Agriculture Reference
In-Depth Information
Table 19.1
A Sample of Diagnostic Horizons in WRB
Surface horizons
Anthropogenic horizons
Surface and subsurface horizons resulting from long-continued anthropedogenic
processes, notably deep working, intensive fertilization, addition of earthy materials,
irrigation, or wet cultivation
Histic horizon
(Peaty) surface horizon, or subsurface horizon occurring at shallow depth, consisting
of organic soil material
Umbric horizon
Well-structured, dark surface horizon with low base saturation and moderate to high
organic matter content
Ye r mic horizon
Surface horizon of rock fragments (desert pavement) usually, but not always, embedded
in a vesicular crust and covered by a thin aeolian sand or loess layer
Subsurface horizons
Cryic horizon
Perennially frozen horizon in mineral or organic soil materials
Duric horizon
Subsurface horizon with weakly cemented to indurated nodules cemented by silica
(SiO
), known as “durinodes”
2
Fragic horizon
Dense, noncemented subsurface horizon that can only be penetrated by roots and water
along natural cracks and streaks
Ver tic horizon
Subsurface horizon rich in expanding clays and having polished and grooved ped
surfaces (slickensides), or wedge-shaped or parallelepiped structural aggregates
formed upon repeated swelling and shrinking
Table 19.2
Descriptive Summary of Sample Diagnostic Properties
Examples of Unique Qualifier Definitions
Albeluvic tonguing
Iron-depleted material penetrating into an argic horizon along ped surfaces
Alic properties
Very acid soil material with a high level of exchangeable aluminium
Permafrost
Indicates that the soil temperature is perennially at or below 0
ο
C for at least two
consecutive years
Stagnic properties
Visible evidence of prolonged waterlogging by a perched water table
summarized in Tables 19.1, 19.2, 19.3, and 19.4. For a comprehensive overview of all WRB
elements, reference is made to FAO World Soil Resources Reports Number 84 (FAO et al., 1998).
Note that the generalized descriptions of diagnostic horizons, properties, and soil materials
given in Tables 19.2, 19.3, and 19.4 are solely meant as an introduction to WRB terminology. For
exact concepts and full deÝnitions, reference is made to FAO Soil Resources Reports Number 84
(FAO/ISRIC/ISSS, 1998).
TWO-TIER APPROACH IN THE WRB
The WRB comprises two tiers of detail (Nachtergaele et al., 2000a):
1.
The ÑReference Base,Ò which is limited to the Ýrst (highest) level, having 30 Reference Soil Groups
2.
The ÑWRB ClassiÝcation System,Ò suggesting combinations of adjectives to the Reference Soil
Groups, which allows precise characterization and classiÝcation of individual soil proÝles
The Reference Base: Accommodating the World Soil Cover in 30 Reference Soil
Groups
In the current text, the WRBÔs 30 Reference Soil Groups are summarized in a simpliÝed key to
the Reference Soil Groups in Table 19.5 (after ISSS/ISRIC/FAO, 1998b).
The following successive steps have to be taken to classify a soil:
