Ortstein and Placic Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 198

significantly less ( p ¼ 0.024) in soils with 50 % ortstein than in soils of the other

two groups.
Despite some differences in mean annual precipitation, the climates of areas
with Spodosols with and without ortstein are comparable (Table 18.6 ). The mean
annual precipitation in areas with the three groups of soils averages between 1,000
and 1,300 mm but ranges from 380 to 2,800 mm. The mean annual air temperature
averages 10 C but ranges from 2.8 to 23 C. As is typical of Spodosols in general,

the vegetation was dominated by coniferous trees in the overstory and ericaceous
vegetation (e.g., Vaccinium ) in the understory and included slash pine ( Pinus
elliottii ) in the “flatwoods” of FL, northern hardwoods ( Acer - Tilia - Fagus - Betula )
and mixed conifers ( Tsuga - Pinus ) in the Great Lakes region and northeastern USA,
and western conifers ( Pseudotsuga - Tsuga - Thuja - Picea ) in western WA and OR
and southeastern AK. The three soil groups were derived from a variety of parent
materials. However, the most prevalent parent materials in soils with ortstein
(79 %) were outwash sands, sandy till, and sandy marine terrace deposits of
crystalline origin.

18.5.2 Placic

The site conditions leading to the formation of placic horizons are poorly understood.
Climate appears to play a major role in that placic horizons are observed in soils with
extremely high precipitation, ranging from 2,400 to 3,900 mm/yr and averaging
3,200 mm Lavkulich et al. 1971 ;(Valen ine 1969 ; Shoji et al. 1988 ); Ping
et al. 1989 ; (Hseu et al. 1999 ; Wu and Chen 2005 ; Pinheiro et al. 2004 ;Schawe
et al. 2007 ). This is the same as the 3,200 mm/yr recorded for the seven soils with
placic horizons in the USA. Soils with a placic horizon generally occur in depressions
on the landscape and have a seasonally high water table (Lapen and Wang 1999 ;
Wu and Chen 2005 ; Pinheiro et al. 2004 ;Crampton 1963 ). In the present study, five
of the seven soil series with a placic horizon have an aquic soil moisture regime
(Table 18.6 ). In Europe placic horizons occurred in areas where the groundwater was
less than 70 cm from the soil surface (Kaczorek et al. 2004 ).
Soils with placic horizons occur in one of three predominant vegetation types:
tropical montane forest (Wu and Chen 2005 ; Pinheiro et al. 2004 ; Schawe
et al. 2007 ), western conifers (Valentine 1969 ; Shoji et al. 1988 ); Ping
et al. 1989 , or bog (Lavkulich et al. 1971 ; Lapen and Wang 1999 ; Crampton
1963 ). Of the seven soil series with a placic horizon examined in this study, three
support western conifers and four feature tropical montane forest (data not shown).
Placic horizons are common in soils derived from volcanic ash (Shoji et al. 1988 );
Ping et al. 1989 ; (Pinheiro et al. 2004 ). One of the soils in the present study is
derived from volcanic ash; three are developed from basaltic or andesitic residuum.
Jien et al. ( 2010 ) reported an argillic horizon below the placic horizon and
suggested that clay migration leads to restricted drainage and the development of
a placic horizon. In the present study, six of seven pedons were underlain by either