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
(No Transcript)
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 propertiesOften
  suffer nutrient deficiency
• Drainage questions Yes? No?CO2 emission,
  biodiversity, shrinking, burning, erosion

20
(No Transcript)
21
Thin section
22
Examp l e
23
Depth (cm) Hori-zon pH (H20) OrganicMatter (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
(No Transcript)
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