Chapter 15: Soil Phase
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:
|Slope class||Percent slope||Approximate degrees||Terminology|
|3||>2-5||>1.1-3||very gentle slopes|
|7||>30-45||>16.5-24||very strong 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.
Class W1, slightly eroded phase
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.
Class W2, moderately eroded phase
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.
Class W3, severely eroded phase
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.
Class W4, gullied land phase
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.
Class D1, slightly wind-eroded phase
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.
Class D2, severely wind-eroded phase
Wind has removed more than 75% of the original A horizon and commomly part of the underlying horizon.
Class D3, blown-out land phase
Wind has removed most of the solum and numerous blowout holes are carved into the parent material. Some areas between blowouts are deeply burried 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.
Class S0, nonstony phase
No stones or too few are present to interfere with cultivation (<0.01% of surface, stones more than 25 m apart).
Class S1, slightly stony phase
Some stones are present that hinder cultivation slightly or not at all (0.01-0.1% of surface, stones 8-25 m apart).
Class S2, moderately stony phase
Enough stones are present to cause some interference with cultivation (0.1-3% of surface, stones 1-8 m apart).
Class S3, very stony phase
There are sufficient stones to handicap cultivation seriously; some clearing is required (3-15% of surface, stones 0.5-1 m apart).
Class S4, exceedingly stony phase
The stones prevent cultivation until considerable clearing is done (15-50% of surface, stones 0.1-0.5 m apart).
Class S5, excessively stony phase
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:
Class R0, nonrocky phase
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.
Class R1, slightly rocky phase
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.
Class R2, moderately rocky phase
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.
Class R3, very rocky phase
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.
Class R4, exceedingly rocky phase
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.
Class R5, excessively rocky phase
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.
Particle size 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.
Substitute classes (particle size) e.g., Dystric Brunisols, cindery phase.
Mineralogy e.g., Black, smectitic phase.
Depth 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).
Reaction e.g., Regosols, alkaline phase.
Calcareous e.g., Rego Dark Brown Chernozem, extremely calcareous phase.
Soil climate 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.
Physical disruption e.g., Humo-Ferric Podzol, turbic phase.
Salinity e.g., Orthic Brown Chernozem, saline phase.
Influence of volcanic ash e.g., Orthic Dystric Brunisol, andic phase.
Secondary carbonates in the A horizon 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.