Argillic, Kandic, and Natric Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 98

In humid environments, argillic horizons are more strongly developed in
well-drained soils than in soils with restricted drainage (Cremeens and Mokma
1986 ; Hopkins and Franzen 2003 ) (Table 11.4 ). In Vertisols under a thermic soil
temperature regime, argillic horizons are more strongly developed in micro-lows
(Sobecki and Wilding 1983 ). Argillic horizons are more strongly developed
on backslopes than on actively eroding shoulders (Young and Hammer 2000 ;
Olson et al. 2005 ; Wilson et al. 2010 ).
In humid environments, argillic horizons require about 12,000 yr to form
(Table 11.4 ). However, in soils with ustic or aridic soil-moisture regimes, the
argillic horizon develops in 9,000 yr (Nettleton et al. 1975 ; Southard and Southard
1985 ; Karlstrom 2000 ). Alexander and Nettleton ( 1977 ) reported a Natrargid
forming in as little as 6,600 yr in Nevada. Kandic horizons may require 1-2 million
years to form (Alexander 2010 ). However, Bt horizons may develop in as few as
2,100 yr (Cremeens 1995 ). These isolated studies do not necessarily imply that
argillic horizons form more rapidly in soils with a ustic or aridic soil-moisture
regime than in those with a udic regime (Rabenhorst and Wilding 1986a , b ;
Gunal and Ransom 2006 ; Khormali et al. 2012 ).
11.6 Genesis of Clay-Enriched Horizons
The evidence for clay enrichment in argillic and related horizons includes (1) cutans
on horizontal and vertical ped faces, (2) clay bridging sand grains, (3) clay lining
pores, (4) an increase in clay from an overlying eluvial horizon that does not
directly reflect stratification or a lithologic discontinuity, (5) lamellae, (6) a wider
fine clay: total clay ratio than in an overlying horizon, (7) thin sections with oriented
clay bodies that are more than 1 % of the section, and (8) a high coefficient of linear
extensibility (COLE) which enables shrinking and swelling clays (Soil Survey
Staff 2010 ).
Birkeland ( 1999 ) recognized four processes that account for clay enrichment in
argillic and related horizons: (1) translocation of clay from eluvial to illuvial hori-
zons, (2) translocation of clay contributed by aeolian processes to illuvial horizons,
(3) weathering of silt-size or coarser particles into clay-size material in situ, and
(4) synthesis of clays from the soil solution, i.e., neoformation. Two additional
processes include parent material stratification and preferential erosion of fine parti-
cles from landform and ped surfaces (Walker and Chittleborough 1986 ). There are
three mechanisms involved in formation of clay-enriched horizons, including (1) dis-
persion, (2) translocation, and (3) accumulation (Eswaran and Sys 1979 ).
The argillic horizon requires decalcification in order for the clay particles to
become dispersed. A sufficient amount of water is required to move the clay from
the eluvial to the illuvial horizon. For this reason, the argillic horizon is generally
found in soils with aquic, udic, ustic, or xeric soil-moisture regimes (Table 11.5 ).
Argillic horizons are common in Aridisols, but most investigators in these regions
attribute them to a previous moister climate (Table 11.4 ). Argillic horizons