Wettability and Osmotic Effects on the Soil Water Retention Curves of Two Soils Dedrick Davis*1, Robert Horton1, and Josh Heitman2 Iowa State University1, North Carolina State University2 Introduction Sand Results Soil water repellency is a widespread phenomenon that can occur due to the presence of organic compounds on the surface of soil particles. Soil water repellency has been found to affect many soil physical properties, but relatively little is known about the extent of these effects. Many hydrological implications occur due to soil water repellency most noticeably the resistance of water moving into the soil matrix. The effect of water repellency on soil water retention is not well known, and osmotic effects on soil water retention are not fully understood. When osmotic effects are coupled together with soil water repellency, the combined effect is not understood, and it is not known which serves as the most influential effect. The soil water retention curve is a useful tool, but many studies have focused on the wet end while relatively few studies have concentrated on the dry end where surface adsorption is vital to the retention of water. Four conditions were applied to the soils: Hydrophilic Low EC Hydrophobic High EC Hydrophilic High EC Hydrophobic Low EC Silt Loam Soil was hydrophobized with dichloromethylsilane to achieve a contact angle ≈ 130° (Bachmann et al., SSSAJ 64:564-567, 2000). Water Content (g g-1) Water Content (g g-1) To obtain the hydrophobic low EC soil, high EC hydrophobic soil was leached with water until the electrical conductivity of the leachate was comparable to that of its hydrophilic counterpart. KCl was added to the hydrophilic low EC soil to obtain an electrical conductivity comparable to that of its high EC hydrophobic counterpart. Vapor equilibration -- Soil samples were placed over eight NaCl solutions mixed to achieve different osmotic potentials and allowed to equilibrate beginning either from relatively dry or relatively wet conditions. Ψm (-MPa) Ψm (-MPa) Vapor Equilibration Controlled Water Content Objective Ψm (-MPa) Ψm (-MPa) Conclusions Vapor Equilibration Controlled Water Content Water droplets visible on the surface of the hydrophobic samples provided clear indication of their hydrophobicity, but note that water remained at the top of the soil samples. Reduction of the electrical conductivity of the hydrophobic samples provided a water content comparable to that of its wettable counterpart clearly indicating the influence of osmotic effects on soil water retention. To examine the dry end of the soil water retention curve and the associated hysteretic behavior for two wettable soils and their matched hydrophobic counterparts. Also to examine combined osmotic and wettability effects on soil water retention. Soil water hysteresis was not obvious in the water potential rage 5-80 MPa for the wettable and hydrophobic soils (data not shown). Controlled water content -- For wetting, water was added to achieve a known water content, and potential was measured with a Decagon WP4 DewPoint Potentiometer. For drying, soil was initially saturated and then dried by evaporation to achieve the desired water content. This study confirms a previous hypothesis that during the hydrophobizing process the osmotic potential increases therefore affecting the amount of water retained. Materials and Methods When coupled together osmotic and wettability effects significantly influence the amount of water retained by a soil compared to osmotic or wettability effects considered separately. Two naturally hydrophilic soils were used: Hanlon sand (Coarse-loamy, mixed, superactive, mesic Cumulic Hapludolls) and Ida silt loam (Fine-silty, mixed, superactive, calcareous, mesic, Typic Udorthents). The authors would like to thank Jörg Bachmann and Susanne Woche for their assistance during the hydrophobizing process. High EC Hydrophobic Silt Loam at -2.26 MPa Financial assistance was provided by National Science Foundation grant no. 0337553