A soil phase is a unit of soil outside the system of soil
taxonomy. It is a functional unit that may be designed
according to the purpose of the survey. Phases of taxa at
any categorical level, from order to series, may be defined.
Also, areas not classified in soil taxonomy such as rockland
and steep slopes may be designated as phases on soil maps. The
two general reasons for differentiating soil phases are
- to recognize and name soil and landscape properties that
are not used as criteria in soil taxonomy, for example,
slope or erosion
- to recognize and name, at a relatively high categorical
level, soil properties that are used as differentiae at
a lower categorical level. For example, depth to a lithic
layer is a family criterion, but it can be used as a
phase criterion at the order, great group, and subgroup
levels such as, Brunisolic soils, very shallow lithic
phase.
The properties recognized above must be associated with areas
of soil or nonsoil as mapped. The major phase differentiae
are listed below.
The slope classes are defined as follows:
| 1 |
0-0.5 |
0 |
level |
| 2 |
>0.5-2 |
0.3-1.1 |
nearly level |
| 3 |
>2-5 |
>1.1-3 |
very gentle slopes |
| 4 |
>5-10 |
>3-5 |
gentle slopes |
| 5 |
>10-15 |
>5-8.5 |
moderate slopes |
| 6 |
>15-30 |
>8.5-16.5 |
strong slopes |
| 7 |
>30-45 |
>16.5-24 |
very strong slopes |
| 8 |
>45-70 |
>24-35 |
extreme slopes |
| 9 |
>70-100 |
>35-45 |
steep slopes |
| 10 |
>100 |
>45 |
very steep slopes |
For example, Dystric Brunisol and rock outcrop, moderate
slopes.
The following water-erosion classes as defined in the Soil
Survey Manual of the U.S. Department of Agriculture (Soil
Survey Staff 1951, pp. 261-264 ) are used as phases.
As much as 25% of the original A horizon may have been removed
from most of the area. In most cases the soils eroded to this
degree are not significantly different in use capabilities and
management requirements from noneroded soils.
Between 25 and 75% of the original A horizon may have been
lost from most of the area. The present Ap horizon consists
of a mixture of the underlying soil and the original A horizon.
Shallow gullies may be present.
More than 75% of the original A horizon and commonly part of
the next underlying horizon have been lost from most of the
area. Shallow gullies are common and a few deep ones may
occur.
The land is dissected by moderately deep to deep gullies with
small areas of intact soil between the gullies. The area is
unsuitable for crop production without reclamation.
The following wind erosion classes, as defined in the
Soil Survey Manual of the U.S. Department of Agriculture
(Soil Survey Staff 1951, p. 267), are used as soil phases.
Wind has removed between 25 and 75% of the original A horizon
and tillage results in mixing of subsurface material with
remnants of the original surface layer.
Wind has removed more than 75% of the original A horizon and
commonly part of the underlying horizon.
Wind has removed most of the solum and numerous blowout holes
are carved into the parent material. Some areas between
blowouts are deeply buried by soil material from the blowouts.
The area is unsuitable for crop production without extensive
reclamation.
The phases for deposition as described in the Soil
Survey Manual of the U.S. Department of Agriculture (Soil
Survey Staff 1951, pp. 295-296), are being used currently in
Canada. Two phases are defined as follows:
Deposits of wind-eroded materials on the soil surface are
great enough to influence management but are not great
enough to destroy the essential characteristics of the
soil series.
Deposits of water-eroded materials on the soil surface are
thick enough to influence management requirements significantly
but are not deep enough to destroy the essential
characteristics of the soil series.
The phases for stoniness are described in the Soil Survey
Manual of the U.S. Department of Agriculture (Soil Survey
Staff 1951, pp. 216-220). Six phases of stoniness are
defined on the basis of the percentage of the land surface
occupied by fragments coarser than 25 cm in diameter.
No stones or too few are present to interfere with cultivation
(<0.01% of surface, stones more than 25 m apart).
Some stones are present that hinder cultivation slightly
or not at all (0.01-0.1% of surface, stones 8-25 m apart).
Enough stones are present to cause some interference with
cultivation (0.1-3% of surface, stones 1-8 m apart).
There are sufficient stones to handicap cultivation seriously;
some clearing is required (3-15% of surface, stones 0.5-1 m
apart).
The stones prevent cultivation until considerable clearing
is done (15-50% of surface, stones 0.1-0.5 m apart).
The land surface is too stony to permit cultivation; it
is boulder or stone pavement (more than 50% of surface,
stones less than 0.1 m apart).
Six phases of rockiness (bedrock exposure) are defined as
follows:
Bedrock exposures do not interfere seriously with tillage.
Exposures, if present, are generally more than 100 m apart
and cover less than 2% of the surface.
The bedrock exposures interfere with tillage but not enough
to make intertilled crops impracticable. Depending on the
pattern and how it affects tillage, rock exposures are roughly
35-100 m apart and cover 2-10% of the surface.
The bedrock exposures make tillage of intertilled crops
impracticable, but the soil can be worked for hay crops
or improved pasture if other soil characteristics are
favorable. Rock exposures are roughly 10-35 m apart and
cover about 10-25% of the surface depending on the pattern.
The rock outcrops make all use of machinery impracticable,
except for small machinery. Where other soil characteristics
are favorable the land may have some use for native pasture
or forests. Rock exposures or patches of soil too thin over
rock for use are roughly 3.5-10 m apart and cover 25-50% of
the surface depending on the pattern.
Sufficient rock outcrop or insufficient depth of soil
over rock makes all use of machinery impracticable. The
land may have some value for poor pasture or forestry. Rock
outcrops are less than 3.5 m apart and cover 50-90% of the
area.
More than 90% of the land surface is exposed bedrock (rock
outcrop).
Any mineral soil having a surface horizon of 15-40 cm of folic
material may be designated as a folic phase.
Any mineral soil having a surface horizon of 15-60 cm of
fibric organic material or 15-40 cm of mesic or humic organic
material may be designated as a peaty phase.
Any noncryoturbated mineral or organic soil having permafrost
below the 1 m depth, or cryoturbated mineral soil having
permafrost below the 2 m depth, may be designated as a cryic
phase.
Any nonpermafrost soil having one or more cryoturbated
horizons may be designated as a cryoturbated phase.
Other differentiae that are used taxonomically at lower
categorical levels are principally family and some series
criteria. These may be used as phase criteria at the order,
great group, and subgroup levels.
Definitions are given in Chapter 14.
e.g., Humo-Ferric Podzols, fragmental phase. The textural
class of the mineral surface layer of a soil series may also
be indicated as a phase, e.g., Breton silt loam.
e.g., Dystric Brunisols, cindery phase.
e.g., Black, smectitic phase.
e.g., Regosols, shallow lithic phase (lithic contact 50-100
cm from the mineral surface); Organic Cryosols, very shallow
cryic phase (permafrost layer at 40-100 cm).
e.g., Regosols, alkaline phase.
e.g., Rego Dark Brown Chernozem, extremely calcareous phase.
e.g., Podzolic, cold, perhumid phase.
Other family criteria for Organic soils that may be used as
phases of higher categories are characteristics of the
surface tier, e.g., Mesisols, fine loamy phase; reaction,
e.g., Typic Fibrisols, dysic phase; and kind of limnic
material, e.g., Limnic Mesisol, diatomaceous phase.
e.g., Humo-Ferric Podzol, turbic phase.
e.g., Orthic Brown Chernozem, saline phase.
e.g., Orthic Dystric Brunisol, andic phase.
e.g., Orthic Brown Chernozem, carbonated phase.
Other series differentiae may also be named as phases of
higher categories.
Also, subgroup differentiae may be used to indicate phases
of classes at the order level, e.g., Podzolic soils, gleyed
phase.
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