1. A proposed master V horizon for near surface horizons with vesicular porosity
Judith Turk
University of California, Riverside
Carrie-Ann Houdeshell
USDA-NRCS, Victorville
Robert Graham

2. Vesicular Horizons

3. Vesicular horizons Silt (%) 10 15 20 25 30 35 40 45 Depth (cm) 60 8060 80
V horizon
V horizon
(Rossi, 2009)

4. Vesicular Horizon Formation

5. Formation in Laboratory
Miller, 1971

6. Why we need a master V horizon:
V horizons are critical to hydrologic and ecological function of arid and semi-arid lands
V horizons are abundant and widespread
Currently there is no NCSS-approved terminology for V horizons
V horizons are designated as A horizons in NCSS terminology, but do not fit the main concept of an A horizon

7. Definition of proposed horizon
V horizons: Mineral horizons that have formed at the soil surface, or below a layer of rock fragments (e.g., desert pavement), physical or biological crust, or recently deposited eolian material. They are characterized by the predominance of vesicular pores and have platy, prismatic, or columnar structure.

8. Hydrologic and ecological function:
Vesicular horizons have low infiltration rates
Young et al., 2004

10. Absent or weak V horizon
Occur primarily in intercanopy spaces
Intercanopy:
Strong V horizon
Undercanopy:
Absent or weak V horizon

11. V horizon formation in intercanopy leads to heterogeneity of hydraulic properties
Schafer et al., 2007

12. Infiltration Rate (cm hr-1)
Infiltration rates in soils with V horizons are consistently lower than those with non-vesicular A horizons
Region
Infiltration Rate (cm hr-1)
Reference
V horizon
A horizon
Sonoran Desert
0.8
Musick, 1975
Central Mojave
6.8-15
Young et al., 2004
8.9
Miller et al., 2009
Northern Mojave
5.5-17
Schafer et al., 2007
Eckert et al., 1975
Great Basin
Blackburn, 1975
Patagonia
0.6
4.1
Rostagno, 1989

13. Runon from intercanopy soils helps to support plant growth in shrub islands

14. Distribution of V horizons
V horizons occur on every continent
In the United States there are 1460 soil series with vesicular horizons
These soil series include Aridisols, Mollisols, Entisols, Alfisols, Inceptisols, Andisols, and Vertisols
The total mapped area of these soil series is 200,000 km2 (52 million acres)

15. Basin and range and intermontane plateaus

16. Current vesicular horizon nomenclature
Terms used in scientific literature
Av horizon
Vesicular layer
Schaumböden (foam soil)
FAO-WRB
Yermic diagnostic horizon (desert pavement and vesicular layer)
USDA-NRCS
Usually designated as A horizons
No terminology specific to the vesicular horizon

17. Does not fit the main concept of A horizon
Limitations of current NRCS nomenclature: Vesicular horizons as A horizons
Does not fit the main concept of A horizon
Not enriched in humified organic matter
Designated as A horizons because they are at the surface
Does not distinguish between surface horizons that support rapid infiltration and those that impede infiltration (V horizons) in desert landscapes

18. Summary
Currently there is no NCSS terminology to distinguish the vesicular horizon from other types of surface horizons.
Adoption of a master V horizon by the NCSS will highlight the presence of a widespread and ecologically important soil feature in arid and semi-arid lands.

19. Acknowledgments Supporting research funded by: USDA-NRCS
Helpful comments on the proposal were provided by:
Joe Chiaretti
Craig Ditzler
Brenda Buck
Patrick Drohan
Les McFadden