Duripan Horizon and Durinodes - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 145

mesic soil-temperature regime. This group of duric soils is polygenetic, having
been influenced by episodic eolian deposition (Blank et al. 1998 ). Volcanic ash and
siliceous loess are weathered comparatively rapidly to opal-A. Microaggregates of
silt and clay act as nuclei for Si precipitation (Blank and Fosberg 1991 ). Unlike
duripans in NV and CA, the duripan in southern ID tends to grow upward.
Whereas duripans on late Tertiary surfaces in Australia often have high concen-
trations of resistant minerals such as anatase and zircon, reflecting intensive
weathering (Milnes and Twidale 1983 ), those in western USA are not enriched in
these constituents (Torrent et al. 1980 ).

14.7 Summary

Soils with durinodes and duripans are widespread in the western part of the USA.
They occur on old geomorphic surfaces mostly in dry climates and in parent
materials that release Si upon weathering. Durinodes may form in 10 ka, whereas
the formation of a mature duripan may take at least 130 ka. Many of the duripans
have been formed in wetter climates than today, and some pedons have multiple
pans. In the USA, parent material seems to be the best predictor for the absence and
presence of durinodes and duripans, followed by climate. The effects of biota and
topography have not been well researched. Durinodes reduce the available soil
volume for plant growth, whereas duripans seriously hamper root growth and air
and water movement through the soil profile.

Acknowledgments Alfred Hartemink contributed to an earlier draft of this manuscript.

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