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et al. 1997 ; Istok and Harward 1982 ;Rabenhorstetal. 1982 ;Hseuetal. 2007 );
(4) ultramafic soils often have a variable charge that is related to the high free Fe and
soil organic C contents (Bonifacio et al. 1997 ; Alexander 2010 ); and (5) weathering
rates of ultramafic materials are rapid when sufficient moisture is available because of
the abundance of serpentine, olivine, and pyroxenes (Alexander 2010 ).
This chapter differs from the previous chapters in that a diagnostic horizon is not
treated; rather soils with an unusual mineralogy are the focus. The extent and
formation of ultramafic soils in the USA using the information from NRCS
SSURGO databases combined with published case studies.
23.2 Properties of Ultramafic Soils
Morphological features of ultramafic soils are the common occurrences of a mollic
or umbric epipedon and a shallow soil depth. Of the 67 ultramafic soil series
examined in the SSURGO database, 25 series (37 %) have an umbric or mollic
epipedon and 18 (27 %) have a shallow depth class or are in a lithic subgroup. The
mean depth to the Cr or R horizon for the 67 ultramafic soil series is 102 cm (data
not shown). An ultramafic landform and soils from the New Idria Formation in the
CA Coast Range is depicted in Fig. 23.1 .
Ultramafic soils have high amounts of exchangeable Mg (average
¼
10.6 cmol c /kg),
narrow Ca/Mg ratios (average
¼
19.5 cmol c /kg), and an average base saturation of 56 % (Table 23.1 ). The pH averages
6.1. Ultramafic soils have high Fe d concentrations (average
¼
0.9), a moderate cation-exchange capacity (average
¼
7.3 %), low Fe o
concentrations (average
1.0 %), and high concentrations of Cr, Ni, Mn, and
Co. The soils commonly have an argillic horizon (35 of 67 soil series). The mean
bulk density of representative pedons is 1.07 g/cm 3 .
From X-ray diffraction analysis, ultramafic soils class contain primarily kaolinite,
smectite, and talc in the clay-size (
¼
m) fraction, with small amounts of goethite,
hematite, and chlorite. From petrographic analysis, the coarse-silt (0.05-0.02 mm)
fraction contains chrysotile, hornblende, pyroxenes, talc, and opaque minerals. The
-1,500 kPa water retention/% clay ratio (WCR) averages 0.7, suggesting an abun-
dance of 0.2
2
<
μ
m pores and short-range-order compounds, especially in the less-
weathered subsoil (Table 23.1 ).
μ
23.3 Classification of Ultramafic Soils
Nine soil orders contain ultramafic soils; they have not been identified in Histosols,
Gelisols, and Aridisols (Table 23.2 ). Most ultramafic soils classify as Alfisols
(22 soil series), followed by Mollisols (17), Inceptisols (16), and Spodosols (4),
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