Ortstein and Placic Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 190

for Fe d and Al d to determine the nature of the cementing agents. Parfitt and Childs
(Parfitt and Childs 1988 ) used M¨ssbauer spectroscopy to estimate forms of Fe and
Al from these extractions. Key findings were as follows: (1) Fe p does not relate to
any particular form of Fe; (2) Al p corresponds to Al in humus complexes; (3) Al o
minus Al p yields allophane and imogolite, if large amounts of ferrihydrite are not
present; and (4) Fe d minus Fe o yields crystalline forms of Fe such as hematite, but
also yields ferrihydrite.
Spodic horizons in all three soil groups contained comparable amounts of
Al-humus (Al p ), amorphous compounds (Al o -Al p ) and Si o , and crystalline forms
of Fe (Fe d -Fe o ) (Table 18.1 ). There were no significant differences in ODOE or any
of the other analytical properties reported in Table 18.2 . Spodic horizons in all three
groups of soils contained abundant organic C (1.7-3.0 %), were very strongly acidic
(pH 4.9-5.0 in water), had abundant exchangeable cations (16-24 cmol c /kg) with
exchangeable Al (52-62 %), and were depleted in base cations (6-9 %). There were
no significant differences in the amount of coarse fragments ( 2 mm) among the

three soil groups. Although the differences were not significant, average bulk
densities could be arrayed:

>

50 % ortstein, 50 % ortstein, and no ortstein

<

(Table 18.1 ).

18.2.2 Placic

Soils with placic horizons generally were shallow (Table 18.1 ). The placic horizons
ranged from 5 to 17 mm in thickness and averaged 12 mm (data not shown).
A spodic horizon occurred in two of the soil series and was considerably shallower
than those for the other three soil groups. The surface of the placic horizon ranged
from 25 to 117 cm in depth and averaged 59 cm (data not shown). Placic horizons
are indurated with laminar ironstone and have a “troweled” upper surface. The
ironstone sheet is impermeable to roots except through cracks. The texture of placic
horizons is highly variable, ranging from clay to extremely gravelly loamy sand.
Extractable Al p averaged 0.93 % (three pedons only) (Table 18.1 ). The placic
horizon from the Kahanui series (HI) and Halbert series (WA) contained 1.0 % and
0.35 % Al, respectively, in allophane or imogolite forms. The dominant Fe minerals
were goethite, hematite, and ferrihydrite, with values of 6.28 % and 2.35 % for the
Kahanui and Halbert soils, respectively.
Placic horizons contained an average of 2.98% organic C, a pH of 4.9, 46 cmol c /kg
exchangeable cations, 68 % Al saturation, and an average base saturation of 4 %
(Table 18.1 ). No bulk density values were available from the NRCS dataset,
but values from the literature ranged widely from 1.00 to 2.20, but most values were
around 1.70 g/m 2 (Lavkulich et al. 1971 ); Ping et al. 1989 ; (Hseu et al. 1999 ;Wuand
Chen 2005 ).