Ortstein and Placic Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 191

18.3 Classification of Soils with Ortstein or Placic Horizons

18.3.1 Ortstein

Aquods accounted for 49 % of the soil series classified with 50 % ortstein,

followed by Orthods (40 %), Cryods (9 %), and Humods (2 %; Table 18.3 ). The
dominant great groups were Duraquods (13 pedons), Haplorthods (10), Alaquods
(8), and Durorthods (6). Soils with

>

50 % ortstein were classified primarily as
Orthods (71 %) and Aquods (29 %). Dominant great groups were Haplorthods
(26 pedons), Alaquods (6), and Endoaquods (6). Geographically associated
Spodosols lacking ortstein were arrayed: Orthods (53 %), Aquods (32 %), Cryods
(12 %), and Humods (3 %). Dominant great groups included Haplorthods
(24 pedons), Alaquods (12), and Fragiorthods (6).
Sixty-two percent of the soils containing some form of ortstein have a udic
soil moisture regime and 38 % have an aquic SMR (Table 18.2 ). Soils with some
form of ortstein were classified primarily into four soil temperature classes,
including frigid (38 soil series), mesic (20), hyperthermic (16), and isomesic
(7). The dominant mineralogy class was mixed (50 soil series), followed by
siliceous (21), isotic (15), and amorphic (3).
All three groups of Spodosols commonly occurred in sandy, sandy-skeletal, or
sandy over loamy particle-size classes (Table 18.2 ). However, the proportion of soil
series in these classes was greater for soils with some form of ortstein. A predom-
inance of soils in the three groups had udic rather than aquic soil-moisture regimes
(SMRs). However, the proportion of soils with an aquic SMR was greatest in soils
with

<

50 % ortstein in the spodic horizon. Ortstein occurs in soils with soil
temperature regimes (STR) ranging from cryic or frigid to hyperthermic as well
as isomesic. However, soils with ortstein most commonly had a frigid STR. Soils
with ortstein occurred in four soil mineral classes, including mixed, siliceous,
isotic, and amorphic, but were most common (56 %) in the mixed class.

>

18.3.2 Placic

The seven soil series with placic horizons occurred in three orders with four great
groups: Petraquepts, Cryaquods, Placorthods, and Placudands (Table 18.3 ). Nearly
three quarters of the placic soils have an aquic soil moisture regime. Soils with
placic horizons are all classified into iso-soil temperature classes, including
isothermic (four pedons), isofrigid (2), and isomesic (one pedon), and into four
mineralogy classes,

including mixed (four pedons), isotic, amorphic, and

parasesquic (one pedon each).