Argillic, Kandic, and Natric Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 100

that is released forms clay films in the lower Bt and BC horizons (Brook and
van Schuylenborgh 1975 ). Argillic horizons commonly show an increase in the
ratio of fine clay to total clay from an overlying eluvial horizon. Although
individual clay lamella do not qualify as argillic horizons, they do qualify as argillic
if the cumulative thickness is

> 15 cm (e.g., Torrent et al. 1980 ; Holliday and

Rawling 2006 ).
Clay illuviation has been successfully reproduced in the laboratory. Bond ( 1986 )
created illuvial bands in a laboratory column of sand, hypothesizing that band
formation resulted from dispersion of clay in the sand and its subsequent deposition,
which was triggered by layers of small pores within the sand column and/or by
exceeding the maximum possible suspension concentration. Gombeer and D'Hoore
( 1971 ) induced migration of clay in the laboratory, reporting that clay movement
was dependent on soil/water dispersion ratio, colloid stability, and “electrophoretic
mobility.” Mel'nikova and Kovenya ( 1971 ) used clay mineral particles irradiated
by thermal neutrons in a reactor to study the effects of chemical and physical soil
properties on clay illuviation. Large amounts of the irradiated clay were
translocated with a weakly acidic solution without destruction of eluvial horizons
in podzols. The rate of clay translocation was dependent on the density and sorption
capacity of clay minerals and was greater in the E horizon than in the B and
C horizons. As the pH of the leaching solution increased, so did the mobility of
the particles, which was attributed to an increase of the electrokinetic potential.
Gagarina and Tsyplenkov ( 1974 ) used open-top chambers containing disturbed
soil to study clay illuviation in the forest-steppe zone of Russia. Clays became
mobile 10 years after the beginning of the experiment following dissolution of
“microcryptogranular” carbonates. During movement, clays filled all the cracks and
fine pores within aggregates that formed following the leaching of carbonates. With
time the clays became more strongly aggregated due to increased orientation of the
clay particles. Circular, striated, and fibrous forms of orientation predominated.
By filling large cracks and pores in the aggregates, the clays formed encrusted or
conchoidal segregations that were characteristically stratified.
Mass-balance studies show that only part of the clay in the argillic horizon of
humid soils originated from translocation out of an eluvial horizon (Smeck
et al. 1968 ; Smith and Wilding 1972 ; Rostad et al. 1976 ). Synthesis of clays from
the soil solution or suspension is an important source of the clay, as well as
weathering in situ. In arid regions, a large portion of the clay in the argillic horizon
may have been contributed by dust deposition (Alexander and Nettleton 1977 ;
Elliott and Drohan 2009 ).
The kandic horizon was introduced into ST to provide an intermediary between
the argillic and oxic horizon with regard to low-activity clays, primarily as a
solution for keeping soils of the southeastern USA from classifying as Oxisols
(Buol and Eswaran 1988 ). The introduction of the kandic horizon addressed the
issue of clay-enriched horizons with a clay “bulge,” few or no argillans, and a
lack of or slight increase in the ratio of fine clay to total clay from an overlying
eluvial horizon (Table 5). In contrast to the argillic horizon, the kandic horizon
contains low-activity clays and generally has a kaolinitic, siliceous, sesquic, ferritic,