Ultramafic Soils - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 272

23.8 Summary

Ultramafic rocks are those containing 45 % SiO 2 rocks, which

upon weathering often give rise to serpentine soils. These soils are characterized by
extreme plant nutrient concentrations and ratios and lead to the formation of
“serpentinite barrens,” which are known for their infertility. Some key features of
ultramafic soils are high exchangeable Mg values, narrow exchangeable Ca/Mg
ratios, shallow depths, and abundant coarse fragments. Of the 67 ultramafic soil
series examined from 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. Ultramafic soils occur in nine orders, primarily Alfisols, Mollisols, and
Inceptisols, 18 suborders, 24 great groups, and 40 subgroups. Seventy-three percent
of the ultrabasic soil series are in the magnesic mineral family.
The total area of ultramafic soils mapped in the USA is 3,031 km 2 ; they occur
primarily in CA and OR, with smaller areas in WA, PR, PA, MD, and DE. The
primary factor influencing the genesis of ultramafic soils obviously is parent
material. Major pedogenetic processes in ultramafic soils include weathering,
oxidation, clay illuviation, and accumulation of Fe oxyhydroxides. Thirty-five of
the 67 ultramafic soil series from the present study contain Bt horizons.

18 % MgO and

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