Ortstein and Placic Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 200

isolated “chunks” of ortstein. The distance between columns or chunks of ortstein
varied from 10 cm to as much as 150 cm. In that the extent of cementation varied
from 5 % to 100 %, it is possible that these chunks eventually coalesce to form a

continuous ortstein horizon.
Most of the soils examined in this study occur on relatively young landforms,

<

ca. 200 ky. These data may suggest that ortstein eventually degrades with

continued pedogenesis. Bockheim et al. (Bockheim et al. 1996 ) examined a
chronosequence of soils on uplifted marine terraces along the OR coast and
provided a model illustrating the stages in development leading to the conversion
of Spodosols containing ortstein to Ultisols. An argillic horizon forms beneath the
ortstein and eventually builds upward, destroying the ortstein. They found relict
pieces of ortstein with clay coatings in Typic Haplohumults ca.

<

600 ky in age.

>

18.6.2 Placic

Textural discontinuities or organic-rich horizons may be important in arresting the
Fe in the placic horizon (Lapen and Wang 1999 ; Wu and Chen 2005 ; Pinheiro
et al. 2004 ). These layers cause perching of the water table which enables the Fe to
become temporarily reduced; with drying the Fe is mobilized and reoxidized at the
textural break. Some investigators suggest that clay eluviation followed by podzol-
ization creates the textural discontinuity that leads to episaturation (Jien
et al. 2010 ). The precipitation of Fe may be due to higher oxidation potential and
pH at depth in the profile (Lapen and Wang 1999 ). The time required for forming
placic horizons is not known.
There were no significant differences in concentrations of Fe, Al, and Si extracted
by Na pyrophosphate, acid NH 4 oxalate, and citrate-dithionite-bicarbonate among
the three groups of Spodosols (Table 18.2 ). However, only small concentrations of
these constituents are necessary to cause cementation (Moore 1976 ). From the
literature Fe and Al extractions yield the following results with regard to ortstein
cementing agents: (1) pieces of orstein do not slake in water but do dissolve in
reagents commonly used to extract Al and Fe (Lee et al. 1988b ); (2) narrow Fe o /Al o
indicate that Al is more abundant in ortstein horizons than Fe (Lee et al. 1988a , b ;
Mokma 1997 ; Lapen and Wang 1999 ; Moore 1976 ); (3) Al p is particularly abundant
in the Fe-Al extractions, suggesting the existence of Al-humus compounds (Barrett
1997 ; Lapen and Wang 1999 ;Kaczoreketal. 2004 ;Farmeretal. 1983 ;Miles
et al. 1979 ); and (4) wide Al o /Al p and Fe o /Fe p indicate that allophane and ferrihydrite,
respectively, may be involved in the cementation (Freeland and Evans 1993 ;Barrett
1997 ) (Table 18.7 ). Based on energy dispersive spectrometry (EDS), scanning
electron microscopy (SEM), differential X-ray diffraction (DXRD), micromorphol-
ogy, and proximate analysis of organic C, ortstein likely is cemented by Al-fulvic-
acid gels or short-range-order compounds containing dissolved organic C, Al, and
some Fe that bridge sand grains (Lee et al. 1988a , b ; Freeland and Evans 1993 ;
Kaczorek et al. 2004 ;Farmeretal. 1983 ; McKeague and Wang 1980 ).