Ultramafic Soils - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 259

Chapter 23
Ultramafic Soils

23.1 Introduction

Ultramafic rocks are those containing 45 % SiO 2 and include

igneous rocks (dunites, peridotites) and metamorphic rocks (talc carbonate and
serpentinite). These rocks generally contain high amounts of Fe and trace metals
such as Cr and Ni (Alexander et al. 2007a ). Ultramafic rocks are found in many
places of the world, such as Greece, South Africa, the Italian Alps, France, Portugal,
Spain, Cameroon, Morocco, Zimbabwe, India, Sri Lanka, Brazil, Costa Rica, New
Caledonia, New Zealand, Indonesia, and Japan. In areas with ultramafic rocks there
is often “edaphic endemism,” i.e., plant species that have a strong fidelity to a
specific soil taxon and that are found nowhere else (e.g., Whittaker et al. 1954 ;
Kruckeberg 1984 ). Some ultramafic rocks give rise to serpentine soils and such
soils are characterized by extreme plant nutrient concentrations and ratios.
“Serpentinite barrens” are known for their infertility (Rabenhorst and Foss 1981 ;
Alexander 1988 ) and for their droughtiness (Alexander 2010 ), sometimes referred
to as “the serpentine factor” (McGahan et al. 2009 ).
In the USA, ultramafic soils have been studied in California (Alexander 1988 ,
2010 ;Grahametal. 1990 ; Lee et al. 2003 ; Alexander et al. 2007a , b ), Oregon (Burt
et al. 2001 ), Alaska (Alexander et al. 1994a , b ), and Maryland (Rabenhorst and Foss
1981 ; Rabenhorst et al. 1982 ). Ultramafic soils also have been studied in Canada,
including British Columbia (Bulmer et al. 1992 ;BulmerandLavkulich 1994 )and
Newfoundland (Roberts 1980 ). Key findings of these and other studies are
(1) exchangeable Ca/Mg ratios of 0.1-0.5 at 30-60 cm depth which is indicative of
ultramafic materials (Rabenhorst and Foss 1981 ; Alexander et al. 1990 ;Balmerand
Lavkulich 1994 ; Bonifacio et al. 1997 ; Lee et al. 2004); (2) ultramafic soils are often
shallow and have abundant coarse fragments (McGahan et al. 2009 ); (3) ultramafic
materials such as serpentinite yields abundant smectite, especially in poorly
drained soils, and vermiculite or vermiculite-chlorite intergrades (Bonifacio

18 % MgO and

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