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SSM - Appendix 4. Web Soil Survey

By Kenneth Scheffe and Soil Science Division Staff.
 

Soil Survey Maps and Map Products

Web Soil Survey (WSS) is the largest natural resource information delivery system in the world. It is the primary delivery mechanism for the maps and data of the National Cooperative Soil Survey and is operated by the USDA Natural Resources Conservation Service. Information can be displayed as maps (fig. A-2) or in tables. The user selects an area of interest on a map and then can view and print a soils map of the area. The user can also access additional soil data for the area. The mapping can be used for natural resource planning and management by landowners, townships, counties, and others. Some knowledge regarding soils data and map scale is necessary to avoid misunderstandings. WSS is updated and maintained as the single authoritative source of soil survey information.

Figure A-2. Soil map showing an area of interest on the Southern High Plains of western Texas and eastern New Mexico. The area is part of Major Land Resource Area 77C in Land Resource Area H. Note the distribution of map unit OcA (Olton clay loam, 0 to 1 percent slopes).The data system supporting WSS is the SSURGO (Soil Survey Geographic) database, which consists of spatial and tabular databases. SSURGO datasets consist of digital map data, tabular data, and information about how the maps and tables were created. The extent of a SSURGO dataset is a soil survey area, which may consist of a single county, multiple counties, or parts of multiple counties.

Soil maps generated in WSS show the soil map unit names and symbols. A legend of conventional and special symbols appearing on the soil maps (fig. A-3 and table A-3) is also generated. The maps are linked to information about the component soils and their properties for each map unit. Each map unit includes up to three major components and some minor components. Web Soil Survey allows map-based display and tabular data for: (1) soil properties and qualities, (2) interpretive ratings (suitabilities and limitations) for various uses, (3) soil reports, and (4) ecological site assessments.

Figure A-3. The map legend and conventional symbols found on soil maps.

Table A-3. Map Unit Symbols and Names Displayed on the Soil Map for the Area of Interest
Curry County and Southwest Part of Quay County, New Mexico (NM669)
Map Unit
Symbol
Map Unit Name Acres
in AOI
Percent
of AOI
AcA Acuff loam, 0 to 1 percent slopes 1,052.6 17.5%
AcB Acuff loam, 1 to 3 percent slopes 373.9 6.2%
AfA Amarillo fine sandy loam, 0 to 1 percent slopes 28.6 0.5%
AfB Amarillo fine sandy loam, 1 to 3 percent slopes 23.6 0.4%
BcA Bippus clay loam, 0 to 2 percent slopes, occasionally flooded 25.5 0.4%
EsA Estacado loam, 0 to 1 percent slopes 86.0 1.4%
EsB Estacado loam, 1 to 3 percent slopes 137.6 2.3%
KmB Kimberson gravelly loam, 0 to 3 percent slopes 80.6 1.3%
OcA Olton clay loam, 0 to 1 percent slopes 4,083.0 67.8%
PsB Posey fine sandy loam, 1 to 3 percent slopes 113.4 1.9%
RcA Ranco clay, 0 to 1 percent slopes, frequently ponded 5.6 0.1%
SpA Sparenberg clay, 0 to 1 percent slopes, occasionally ponded 10.8 0.2%
Totals for Area of Interest 6,021.1 100.0%


Soil properties and qualities are presented as aggregate values or classes that were measured, observed, or estimated for each soil component of map units in the survey area. A broad array of physical and chemical properties, as well as soil qualities and features (such as depth, drainage class, and hydrologic soil group) are displayed on a thematic map and in tabular format.

Suitabilities and limitations are soil ratings for various uses, such as agricultural production, engineering, urban development, and waste and water management. Tables list the properties or qualities that limit a soil’s suitability for given uses. The interpretations are displayed as thematic maps with a summary table for the soil map units in the selected area of interest.

For each map unit, a single value or rating is generated by aggregating the interpretive ratings of individual map unit components. This aggregation process is defined for each interpretation. Aggregation is the process by which a set of component values is reduced to a single value that represents the map unit as a whole. Once a single value for each map unit is derived, a thematic map for soil map units can be rendered. Aggregation is necessary because map units are delineated but individual components are not. For each component in a map unit, a corresponding percent composition is recorded. For example, a percent composition of 60 indicates that the corresponding component typically makes up approximately 60 percent of the map unit. Percent composition is a critical factor in some, but not all, aggregation methods. Table A-4 lists the various aggregation methods.

Table A-4. Aggregation Methods
[These methods determine the attribute value for thematic maps of soil properties and interpretative ratings in WSS.]
Method Description
Dominant Condition Groups components in a map unit based on like-values for the attribute. For each group, percent composition becomes the sum of the percent composition of all components in the group. These groups therefore represent conditions rather than components. If more than one group shares the highest percent composition, a corresponding tie-breaker rule determines which value is returned.
Dominant Component Returns the attribute value associated with the component that has the highest percent composition in the map unit. If more than one component shares the highest percent composition, a corresponding tie-breaker rule determines which value is returned.
Most Limiting Suitable only for attributes used to generate a soil suitablity rating for a particular use. The most limiting result among all components of the map unit is returned. This method may or may not represent the dominant condition. The result may be based on the limitations of a map unit component of minor extent.
Least Limiting Suitable only for attributes used to generate a soil suitablity rating for a particular use. The least limiting result among all components of the map unit is returned. This method may or may not represent the dominant condition. The result may be based on the limitations of a map unit component of minor extent.
Weighted Average Computes a weighted average of the value for all components in the map unit. Percent composition is the weighting factor.
All Components Returns the lowest or highest attribute value among all components of the map unit, depending on the corresponding tie-breaker rule. In this case, the tie-breaker rule indicates whether the lowest or highest value among all components is returned. For this aggregation method, percent composition ties cannot occur. The result returned represents either the minimum or the maximum value of the corresponding attribute throughout the map unit. The result may be based on a map unit component of minor extent.
Absence/ Presence Returns a value, for all components of a map unit, that indicates if a condition is always present, never present, or partially present or whether the condition’s presence or absence is unknown.
No Aggregation Necessary Although the majority of soil attributes are associated with a component of a map unit, some are associated with a map unit as a whole. An attribute of a map unit does not have to be aggregated in order to render a corresponding thematic map. Therefore, the “aggregation method” for any attribute of a map unit is referred to as No Aggregation Necessary.
Component Percent Cutoff Components whose percent composition is below the cutoff value are not considered. If no cutoff value is specified, all components in the database are considered.
Tie-Break Rule Indicates which value should be selected from a set of multiple candidate values, or which value should be selected in the event of a percent composition tie.
Interpret Nulls as Zero Indicates if a null value for a component should be converted to zero before aggregation. This conversion is done only for map units that have at least one component for which the attribute value is not null.
Layer Options For an attribute of a soil horizon, a fixed depth range must be specified. Either centimeters or inches may be used, but the bottom depth must be greater than the top depth. The top depth can be greater than zero. When “Surface Layer” is specified, only the surface layer or horizon is used to derive a value for a component. When “All Layers” is specified, all layers recorded for a component are considered when deriving the value for that component. Whenever more than one layer or horizon is considered, a weighted average value is returned based upon layer or horizon thickness.
Month Range For an attribute that is recorded by month, a range of months must be specified.


Soil reports include various formatted tabular and narrative reports (tables) containing data for each soil map unit in the selected area of interest and for each component of each soil map unit. The reports contain soil interpretive information as well as basic soil properties and qualities, but do not require aggregation of data. Soil reports are organized by category, such as “Recreational Development.” A description of each report (table) is available.

Examples of Maps and Reports

The following pages demonstrate a few of the many maps that can be generated in Web Soil Survey with the full integration of spatial and tabular databases. Over 100 thematic maps for various suitability or limitation ratings have been developed as well as almost 50 thematic maps of soil properties and qualities. Each thematic map includes a tabular report by map unit and component for the thematic data. In addition, over 60 tabular reports of various combinations of soil interpretations, properties, and features can be generated.

Land Capability Class

Land capability classification shows, in a general way, the suitability of soils for most kinds of field crops (fig. A-4). Crops that require special management are excluded. The soils are grouped according to their limitations for field crops, the risk of damage if they are used for crops, and the way they respond to management. The criteria used in grouping the soils do not include major and generally expensive landforming that would change slope, depth, or other characteristics of the soils, nor do they include possible but unlikely major reclamation projects. Capability classification is not a substitute for interpretations that show suitability and limitations of groups of soils for rangeland, for woodland, or for engineering purposes.

Figure A-4. Map showing land capability class.Capability classes, the broadest groups, are designated by the numbers 1 through 8. The numbers indicate progressively greater limitations and narrower choices for practical use. The classes are defined as follows:

Class 1 soils have few limitations that restrict their use.

Class 2 soils have moderate limitations that reduce the choice of plants or that require moderate conservation practices.

Class 3 soils have severe limitations that reduce the choice of plants or that require special conservation practices, or both.

Class 4 soils have very severe limitations that reduce the choice of plants or that require very careful management, or both.

Class 5 soils are subject to little or no erosion but have other limitations, impractical to remove, that restrict their use mainly to pasture, rangeland, forestland, or wildlife habitat.

Class 6 soils have severe limitations that make them generally unsuitable for cultivation and that restrict their use mainly to pasture, rangeland, forestland, or wildlife habitat.

Class 7 soils have very severe limitations that make them unsuitable for cultivation and that restrict their use mainly to grazing, forestland, or wildlife habitat.

Class 8 soils and miscellaneous areas have limitations that preclude commercial plant production and that restrict their use to recreational purposes, wildlife habitat, watershed, or esthetic purposes.

Hydrologic Soil Group

Hydrologic soil groups are based on estimates of runoff potential (fig. A-5). Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long duration storms. The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D) where the first letter is for drained areas and the second is for undrained areas. Only the soils that are in group D in their natural condition are assigned dual classes.

Figure A-5. Map showing hydrologic soil groups. Soils in group A are most permeable and soils in group D least permeable. Dual classes (e.g., C/D) indicate hydrological soil groups for both the drained and undrained conditions.

Ecological Site Assessments

Ecological site assessments document the ecological conditions and plant communities correlated to components of the soil map units. They provide maps (fig. A-6), descriptions, tables, illustrations, and photographs (fig. A-7). They include information on species composition, annual production, and growth curves and a state-and-transition diagram (fig. A-8).

Figure A-6. Map showing ecological sites. The dominant ecological site is Deep Hardland, 16-21” PZ (R077CY022TX).Figure A-7. Shortgrass/blue gramma dominant community of the Deep Hardland ecological site (R077CY022TX).Figure A-8. State-and-transition model showing pathways and causes of change in the plant communities.An ecological site is the product of all the environmental factors responsible for its development. It has characteristic soils that have developed over time; a characteristic hydrology, particularly infiltration and runoff, that has developed over time; and a characteristic plant community (kind and amount of vegetation). The vegetation, soils, and hydrology are all interrelated. Each is influenced by the others and influences the development of the others. For example, the hydrology of the site is influenced by development of the soil and plant community. The plant community on an ecological site is typified by an association of species that differs from that of other ecological sites in the kind and/or proportion of species or in total production.

An ecological site name provides a general description of a particular ecological site. For example, “Loamy Upland” is the name of a rangeland ecological site. An ecological site ID is the symbol assigned to a particular ecological site.

The “Dominant Ecological Site” map identifies the dominant ecological site for each map unit, aggregated by dominant condition. Other ecological sites may occur within each map unit. Each map unit typically consists of one or more components (soils and/or miscellaneous areas). Each soil component is associated with an ecological site. Miscellaneous areas, such as Rock outcrop, Sand dunes, and Badlands, have little or no soil material and support little or no vegetation. These areas are not linked to an ecological site. The table “Ecological Sites by Map Unit Component” lists all of the ecological sites for each map unit component in the area of interest.

Engineering Properties

Table A-5 gives the engineering classifications and the range of engineering properties for the layers of each soil in the survey area. Included are hydrologic soil group, USDA texture, Unified and AASHTO classification, coarse fragments, percent of soil passing standard sieves, liquid limit, and plasticity index.

Table A-5. Engineering Properties and Classifications.

Soil Chemical Properties

Table A-6 shows estimates of some chemical characteristics and features that affect soil behavior. These estimates are given for the layers of each soil in the survey area. The estimates are based on field observations and on test data for these and similar soils.

Table A-6. Soil Chemical Properties.