Geoscience Reference
In-Depth Information
In the World Reference Base for Soil Resources (IUSS Working Group WRB
2006 ), clay illuviation is recognized in the argic horizon , which is a subsurface
horizon with distinctly higher clay content than the overlying horizon. The textural
differences may be caused by illuviation, neoformation of clay in the subsoil,
destruction and erosion of clay in the topsoil, upward movement of coarse particles,
biological activity, or a combination of these processes. Diagnostic criteria include
a texture of loamy sand or finer and 8 % or more clay in the fine earth fraction,
but this depends on the clay content of the overlying horizon and the thickness
of the soil. Textural differentiation is the main feature for recognition of argic
horizons, and the illuvial clay may be observed using a hand-lens if clay skins occur
on ped surfaces, in fissures, in pores, and in channels. Illuvial argic horizon should
show clay skins on at least 5 % of the ped faces and in the pores. According to
WRB the best identification for an argic horizon is by thin sections. Argic horizons
are normally found below eluvial horizons from which clay and Fe have
been removed. Some clay-increase horizons may have the properties that charac-
terize the ferralic horizon, i.e., a low CEC and effective CEC, a low content
of water-dispersible clay, and a low content of weatherable minerals. In WRB,
argic horizons lack the sodium saturation characteristics of the natric horizon.
Argic horizons that occur in cool and moist, freely drained soils of high plateaus
and mountains in tropical and subtropical regions are often found in association
with sombric horizons (IUSS Working Group WRB 2006 ). The nitric horizon is
a special type of argic horizon, and also the natric horizon may have increased
clay content. Reference Soil Groups that may have an argic horizon in WRB are
Albeluvisols, Alisols, Acrisols, Luvisols, Lixisols, Chernozems, Kastanozems,
and Phaeozems.
11.3 Soil Taxa Containing Taxonomic Clay Enrichment
Clay illuviation is recognized in ST in 10 of the 12 orders (Table 11.2 ). Clay
illuviation does not occur in Histosols or in Entisols. Two of the orders, Alfisols and
Ultisols, require an argillic or kandic horizon (or natric horizon, in the case of some
Alfisols). In Aridisols, clay enrichment is recognized at the suborder level (Argids)
and at the great-group (Argi-, Natri-) and subgroup (Argic, Natric, Ustalfic) levels.
In Mollisols argillic and natric horizons are recognized at the great-group level
(Argi-, Natri-) and at the subgroup level (Argic, Natric). Soils of the Argiorthels
great group in Gelisols contain an argillic horizon. In Oxisols soils of the
Kandiudox and Kandiustox great groups and in Kandiudalfic subgroups contain
kandic horizons. There is one great group, the Natraquerts within the Vertisols,
which contains a natric horizon. Spodosols may contain argillic or kandic-like
horizons or lamellae, which are recognized only at the subgroup level (Alfic,
Ultic, Lamellic, etc.). Andisols contain Alfic and Ultic subgroups (and occasionally
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