Chapter 4: Brunisolic Order
A diagrammatic representation of profiles of some subgroups of the Brunisolic order is given in Figure 28. Individual subgroups may include soils that have horizon sequences different from those shown. In the description of each subgroup, presented later in this chapter, a common horizon sequence is given; diagnostic horizons are underlined and some other commonly occurring horizons are listed.
Soils of the Brunisolic order have sufficient development to exclude them from the Regosolic order, but they lack the degree or kind of horizon development specified for soils of other orders. The central concept of the order is that of soils formed under forest and having brownish-colored Bm horizons, but the order also includes soils of various colors with both Ae horizons and B horizons having slight accumulations of either clay (Btj), or amorphous Al and Fe compounds (Bfj), or both. Soils having a Bf horizon less than 10 cm thick are a part of this order.
A Bm horizon may have any or all of the following: stronger chroma and redder hue than the underlying material, partial or complete removal of carbonates, slight illuviation based mainly on the occurrence of an overlying Ae horizon, a change in structure from that of the original material.
A Bm horizon may develop in materials of any color, such as gray, brown, black or red, and which vary in texture from gravel to clay.
Brunisolic soils include some that are calcareous to the surface and very slightly weathered, and others that are strongly acid and apparently weathered to about the same extent as the associated Podzolic soils. Most Brunisolic soils are well to imperfectly drained, but some that have been affected by seepage water are poorly drained although not strongly gleyed. They occur in a wide range of climatic and vegetative environments including Boreal Forest; mixed forest, shrubs, and grass; and heath and tundra.
Brunisolic soils have a Bm, Bfj, thin Bf, or Btj horizon at least 5 cm thick. They lack the diagnostic properties specified for soils of other orders. Thus they do not have any of the following: a solonetzic or podzolic B horizon, a Bt horizon, evidence of gleying as specified for soils of the Gleysolic order, organic horizons thicker than 40 cm if mesic or humic or 60 cm if fibric, permafrost within 1 m of the surface or 2 m if cryoturbated. Some Brunisolic soils have an Ah horizon, but it does not meet the specifications for a chernozemic A either because of its inherent properties or the associated soil climate.
Distinguishing Brunisolic Soils from Soils of Other Orders
Guidelines for distinguishing Brunisolic soils from soils of other orders with which they might be confused follow. To a degree the Brunisolic order can be considered as an intergrade order between Regosolic soils and soils of several other orders. The distinctions are based more upon the degree than the kind of development.
Chernozemic Some Brunisolic soils and many Chernozemic soils have an Ah or dark- colored Ap horizon and either a Bm or a Btj horizon. These soils are classified as Chernozemic only if they have a chernozemic A horizon as defined in Chapter 2. For example, Melanic Brunisols of the St. Lawrence Lowlands have a chernozemic-like A horizon but are excluded from the Chernozemic order because their soil moisture regime is humid.
Soils of subalpine, alpine, northern boreal, and subarctic areas having Ah and Bm horizons are classified as Chernozemic if they have a chernozemic A horizon. Many similar soils in these areas do not have a chernozemic A either because of a soil moisture regime that is humid or wetter, a mean annual soil temperature that is colder than 0oC, or because of some inherent property of the Ah horizon such as low base saturation. These soils are classified as Brunisolic. Further studies of these soils may lead to improved criteria for differentiating Brunisolic and Chernozemic soils. Current information indicates that many Ah horizons of soils at high altitudes and latitudes have the following characteristics that differ from the Ah horizons of Chernozemic soils:
- Low degree of incorporation of organic matter with mineral material, moder. This includes the turfy A horizons of some alpine soils with bulk density less than 1.0 Mg m-3.
In addition, some of these soils at high elevations have the following properties that differ from those of Chernozemic soils:
- Appreciable content of pyrophosphate-extractable Al and Fe in the Ah horizon, especially in soils containing volcanic ash.
- More than 1% organic C in the B horizon.
Luvisolic Luvisolic soils must have a Bt horizon, but Brunisolic soils do not. However, it is difficult to distinguish a Bt from a Btj horizon and micromorphological examination may be required.
Podzolic Podzolic soils must have a Podzolic B horizon and Brunisolic soils do not. However, the colors of some Bm and Bfj horizons are within the range of that of Podzolic B horizons, and some of these horizons contain concentrations of amorphous complexes of Al and Fe with organic matter close to the minimum concentration diagnostic for Bf horizons. Therefore, chemical analysis is required to differentiate some Brunisolic soils from Podzolic soils. Soils having a Bf horizon thinner than 10 cm are classified as Brunisolic.
Regosolic Brunisolic soils must have a Bm, Bfj, thin Bf, or a Btj horizon at least 5 cm thick; Regosolic soils do not.
Cryosolic Cryosolic soils have permafrost within 1 m of the mineral surface or 2 m if strongly cryoturbated, but Brunisolic soils do not.
Vertisolic Vertisolic soils have both a vertic horizon (v) and a slickenside horizon (ss), but Brunisolic soils do not.
The Brunisolic order is divided into four great groups: Melanic Brunisol, Eutric Brunisol, Sombric Brunisol, and Dystric Brunisol based on soil reaction and the presence or absence of an Ah horizon, as indicated in the Brunisolic order chart.
Subgroups are separated on the basis of the kind and sequence of the horizons. Some former subgroup features (lithic, andic, turbic, and cryic) are now recognized taxonomically at either the family or series level, but different names are used for some of them. Alternatively, these features may be indicated as phases of subgroups, great groups, or orders.
|Melanic Brunisol||Eutric Brunisol||Sombric Brunisol||Dystric Brunisol|
|Diagnostic horizons are underlined|
|Horizon||thick Ah or Ap (≥10 cm)||no (or thin) Ah or Ap (<10 cm)||thick Ah or Ap (≥10 cm)||no (or thin) Ah or Ap (<10 cm)|
|Ap color value||moist <4||moist ≥4||moist <4||moist ≥4|
|Melanic Brunisol||Orthic Melanic Brunisol O.MB|
|Eluviated Melanic Brunisol E.MB|
|Gleyed Melanic Brunisol GL.MB|
|Gleyed Eluviated Melanic Brunisol GLE.MB|
|Eutric Brunisol||Orthic Eutric Brunisol O.EB|
|Eluviated Eutric Brunisol E.EB|
|Gleyed Eutric Brunisol GL.EB|
|Gleyed Eluviated Eutric Brunisol GLE.EB|
|Sombric Brunisol||Orthic Sombric Brunisol O.SB|
|Eluviated Sombric Brunisol E.SB|
|Duric Sombric Brunisol DU.SB|
|Gleyed Sombric Brunisol GL.SB|
|Gleyed Eluviated Sombric Brunisol GLE.SB|
|Dystric Brunisol||Orthic Dystric Brunisol O.DYB|
|Eluviated Dystric Brunisol E.DYB|
|Duric Dystric Brunisol DU.DYB|
|Gleyed Dystric Brunisol GL.DYB|
|Gleyed Eluviated Dystric Brunisol GLE.DYB|