1. Definition
A map unit is a collection of areas defined and named the same in terms of their soil components or miscellaneous areas or both. Each map unit differs in some respect from all others in a survey area and each map unit has a symbol that uniquely identifies the map unit on a soil map. Each individual area, point, or line so identified on the map is a delineation. The MLRA SSO or soil survey project office specially designs map units to meet the needs of the major users in each major land resource area. Map units in adjoining survey areas are comparable especially within the same major land resource area.
Use any category of soil taxonomy, miscellaneous areas, and accompanying terms to name map units. A map unit has specified kinds of soil or miscellaneous areas (map unit components), each with a designated range in proportionate extent. Map units include one or more kinds of soil or miscellaneous area. See Section 627.04 for more information on miscellaneous areas.
2. Design of Map Units
2.1.Design map units to meet the objectives of the soil survey as stated in the memorandum of understanding. Consider the following items in designing a map unit:
- Kinds of map units;
- Phase criteria used to identify map units;
- Kind and intensity of field investigation and documentation;
- Soil properties for which data are required;
- Minimum size management unit relevant to the various uses; and
- Characteristic signature in the landscape that can be recognized from aerial photography, topographic maps, or field observation.
2.2.When map units consistently associate with landforms, landform segments, vegetation, slope gradient, slope aspect, geomorphic position, or other surface observable feature, the consistency of delineations improves. The correlation of map units with these surface features reduces the number of observations and samples needed to obtain a stated degree of confidence.
2.3.The design of map units is flexible but should correspond to the other surveys within the MLRA. A map unit is defined by the important different kinds of soil and miscellaneous areas (components) and their proportionate extent within delineations of the map unit. Map units can have a single component or they can have many components. Chapter 2 of the Soil Survey Manual further discusses the design of map units.
2.4.The components of a map unit are soils or miscellaneous areas. For naming map units the components are often grouped. The following groups of components can be help ful in describing and naming map units.
o The named soils or miscellaneous areas that are dominant and co-dominant in extent;
o Similar soils or miscellaneous areas that may be extensive but not as extensive as the named components; and
o Dissimilar soils or miscellaneous areas that are minor in extent.
Similar soil or miscellaneous area components are those that differ so little from the named components that their soil interpretations for most uses are very similar. The differences for management are small. Dissimilar soil or miscellaneous area components are those that differ enough from the named components to affect major interpretations. The differences for management are large. Soil components are minor in extent when they occupy a small percentage of the map unit. The percentage varies depending on how they effect the use and management of the map unit.
Generally, dissimilar components are considered minor if they are less than 15 percent and limiting to management of the map unit. If they are not limiting to the management, they can occupy up to 25 percent of the map unit and still considered minor in extent. A single component that is dissimilar and limiting should not exceed 10 percent and remain as minor extent. Also, see Chapter 2 of the Soil Survey Manual.
Components, whether major or minor, meet the following criteria:
o Exist in most delineations;
o Add to the understanding of the map unit;
o Are contrasting to all other components in the map unit (do not list similar soils as separate components unless it helps in understanding the map unit); and
o Allow for useful and significant soil data and interpretations to the users.
Documented components that do not meet the above criteria are similar or nonrecurring or isolated features of the map unit. If appropriate, recognize nonrecurring, contrasting components with special or ad hoc features, or point or linear map unit delineations.
2.5.The composition and purity of map units are important in the interpretation of soil maps. Most delineations of a map unit include dissimilar soils or miscellaneous areas of minor extent that are not identified in the map unit name but may be included in the database for the unit. Practical field mapping methods cannot delineate these components at the selected scale of mapping. But they may be associated with a specific landform segment different from that of the named components of the map
unit. Some of these components could be delineated if smaller management units wereneeded.
2.6.Incorporate soils that have properties similar to the named components (similar soils) with the named soils. Likewise, minor components that are contrasting with the named major components, but that are similar to one another, should be correlated to one minor soil component. By doing so, the number of components listed for any map unit is kept to a minimum.
2.7.Attain a defined standard or level of confidence in the interpretative purity of map unit delineations by adjusting the kind and intensity of field investigations. If the objective of the survey requires delineation of areas of dissimilar soils as small as 2 acres in size, the soil scientist must carry out the field investigations in sufficient detail to identify accurately and consistently map 2-acre areas. Investigations that observe map unit boundaries directly and thoroughly provide greater control than those that observe map unit boundaries at moderately spaced intervals.
3. Minimum Size Delineation
The memorandum of understanding for the survey area states the minimum size map unit delineation. It represents the size of an area most users would agree is the smallest area that is managed for an intended land use. The memorandum of understanding also states the map scale. The scale must accommodate legible delineations of the smallest size map unit. A legible delineation is the smallest area on the map that reasonably accommodates a map unit symbol (about 1/2 cm square).
4. Kinds of Map Units
Soils differ in the size and shape of their areas, in their degree of contrast with adjacent soils, and in their geographic relationships due to soil formation or land use. Soil surveys use four kinds of map units to distinguish the different relationships: consociations, complexes, associations, and undifferentiated groups. Table 1 describes and compares these relationships.
4.1.Consociations
In a consociation, delineated areas use a single name from the dominant component in the map unit. Dissimilar components are minor in extent. The soil component in a consociation may be identified at any taxonomic level. Soil series is the lowest taxonomic level. A consociation map unit that is named for a miscellaneous area is dominantly that kind of area and any minor components present do not significantly affect the use of the map unit.
4.2.Complexes and associations
Complexes and associations consist of two or more dissimilar components that occur in a regularly repeating pattern. The total amount of other dissimilar components is minor in extent. The following arbitrary rule determines whether “complex” or “association” is used in the name. The major components of a complex cannot be delineated separately at the scale of mapping. The major components of an association can be delineated separately at the scale of mapping. In either case, because the major components are sufficiently different in morphology or behavior, the map unit cannot be called a consociation. In each delineation of a complex or an association, each major component is normally present though their proportions may vary appreciably from one delineation to another
4.3.Undifferentiated groups
Undifferentiated groups consist of two or more components that are not consistently associated geographically and, therefore, do not always occur together in the same map delineation. These components are included in the same named map unit because their use and management are the same or very similar for common uses. Generally, they are grouped together because some common feature, such as steepness, stoniness, or flooding, determines their 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 included in the same unit. Each delineation has at least one of the major components, and some may have all of them. The same principles regarding the proportion of minor components that apply to consociations also apply to undifferentiated groups.
Table 1. Description of Kind of Map Unit
Type of map unit | Map unit name from: (name soil at any taxonomic level) | % Dissimilar soils not included in map unit name | Other criteria |
Consociation | One soil or misc. area (similar soils or similar misc. areas included with named component) | 15% limiting, 25% nonlimiting, <10% of any one, limiting, very contrasting soil | |
Complex | Two or more soils or misc. areas (similar soils or similar misc. areas included with named component) | 15% limiting, 25% nonlimiting, <10% of any one, limiting, very contrasting soil | Cannot separate the named soils or misc. on the map at the scale used |
Association | Two or more soils or misc. areas (similar soils or similar misc. areas included with named components | 15% limiting, 25% nonlimiting, <10% of any one, limiting, very contrasting soil | Can separate the named soils or misc. on the map at the scale used |
Undifferentiated | Two soils or misc. area (similar soils or similar misc. areas included with named components | 15% limiting, 25% nonlimiting, <10% of any one, limiting, very contrasting soil | A limitation, such as slope or salinity, overrides the primary use to such an extent that a separate map unit is not used for each soil |