ReferencesBachmann, J., Woche, S., Goebel, M. –O., Kirkham, M. & Horton, R. 2003. Extended methodology for determining wetting properties of porous media. Water Resources Research, 39, 1353-1367.

Diehl, D., Ellerbrock R.H. & Schaumann, G.E. 2009. Influence of drying conditions on wettability and DRIFT spectroscopic C–H band of soil samples. European Journal of Soil Science, 60, 557-566.

Doerr, S.H. & Thomas, A.D. 2000. The role of soil moisture in controlling water repellency: new evidence from forest soils in Portugal. Journal of Hydrology, 231-232, 134-147.

Horne, D.J. & McIntosh, J.C. 2000. Hydrophobic compounds in sands in New Zealand—extraction, characterization and proposed mechanisms for repellency expression. Journal of Hydrology, 231-232, 35-46.

Kleber, M., Sollins, P. & Sutton, R. 2007. A conceptual model of organo-mineral interactions in soils: self-assembly of organic molecular fragments into zonal structures on mineral surfaces. Biogeochemistry, 85, 9-24.

Hydrophobic Soils: Exploring the Mechanism for Reversible HydrophobicityPriscilla Woolverton, Maria Dragila, Markus Kleber, Don Horneck

Department of Crop and Soil Science, Oregon State University, Oregon, USA

The New View

Present understanding:Water repellency is thought to be caused by organic hydrophobic compounds, which are present as coatings on soil particles or as interstitial matter between soil particles. 

http://www.water-repellency.alterra.nl/

The new view:

Water repellency can occur in any high stress environment. Depending on the ability of organic matter to reorient upon rewetting, hydrophobicity can be permanent or reversible.

q > 90

Scientific goal: • Determine the precise mechanisms that link molecular structure of soil OM to

particulate surface energy (wettability).

Hypothesis:

• The orientation of individual amphiphilic molecular fragments of soil organic matter (OM) determines the occurrence and the extent of water repellent properties in the soil,

• this orientation changes as a function of moisture content.• Irrigation treatments enhance OM decomposition and preferentially deplete the soil of

mobile amphiphilics.• As a consequence, the amphiphiles in the kinetic zone are lost and permanent hydrophobicity

may develop.

Conceptual model

ProblemA serious consequence of irrigation agriculture, in high-stressenvironments is that, in time, soil will degrade and develop water repellency, i.e., hydrophobic soil. Water repellency is a worldwide problem.

Soil texture Soil management

Climate

Low input

High input

High H 2

O

holdi

ng ca

pacit

y

Low

H 2O

hold

ing

capa

city

Moist

Arid

Marginal soils, i.e. sandy soils in arid climates under low OM input systems.

Figure 4. Definition of high-stress soils. Schematic of conditions leading to permanent hydrophobicity in soils.

Three parameters drive soil hydrophobicity: Soil texture; Climate; OM management.While reversible hydrophobicity can occur in many environments, it is the particular intersection of conditions shown above (triangle) that lead to permanent hydrophobicity.Of the three conditions, growers can only control soil OM.

Repercussions of hydrophobicity: Decreased crop productivity, increased water and fertilizer use, run-off and soil erosion, preferential flow and transport of contaminants to groundwater.

Circle 3

Circle 2

Circle 1

Circle Cf2

Circle An

Circle E

Water Content (%)

Dyn

amic

Con

tact

Ang

le (d

egre

es) 100

90

80

70

60

50

40

30

20

10

0

0 2 12 14 1610864

Dynamic Contact Angle vs. Water Content

Confirmation

Figure 5. Graph of CA (measure of severity of hydrophobic phenomenon) vs. water content.

• If hydrophobic coatings were the only reason for poor wettability, all contact angles should be high (above 60 degrees)

• If water content was the only parameter controlling hydrophobicity, all data would fall within the blue region of the graph.

• However, two distinct responses to water content are observed: 1. soils that retain hydrophobic behavior even at higher water contents

(orange region) and 2. a dynamic hydrophobic characteristic of soil that changes in response to

soil water content (reversible hydrophobicity).

AcknowledgementsThis work is supported by the AFRI-USDA program on Soil Processes grant #2009-65107-05928, and National Science Foundation Hydrologic Sciences Program grant # 0449928.

Study approach

Quincy soils

Associated soils

An

As 23

1 E4

Cf2

Cf3

Csw

Figure 2:a. Location of Quincy (red) and associated soils in

Morrow County, Oregon. White circles indicate fields sampled and analyzed.

b. Study site location in E. Oregon (45”47’15.21 N, 119”31’03.47 W).

Figure 3a. Even though, this is sandy soil with high hydraulic conductivity, water sits on surface of non-wettable soil and ponds

between plantsb. Schematic of preferential water infiltration between rows that is unavailable for use by plantsc. Soil erosion as an effect of water- ponding and subsequent run-off

b.

a. c.

a.

b.

• To test this model we selected 10 125-acre circles (with same soil texture and climate) in agricultural production and representing a range of soil management strategies. We are quantifying the impact of management on OM structure and resulting laboratory and field scale hydrophobicity.

• Soil sample analysis: surface energy, C and N content and OM structure. 1. 2.

3. 4.

(Based on Horne & McIntosh 2000; Kleber et al. 2007; Diehl et al. 2009)

OREGON

WA

A1

C2C1 P1 P2 A2 H3

H2

OREGON

WA

OREGON

WA

A1

C2C1 P1 P2 A2 H3

H2

95-3