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. www.fao.org/.org/ag/agl/agll/wrb/
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.
SET # 1 Organic soils. Soils with more than a defined quantity of 'organic soil materials'. Reference Soil Group: the HISTOSOLS. LN: Page 7
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.
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
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.
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.
SET # 1 Soils with more than a defined quantity of 'organic soil materials'. Reference Soil Group: the HISTOSOLS.
HISTOSOLS
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
Definition 1.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 2.having no andic or vitric horizon starting within 30 cm from the soil surface. HISTOSOLS (HS)
A histic horizon must: Histic horizon LN page: 317 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
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.
A folic horizon must: Folic horizon 1. have more than 20 percent (by weight) organic carbon (35 percent organic matter); and 2. not be saturated with water for more than one month in most years; and 3. 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.
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
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.
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
Characteristics of Histosols Profile development: Most Histosols H or HCr Large total pore volume (typically > 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? CO 2 emission, biodiversity, shrinking, burning, erosion
E x a m p l e
Depth (cm) Horizon 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
E x a m p l e Histic horizon OM > 20% > 40 cm HISTOSOLS (HS)
HISTOSOLS CRYOSOLS ANTHROSOLS LEPTOSOLS VERTISOLS Cryic Histic Hydragric Lithic Thionic Glacic Lithic Irragric Gleyic Salic Salic Leptic Terric Rendzic Natric Gelic Turbic Plaggic Umbric Gypsic Thionic Salic Hortic Yermic Duric Folic Natric Gleyic Aricic Calcic Fibric Gleyic Stagnic Vertic Alic Sapric Andic Spodic Gelic Gypsiric Ombric Mollic Ferralic Hyperskeletic Pellic Rheic Gypsic Luvic Mollic Grumic Alcalic Calcic Arenic Humic Mazic Toxic Umbric Regic Gypsiric Chromic Dystric Yermic Calcaric Mesotrophic Eutric Aridic Dystric Hyposodic Glacic Eutric Eutric Thionic Haplic Haplic Oxyaquic Stagnic Haplic
Dystri-sapric Histosol Glacic Thionic Cryic Gelic Salic Folic Fibric Sapric Ombric Rheic Alcalic Toxic Dystric Eutric Haplic Less than one-sixth of recognizable plant tissue (after the material is rubbed) BS% <50%
HISTOSOLS
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
The end of Histosols