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Chapter 15 Soil inventories and mapping.


Author unknown

"Maps are a way of organizing wonder." Peter Steinhart, 1986


Soil classification is a useful endeavor because without it you would not have a full understanding of how the soil in a particular location is constructed, and therefore you would not have a good idea about how it can be managed. In previous chapters the management practices for erosion, organic matter, water, and fertility were all based on fundamental properties of the soils: what they were made from, how the soil particles were held together and influenced water flow, and the potential nutrient-holding and nutrient-releasing capacity.

Surveying and mapping build on that classification knowledge to provide information that will identify the appropriate uses for soils. Once the identity of different soils is determined by survey, maps can be generated to show their distribution throughout landscapes, and then the properties of the soils within these landscapes can be used to make decisions about sustainable land use.

The objectives of this chapter are to briefly describe how soil surveys are conducted and the products of those surveys, and then to discuss the types of information gathered from soils within a mapping unit that are significant in determining its use.


After reading this chapter, you should be able to:

* Understand how soil surveys are conducted.

* Appreciate the difference between individual soil units and associations.

* Describe how the presentation of soil survey data has changed with time.

* Interpret some basic information from a soil survey such as the soil series name and slope.

* List some basic soil characteristics that will determine land use.




generalized soil maps


land capability class

mapping unit


soil association

soil survey


undifferentiated soils

The NRCS, Forest Service,
and BLM all carry out soil

The Natural Resources Conservation Service (NRCS) has the overall responsibility for making soil surveys in the United States and developing the inventory of the nation's soil resources. In addition, soil surveys are conducted by the U.S. Forest Service and the Bureau of Land Management (BLM) for the soils over which they have jurisdiction. Soil surveys are published on a county-by-county basis, and the most recent surveys have been prepared in digital versions and can be obtained electronically. The soil survey will contain a map of soil locations, descriptions of the soils, and interpretations for their agronomic and engineering use.

Development of Survey Maps
Soil mapping in the
United States is not yet

Early soil survey maps were usually published as a single sheet at a scale of 1:63,650 (1 inch per mile). Soils were color-coded and contained letter symbols for identification. Obvious types of drainageways and relief were noted, as well as the location of such cultural features as roads, cities, bridges, schools, churches, and cemeteries. Since 1935 most surveys have been based on aerial photography, and are much more detailed with scales of 1:15,840 (1 inch per quarter mile). These maps are published on separate pages and contain much more information about the location of individual soils.
Soil surveys have become
more detailed and
rely more on aerial
photography than surveys
of the past.

The essential tools of the soil surveyor have been the base map (either a geologic map or an aerial map), a spade, an auger or probe, a clinometer to determine slope, a soil color chart or set of color vials, and experience (Figure 15-1). Experience is required to accurately determine texture-by-feel in the field, predict where changes in soil type occur in the landscape so that the number of soil test probes can be minimized, and identify the soil profile with that of previously described soil series. Because mappers are expected to survey large areas on a daily basis (approximately 300 acres or 121 ha per day), speed and efficiency are essential. Soil mapping in the United States continues today because not every county has a published soil survey, and changing information, due to land use and other factors, requires updating existing surveys.


Mapping Units
Map units represent
phases of a soil series.

The areas delineated on a soil map are called mapping units. Most soil-mapping units represent phases of soil series. A phase of a soil series is a further division of that series to a level that has practical management applications. For example, a soil series may occur across a range of slopes that have implications for the way a soil is managed for erosion control. Or a soil series may have been eroded through past use. The erosion does not change the soil series, but it does change how it can be used and how it should be managed.

Soil-mapping units may have inclusions. Inclusions are small areas of one type of soil within a greater expanse of a different soil series. As long as the inclusions don't exceed 15 percent of a mapping unit they are generally not identified. A soil complex is used to distinguish a location where two or more soil series are so intermixed that they cannot be reasonably separated for mapping purposes. In contrast, undifferentiated map units are used when two soil series could be distinguished but are not because their differences are not important.
Soil associations represent
groups of soils that occur
in repeating patterns in

Soil associations are groups of soils that occur in repeating patterns in a landscape. A typical pattern is soil that forms on a ridge, another on the hillside, and a third in the valley. In the McAfee-Maury-Braxton association, for example, the Maury series occurs on ridgetops, the McAfee in moderately sloping hillsides, the Braxton on steeper sloping hillsides, and the Huntington series in the toe slope positions. As you might expect, Huntington soils are colluvial and alluvial soils that have a tendency to flood.

The varied soil series in a soil association can be quite different, but the important thing is that they occur together regularly in the same positions in a landscape. Because soil associations tend to consistently appear, they are used to prepare generalized soil maps. Generalized soil maps are used to characterize large areas, such as a county, and provide a quick overview of what the soils look like in a location.
Catenas are
toposequences forming
from the same parent

A toposequence is a soil association that has differences related to the topography of a site. In a toposequence the soils developed in the same climate and vegetation, but did not necessarily develop with the same parent material. In contrast, a catena is a toposequence that formed entirely from the same parent material.

Interpreting Soil Survey Mapping Units

The soil survey map contains important information in the legend and also in the map units themselves (Figure 15-2). Once the code is understood, it makes reading the survey map much easier, and one can start developing an internal image of what the landscape looks like based on clues about each soil's position and slope. The example from Figure 15-2 is actually very easy to interpret. The letter code for each soil is followed by a letter code for slope and a numerical code for erosion if those are significant characteristics. In some cases the letter code for the soil is replaced by a numerical code for the soil. The same soil series can have a different letter code. For example, MnB and MpC2 both refer to the McAfee soil series. But the former (Mn) has a silt loam texture and the latter (Mp) has a silty clay loam texture. The MpC2 soil also has a steeper slope (C, 6-12 percent vs. B, 2-6 percent) (Table 15-1), and shows evidence that erosion has occurred (the "2" designation), which is not surprising considering the slope (Table 15-2). Knowing how to interpret soil survey maps is a powerful tool in being able to manage land resources.

Letter codes indicate soil
type, slope, and extent
of erosion.

Information in the Soil Survey

There is a wealth of information in a soil survey beyond the distribution of different soil series in the landscape (Figure 15-3). Most soil surveys have four basic components:

1. General soil map unit descriptions.

2. Detailed but nontechnical descriptions of each map unit (i.e., soil series).

3. Use and management descriptions for agriculture and engineering purposes.

4. Technical descriptions of the soil series and its morphology.
Most soil surveys have
four basic components in
addition to the maps of
soil location.


In addition, you can find useful information about specific items such as:

* Suitability ratings for engineering projects.

* Suitability ratings for water management projects such as building reservoirs or installing drainage.

* Suitability ratings for recreational development.

* Potential for cropping and typical yields.

* Woodland suitability and potential trees adapted to the soils.

* Potential for wildlife habitat.


1. Who has authority for collecting and organizing soil survey data in the United States?

2. What was the scale of early soil survey maps? How were the different soil series represented?

3. What is one of the biggest differences between soil survey maps prior to 1935 and those of today?

4. What is a mapping unit?

5. What feature of soil associations makes them useful for developing generalized soil maps?

6. What are the basic components of a soil survey report?

7. What kinds of information are found in a soil survey report?

8. What key information do you typically find in a mapping unit code?

The NRCS also classifies
soils in terms of land
capability class.

The lowest level of classification used by the NRCS is the soil series. However, the NRCS also classifies soils at a higher level of classification, distinct from a taxonomic classification, called a land capability class. The land capability class will group soils on the basis of similar hazards and limitations for use such as their erosion hazard. This type of functional classification may group soils of different taxonomies together. It is more subjective than a soil series classification, but more practical from the perspective of actual land use.

Types of Land Use Classification

There are eight land use classifications:

* Class I: These lands can be cultivated safely with long-term productivity and good yields for adaptable crops without the need for special practices or treatments.

* Class II: These lands cannot be cultivated with long-term productivity to produce moderate to good yields unless some simple practices or treatments are made.

* Class III: The lands require extensive practices or treatments such as contour cultivation, strip cropping, terracing, tile drainage, fertilization, or systematic rotation. The practices listed indicate the sort of limitations these lands have-steeper slopes and potential erosiveness, drainage problems, and low native fertility and/or CEC.

* Class IV: These lands cannot be cultivated safely under any plan for continuous use, but they can be used safely for hay and pasture, which retain a continuous soil cover.

* Class V: These lands cannot be cultivated safely at any time and are only suitable for permanent cover.

* Class VI: These lands have extreme limitations that restrict their use to pasture, range, woodland, or wildlife.

* Class VII: These lands have even more severe limitations than Class VI soils and are primarily restricted to grazing, woodland, and wildlife.

* Class VIII: No plant production should occur on these lands. They should be reserved for recreation, wildlife, plant supply (seed banks), and aesthetic purposes (greenspace).
Land use classes range
from I to VIII, with Class
VIII having the most

Figure 15-4 illustrates how these lands may be positioned within a landscape. You can see how plant use varies (cultivated row crops in the foreground shading into pasture and forest in back) and you can also see the major limitation that controls land use classification at this site: slope.
There are four land use
capability subclasses that
reflect specific
restrictions, such as
erosion potential.

There are also four land capability subclasses that reflect specific limitations to each soil group (Table 15-3). Land use capability units (as opposed to soil-mapping units) have their own distinct code, which is made by adding the subclass code to the capability class designation. For example, a Class II capability soil that is most limited by the potential for erosion would be designated Ile; one that was most limited by moisture would be designated IIw. Table 15-4 illustrates how the soil-mapping units for Mason County, Kentucky are further classified by land use capability.


The NRCS has a color-coding system for land use capability that it uses during the preparation of conservation plans:

* Class I, Light Green

* Class II, Yellow

* Class III, Red

* Class IV, Blue

* Class V, Dark Green or White

* Class VI, Orange

* Class VII, Brown

* Class VIII, Purple


Soil Properties Affecting Land Use

Part of constructing the soil survey involves evaluating the physical and chemical properties of the soil, particularly with respect to their effect on agricultural activity. An outline of some of these measured soil properties is given in Figure 15-6.

Available water-holding capacity is important because it influences the amount of water plants can acquire during the growing season. Water-holding capacity is influenced by texture, organic matter content, compaction, and the depth to restrictive layers. There are at least eighteen types of restrictive layers recognized in soil surveys. Soils are considered restricted because they physically impede root growth or because they inhibit root growth. An example of the latter are saline or alkaline layers in soil.
Some of the properties
measured for land use are
water-holding capacity,
restrictive layers, and
depth to bedrock.

Examples of soil layers that physically impede root growth are cemented pans, permafrost, fragipans, clay pans, and plowpans. Changes in soil texture and bulk density can also affect root growth. Table 15-5 provides some guidance for the maximum bulk density that should occur in different soil textures before root penetration is affected.

The soil survey recognizes five different categories of soil depth to bedrock from very shallow (< 10 inches, 25 cm) to very deep (> 60 inches, 152 cm; Table 15-6). However, the effective rooting depth can be significantly affected by any of these other restrictive layers, which can play a critical role in such things as the siting of on-site waste disposal.

The calcium carbonate (CaC[O.sub.3]) content, as you saw in Chapter 14, influences pH, and sensitive plants can be affected by micronutrient deficiencies (or toxicity in the case of Mo) when there is as little as 0.5 to 2.0 percent CaC[O.sub.3] in soil.

There are seven natural drainage classes used in soil surveys that range from very poorly drained to excessively well-drained (Table 15-7). These drainage classes are closely related to permeability rates in soils, which range from impermeable to very rapid (Table 15-8). Drainage class is an important feature for determining whether soils are suitable for on-site waste disposal from septic systems (Figure 15-7).

Another classification having to do with water is the frequency and duration of flooding that occurs in particular soil groups (Table 15-9). The flooding can be rare or frequent and the duration short- or long-term.



1. In what ways does soil use classification differ from taxonomic classification?

2. How many different land use classifications are there? Which are the most restrictive?

3. What do land use subclassifications indicate?

4. Does mapping on the basis of land use classification create more, or less, heterogeneity of mapping units?

5. What sorts of information contained in the soil survey are useful for agronomic purposes?


What is the value of soil mapping and classification? As you have seen, mapping provides an inventory of where soils are in relationship to one another. Land use classification provides a better sense of a landscape's capability, which is vital for appropriate management and determining whether a particular location is suitable for an intended purpose. Land use classification is useful for revenue because potentially more productive lands can be taxed at a higher rate.

In this chapter you examined how the soil survey evolved and some of the very basics of creating a survey. You examined the different types of mapping units that will be found in a soil survey and how to interpret the codes associated with mapping units. In addition to the information on mapping units, you looked at other types of information that are found in a typical soil survey, which range from a taxonomic description of each soil series in the survey to an evaluation of potential agronomic and engineering uses of each soil.

The second part of this chapter was devoted to land use classification, which is a parallel system for evaluating soils that the USDA-NRCS employs. Land use classification may collect various taxonomically different soils together because it bases associations on criteria such as susceptibility to erosion, drainage, and soil depth. Of the eight land use classifications employed, the first three classes (I-III) are suitable for row crop production with increasing restrictions. The next two classes (IV and V) are suitable for pasture and grazing. The last three classes (VI-VIII) have such severe restrictions on use that they should only be used recreationally or for forest and wildlife.

Much of the soil survey information is devoted to soil characteristics that will affect agronomic use. This information for a particular location is extremely useful in terms of determining management schemes for a particular soil. The data is collected in various tables found in the soil survey that allow different soils to be compared.

In the next chapter you will examine the perception of soil as a commodity, and how social and environmental changes have influenced the way people deal with soils.


1. What are several reasons for conducting soil surveys?

2. Published soil surveys after 1935 are very different from earlier surveys for two reasons. Can you name them?

3. How has the scale of published soil surveys changed with time?

4. Is a soil surveyor expected to map every different soil in a landscape?

5. How do toposequences differ from catenas?

6. What is useful to know about soil associations?

7. How much of a landscape can consist of inclusions before they have to be mapped?

8. What are the basic tools of a soil mapper?

9. What knowledge about soil formation helps a soil mapper reduce the number of soil probes that are made?

10. What's the smallest unit described in a soil survey?

11. In the code LyD3, what does each component of the code indicate?

12. Which soil, LoB, LoC, or LoC2, has the steepest slope? Which is most eroded?

13. Does erosion change soil classification?

14. If the soil survey indicates a soil is alluvial, what should you predict about the slopes of soil series on either side of it?

15. Should you grow corn on Class IV soil? Why or why not?

16. What do the subclassification terms for land use classification tell you? (The following questions refer to Table 15-4.)

17. According to land use classification, what is the best soil in Mason County, Kentucky?

18. Which soil series are suitable for row crop production?

19. Which soil series are only suitable for pasture or grazing?

20. What feature of land use capability in these soils probably most limits use?

21. What are the most likely major problems with the Pits soil series in terms of land use? (The following questions refer to Tables 15-5 to 15-9.)

22. If the bulk density exceeds 1.7 in a loam soil, should you be concerned? Why or why not? What other soil properties in this soil might be affected?

23. Is a soil with a depth of 40 inches to bedrock considered very deep?

If not, what should be its classification?

24. If you observe mottles in the lower A, B, and C horizons, what would you predict is the drainage class of this soil?

25. Explain how a soil can be too well-drained.

26. How does the position of the soil in a landscape affect its drainage? (See Figure 15-12 for clues.)

27. If the permeability of a soil is moderately slow, will 2 inches of rain likely cause runoff? Why or why not?

28. Soils that have occasional or frequent flooding are likely to be what type of soils?

29. If the duration of flooding in a soil exceeds one month, what might be its land use classification, and to what use might you put it?

30. For soils that have a flooding duration greater than seven days, what color might you expect to see in the subsoil?


A very short and succinct introduction to the topics in this chapter can be found in the USDA-NRCS publication From the Surface Down: An Introduction to Soil Surveys for Agronomic Use by W. Broderson (2000). Many of the figures from this chapter were drawn from this source.


Broderson, W. 2000. From the surface down. An introduction to soil surveys for agronomic use. Washington, DC: USDA-NRCS.

Harpstead, M. I., T. J. Sauer, and W. F. Bennett. 1997. Soil science simplified, 3rd ed. Ames: Iowa State University Press.

Troeh, F. R., and L. M. Thompson. 1993. Soils and soil fertility, 5th ed. New York: Oxford University Press.

USDA-NRCS. 1968. Soil survey: Fayette Co. Kentucky. Washington, DC.

USDA-NRCS. 1988. Soil survey of Mason County Kentucky. Washington, DC.

USDA-NRCS. 2004. National soil survey handbook. Washington, DC.
TABLE 15-1 Designation of slope characteristics
for soil-mapping units.

Map Unit Code     %     Slope Description

      A          0-2    Nearly level
      B          2-6    Gently sloping
      C         6-12    Sloping
      D         12-18   Strongly sloping
      E         18-30   Severely sloping
      F         30-60   Steep
                 > 60   Very steep

TABLE 15-2 Designation of erosion characteristics
for soil-mapping units.

Map Unit Code   Description

  0 or none     No erosion
  1 or P        Slight, 0 to 1/3 of the topsoil gone
  2 or R        Moderate, 1/3 to 2/3 of the topsoil gone
  3 or S        Severe, 2/3 or more of topsoil gone, up to 1/3 of
                subsoil gone
  4             Heavy subsoil erosion and deposition of eroded soil

TABLE 15-3 Land capability subclasses.

Code   Description

 e     Eroded
       Existing or potential erosion and runoff
       Lands with slopes > 2%
       Some form of runoff control is needed

 w     Wetness
       Poorly drained or occasionally flooded
       Some soils may be drained, some have been prior converted
       Some soils are statutory wetlands and must be maintained
       as such

 s     Shallow
       Shallow root zone and tillage problems
       Soils tend to be stony, droughty, infertile, or saline
       Some potential for wind and water erosion

 c     Climate
       Rainfall or temperature extremes make farming difficult

TABLE 15-4 Land use capability designations of soil-mapping
units from Mason Co., Kentucky.

Soil Name         Map Symbol   Land Use Capability

Beasley              BaB              IIe
                     BaC2             IIIe
                     BeE3             VIe

Boonesboro           Bo               IIIw

Chavies              ChB              IIe
                     ChC              IIIe

Dumps                Du               VIIIs

Eden                 EdD2             IVe
                     EfE2             VIIe

Elk                  EkB              IIe
                     EkC              IIIe

Fairmount            FrF              VIIe

Faywood              FwB              IIe

Lowell               LoB              IIe
                     LoC              IIe
                     LoD              IVe

Nicholson            NcB              IIe

Nolin                No               IIw

Otwell               OtB              IIe

Pits                 Pt               VIIs

Wheeling             WhA              I
                     WhC              IIIe

TABLE 15-5 Root restriction guide for soil classification
based on texture and bulk density characteristics.

Applicable Textures                               Average Bulk Density

Coarse sand, loamy coarse sand,                          > 1.85
loamy sand, fine sand, loamy fine sand

Very fine sand, loamy very fine sand,                    > 1.80
fine sandy loam, coarse sandy loam,
very fine sandy loam, sandy loam,
loam with < 18% clay

Loam, sandy clay loam, clay loam that                    > 1.70
has 18-35% clay

Silt, silt loam, silty clay loam that has                > 1.60
< 35% clay

Clay loam, sandy clay, clay, silty clay loam,            > 1.50
silty clay with 35-39% clay (> 30% in ertisols)

Clay soil with > 60% clay (except Vertisols)             > 1.36

TABLE 15-6 Designation of depth classes for land use classifications.

Depth (in.)   Designation

<10           Very shallow
10-20         Shallow
20-40         Moderately deep
40-60         Deep
>60           Very deep

TABLE 15-7 Drainage classes used for land use characterization.

Class                      Description

Very poorly drained        Water table at or near the soil surface
                           most of the year
                           Histic epipedons
                           Too wet to support most crops
                           Obvious gleying

Poorly drained             Usually wet
                           Water table close to the soil surface much
                           of the year
                           Obvious gleying

Somewhat poorly drained    Wet for significant periods
                           Mottles in lower A, B, and C horizons
                           A horizon can be thick
                           Crop growth possible, but improves with

Moderately well-drained    Wet for a small but significant part of the
                           Influences crop choice, timing of management
                           Mottles restricted to B horizon

Well-drained               No water table in the soil profile
                           No mottles in the soil solum
                           Optimal for plant growth

Somewhat excessively       Sandy and/or gravely
well-drained or            Poor water-holding capacity
excessively well-drained   No mottling, but plant productivity limited
                           by available water

TABLE 15-8 Permeability classes for land use characterization.

Class                     Permeability Rate

                     in./hr          cm/min

Impermeable        < 0.0015      6.35 x [10.sup.-5]
Very slow          0.0015-0.06   6.35 x [10.sup.-5]-0.003
Slow               0.06-0.20     0.003-0.008
Moderately slow    0.20-0.60     0.008-0.025
Moderate           0.60-2.0      0.025-0.085
Moderately rapid   2.0-6.0       0.085-0.254
Rapid              6.0-20        0.254-0.85
Very rapid         > 20          > 0.85

TABLE 15-9 Designation of flooding frequency and duration
classes for land use classification.

Classification                        Description

Flooding frequency (per 100 years)
  None                                Near 0
  Rare                                0-5
  Occasional                          5-50
  Frequent                            > 50

Flooding duration
  Very brief                          < 2 days
  Brief                               2-7 days
  Long                                7 days to 1 month
  Very long                           > 1 month

FIGURE 15-6 Soil survey information that will influence
agronomic use. (Diagram adapted from Broderson, 2000)

Soil Properties

 Agronomic      Organic     Flooding     Texture     Bedrock
    Use          Matter                              or Pan

  Tillage       [check]      [check]     [check]     [check]

Erodibility     [check]                  [check]

 Irrigation                  [check]     [check]     [check]

  Drainage                   [check]     [check]     [check]

Productivity    [check]      [check]     [check]     [check]

  Conser-       [check]      [check]     [check]     [check]

  Land Use                   [check]     [check]     [check]

   Plant        [check]      [check]     [check]     [check]

 Agronomic         pH        Subsi-        CEC         CaC
    Use                       dence                 [O.sub.3]


Erodibility                                          [check]

 Irrigation     [check]      [check]                 [check]

  Drainage                   [check]

Productivity    [check]                  [check]     [check]


  Land Use      [check]                              [check]

   Plant        [check]                  [check]     [check]

 Agronomic        Bulk       Permea-      Frost     Available
    Use         Density      bility     Potential     Water

  Tillage       [check]      [check]

Erodibility     [check]      [check]

 Irrigation     [check]      [check]                 [check]

  Drainage      [check]      [check]

Productivity    [check]                  [check]     [check]

  Conser-       [check]      [check]                 [check]

  Land Use                   [check]                 [check]


 Agronomic      Salinit/      Water       Wind
    Use        Alkalinity     Table      Erosion

  Tillage                    [check]

Erodibility                  [check]     [check]

 Irrigation     [check]      [check]     [check]

  Drainage      [check]      [check]

Productivity    [check]      [check]

  Conser-       [check]      [check]     [check]

  Land Use      [check]      [check]     [check]

   Plant        [check]      [check]

 Agronomic      Erosion       Slope
    Use         Factors
                  K, T

  Tillage                    [check]

Erodibility     [check]      [check]

 Irrigation     [check]      [check]

  Drainage                   [check]

Productivity                 [check]

  Conser-       [check]      [check]

  Land Use      [check]      [check]

   Plant                     [check]

[check] Indicates that the soil properties listed in the soil
interpretations data base affect the selected agronomic concerns.
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Title Annotation:Section 6 Integrating Soil with Other Resources
Publication:Fundamental Soil Science
Date:Jan 1, 2006
Previous Article:Chapter 14 Soil fertility and nutrient management.
Next Article:Chapter 16 Soil as a natural resource.
