Ortstein and Placic Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 199

saprolite or dense till. There were insufficient data to identify key factors influenc-
ing distribution of soils with placic horizons in the USA (Table 18.3 ). However,
these soils tended to be in areas with abundant precipitation (ca. 2,400 mm/year),

restricted internal drainage because of a lithic or paralithic contact, and textural
discontinuities.

>

18.6 Genesis of Ortstein and Placic Horizons

18.6.1 Ortstein

There are at least four conditions necessary for the formation of ortstein: (1) a
supply of Fe and Al from weathering of ferromagnesian minerals; (2) a supply of
dissolved organic C, particularly fulvic acids, for complexing with the Al and Fe;
(3) restricted drainage in the profile for at least part of the year to enhance the
complexation process; and (4) a period of drying whereby the cementing com-
pounds dehydrate and harden (Lapen and Wang 1999 ; Kaczorek et al. 2004 ;
Moore 1976 ; McKeague and Wang 1980 ). To these conditions Lapen and Wang
(Lapen and Wang 1999 ) would add the condition of a strong soil pH gradient that
would enable hydrolysis and precipitation of Al compounds.
In addition to being Spodosols, the majority (89 %) of the soils in this study
containing ortstein had one or more of three conditions that cause restricted internal
drainage, including (1) a textural discontinuity at or immediately below the ortstein,
(2) a seasonally or permanently high water table immediately below the ortstein, or
(3) bedrock or hard till several decimeters below the ortstein layer. More than half of
the pedons have a textural discontinuity, including a coarse layer over a finer layer or
a fine layer over a coarser layer. Textural discontinuities are important for holding up
water and increasing the time for soil particles to react with the soil solution. About a
third of the pedons had either a seasonally or semipermanently high water table.
The presence of a water table at depth enables coprecipitation in the Bh horizon of
organic compounds and Al, especially in soils containing ortstein in FL (Lee
et al. 1988a , b ) and in tropical regions (Farmer et al. 1983 ). Four of the pedons with
ortstein in the present study had bedrock with 100 cm of the surface. Since these soils
were somewhat excessively or well drained, the bedrock serves to temporarily restrict
internal drainage to increase the time for soil particles to react with the soil solution.
Eight of the soils are developed under excessively to well-drained conditions in
relatively uniform sandy outwash, and yet they contain ortstein. These include the
Copemish, Crowell, Garlic, Pullup, Wallace, Kaleva, Kalkaska, and Netarts series.
All of these series occur in northern MI except for the Netarts which exists along the
OR coast. These soils warrant further investigation as they do not reflect the
drainage conditions for ortstein formation.
The NRCS data suggest that ortstein begins to form in discrete areas of the
spodic horizon as nodules, “bodies,” or “pockets” and that eventually they form