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
|
