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Soil Reference Groups of WRB (and related diagnostic criteria)

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Title: Soil Reference Groups of WRB (and related diagnostic criteria)


1
Soil Reference Groups of WRB (and related
diagnostic criteria)
  • Discussion of Reference Groups will be based on
    the
  • FAO World Soil Resources Report 94 LECTURE
    NOTES ON THE MAJOR SOILS OF THE WORLD
  • available also http//www.fao.org/ag/agl/agll/
    wrb/

2
  • The 30 Reference Soil Groups are aggregated in 10
    sets on the basis of 'dominant identifiers',
    i.e. those soil forming factor(s) which most
    clearly conditioned soil formation.

3
SET 1
  • Organic soils. Soils with more than a defined
    quantity of 'organic soil materials'.
  • Reference Soil Group the HISTOSOLS.

LN Page 7
4
SET 2
  • Soils of set 2 vary widely in properties and
    appearance and can occur in any environment but
    have in common that their properties are strongly
    affected by human activity.
  • Reference Soil Group ANTHROSOLS.

5
SET 3
  • Mineral soils whose formation is conditioned by
    the particular properties of their parent
    material.
  • Reference Soil Groups (3)
  • ANDOSOLS of volcanic regions
  • the sandy ARENOSOLS
  • the swelling and shrinking heavy clayey
    VERTISOLS

6
SET 4
  • Mineral soils whose formation was influenced
    mainly by their topographic/physiographic
    setting.
  • Reference Soil Groups (4)
  • In low terrain positions
  • Young alluvial FLUVISOLS, which show
    stratification recent sedimentation
  • Non-stratified GLEYSOLS in waterlogged areas.
  • In elevated and/or eroding areas
  • Shallow LEPTOSOLS over hard rock or highly
    calcareous material,
  • Deeper REGOSOLS, which occur in unconsolidated
    materials.

7
SET 5
  • Soils that are only moderately developed on
    account of their limited pedogenetic age or
    because of rejuvenation of the soil material.
  • They have not more in common than 'signs of
    beginning soil formation'
  • Reference Soil Group the CAMBISOLS.

8
SET 1
  • Soils with more than a defined quantity of
    'organic soil materials'.
  • Reference Soil Group the HISTOSOLS.

9
HISTOSOLS
10
HISTOSOLS (Gr. histos, tissue)
  • Soils formed in organic soil material
  • Common international names are
  • peat soils', muck soils', bog soils,
    organic soils'.
  • Histosols are found at all altitudes but the vast
    majority occurs in lowlands.

LN Page 21
11
Definition
  • Soils having a histic or folic horizon, - either
    10 cm or more thick from the soil surface to a
    lithic or paralithic contact - or 40 cm or more
    thick and starting within 30 cm from the soil
    surface and
  • having no andic or vitric horizon starting within
    30 cm from the soil surface.
  • ? HISTOSOLS (HS)

12
Histic horizon
LN page 317
  • A histic horizon must
  • 1.
  • have 18 (by weight) organic carbon (30
    organic matter) or more if the mineral fraction
    have 60 or more clay or
  • have 12 (by weight) organic carbon (20 organic
    matter) or more if the mineral fraction has no
    clay or
  • have a proportional lower limit of organic
    carbon content, between 12 and 18 , if the clay
    content of the mineral fraction is between 0 and
    60 . and

13
Histic horizon (2)
  • 2.
  • be saturated with water for at least one month in
    most years (unless artificially drained) and
  • 3.
  • have a thickness of 10 cm or more. A histic
    horizon less than 20 cm thick must have 12
    percent or more organic carbon after mixing down
    to a depth of 20 cm.

14
Folic horizon
  • A folic horizon must
  • have more than 20 percent (by weight) organic
    carbon (35 percent organic matter) and
  • not be saturated with water for more than one
    month in most years and
  • have a thickness greater than 10 cm if a folic
    horizon is less than 20 cm thick, the upper 20 cm
    of the soil (after mixing) must contain 20
    percent or more organic carbon.

15
Additional diagnostics (qualifiers)
  • Lithic
  • having, within 10 cm from the soil surface,
    continuous hard rock.
  • Paralithic having, within 10 cm from the soil
    surface, a broken rock contact with fissures less
    than 10 cm apart, which allow roots to penetrate
    the underlying rock.

LN page 317
16
Genesis of Histosols
  • Histosols are formed in organic soil material'
    with physical, chemical and mechanical properties
    that differ strongly from those of mineral soil
    materials.
  • Organic soil material accumulates in conditions
    where plant matter is produced by an adapted
    (climax') vegetation, and where decomposition of
    plant debris is slowed by
  • -low temperatures,
  • -water saturation of the soil body,
  • -extreme acidity and/or
  • -high levels of electrolytes or organic toxins.

17
The degree of decomposition of organic soil
material has direct implications for the
management of Histosols and is an important
diagnostic criterion Sapric' peat less than
one-sixth of recognizable plant tissue after the
material is gently rubbed. Fibric' peat more
than two-thirds of recognizable plant tissue
(after rubbing) Hemic' peat is intermediate
between fibric and sapric peat.
LN page 317
18
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19
Characteristics of Histosols
  • Profile development Most Histosols H or HCr
  • Large total pore volume (typically gt 85),
  • Low bulk-density (typically between 0.05 and 0.15
    Mg/m3),
  • Variable (often poor) chemical properties Often
    suffer nutrient deficiency
  • Drainage questions Yes? No? CO2 emission,
    biodiversity, shrinking, burning, erosion

20
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21
Thin section
22
Examp l e
23
Depth (cm) Hori-zon pH (H20) Organic Matter (LOI - ) Clay () CEC (cmolc kg-1) B
0 20 H1 6.8 45.7 --- 55.5 45
20- 40 H2 6.1 51.5 --- 63.8 42
40 60 H3 5.7 70.5 --- 68.1 39
60 80 H4 5.1 73.7 --- 50.4 35
90 - 110 H5 4.9 41.6 --- 37.8 32
24
Examp l e
Histic horizon OM ? 20 ? 40 cm
? HISTOSOLS (HS)
25
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26
Less than one-sixth of recognizable plant tissue
(after the material is rubbed)
BS lt50
Dystri-sapric Histosol
27
HISTOSOLS
28
Associations with other Reference Groups
Cryosol
  • Permafrost-affected Histosols are associated with
    Cryosols
  • soils that have gleyic or stagnic properties
    with Gleysols in
  • associations with Podzols are also common

29
The end of Histosols
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