**SIMILAR/DISSIMILAR GUIDE FOR USE IN REGION 10 (8/11/2005)**

*Use this guide to assign similar/dissimilar concepts to components and
map units. Although the guide is comprehensive and will cover most
situations, soil components may be encountered that are not addressed in
the guide. If these circumstances arise, please contact your Regional
Soil Data Quality Specialist. An effort will be made to incorporate new
criteria in the guide as necessary to cover these circumstances. **A
report is now available in NASIS to evaluate components based on the
criteria in this guide. The report is available on the Region 10 site
and is named: CORR-MO10 Component Similar/Dissimilar Comparator.***

1.  This similar/dissimilar key is based mostly on soil properties and
    on some soil interpretations. Similar/dissimilar status is based on
    the differences encountered in properties and interpretations,
    either individually or combination.

2.  We identified the following properties and interpretations to be
    used in a key:

> Difference in representative (rv) depth to the top of wet soil
> moisture status (saturation)
>
> Family particle size class
>
> Depth and Kind of Component Restriction
>
> Surface texture
>
> Surface layer rock fragment class
>
> Surface stones and boulders
>
> Erosion class
>
> Slope
>
> Flooding frequency
>
> Surface calcium carbonate content
>
> Organic soils (Dysic versus Euic reaction classes)
>
> Epipedon thickness

**Difference in representative (rv) depth to the top of wet soil
moisture status (saturation):**

> Since the population of soil moisture status based on measured data
> and soil morphology is somewhat variable throughout the Region the
> soil scientist may define the depth differences that represent
> dissimilar and very contrasting. In the ***CORR-MO10 Component
> Similar/Dissimilar Comparator*** report depth differences are defined
> by the soil scientist in the parameter boxes that display at the
> beginning of the report.

**Family particle size class:**

> Sandy or sandy skeletal (sandy and sandy skeletal components with
> average rock fragment content in the series control differing by \>=
> 10 percent would be considered dissimilar)
>
> coarse-loamy, coarse silty, loamy-skeletal, or loamy (\< 18 percent
> clay)
>
> fine-loamy, fine-silty, or loamy (\>= 18 percent clay)
>
> fine or clayey \< 60 percent clay
>
> very fine or clayey \>= 60 percent clay

Difference of one group is dissimilar and 2 groups is very contrasting.

For components with a contrasting particle size class, only the class
for the upper part of the control section will be used. Dissimilar
components will be non-limiting. Components with organic control
sections are very contrasting to components with mineral particle size
control sections.

**Kind and Depth of Component Restriction:**

> Kind of Component Restrictions
>
> Lithic and Paralithic Contacts
>
> Dense Till
>
> Fragipans
>
> Strongly contrasting textural stratification
>
> Sand and or gravel (although not considered a component restriction)

Any difference in kind of component restriction is dissimilar regardless
of the depth.

> Depth classes (in inches): 0 - 10 very shallow
>
> 10 - 20 shallow
>
> 20 - 40 moderately deep
>
> 40 - 60 deep
>
> \> 60 very deep
>
> Difference of one class is dissimilar and 2 classes is very
> contrasting. Components in adjacent (shallow, moderately deep, or
> deep) classes with representative (rv) values that differ by 10 inches
> or more are also dissimilar. Shallower components are limiting. Deeper
> components are non-limiting. Very shallow components are very
> contrasting to components in all other depth classes.

**Surface texture:**

> coarse (s, ls, cos, lcos, fs, lfs, vfs, lvfs)
>
> moderately coarse (sl, cosl, fsl)
>
> medium (l, sil, si, vfsl)
>
> moderately fine (scl, cl, sicl)
>
> fine (sc, c, sic)
>
> organic (muck, peat, mucky peat)
>
> Difference of one group is dissimilar and 2 groups is very
> contrasting. All dissimilar components are limiting.

**Surface layer rock fragment class (percent by volume):**

> Less than 15%
>
> 15 - 35%
>
> 35 - 60%
>
> 60% +
>
> Difference of one class is dissimilar and 2 classes is very
> contrasting. Components with more rock fragments are limiting and
> those with less rock fragments are non-limiting. Components in
> adjacent classes with representative (rv) values that differ by 10
> percent or more are also dissimilar, and by 20 percent or more are
> also very contrasting.

**Surface stones and boulders:**

> Class 1
>
> Class 2
>
> Class 3
>
> Class 4
>
> Class 5
>
> Difference of one class is dissimilar and 2 classes is very
> contrasting. The more stony components are limiting the less stony
> components are non-limiting.

**Erosion class:** Class 1 (slight)

Class 2 (moderate)

Class 3 (severe)

Class 4 (very severe/gullied)

> Difference of one class is dissimilar and classes 1 or 2 and class 4
> are very contrasting. More severe erosion is limiting, except that
> class 2 is not limiting to class 1. Less severe is non-limiting.

**Slope:**

**Slope Class Table**

  ----------------- ----------------- ----------------- -----------------
  Slope range       Similar           Dissimilar        Very Contrasting

                                      Absolute          
                                      Difference        

  0 to 1            \< 3              3                 4

  2 to 3            \< 4              4                 6

  4 to 8            \< 5              5                 8

  9 to 16           \< 7              7                 10

  17 to 30          \< 9              9                 14

  \> 30             \< 12             12                20
  ----------------- ----------------- ----------------- -----------------

> A component must have a slope which differs by the indicated amount
> from the high or low slope of the map unit to be dissimilar.
> Dissimilar components with greater slopes are limiting.
>
> Example 1 - A component with 20% slope within a map unit with a slope
> range of 6-12%. Since the component has slopes greater than the upper
> limit of the map unit slope range, compare the component with the
> upper limit of the map unit slope range. The upper limit of the map
> unit slopes is 12%, which fits within the 9-16% slope group. To be
> dissimilar the component must have at least 7 % more slope (7% + 12% =
> 19%) than the map unit. The component on 20% slope, therefore, is
> dissimilar. To be very contrasting, the component must have at least
> 10% more slope (10% + 12% = 22%) than the map unit. The component on
> 20% slope, therefore, is not very contrasting. Since the component has
> greater slopes than the upper slope limit of the map unit, it is
> limiting.
>
> Example 2. Compare components on 3% and 7% slopes. Find the difference
> required for the lower slope component. The 3% component fits into the
> 2-3% group, which requires a difference of 4% to be dissimilar,
> therefore, the 7% component is dissimilar. However, the difference
> required for the 7% component, which fits into the 4-8% group is 5%.
> Therefore, the 3 % component is not dissimilar to the 7% component.
>
> Example 3. Compare slopes of 5-10% with slopes of 8-16%, using the
> lower slopes for each component (5 & 8) and the upper slopes for each
> component (10 & 16). A slope of 5% requires a difference of 5%,
> therefore, 8% is not dissimilar. A slope of 10% requires a difference
> of 7%, therefore, 16% is not dissimilar. Therefore, components with
> 8-16% slopes are not dissimilar.

**Flooding frequency:**

> Flooding classes: none
>
> very rare and rare
>
> occasional
>
> frequent
>
> The adjacent groups above are dissimilar. None and occasional and none
> and frequent are very contrasting. Very rare and rare are similar.
> Very rare and rare are very contrasting to frequent. More frequent
> flooding is limiting.

**Surface calcium carbonate content:**

> Well drained soils 5% or more CaC0~3~ (high value) in the surface
> layer are dissimilar and limiting to well drained soils with no
> calcium carbonate in the surface layer.

**Organic soils:**

> Organic soils with Dysic reaction classes are dissimilar (but not
> limiting) to those with Euic reaction classes.

**Epipedon thickness:**

The relative difference in representative (rv) thickness of the
epipedons (ochric, mollic) of mineral soils is as follows:

-   50 to 75 percent (of the thickness of the thinner epipedon) for
    dissimilar

-   75 percent or greater (of the thickness of the thinner epipedon) for
    very contrasting