Argillic, Kandic, and Natric Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 89

In the literature, there is some confusion in distinguishing between the Bt
horizon and diagnostic subsurface horizons featuring clay enrichment. A Bt horizon
“indicates an accumulation of silicate clay that either has formed within a horizon
or has been moved into the horizon by illuviation, or both” (Soil Survey Staff 2010 ,
p. 318). The definition further states: “at least part of the horizon should show
evidence of clay accumulation either as coating on surfaces of peds or in pores, as
lamellae, or as bridges between mineral grains.” However, not all Bt horizons meet
the thickness or depth-distribution of clay requirements of diagnostic subsurface
horizons with clay enrichment (see below).
In Soil Taxonomy (ST), the argillic horizon is a subsurface horizon that contains
“a significantly higher percentage of phyllosilicate clay than the overlying soil
material” and “shows evidence of clay illuviation” (p. 10). The thickness require-
ment ranges between 7.5 and 15 cm, depending on the particle-size class. There
must be evidence of clay illuviation in at least one of the following forms:
(i) oriented clay bridging sand grains, (ii) clay films lining pores, (iii) clay films
on both vertical and horizontal surfaces of peds, or (iv) thin sections with oriented
clay bodies that comprise more than 1 % of the section. In addition to a thickness
requirement and evidence for clay illuviation, the argillic horizon must have a
greater amount of clay than an overlying eluvial horizon; the amount of clay
depends on the clay content of the eluvial horizon and ranges from at least 3 %
(absolute) for eluvial horizons with 15 % clay to at least 8 % (absolute) for eluvial

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horizons with 40 % clay.

The kandic horizon is a subsurface horizon defined in ST on the basis of its
thickness (minimum of 15-30 cm, depending on soil depth), the depth interval
at which the clay increases from an overlying eluvial horizon (50-200 cm),
the amount of clay increase from an overlying eluvial horizon, an apparent CEC
of 16 cmol(+)/kg clay (by 1 M NH 4 OAc, pH 7), and an apparent effective CEC of

< 12 cmol(+)/kg clay (sum of bases extracted with 1 M NH 4 OAc, pH 7, plus 1 M

KCl-extractable Al). The amount of clay increase ranges from 4 % (absolute) for
eluvial horizons with

20 % clay to at least 8 % (absolute) for eluvial horizons with
> 40 % clay. It is noteworthy that the kandic horizon does not require evidence for
clay illuviation.
In ST the natric horizon is comparable to the argillic horizon except that it
shows evidence of accelerated clay illuviation by the dispersive properties of
Na. The natric horizon has a thickness requirement (7.5-15 cm) and evidence for
clay illuviation and a clay increase from an overlying eluvial horizon that are
comparable to the argillic horizon. In addition, the natric horizon must have either
a columnar or a prismatic structure in some part and an exchangeable Na percentage
of 15 % or more. The argillic, kandic, and natric horizons are illustrated in Fig. 11.1 .
In addition to diagnostic subsurface horizons, there are two diagnostic soil
characteristics that reflect clay movement: (1) abrupt textural change and (2) lamel-
lae. An abrupt textural change is “a specific kind of change that may occur
between an ochric or an albic horizon and an argillic horizon” (Soil Survey Staff
2010 , p. 15) and is characterized by a considerable increase in clay content within
a very short vertical distance. In Australia such soils are called “duplex” and

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