Similar and Dissimilar Soils

1
Similar and Dissimilar Soils
Soil Scientists Workshop March 17-19, 2009
Tom Hahn, MO 6 Lakewood, CO
2
Similar and Dissimilar Soils

• Dissimilar soils are synonymous with
  contrasting
• From chapter 2, page 30 of the Soil Survey
  Manual
• Similar components are alike or much alike in
  most properties and share limits of those
  diagnostic properties in which they differ.
  Differences are beyond the limits of the
  reference taxon or phase class, but they
  generally are within or slightly beyond normal
  errors of observation. Because only a few limits
  are shared or the range is small, interpretations
  for most common uses are alike or reasonably
  similar and the interpretative value of a map
  unit is not affected.
• Dissimilar components on the other hand, differ
  appreciably in one or more properties, and the
  differences generally are great enough to affect
  major interpretations. . . .

3
Determining Similar Soils

• The determination is based on two things
• Similarity of soil properties
• Similarity of interpretations
• For similarity of soil properties, the key point
  is sharing the same limits. A limit is the
  value of a measured soil property that separates
  categories. The categories can be taxonomic
  classes, depth classes, textures, or any other
  class.

4
Examples of shared limits

• The limit of 35 percent clay - separates fine
  from fine-loamy families.
• The 10-inch limit for mollic thickness -
  separates ochric from mollic epipedons and
  Mollisols from other orders
• The 15 CCE limit for calcic horizons separates
  Calcic subgroups or Calci- great groups

5

• Soils on either side of the limit are similar
• Dissimilar soils have an intervening category
• A moderately deep soil is dissimilar to a very
  deep soil because the deep class intervenes
• A fine family is dissimilar to coarse-loamy
  because fine-loamy intervenes.

6
Examples of Similar Taxonomic classes

• Pachic Haplustolls vs. Typic Haplustolls
• share 16 inch limit for mollic thickness
• Typic Palecryolls vs. Typic Argicryolls
• share 24-inch depth to argillic
• Vertic Haplargids vs. Typic Haplargids
• share 6.0 limit for LE
• Lithic Argiustolls vs. Typic Argiustolls
• similar only if a lithic (or paralithic) contact
  within 40 inches

7
Examples of Dissimilar classes

• Typic Argiustolls vs. Aquic Argiustolls
• Oxyaquic intervenes
• Typic Haplocryolls vs. Typic Haplocryalfs
• Mollic Haplocryalfs intervenes
• Typic Haplargids vs. Aridic Argiustolls
• Ustic Haplargids intervenes

8
Confirm with Use and Management

• Once the soils are determined to be similar, this
  must be confirmed with an interpretations test
• If the differing property significantly affects
  use and management, then the soils are considered
  dissimilar.
• Example
• Shallow and moderately deep soils can share the
  same limit (20 inch depth) but the ecological
  site may be significantly different, hence may be
  dissimilar

9
Limiting vs. Nonlimiting Soils

• Interpretations and the intended use must be
  considered. Even for very dissimilar soils, it
  is important to distinguish between those soils
  that impose more and those that impose fewer
  restrictions on soil performance under various
  uses.
• If a dissimilar soil has less severe restrictions
  than the dominant component it is considered
  nonlimiting

10
Examples

• Limiting dissimilar soils
• Moderately deep soils in areas of very deep soils
• Aquic subgroups in areas of Typic subgroups
• Nonlimiting dissimilar soils
• Very deep soils in areas of moderately deep soils
• Typic subgroups in areas of Aquic subgroups

11
Use of Similar/Dissimilar and Limiting/Nonlimiting
in Map Unit Design

• Major components in complexes and associations
  should be dissimilar soils.
• The percent composition attributed to a named
  soil includes the percent of all soils that are
  similar to the named soil.
• There are prescribed limits for the amount of
  dissimilar limiting and nonlimiting minor
  components (inclusions) in a map unit
• Only dissimilar soils are named as minor
  components (inclusions) in map unit descriptions
• The minor components given in a map unit
  description can be either limiting or nonlimiting

12

• Transect Summary (assume all other soil
  properties are the same, and potential use and
  management is not intensive, rangeland, for
  example)
• Stop Depth Component
• 1 shallow
• 2 mod deep
• 3 deep
• 4 deep
• 5 mod deep
• 6 shallow
• 7 shallow
• 8 deep
• 9 deep
• 10 very deep
• Map Unit Composition
• Named component(s)
• soil percent
• soil percent
• Similar soils percent
• Dissimilar nonlimiting percent

13

• Transect Summary (assume all other soil
  properties are the same, and potential use and
  management is not intensive, rangeland, for
  example)
• Stop Depth Component
• 1 shallow major component
• 2 mod deep similar soil
• 3 deep major component
• 4 deep major component
• 5 mod deep similar soil
• 6 shallow major component
• 7 shallow major component
• 8 deep major component
• 9 deep major component
• 10 very deep similar soil
• Map Unit Composition
• Major component(s)
• Deep soil 40 percent Dissimilar
  nonlimiting 0 percent
• Shallow soil 30 percent Dissimilar
  limiting 0 percent
• Similar soils 30 percent

14

• Transect Summary (assume all other soil
  properties are the same, and potential use and
  management is not intensive, rangeland, for
  example)
• Stop Particle-Size Wtd Ave Rock Kind of
• Number Family Class Frags in PSCS Component
• 1 loamy-skeletal 40
• 2 clayey-skeletal 45
• 3 clayey-skeletal 50
• 4 fine 0
• 5 fine-loamy 0
• 6 clayey-skeletal 35
• 7 clayey-skeletal 45
• 8 clayey-skeletal 50
• 9 clayey-skeletal 45
• 10 fine 30
• Map Unit Composition
• Major component(s)
• soil percent
• soil percent
• Similar soils percent
• Dissimilar nonlimiting percent

15

• Transect Summary (assume all other soil
  properties are the same, and potential use and
  management is not intensive, rangeland, for
  example)
• Stop Particle-Size Wtd Ave Rock Kind of
• Number Family Class Frags in PSCS Component
• 1 loamy-skeletal 40 similar soil
• 2 clayey-skeletal 45 major component
• 3 clayey-skeletal 50 major component
• 4 fine 0 dissimilar nonlimiting soil
• 5 fine-loamy 0 dissimilar nonlimiting
  soil
• 6 clayey-skeletal 35 major component
• 7 clayey-skeletal 45 major component
• 8 clayey-skeletal 50 major component
• 9 clayey-skeletal 45 major component
• 10 fine 30 similar soil
• Map Unit Composition
• Major component(s)
• Clayey-skeletal soil 60 percent Dissimilar
  nonlimiting_____20 percent
• soil percent
  Dissimilar limiting_______0 percent
• Similar soils ____20 percent