[1] Because most methods for assessing the wettability of porous materials are restricted in their applicability, we developed two new methods for measuring contact angles and particle surface energy. The proposed methods (the Wilhelmy plate method (WPM) and the modified capillary rise method (MCRM)) were tested on 24 soils. For comparison, the water drop penetration time test (WDPTT) and the sessile drop method (SDM) were also applied. It was found that advancing contact angles, measured either with WPM or MCRM, agreed well in the range of 0°to 142°. Sessile drop contact angles were within the domain enclosed by the range of advancing and receding contact angles as determined with WPM. WDPTT, however, was only sensitive in the narrow range of 85°to 115°. We conclude that both WPM and MCRM are reliable methods for determining contact angles and particle surface energy over a wide range of porous material wettabilities.

Existing methods for determining the soil–water contact angle as a measure of water repellency are either indirect, cumbersome, or time‐consuming. Our objective was to develop a method that is simpler than existing procedures and that still yields accurate results. The proposed method represents a modified sessile drop method for measuring the initial contact angle of powdered or granular material. The measurements are made by placing a layer of uniform soil particles onto adhesive tape, adding droplets of deionized water, and reading off the contact angle at the three‐phase boundary line with a goniometer‐fitted microscope immediately after placing the drops on the soil sample. Sieved soil fractions <63 μm, 63 to 100 μm, and 100 to 200 μm were used to ensure particle layer uniformity. The method was tested on 10 samples from different depths of a sandy soil profile. The contact angles measured on these soil fractions were compared with the water drop penetration time (WDPT) test and the capillary‐rise method. The contact angles of the fractions <63 μm and 63 to 100 μm compared reasonably well with those measured with the capillary‐rise method and their rank order agreed in general with that of the WDPT test. We conclude that the new method appears to be promising for the simple, rapid, and reproducible determination of the contact angle of sandy soils. The sessile drop method can be used in a wider range of water repellency conditions compared with either the capillary‐rise or the WDPT method.

Water repellency can be a significant factor in soil physical behaviour, but little is known about the depth dependence of the contact angle of field soils. We investigated contact angles and wetting properties as a function of depth for a wide range of agricultural and forest soils in Germany. The agricultural soils ranged from silty to sandy texture (six profiles), and the forest soils ranged from sandy to loamy texture (eight profiles). Contact angles (CA) were measured with the Wilhelmy plate method (WPM). In most of the soils, advancing WPM contact angles were considerably greater than 0 and they varied irregularly with depth. In general, sandy soils had larger WPM contact angles than silty soils. From the relation of the contact angle with texture and pH the quality of soil organic matter (SOM) was considered as more important for the wetting properties than the total amount of soil organic carbon (SOC). Finally, it was found that for soils with intermediate sand contents either under agricultural or forest use, the kind of land use seemed not to influence CA. Coarse-textured sandy soils that were used only as forest sites were more hydrophobic than silty soils which were exclusively used as agricultural soils. We conclude that a coarse texture favours, in combination with other factors (mainly pH), hydrophobic SOM.

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Soil wettability affects hydrological processes like infiltration, percolation, preferential flow, and surface runoff. Wettability is related to the soil‐water contact angle, which in turn depends on the solid surface free energy. Little is known, however, about contact angles and their dependence on soil water potential. The main objective of this study was therefore to investigate the dynamics of contact angle due to variation of the water potential. Aggregate fractions of 2‐ to 4‐, 1‐ to 2‐, and <1‐mm diameter and corresponding homogenized material of a subcritical water repellent Orthic Luvisol were studied at water potentials of −1000, −154, −30, and −0.14 MPa. Wettability was assessed in terms of the advancing contact angle by the capillary rise method (CRM). Additionally, we calculated the surface free energy. Results showed, that the contact angle increased as water potential increased to a specific level. It was found for several soil samples, that above this water potential level, the contact angle decreased again. The change of contact angle due to variation of water potential reached nearly 90° for one sample. Contact angles of homogenized fractions were slightly larger than those measured for the aggregate surfaces. Surface free energy was consistently between 55 and 65 mJ m−2 with relative contributions of the dispersion and polar components to surface free energy of approximately 1/3 and 2/3, respectively. We conclude, that the assessment and physical description of the specific water potential for which a surface becomes wettable is a key factor for a better understanding of soil wetting.