A proposed master V horizon for near surface horizons with vesicular porosity

Judith TurkUniversity of California, Riverside

Carrie-Ann HoudeshellUSDA-NRCS, Victorville

Robert GrahamUniversity of California, Riverside

Vesicular Horizons

Silt (%)10 15 20 25 30 35 40 45

De

pth

(cm

)

0

20

40

60

80

V horizon

V horizon

Vesicular horizons

(Rossi, 2009)

Vesicular Horizon Formation

Formation in Laboratory

Miller, 1971

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

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.

Hydrologic and ecological function:Vesicular horizons have low infiltration rates

Young et al., 2004

Occur primarily in intercanopy spaces

Intercanopy:Strong V horizon

Undercanopy:Absent or weak V horizon

V horizon formation in intercanopy leads to heterogeneity of hydraulic properties

Schafer et al., 2007

Region Infiltration Rate (cm hr-1) Reference

V horizon A horizon

Sonoran Desert 0.8 6.0-9.6 Musick, 1975

Central Mojave 0.3-0.8 6.8-15 Young et al., 2004

Central Mojave 1.3-4.6 8.9 Miller et al., 2009

Northern Mojave 1.2-4.5 5.5-17 Schafer et al., 2007

Northern Mojave 0.4-1.4 3.1-3.2 Eckert et al., 1975

Great Basin 1.7-3.2 5.8-7.2 Blackburn, 1975

Patagonia 0.6 4.1 Rostagno, 1989

Infiltration rates in soils with V horizons are consistently lower than those with

non-vesicular A horizons

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

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)

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

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

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.

AcknowledgmentsSupporting research funded by:

USDA-NRCS

Helpful comments on the proposal were provided by:

Joe Chiaretti

Craig Ditzler

Brenda Buck

Patrick Drohan

Les McFadden