Argillic, Kandic, and Natric Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 101

or ferruginous soil-mineral class. Much of the clay in the kandic horizon has
originated from “clay decomposition,” or weathering in situ (Eswaran and Sys
1979 ; Okusami et al. 1997 ; Shaw et al. 2004 ).
The natric horizon is a type of argillic horizon that is dispersed by abundant
sodium; therefore, it has a high sodium adsorption ratio (SAR). The clay activity
tends to be high, and dominant mineral classes are mixed or smectitic (Table 11.5 ).
Although soils with a natric horizon show a distinct clay accumulation in the Bt or
Btn horizon, there generally are few argillans because they are destroyed by
shrinking and swelling (Nettleton et al. 1969 ; Alexander and Nettleton 1977 ).
Soils with a natric horizon often have a high COLE. The dominant source of the
clay in natric horizons is from weathering in situ (Nettleton et al. 1969 ), although
dust deposition can be a major source (Alexander and Nettleton 1977 ; Elliott and
Drohan 2009 ).
The genesis of clay-enriched horizons is complicated. In his discussion of the
origin of texture-contrast soils, Phillips ( 2001 ) states: “Multiple causality is likely,
and attempts to apply any single explanation to a county-size area (and sometimes
to a pedon) are not likely to be successful. The implication is not that pedologists
should abandon the search for generalizations but that the context in which
laws and generalizations are developed needs rethinking. Explanatory constructs
should be formulated not with the notion that a single explanation is likely to be
applicable to most soils, but with the idea that multiple causality and polygenesis
are likely, and that location-specific characteristics cannot be ignored” (p. 347).
In Australia about 20 % of the soils have pronounced differences in texture between
the A and B horizon that was envisaged as progressing from an initial translocation
of the clay inherited from parent materials to intensive weathering and size reduc-
tion of clay particles in response to strong seasonal fluctuations in soil moisture
(Walker and Chittleborough 1986 ).
11.7 Summary
• There are three diagnostic subsurface horizons in ST that are defined on the basis
of clay illuviation of silicate clays: (1) the argillic horizon, (2) the kandic
horizon, and (3) the natric horizon. In addition to diagnostic subsurface horizons,
there are two diagnostic soil characteristics that are based on clay movement:
(1) abrupt textural change and (2) lamellae.
• The analysis suggests that clay illuviation is recognized in ST at some level in
10 of the 12 orders, including order (Alfisols, Ultisols), suborder (Aridisols),
great group (Aridisols, Gelisols, Mollisols, Oxisols, Vertisols), and subgroup
(Andisols, Aridisols, Inceptisols, Mollisols, Oxisols, Spodosols).
• Forty-four percent of the soil series in the USA contain taxonomically defined
argillic, nitric, or kandic horizons. Other soils contain a Bt horizon so that more
than half of the soils of the country feature clay illuviation.