the survey will not be met.

While studying the soil patterns in different landscapes, soil 

scientists must keep in mind how best to relate the patterns observed 

to the appropriate map units. They must determine the kinds of map 
Soil Survey Manual 263

units, the level of soil taxa, and the phases needed to satisfy the survey 

objectives. By definition, a map unit differs from all others in the survey 

area and should be uniquely identified. This requires many judgements 

and hypothesis testing. Every map unit that is tentatively identified is 

evaluated by three tests: (1) Does it capture the characteristic signature 

in the landscape that can be recognized from remote sensing imagery 

or field observation? (2) Is it recognizable and repeatable for consistent 

mapping? (3) Is it needed to meet the objectives of the survey?

Delineating the Areas

The landscape is partitioned either in the field or using remotely sensed 

data. The first step is to group areas having the same soil-forming factors 

(chapter 1) and known catenas or conceptual models of related soils. 

This premapping step groups defined landscapes, landforms, geology, 

vegetation, and climatic areas. Areas that have these same repeating 

patterns are delineated and labeled as the same map unit. It is recommended 

that broad groupings are established first. The lines can be adjusted as the 

survey team completes fieldwork to verify map units and refine concepts.

Most Important Soil Line

Designing map units to indicate significant differences in behavior 

among soils is particularly important for meeting the current objectives 

of a survey. Map units separated according to differences in geomorphic 

processes (e.g., parent material, relief, and time) are considered the most 

important soil lines on the landscape. These lines should be the first 

delineated on a map. Indicating differences in geomorphic processes is 

important, even if no immediate differences in interpretations are known. 

Differences in soil properties that do not affect current interpretations 

may be important in the future. Too many delineations may greatly reduce 

the immediate usefulness of a soil map. The potential benefit of extra 

delineations (the value of the additional information) must be weighed 

carefully against the costs incurred in making additional separations. 

Every soil survey is designed to record knowledge about soils; however, 

this does not mean that the soil map must show the location of every kind 

of soil in a survey area or that the publication must record everything that 

is known about the soils. Capturing and managing all observations of soil 

data on maps, even if the data is not used for publication, is invaluable in 

later analysis to develop new maps or update soil information.

Defining the Components

The objectives of a survey determine the kind of map units and 

the kind of components used to define the map units (see table 4-1). 
264 Chapter 4

Taxonomic classes provide the framework upon which the basic 

sets of soil properties distinguishing soil map units are defined. They 

summarize an immense amount of research and experience related to 

the significance of soil properties and combinations of properties. They 

provide predefined sets of soil properties that have been tested for 

genetic relationships and interpretative value. Taxa provide a firm base 

for recognizing the components of potential map units in an unfamiliar 

area. Using established taxa is much easier than independently sorting 

out sets of properties and determining significant class limits.

Within each survey, soil maps can be designed with components 

correlated to a taxonomic level that reflects narrowly or broadly defined 

ranges of soil properties. In addition, map units can be designed with 

different compositions of major and minor components. Design flexibility 

allows the development of map units that will be useful for the purposes 

of a specific survey while maintaining as much uniformity in mapping 

as possible.

Traverses

Traverses are used to identify different components on a landform. 

The observation points along a traverse can be any distance apart. The 

distance is adjusted to the direction and scale of the soil boundaries and 

the variability of the important properties in each component. Sites for 

each observation are chosen to represent specific areas on a landform. 

For example, one recommended way to lay out a traverse in a field is 

to travel in a direction perpendicular to local drainage patterns. The 

soil components can then be documented in a swale where it is darkest, 

on a ridge where it is lightest in color, and also in the footslope and 

backslope positions (for observations of transitions between areas). Once 

the soil component is known for each position, a landform model can be 

developed for use in similar areas throughout the survey area.

Transects

Transects are used to determine the composition and design of map 

units. They have fixed length intervals between observation points. 

Observations made at points along a transect are typically identified as 

belonging to a particular taxon, or soil component, but can also be a 

combination of properties, such as depth, thickness, color, or vegetation. 

When selecting delineations for transecting, it is essential to eliminate 

bias by stratifying transects randomly. One simple method is to separate 

the survey into several subdivisions and conduct transects within each 

subdivision. The different land uses (e.g., cropland and forestland) in each 
Soil Survey Manual 265

delineation of a map unit should be considered, and transects should be 

conducted in map units under each land use. Transects must be positioned 

to encounter the maximum variation in each delineation. The transect 

should be oriented so that the line does not follow a contour around a hill. 

Transects should go up and down the hill and across drainages. 

Systematic variation is quantified and more easily understood with a 

transect. A map unit complex consisting of a soil component in concave 

positions and another soil component in convex positions is an example 

of a map unit exhibiting systematic variation. Interval spacing in the 

transect must be small enough to capture the variability of the landform. 

Narrow map units can be problematic because a straight line transect 

may not fit or may miss the variability visually observed. In such cases, 

measured line segment transects can be used to ensure all components are 

captured and quantified. Typically, only a small percentage of the map 

unit delineations contain transects. As the order of survey increases, the 

length and intervals in a transect also increase. In all cases, transecting 

is not the same as line mapping used to determine the placement of map 

unit boundaries on a landform.

Random variation (i.e., variation that is not understood and therefore 

cannot be readily explained) can also be quantified using transects. Areas 

containing soil properties that are not readily observed or explained, as 

in stratified layers of alluvium or depth to bedrock on a loess-covered basalt plain, are well suited to grid or stratified random transects. Random 

variation methods for transecting can also be used to accurately quantify 

components having systematic variation. Methods designed specifically 

to document systematic variation transecting should not be used to quantify components having random variation.

Other statistical methods to determine composition, such as Latin 

hypercube sampling, are employed in digital soil mapping techniques 

(see chapter 5). Site selection using digital techniques can be used when 

digital mapping is performed. Latin hypercube sampling is especially 

useful if the field investigator is inexperienced or lacks the intrinsic 

knowledge of the landscape and soil patterns.

Naming Map Units

A map unit is uniquely named to distinguish it from all others in the 

survey area. Different conventions are used for each of the four kinds 

of map units so that the kind of unit is easily recognizable. In general, 

names are as short as possible. Map unit names typically include the 

named major components, both soil and nonsoil, that occur in the map 
266 Chapter 4

unit. Miscellaneous areas are named if they occur as a major component. 

Commonly an extra term, such as surface texture, which is not needed 

to distinguish a phase from all others in the survey, is used so that 

comparable phases in other areas have the same name.

Phases are groupings created to serve specific purposes in individual 

soil surveys. They can be defined for any class or classes of any category. 

Table 4-3 lists some commonly used phase terms. Other terms can be 

developed as necessary. The phase classes are helpful in describing 

the soil phases that are important for the survey. Differences in soil or 

environmental features that are significant to use and management or soil 

behavior are the basis for designating soil phases.

Any property or combination of properties that does not duplicate 

class limits for a taxon can be used to differentiate phases, and any value 

of a property can be set to divide phases. The choice of properties and 

limits are determined by the purpose of the survey and by how consistently 

the phase criteria can be applied. Because objectives differ from one soil 

survey to another, limits and ranges of a property or attribute may also 

differ from one survey to another. In general, phase criteria are given 

a smaller range where soil use is intensive (as for irrigated farming or 

urban development) and a larger range where soil use is extensive (as for 

forestry or grazing).

For detailed surveys, decisions must be made about what criteria to 

use to recognize phases of soil components, how broadly or narrowly to 

define the phases, and whether similar phases of different components 

have interpretations similar enough that they can be combined. Phases are 

used to convey important information about a map unit and to differentiate 

it from other map units in the legend. For less-detailed surveys, decisions 

must be made about how the complexities of soil in large areas can be best 

identified and represented for purposes of the survey, what combinations 

of soils characterize useful and mappable units, what taxonomic level 

should be used in naming map unit components, and which phases 

contribute to the usefulness of the map units.

The names of soil taxa, along with one or more modifying terms, 

are used to identify the soils in map units. For example, the name “Tama 

silt loam, 2 to 5 percent slopes” indicates that soils of the Tama series 

(an Udoll) are dominant in that map unit. The names of taxa of higher 

categories are also used in map unit names, especially on small-scale 

maps. For example, “Udolls, rolling” identifies a map unit consisting 

dominantly of soils of the Udoll suborder, which includes Tama and 

other series. 

As methods of measuring soil properties are refined, as experience 

in the field increases, and as use and management requirements are 
Soil Survey Manual 267

Table 4-3

Phases Most Commonly Used in Naming Soil Map Units

[Terms are listed in preferred order of occurrence if more than one 

phase term is used.]

Phase group Phase usage

Surface texture USDA surface texture name

Deposits on the 

surface

Overblown, wind hummocky, and overwash

Fragments Size and quantity classes, including gravelly, cobbly, 

stony, and rocky, or artifactual (anthropogenic) and 

appropriate modifier of non, very, or extremely

Slope Expressed as percent or a descriptive slope class, 

such as nearly level, gently sloping (undulating), 

strongly sloping (rolling), moderately steep (hilly), 

steep, and very steep

Depth Shallow or deep, and appropriate modifier (such as 

moderately or very)

Substratum Contrasting material as base of named soil (e.g., 

sandy substratum, gravelly substratum, saline 

substratum)

Soil water state Reference to water table, drainage classes, wetness, 

flooding, or ponding or to artificial drainage 

(drained)

Salinity Nonsaline through strongly saline

Sodicity Sodic, with modifier as needed (e.g., strongly sodic)

Physiography Landscape or physiographic term, as appropriate 

Erosion Degree of erosion, from slightly eroded through

severely eroded, and gullied 

Thickness Thick or thin surface horizon or subsoil (e.g., thick 

surface, thin solum)

Climate Precipitation and temperature variation (e.g., high 

precipitation, cool) 

intensified, progressively narrower ranges in soil properties can be 

recognized or established. Narrow ranges of properties should not be 

established just because methods permit it. Unnecessary separations 
268 Chapter 4

take time to delineate consistently, and they make the survey difficult 

to use. However, not separating two significantly different, mappable 

units makes a survey less useful. The significance of each map unit in 

meeting the objectives of the survey must be constantly evaluated during 

the mapping process. 

Orders of Soil Surveys

All soil surveys are made by examining, describing, and classifying 

soils in the field and delineating their areas on maps. Some surveys are 

made to serve users who need precise information about the soil resources 

of areas a few hectares or less in size. These surveys require refined 

distinctions among small, homogeneous areas of soil. Others are made 

for users who need a broad perspective of heterogeneous, but distinctive, 

areas thousands of hectares in size. A soil survey made for one group of 

users may not be useful for another group. 

The elements of a soil survey can be adjusted to provide the most 

useful product for the intended purposes. Different intensities of field 

study, different degrees of detail in mapping, different phases or levels 

of abstraction in defining and naming map units, and different map unit 

designs produce a wide range of soil surveys. Adjustments in these 

elements form the basis for differentiating five orders of soil surveys. 

Table 4-4 is a key for identifying orders of soil surveys.

Recognition of these different levels of detail is helpful in 

communicating information about soil surveys and maps, even though the 

levels cannot be sharply separated from each other. The orders are intended 

to convey the level of detail used in making a survey, the scale used to 

delineate map units, and how general the map units are. They also indicate 

the general levels of quality control that are applied during surveys. These 

levels affect the kind and precision of subsequent interpretations and 

predictions. The orders differ in the following elements:

• The soil survey legend, including: 

○ the kinds of map units (consociations, complexes, associations, and undifferentiated groups) and

○ the kinds of soil taxa used in identifying the map unit 

components (soil series, families, subgroups, great 

groups, suborders, orders, and phases of them);

• The standard for purity (in composition or probability) of delineated soil areas, including: 
Soil Survey Manual 269

○ the minimum area of a limiting dissimilar soil that must 

be delineated separately and thus excluded from areas 

identified as another kind of soil, and

○ the maximum percentage of limiting dissimilar minor 

components that is permissible in a map unit;

• The field operations necessary to identify and delineate areas of 

the map units within prescribed standards; and

• The minimum map scale required to accommodate the map units 

of the legend, the standards of minimum composition, and the 

map detail justified by field methods.

Mapping legends are designed to provide the degree of refinement 

of map units required by the objectives of the survey. A map unit can be 

identified as a consociation (an area dominated by a soil component of a 

single taxon, such as a series or suborder) or as a group (geographic mixture) of taxa, such as an association or complex. A group may be more 

heterogeneous, and less refined, than a consociation at the same level of 

classification. A soil series has a much more narrowly defined set of soil 

properties than a suborder and, therefore, is a more refined distinction. 

Thus, phases of soil series are used as map unit components if users 

need more precise information about small areas of soils. Phases of any 

category in Soil Taxonomy might be used as soil map unit components 

if only a very broad perspective of the soil resources of very large areas 

is needed.

Scale and Order of Mapping

The order of a survey is commonly reflected in the scale of mapping, 

but not determined by it. Rather, the order of a survey is determined by 

the field procedures used to identify soil components and place map unit 

boundaries, the minimum permissible size of map unit delineation, and 

the kind of map unit to which soil components are aggregated. Where 

soil maps are available in digital form, computer software allows users to 

change the map scale for display purposes. The ability to enlarge a map 

in this way can lead to misunderstanding the accuracy and level of detail 

on the soil map. The scale used to make the survey is the scale that must 

be used to display the mapping. See the 1993 Soil Survey Manual (Soil 

Survey Division Staff, 1993) for a discussion on scale and map legibility.

Digital soil mapping techniques (discussed in chapter 5) augment 

field procedures and remote sensing in identifying soil map components 

and placement of map unit boundaries. The benefits of digital soil 

mapping increase with increasing order of soil mapping. 
Order and level of data needed Field procedures Min. size of map units (ha)1 Typical components of map units Kind of 

Order 1 —Very 

intensive (e.g., experimental plots, individual building sites, required reviews and permits from regulatory agencies)

The soils in each delineation 

are identified by transecting or traversing or even grid mapping. Soil boundaries are observed throughout their length. Remotely sensed data are used as an aid in boundary delineation.

1 or less Phases of soil series; 

misc. areas

Mostly c

some cmisc. a

Order 2 —Intensive 

(e.g., general agriculture, urban planning)

The soils in each delineation are 

identified by field observations and by remotely sensed data. Boundaries are verified at closely spaced intervals.

0.6 to 4 Phases of soil series; 

misc. areas; few components named at a level above the series

Consocia

complassociaundiffegroups

Order 3 —Extensive 

(e.g., range, community planning)

Soil boundaries are plotted by 

observation and interpretation of remotely sensed data. They are verified by traversing representative areas and by some transects.

1.6 to 16 Phases of soil series 

or taxa above the series; misc. areas

Mostly a

or comconsocundiffegroups
Order and level of data needed Field procedures Min. size of map units (ha)1 Typical components of map units Kind of 

Order 4 —Extensive (e.g., general soil information for broad statements concerning land 

use potential and general land management)

Soil boundaries are plotted by 

interpretation of remotely sensed data. They are verified by traversing representative areas and by some transects.

16 to 252 Phases of soil series 

or taxa above the series; misc. areas

Mostly a

some cconsocundiffegroups

Order 5 —Very 

extensive (e.g., regional planning, selection of areas for more intensive study)

The soil patterns and composition 

of map units are determined by mapping representative ideas and like areas by interpretation of remotely sensed data. Soils are verified by some onsite investigation or by traversing.

252 to 4,000 Phases of levels 

above the series; misc. areas

Associati

consocundiffegroups

1 This is about the smallest delineation allowable for readable soil maps. In practice, the minimum size of 

than that shown in table.

2 Where applicable, all kinds of map units (consociations, complexes, associations, and undifferentiated g

of soil survey.
272 Chapter 4

Order 1 Surveys

Order 1 (or first order) surveys are made if very detailed information 

about soils, generally in small areas, is needed for very intensive land 

uses. These land uses commonly require reviews and permits from 

regulatory agencies, engineers, and other professionals. Order 1 surveys 

are also conducted for specialized information, such as for critical 

habitat or cultural resources. The information can be used to plan for 

irrigation, drainage, truck crops, citrus or other specialty crops, and 

experimental plots; to site individual building lots; to locate disturbed 

areas or anthropogenic landforms (see chapter 11); to delineate wetlands 

and special habitat; and for other uses that require a detailed and very 

precise knowledge of the soils and their variability. Order 1 surveys are 

also referred to as high-intensity soil surveys.

Transecting, traversing, and, in some cases, grid mapping are used 

for accurate placement of soil boundary lines over small distances. Soil 

boundaries can be marked in the field with flagging for accurate location 

by GPS or standard land surveying methods and later transferred to a 

base map using mapping software. Remotely sensed data and digital 

techniques using LiDAR and ground-penetrating radar (chapter 6) can 

aid in soil boundary delineation. Typically, soil pits are used to determine 

parent material, bedrock, and drainage classes. They are mechanically 

dug with small excavators or backhoes.

Order 1 surveys have high map unit purity. Map units are typically 

consociations, containing no more than 15 percent dissimilar minor 

components. Complexes are seldom used. Map unit components can 

be phases of soil series, taxonomic categories above the soil series, or 

miscellaneous areas. Some map units may be named at a categorical level 

above the series or named for the type of material (e.g., “excavated,” 

“regraded”). Soil mapping legends may use taxonomic categories or 

connotative terms that are customized for users. Delineation size is 

designed to meet the detailed needs of the survey. Many order 1 surveys 

use a minimum size of about 1 hectare (2.5 acres). Depending on scale, 

environmental concerns, and needs of the survey, as small as 2,000 square 

feet may be used. Base map scale is generally 1:15,840 or larger and may 

be as large as 1 cm = 15 m (1 inch = 20 feet). Order 1 base maps may 

also have perimeter surveys determined by a professional land surveyor 

and show detailed topography with less than 2-foot interval contour 

lines. Order 1 surveys may employ significantly different methodologies 

than traditional order 2 and 3 surveys, such as a connotative legend. A 

connotative legend has map unit symbols that notate specific interpretive 

or inherent properties of the taxonomic component (e.g., drainage class, 

texture, hydrologic soil group) or any aspect of a component that is of 
Soil Survey Manual 273

interest to the user. Typical end users of a high-intensity soil survey, 

such as engineers, regulatory agency staff (Federal, State, and local), 

land developers, wetland scientists, site evaluators (e.g., septic system 

designers), and other professionals, generally are not familiar with the 

named soil series on a map.

Due to the deviations from normal soil survey standards to 

accommodate unique user needs and the lack of a formal soil correlation 

process, order 1 surveys in the U.S. are treated as special types of onsite 

investigations and are not part of the official soil survey for the National 

Cooperative Soil Survey. Order 1 surveys can differ from order 2 and 

3 surveys in the landscape models used to explain soil and landform 

distribution. It is useful to view order 1 components at an order 2 or 3 

level to better understand landscape patterns.

Order 2 Surveys

Order 2 (or second order) surveys are made if detailed information 

about soil resources is needed to make predictions of soil suitability and 

treatment needs for intensive land uses. The information can be used 

in planning for general agriculture, construction, urban development, 

and similar uses that require precise knowledge of the soils and their 

variability.

Field procedures allow plotting of soil boundaries by observation and 

by interpretation of remotely sensed data. The soils in each delineation 

are identified primarily by traversing and transecting. Observations 

and remotely sensed data are secondary types of documentation. 

Boundaries are verified at closely spaced intervals. Map units are mostly 

consociations and complexes but may also include undifferentiated 

groups or associations. Map unit components are phases of soil series 

or phases of miscellaneous areas. Map units may also be named for a 

taxonomic category above the series. Delineations are variable in size, 

with a minimum of 0.6 hectare to 4 hectares (1.5 to 10 acres), depending 

on landscape complexity and survey objectives. Contrasting minor 

components vary in size and amount within the limits permitted by the 

kind of map unit used. Base map scale is generally 1:12,000 to 1:31,680, 

depending on the complexity of the soil pattern within the area.

Order 3 Surveys

Order 3 (or third order) surveys are made where land uses do not 

require precise knowledge of small areas or detailed soil information. 

The survey areas are commonly dominated by a single land use and 

have few subordinate uses. The soil information can be used in planning 

for range, forest, and recreational areas and in community planning.
274 Chapter 4

Field procedures allow plotting of most soil boundaries by observation 

and by interpretation of remotely sensed data. Boundaries are verified 

primarily by field observations, transecting, and remotely sensed data. 

Secondary types of documentation include traversing representative 

areas and applying the information to like areas. Map units include 

associations, complexes, consociations, and undifferentiated groups. 

Components of map units are phases of soil series, taxa above the series, 

or miscellaneous areas. Delineations have a minimum size of about 

1.6 to 16 hectares (4 to 40 acres), depending on the survey objectives 

and complexity of the landscapes. Contrasting minor components vary 

in size and amount within the limits permitted by the kind of map unit 

used. Base map scale is generally 1:20,000 to 1:63,360, depending on the 

complexity of the soil pattern and intended use of the maps.

Order 4 Surveys

Order 4 (or fourth order) surveys are made if general soil information 

is needed about the potential and general management of land for extensive 

uses. The information can be used in locating, comparing, and selecting 

suitable areas for major kinds of land use, in regional land use planning, 

and in selecting areas for more intensive study and investigation.

Field procedures permit plotting of soil boundaries primarily by 

interpretation of remotely sensed data and transecting. Secondary 

documentation types are field observations. Traverses are made in 

representative areas to determine soil patterns, and the information is 

applied to like areas. Transects are made in selected delineations to 

estimate map unit composition. Most map units are associations, but 

some surveys have consociations and undifferentiated groups. Map 

unit components are phases of soil series, taxa above the series, or 

miscellaneous areas. Minimum size of delineations is about 16 to 252 

hectares (40 to 640 acres). Contrasting minor components vary in size 

and amount within the limits permitted by the kind of map unit used. 

Base map scale is generally 1:63,360 to 1:250,000.

Order 5 Surveys

Order 5 (or fifth order) surveys are made to collect soil information 

in very large areas at a level of detail suitable for planning regional land 

use and interpreting information at a high level of generalization. The 

primary use of this information is selection of areas for more intensive 

study.

Field procedures consist of mapping representative areas 39 to 

65 square kilometers (15 to 25 square miles) in size to determine soil 

patterns and composition of map units. This information is then applied 
Soil Survey Manual 275

to like areas by interpretation of remotely sensed data. Soils are identified 

by a few onsite observations or by traversing. Map units are typically 

associations but may include some consociations and undifferentiated 

groups. Map unit components are phases of taxa above the series level 

and miscellaneous areas. Minimum size of delineations is about 252 to 

4,000 hectares (640 to 10,000 acres). Contrasting minor components 

vary in size and amount within the limits permitted by the kind of map 

unit used. Base map scale ranges from about 1:250,000 to 1:1,000,000 

or smaller.

Two Orders of Soil Survey in the Same Project

Some soil survey areas have two or more separate and distinct parts 

with different needs. For example, one part may be mapped to make 

predictions related to irrigation and the other may be mapped to make 

predictions related to range management. For the irrigated part, areas are 

mapped at the intensity required for an order 2 soil survey and map unit 

components are mostly consociations of narrowly defined phases of soil 

series. For the rangeland part, areas are mapped as an order 3 survey and 

map units are associations, complexes, and some consociations of more 

broadly defined phases of soil series or of taxa above the series. Some 

map units of the two parts will consist of the same kinds of soil, but it is 

essential that map units for the two different orders of soil survey maps 

do not have the same names or symbols.

Large, separate, and distinct areas that are within the same project 

but are surveyed by different methods need to be distinguished clearly by 

boundaries on the published soil map or on a small-scale inset map. Each 

part should be identified by a note printed parallel to the line separating 

the areas of each survey order. The two parts need separate legends. The 

parts are considered as distinctly different orders of soil survey, but the 

results are reported in the same publication. The same or different map 

scales may be used for the different survey orders, depending on the 

intended uses.

Many order 2 surveys delineate some map units by methods that are 

less intensive, even though the areas mapped at different intensities are 

intermingled on the map. For example, within an otherwise detailed soil 

map, the delineations of very steep or very stony soils are commonly 

investigated at the intensity normally used for an order 3 survey. 

Other soil surveys include areas consisting of two or more distinctive 

soils that could be mapped separately by detailed soil survey methods 

but are not, because the cost of making the separation cannot be justified. 

For example, a survey area that is mostly productive soils suitable for 
276 Chapter 4

general farming may contain large areas of unproductive sandy soils 

covered with thick brush. Although the sandy areas contain contrasting 

kinds of soil that could be delineated separately, the cost of detailed 

mapping to separate the kinds of soils may outweigh the expected 

benefit. The outer boundaries of the sandy areas are plotted in as much 

detail and with as careful investigation as any other boundaries of the 

soil survey, but the sandy areas themselves are mapped using order 3 or 

order 4 methods. Traverses are made, and the composition of the areas is 

defined in terms of the kinds, proportions, and patterns of the individual 

soils. The delineations are described in the text of the published soil 

survey as soil associations mapped by methods of the appropriate survey 

order.

It is important to note that many soil survey areas in the U.S. were 

recompiled from their original mapping and publication base scale 

during soil survey digitizing in the 1990s. Surveys containing intensively 

managed or populated areas and also vast remote areas or wild lands 

were mapped using multiple map scales that were recompiled and 

digitized as a single base layer at a scale of 1:24,000. Original mapping 

and publication scales can be determined by referencing the original 

printed maps and correlation data.

Correlation Steps

Soil correlation is a multi-step quality assessment process (fig. 4-6) 

that ensures accuracy and consistency both within and between soil 

surveys on both local and regional bases. It involves classifying soils, 

naming map units, and providing accurate interpretations. The purpose 

of correlation is to provide consistency in designing and naming map 

units, provide effective transfer of information to and between users, 

and allow flexibility between the standards used in soil survey and the 

variability scientists observe and document geographically. Correlation is 

a continuous process, from the initial descriptions at the start of mapping 

through the final manuscript, tables, map development, and certification. 

It is the responsibility of all survey team members, and the decisions are 

based primarily on the standards used to create the survey (see table 4-5). 

The correlation process is an integral part of soil survey. It is carried 

out on a continuing basis throughout the course of the project. Soil 

correlation can be described by the following steps: (1) design of map 

units, (2) characterization of map unts, (3) classification of map unit 

components, (4) correlation of map units, and (5) certification.
Soil Survey Manual 277

Figure 4-6

Diagram that illustrates soil correlation is a continuous process, not a single event. The 

process is used to facilitate consistent collection, identification, grouping, and transfer 

of soil information.

Design of Map Units

Every soil survey must begin with a clear understanding of the purpose 

and needs for the project. At a minimum, a project plan must be developed 

to outline the needs of a soil survey. Preferably, all partners in a survey 

create a memorandum of understanding and agree to it. These documents 

outline the scale to be used in making the survey, minimum delineation 

size for map units, kinds of map units, documentation requirements, and 

interpretation needs of the soil survey users. Commonly, there is agreement 

on soil-landscape models to be used and the important soil-forming factors 

and soil orders known in a project area. These documents are essential to 

balancing survey detail, survey costs, and time frames for a project. 

A map unit can be tentatively correlated as soon as it has been 

accurately described and mapped. Map units in a survey are correlated to 

ensure consistency in design and level or order of mapping in a survey 

area. 

Characterization of Map Units

Map unit characterization includes identifying the kind of components 

(see table 4-1 in the “Soil Map Units” section) and the kind of map units to 
278 Chapter 4

use and what data to collect for the soil database. Surveys in high-value, 

heavily used areas may require that most map units be consociations with 

components identified to the series level. Surveys in remote areas used 

primarily for watershed protection or wildlife habitat may only require 

that map units be complexes and associations with components named 

for taxonomic categories above the series.

Classification of Map Unit Components

Soil pedons representing the components of the map unit are described 

and classified to the appropriate taxonomic level (series or higher). In 

addition to pedon description and classification, laboratory characterization 

data are collected, and interpretive features (such as ecological site 

descriptions; see appendix 4) may be developed. The importance of this 

information cannot be overemphasized. The descriptions and data provide 

the basic information needed for complete and accurate interpretation. 

Working from the soil descriptions, supervisory soil scientists can give 

maximum help to the survey team.

Soil taxa (series or higher category) are used to name the components 

making up the map unit. Soil map unit components are correlated internally 

to ensure that classification is consistent and that the recorded properties 

coincide with established taxonomic limits. Property ranges documented 

for the component that extend slightly beyond the taxonomic ranges are 

used to document and interpret the map unit component. Laboratory data 

supports the aggregation or grouping of pedons as well as soil database 

population. Pedons described in U.S. surveys use the Soil Taxonomy 

system of classification. 

There are four main purposes of Soil Taxonomy. The first is to 

facilitate communication among soil scientists. Soil Taxonomy allows 

scientists to group and sort thousands of soil series in a meaningful way. 

Groupings can be made at various map scales by using different levels 

of the classification hierarchy. For the most detailed surveys, soils are 

grouped into the series level of components. Some order 4 surveys may 

use families or phases of higher taxa. Small-scale surveys may use the 

order or suborder level in Soil Taxonomy.

The second purpose of Soil Taxonomy is to provide names for taxa 

that are based on formative elements. For example, soil orders (such as 

Aridisols) begin with a formative element and end in “sol,” the Latin 

word for soil. The taxa names quickly convey information about soil, 

including diagnostic horizons and features, moisture and temperature 

regimes, and natural fertility. Use of the formative elements helps 

organize knowledge about soils.
Soil Survey Manual 279

The third purpose is to provide a link between the conceptual classes in Soil Taxonomy and actual natural bodies of soils. The conceptual 

classes embody current understanding of soil genesis and geographic 

distribution of soils worldwide. Through the correlation process, the 

specific natural soil bodies (components) making up the map units depicted in soil survey maps are assigned names using the taxonomic 

system.

Lastly, Soil Taxonomy provides a way to transfer information and 

technology. It allows the transfer of information about soil properties and 

performance gathered at one location to other locations where the same 

soil occurs.

Correlation of Map Units

The correlation of map units impacts many subparts of a soil survey. 

Similar and dissimilar soils should be consistently and objectively 

evaluated and listed in map unit descriptions and databases to properly 

account for the complexity in a survey. A system of analyzing this 

information should be developed and followed. Analysis methods might 

include the use of spatial analysis software or tabular information in 

databases to identify correct groupings. 

Taxonomic unit descriptions represent the range in characteristics 

of the dominant soils in a survey area. Each map unit should reference 

a typical pedon that describes the range of characteristics for that taxa 

within the survey. The typical pedon, and commonly the taxonomic unit 

description, represent only a portion of the full range in characteristics 

for a given soil series.

Soil interpretations and ratings are correlated to ensure that soil 

suitabilities or limitations are evaluated equitably across the survey. 

Correlation ensures consistency within the map unit descriptions, 

including consistent wording to describe important features and 

consistent use of performance data among map units having the same 

use and management. 

Map unit correlation also includes the proper documentation of the 

map unit history. This includes conceptual changes that may occur over 

the course of a survey project as new areas are identified for use of the 

same map unit. Current surveys maintain and track this history in a soil 

database.

Certification

Soil surveys typically have a formal, final correlation document 

that summarizes all correlation decisions within a survey project. This 
280 Chapter 4

document lists the final versions of map unit and taxonomic legends and 

explains the reasons for combining soils into map units, any classification 

anomalies, and any geographical exceptions. An explanation of the 

correlation of map units and components between adjoining survey areas 

ensures consistency between surveys. Both initial and update surveys 

use a similar process to explain correlation decisions and to present new 

information and data collected to support those decisions.

Correlation documents certify that a soil survey product has followed and met the standards used to make a survey. This certification is 

essential for product delivery. Current delivery of U.S. soil surveys uses 

the publicly available Web Soil Survey (Soil Survey Staff, 2016b).

Quality Control and Quality Assurance

Quality control and quality assurance provide consistency in a survey, 

for mapping, classifying, and naming soils. They also include joining 

maps, database population, and developing interpretations. Survey 

activities that ensure consistency include field visits, field reviews, and 

survey team communication. Quality control is the process of evaluating, 

prioritizing, and coordinating survey activities to ensure that products 

meet the agreed-upon standards and user needs. It is carried out on a 

daily basis by each member of the soil survey team. It requires that each 

member be aware of the standards used in making the soil survey and 

adhere to those standards in their daily activities. Quality assurance is a 

review and assessment process, commonly led by senior soil scientists 

and carried out on a periodic basis. This process provides review of 

completed work and training of staff members to support and ensure 

soil survey quality for users. Progressive reviews of completed work are 

performed to discover and correct errors or inconsistencies within the 

survey and ensure consistent use of standards throughout a soil survey 

project. Problems identified during reviews are corrected and can be used 

to provide training to the survey team. 

It is essential that everyone involved in making a soil survey has a 

thorough understanding of the standards used to conduct it. Table 4-5 lists 

ways standards are used in creating soil surveys. Standards are dynamic, 

changing to meet current needs of users and keep pace with technology. 

It is important to know what the standards were when a soil survey was 

initially made because they will impact soil survey maintenance and 

update.
Soil Survey Manual 281

Table 4-5

Major Applications of Soil Survey Standards

• Designing and controlling map legends

• Identifying, describing, and classifying soils in the field

• Delineating soil boundaries on a map

• Determining map unit composition

• Populating soil databases

• Preparing map unit descriptions

• Selecting and classifying representative pedons

• Naming map units

• Conducting special studies

• Preparing or testing interpretations

• Preparing soil survey manuscript and database

• Preparing correlation documentation

• Making documentation requirements

• Evaluating data

• Developing analytical procedures

Records and Documentation

Keeping definitions and names of soil taxa up to date is essential 

for identification of map units, for correlation of soils nationwide, and 

for transfer of information about soils at one place to similar kinds of 

soil elsewhere. Different methods can be used and periodically modified. 

Some kind of centralized system is needed to obtain a nationwide 

perspective, to maintain standards for defining soil taxa, to assemble 

field and laboratory data, and to disseminate information to the field. See 

chapter 7 for more information.

Soil Series Definitions

Soil series are used for naming most map units in U.S. soil surveys. 

There are currently more than 22,000 series defined and named. Soil 

series definitions are the framework within which most of the detailed 

information about U.S. soils is identified with soils at specific places. They 

also provide the principal medium through which detailed information 

about the soil and its behavior at one place is projected to similar soils at 
282 Chapter 4

other places. The concepts of the series category and of individual series 

have changed over time.

Rigorous standards for definitions of soil series ensure that names 

and descriptions for the same kinds of soils are consistent from survey to 

survey. Consistency is a major objective of the correlation process. The 

classes of the soil series category are not static. As new knowledge is 

acquired, definitions of some established series must be modified. New 

series are defined for newly recognized kinds of soils. Changes in criteria 

or limits of taxa in higher categories commonly require modification of 

definitions of member series.

Keeping records of series names and updating definitions of series 

is a continuous process. Changes should be made in ways that detract 

the least from the predictive value associated with the earlier definitions 

and names. A centralized system for keeping records of soil series names 

and definitions ensures that names and definitions of soil series meet the 

rigorous standards needed in a national soil survey program.

Official Soil Series Descriptions

Each soil series must be defined as fully and accurately as existing 

knowledge permits. This applies to proposed soil series used in an 

individual survey as well as to established series. To ensure the inclusion 

of essential information and to permit comparison of series definitions, a 

standard format for recording specific kinds of information is used.

Official soil series descriptions (OSD) record definitions and other 

relevant information about each series. The format and the kind and 

amount of detail may change from time to time, but a detailed definition 

is essential. Generally, descriptive information is also needed to aid the 

reader in identifying the soil in the landscape and relating it to other 

kinds of soil. 

Since soil series are not listed and described in Soil Taxonomy, 

a system is required to record and store this information in an easily 

accessible format. In the United States, soil series are maintained in the 

Official Soil Series Descriptions database (Soil Survey Staff, 2016a). 

An official soil series description in the U.S. includes the features and 

sections listed below, in the order shown. Some items (such as 1, 2, 4, 16, 

and 17) are partly or wholly applicable only in the U.S. system designed 

to store series descriptions. This list can be modified for other systems. 

See appendix 1 for an example of an official soil series description. 

Features and sections of an OSD:

1. Location line with first instance of series name and the States 

using it (FIPS code) 
Soil Survey Manual 283

2. Status of soil series (tentative, established, or inactive) 

3. Initials of authors (up to three sets of initials; those of the original 

author are listed first) 

4. Date of latest revision in mm/yyyy format (auto-generated if 

using the SC-OSD maintenance tool) 

5. Name of soil series 

6. Introductory paragraph. It includes information on the general 

nature of the soil (including soil depth, drainage, and parent 

material and its probable sources); landscape information 

(including position on landform and slope ranges); and climatic 

information (such as average annual air temperature, annual 

precipitation, frost-free season, and elevation). It may also include 

specific information important to the pedogenic processes and 

landscape evolution for the series.

7. Taxonomic class. The full taxonomic name of the family taxon 

is given. It indicates the classes that provide limits of properties 

that are diagnostic for the series at all categorical levels, except 

for those between series of the same family.

8. Typical pedon. A typical pedon and its horizons are described in 

as much detail as necessary to recognize taxonomic class. Horizons and features that are diagnostic for the pedon are described.

9. Type location. The location at which the pedon was described 

is specified. It is given in geographic unit coordinates and is 

descriptively accurate enough that it can be identified in the field.

10. Range in characteristics. The ranges of properties of the series 

are described. This section also contains statements about the 

relationship of the series control section and diagnostic horizons 

to vertical subdivisions of the typical pedon.

11. Competing series. The series is distinguished from the other 

series within the same or similar class with which it might be 

confused. Competing series commonly share limits with the 

series described or are members of the same family.

12. Geographic setting. The physiography and landscape in which 

the soil occurs are described.

13. Geographically associated soils. Other soils with which the 

series is closely associated geographically are identified.

14. Drainage and saturated hydraulic conductivity (“permeability” in 

older series). Drainage of the soil is described by drainage class 

or other means of description relative to soil moisture regimes 

and the rate of water movement through the soil. Seasonal 

wetness or dryness, if important, is also described.
284 Chapter 4

15. Use and vegetation. Major uses of the soil and dominant kinds of 

vegetation that grow on it are described. Native plants, if known, 

are identified.

16. Distribution and extent. The known geographic distribution 

(generally physiographic areas, States, and MLRAs) is given 

along with whether the soil occupies a small, intermediate, or 

large aggregate area.

17. Soil survey regional office (SSRO) responsible. This is the 

NRCS soil survey regional office that provides quality assurance 

review and maintenance of soil series records.

18. Series proposed or series established. Date and location of the 

soil survey project in which the series was established are given.

19. Remarks on diagnostic horizons and features recognized in the 

pedon. All of the diagnostic horizons and features, including 

thickness, depth, and horizons needed in determining the taxonomic class, are listed.

20. Additional data as needed. This section is generally used to 

document sampling and laboratory analysis associated with the 

series and information specific to the survey or investigation 

project. 

Other items that may enhance the official soil series descriptions for 

a broader audience include:

• Pictures of the profile and landscape setting for individual series, 

and

• Ability to search archived series descriptions for diagnostic 

features, horizon thicknesses, and other soil characteristics that 

need to be interpreted.

Soil Handbook

The descriptive legend is the main document governing field 

operations, but it is only part of the information compiled during a 

survey. The descriptive legend—the basic document of a soil survey—

consists of four parts: (1) description and classification of the soils, (2) 

identification legend, (3) conventional and special symbols legend, and 

(4) general soil map and legend. The descriptive legend and the other 

information about the soils in the survey area are organized into a soil 

handbook (not to be confused with the National Soil Survey Handbook,

which is the repository of NCSS policy and guidance). The soil handbook 

is used by the field team and by engineers, agronomists, planners, and 
Soil Survey Manual 285

others who need information about the soils of the area before the survey 

is completed.

The soil handbook contains everything needed for the published 

soil survey, plus material that is important to the soil scientists making 

the survey. A detailed outline for the text of the published soil survey 

should guide development of the handbook. The descriptive legend and 

soil handbook should follow the same format that will be used for the 

published soil survey. A soil handbook that is kept up to date as mapping 

progresses will require a minimum amount of editing after the mapping 

has been completed.

In addition to the mapping legend, a soil handbook includes interpretations and general sections covering topics related to the kinds of soil 

in the area, such as climate, physiography, relief, drainage, geology, and 

vegetation. This information improves the understanding of the properties, distribution, use, and management of the soils.

A record of the extent of each map unit is also maintained. In some 

surveys, the map unit extent is recorded progressively as the field sheets 

are completed. In other surveys, progressive extent records of each map 

unit are kept only until the unit is deemed extensive enough to keep in 

the legend. The final tally is made after the survey has been completed.

Some items prepared for the mapping legend or handbook may be 

incorporated into different sections of the publication. For example, the 

genetic key and classification table could become part of the section on 

how the soils formed and how they are classified. Some diagrams could 

be used in that section as well as in the section on the general soil map.

Description and Classification of the Soils

The descriptive legend includes descriptions of the soil taxa as they 

occur in the survey area and descriptions of map units delineated on 

field sheets. These descriptions form the primary reference document for 

identifying kinds of soils and miscellaneous areas and provide the information needed for proper classification, correlation, and interpretation. 

They also provide the information needed to recognize the map units in 

the survey area. Descriptions of the taxa and the map units, including the 

ranges in characteristics within the survey area, ensure that all members 

of the field team classify and map the soils consistently. Creating a clear, 

concise, accurate, and complete set of descriptions of the soils is a difficult and important job.

An up-to-date record of what has been learned about the soils is 

especially important when members of a survey team change. If the 

project leader leaves before completion of the survey area, an up-to-date 
286 Chapter 4

descriptive legend of how the soils have been classified and mapped 

ensures continuity in survey operations.

The project leader organizes the information that has been gathered 

about the soils in an area. While preparing the descriptions, the project 

leader may discover items that need clarification or supporting field 

data. Field studies can then be planned to clarify concepts and improve 

knowledge of the soils.

Guidelines for describing soils presented earlier in this chapter 

emphasize individual pedons and polypedons used to define soil map 

unit components. The soil descriptions in the descriptive legend give the 

properties of pedons and polypedons plus the extent of the components 

in each map unit, the variations in properties and in extent of components 

from one delineation to another throughout the survey area, and the 

geographic relationships of components within each map unit and of 

map units to each other. Compete descriptions of the soils in the survey 

are made from detailed field descriptions of pedons and polypedons, 

laboratory data, brief notes about internal properties and surface features, 

and summaries of transects. Appendix 2 gives an example of a map unit 

description.

As the descriptions of the soils are prepared, each map unit 

description is compared with the standard definition of the soil for 

which it is named and with the descriptions of closely related soils. The 

classification of the soils must be consistent with the descriptions of the 

components in the map units and also with the standard definition of 

series or other taxa.

A table of classification is included in the descriptive legend and 

shows how soils in the survey area fit into the national system of soil 

classification as presented in Soil Taxonomy. If soil series are used in 

naming map unit components, the table can list the series alphabetically, 

followed by the classification. Otherwise, the soils can be arranged 

according to the appropriate families, subgroups, etc.

The nature, kind, position, and amount of minor components are 

also described for every map unit. The extent, position, and significant 

differentiating characteristics of soils that are dissimilar to the named 

components of the map unit are particularly important. The extent and 

nature of minor components that have interpretative or management 

characteristics similar to those of the major components should also be 

determined.

Written descriptive records of the soils are references for an ongoing 

soil survey. The properties of a soil commonly vary from one part of a 

survey area to another and may be evaluated differently as a result of 
Soil Survey Manual 287

increased experience in the area. The soil descriptions are continually 

revised and updated as mapping progresses. During mapping, new map 

units and taxonomic units are commonly added and units determined to 

be of limited extent are discontinued.

As mapping progresses, kinds of soils that do not fit any map units 

in the legend commonly are discovered. If the kind of soil is extensive 

and uniquely different from the soils in other map units, it is added to the 

legend after it has been defined by a project member and approved by 

supervisory soil scientists of the cooperating agencies. Some new kinds 

of soil can be treated by redefinition of existing map units, and others 

can be treated as minor components. New, approved map units must 

be promptly listed in the legend and defined so that all members of the 

project team can use them correctly.

Some soils are so limited in extent that they should be included 

in other map units. It may be best to combine two or more soils that 

have similar use and management in one map unit. Soils that are so 

closely intermingled that they cannot be delineated separately should 

be mapped as complexes. Deletions and other changes are not made 

formally until the supervisory soil scientists have reviewed the proposed 

legend changes and deemed them acceptable. If proposed changes are 

not acceptable, the agency representatives and the project leader resolve 

any issues. A complete record is kept concerning changes in map units 

and the disposition of any discontinued map unit. Any changes made 

between field reviews are recorded in the report of the next field review.

Distinctions between map units must be larger than the ranges that 

normally occur in measuring diagnostic properties and locating soil 

boundaries. The soil descriptions must be tested to ensure that the map 

units are recognized and delineated consistently.

Progressive mapping by the field team is a continuing test of the 

legend. Inadequacies are evaluated, and any necessary changes are 

made in the legend. Changes are recorded on all copies of the legend, 

and each soil scientist in the team must clearly understand the new 

concepts.

Field notes are summarized periodically, and the summary is recorded 

in the revisions of the soil descriptive legend. If observations are not 

summarized and recorded promptly, they may be lost or not used by other 

members of the survey project.

Field reviews also test the legend and its use in mapping to determine 

whether survey objectives and requirements are being met. Such reviews 

typically involve supervisory soil scientists and representatives of cooperating agencies.
288 Chapter 4

Identification Legend

A symbol is placed in each delineation on the map for identification. 

The identification legend is a list of these symbols and the names of the 

map units they represent. In some legends, the names of the map units 

are listed alphabetically, followed by their symbols. This list of names 

is used by soil scientists as they map. In other legends, the symbols are 

listed numerically or alphabetically, followed by their names. This list is 

used by everyone who reads the maps. Typically, both lists are prepared.

The identification legend links names of map units to delineations on 

the soil maps through the map unit symbols. Many conventions and systems 

are used for selecting symbols. The choice of symbols is unimportant 

provided that the symbols are short, that each symbol is unique, and that 

the map unit that each symbol represents is named and described.

All symbols must be legible on the maps when viewed on a computer 

screen or in hard-copy printouts. Long symbols are problematic. If they 

are made small enough to place on the map they may be illegible. They 

commonly must be placed outside small delineations and arrowed to 

them. This increases the chance of error. Experience and tests have 

shown that map users have difficulty reading field sheets that have many 

symbols placed outside the areas to which they correspond. If the symbol 

is arrowed from a large delineation to a small one, many users assume 

that it represents the large delineation. In addition, potentially confusing 

combinations in symbols should be avoided. They include the lowercase 

letter l with the number 1 and the capital letter O with the number 0 (zero). 

While map unit symbols consisting of numbers are simplest to manage, 

care is needed to ensure that they are distinguished from other numbers, 

such as coordinates, grid numbers, and other numeric attribution that 

may appear on finished maps.

The map symbols are primarily used to identify map units delineated 

within the polygons. Annotation using soil map symbols connotative 

with a particular soil or property should be avoided. Connotative symbols 

typically result in a legend that fails to achieve its primary purpose. Any 

connotative value of symbols may be offset by decreased legibility of the 

map. Use of connotative map symbols can lead to confusion and mistaken 

association of map symbols to soil component names, especially when 

map legends from adjoining survey areas are viewed together. Map users 

must not assume that connotative symbols or even the map unit names 

describe all of the important soil properties. The set of soil descriptions 

(map unit and taxon descriptions) is essential to the purpose of the 

soil survey and should be used by mappers and by those who need the 

information while the survey is in progress.
Soil Survey Manual 289

Using the same or similar symbols during the mapping process 

and on published maps accelerates map compilation because it reduces 

the amount of time compilers spend converting one set of symbols to 

another. It also reduces the amount of errors. It is most practical in areas 

where soils are well known. It is less practical if soils are not well known 

at the start of the survey, because symbols can change during mapping 

and correlation.

Taxonomic Legend

Taxonomic legends list all soil component names appearing in map 

unit names for a survey area followed by their full taxonomic classification. 

The names of series that are proposed are typically followed by “(P).” 

The names of series used for a soil taxadjunct are followed by “(T).” The 

taxonomic classification as observed and described in the survey area is 

used for the taxadjuncts. 

Conventional and Special Symbols Legend

Conventional symbols on soil maps show many natural and cultural 

features other than map units and their boundaries. They help users 

locate delineations. Special symbols identify some areas of soils or 

miscellaneous areas that are too small to be delineated at the scale of 

mapping. All symbols must be defined. Definitions of special symbols 

should specify the size of area that each represents.

General Soil Map and Legend

The general soil map helps the field team in mapping and organizing 

fieldwork. It also provides, for any user of the soil survey, a general overview and introduction to the major soils and their pattern of occurrence 

in the survey area. The draft of the general soil map prepared during 

preliminary field studies is refined as more is learned about the soils. The 

properties, distribution, and extent of the soils in each general area and 

their suitabilities, limitations, and potentials are described. Significant 

differences in soil moisture or soil temperature between areas can also be 

shown on the general soil map.

Soil Maps Made by Other Methods

Although most soil maps published in the U.S. by the National 

Cooperative Soil Survey are made from field investigations, some are 

compiled from other sources. These kinds of soil maps are described below.
290 Chapter 4

Generalized Soil Maps

Some users need soil information about areas larger than individual 

fields or tracts, perhaps as large as several square kilometers. A detailed 

map tends to obscure the broad relationships. Generalized soil maps are 

made to reveal geographic relationships that cannot be seen readily on 

detailed maps. Most soil survey reports include a general soil map for the 

area. The scale of these maps depends on the intended uses. 

Generalized soil maps are made by combining the delineations of 

existing soil survey maps to form broader map units. A detailed map is 

generalized by enclosing those larger areas within which a few kinds of 

soil predominate in relatively consistent proportions and patterns. On the 

generalized soil map, detailed map units are commonly grouped based 

on repeating landscape segments and broader physiographic areas. The 

larger areas are described in terms of the dominant soils. The map is 

interpreted to show the combined effects of the constituent soils of each 

map unit.

Generalized soil maps are commonly used to appraise the basic soil 

resources of whole counties, to guide commercial interests, and to assist 

farm advisors. They serve as a basis for targeting and implementing 

agricultural and conservation programs. These maps are increasingly 

becoming the base maps for county and regional land use planning and 

for predicting the general suitability of large areas of soils for residential, 

recreational, wildlife, and other nonfarm uses. 

The Digital General Soil Map of the United States (STATSGO2) is 

the nationally coordinated State-level general soil map of the U.S. It is 

produced at a scale of 1:250,000 for most of the U.S. and its territories 

and at a scale of 1:1,000,000 for Alaska. The level of mapping is designed 

for broad planning and management uses covering State, regional, and 

multi-State areas. STATSGO2 is comprised of general soil map units and 

is maintained and distributed as a spatial and tabular dataset.

Schematic Maps

Schematic maps (also called reconnaissance maps) differ from 

generalized soil maps in being compiled from information other than preexisting soil maps. Scale is commonly 1:1,000,000 or smaller, although 

maps made at larger scales can be useful in some cases. Schematic 

soil maps are commonly made as a preliminary step in locating areas 

where further investigation is justified. For many areas, especially 

in undeveloped regions, a schematic soil map is useful in advance of 

an organized field survey. Some maps serve as the only source of soil 

information in areas where more intensive studies are not feasible.
Soil Survey Manual 291

Schematic soil maps are made by using many sources of information 

to predict the geographic distribution of different kinds of soil. Information about climate, vegetation, geology, landforms, and other factors 

related to soil are gathered and studied. Data obtained by remote sensing 

techniques, including aerial photography and satellite multispectral band 

imagery, may be useful. Any available information about the soil is used 

to the extent justified by its quality. Some soil information is available 

for most parts of the world, but the information for remote areas may be 

mainly notes by travelers and rough maps interpreted from aerial photographs and never verified on the ground.

Thematic Maps

Thematic maps are created by combining delineations of soil maps 

based upon a singular property theme, including soil features (e.g., surface 

texture, depth to water table, or salinity). They commonly represent 

interpretative qualities, such as suitability for septic tank absorption 

fields, land capability classification for farming, or hazards to use (such as 

flooding). Thematic maps provide a geographic comparison of a singular 

soil quality or feature across broad land areas. The use of digital soil map 

products and GIS systems together with soil attribute databases enables 

rapid creation and manipulation of thematic soil maps that can be easily 

understood by land managers and policy makers. 

GIS technologies and digital mapping techniques (see chapter 5) are 

extremely valuable in developing generalized, schematic, and thematic 

maps. Combining digital data drawn from other sources with known soil 

information can increase the precision of map line placement as well as 

improve the purity of the composition or consistency in identification of 

soil components. 

Supporting Data

Data collected during the course of a soil survey is recorded and 

analyzed, and then integrated in mapping, interpretation, and correlation 

decisions. The most notable types of supporting data and information 

developed are transects, field notes, photographs, laboratory analyses, 

investigations, special interpretations, climatic data, geology maps, vegetation maps, and research reports. 

Notes are indispensable parts of the mapping legend. Some notes 

are used in revising the descriptive legend, which becomes incorporated 

into the manuscript for publication. Notes help make mapping faster and 
292 Chapter 4

more accurate. They may record tonal patterns on aerial photographs 

that are peculiar to a certain map unit, the relationship between minor 

but key indicator plants, or surface configurations that have little bearing 

on use or management but help the mapper locate significant soil areas. 

Notes and other information needed in mapping, but not intended for 

publication, can be kept on separate sheets after each taxon or map unit 

description in the descriptive legend.

Photographs of soil profiles can be very effective in illustrating some 

soil features. Photographs or diagrams of soil systems and landscapes 

show the relationships of soils to various landscapes. Cross-sectional and 

three-dimensional diagrams of parts of the survey area are also helpful.

References

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Gile, L.H., R.J. Ahrens, and S.P. Anderson (eds.). 2003. Supplement to 

the Desert Project Soil Monograph: Soils and landscapes of a desert 

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Platt, J.R. 1964. Strong inference. Science 146:347-353.

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