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a Institute of Soil Science, Univ. of Hannover, Herrenhaeuser Str. 2, 30419 Hannover, Germany
b Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
* Corresponding author (goebel{at}ifbk.uni-hannover.de).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONTHEORYMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CRM, capillary rise method • RH, relative humidity • SOM, soil organic matter • WC, gravimetric water content • WDPT, water drop penetration time
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONTHEORYMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The existence of water repellent soils has been known for many years, but there are indications that under certain conditions all soils may display water repellency to some degree (Doerr et al., 2000). In the past, water repellency was often attributed only to sandy soils. However, recently it has also been observed in loam, clay, peat, and volcanic ash soils (Wallis and Horne, 1992; Ellies and Hartge, 1994; Ritsema et al., 1997; Jaramillo et al., 2000). For many soils distinct stages of restricted wettability have been found. The phenomenon covers a wide range of severity and is often too weak to be detected by visual diagnosis. This less extreme manifestation of water repellency is referred to as subcritical repellency (Tillman et al., 1989). Strong repellency, that is, no infiltration into the soil matrix, can be recognized visually only for contact angles >90°, and is then called hydrophobicity. So far, little is known about the dependence of the contact angle on the moisture status of the soil.
Water repellency has an impact on a wide range of physical processes in the vadose zone, like infiltration, preferential flow, and the three-dimensional distribution and dynamics of soil moisture (Dekker and Ritsema, 1994; Bauters et al., 1998; DeBano, 2000). For this reason, it also has implications for root water uptake and the leaching of agrochemicals (Doerr et al., 2000). Water repellent topsoils resist or retard water infiltration and therefore favor surface runoff and erosion (Clothier et al., 2000). Moreover, water repellency enhances aggregate stability against slaking and dispersion (Zhang and Hartge, 1992) and may in this way increase the stability of soil organic matter (SOM) against microbial decomposition (Giovannini et al., 1982; Tisdall, 1996; Hassink and Whitmore, 1997; Piccolo and Mbagwu, 1999).
Water repellency is caused by low surface free energy of the soil particles resulting in a weak attraction between the solid and the liquid phase (Roy and McGill, 2002). Soil minerals, in general, have high-energy surfaces, but under natural conditions they are often covered by films of adsorbed organic molecules with low energy (Doerr et al., 2000). These substances may originate from fungal hyphae (Sun et al., 1999), humic acids (Roberts and Carbon, 1971), or partly decomposed plant materials. This can lead to a large number of nonpolar sites on the solid surface (Tschapek, 1984; Drehlich, 1997). Van't Woudt (1959) showed that water repellency is a time-dependent property. It can be attributed to water–soil surface interactions, leading to a reduction of the liquid surface tension. Agricultural management influences the amount and quality of SOM and may therefore affect the wettability of the soil particles (King, 1981). Another factor, which influences the wettability, is the soil water content or soil water potential (King, 1981; Jex et al., 1985; de Jonge et al., 1999; Doerr and Thomas, 2000; Dekker et al., 2001; Doerr et al., 2002). While authors like Dekker and Ritsema (1994) reported an inverse relation between soil water content and water repellency and concluded that air-dry soils repel water the most, others, like Doerr et al. (2002) found that water content is directly related to water repellency. Decreasing water repellency with increasing water content is explained by the detachment of hydrophobic molecules from the mineral particles leading to nonrepellent polar surfaces (Doerr and Thomas, 2000). King (1981) and de Jonge et al. (1999), however, found a more complex behavior with one or two water repellency maxima in relation to water content.
Previous studies indicate that it is not possible to describe the wettability of soils only as a static property but rather as a range of possible states of wettability depending on the existing soil water condition. Therefore, the investigation of the dynamics of water repellency is the main objective of this study. Different sizes of aggregates taken from soil considered to have a low degree of water repellency are used to study the effect of soil water potential on soil wettability. Special emphasis is put on the range of high (less negative) soil water potentials. Measurements of wettability are made on intact aggregates and on homogenized aggregate material to analyze the difference in water repellency that may exist between the outer surfaces and the interior of aggregates. This could provide an indication of possible effects on the stabilization of encapsulated SOM.
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THEORY |
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Most approaches start with the Young equation (Young, 1805), which describes the energy balance of a water drop on a solid surface. This equation can be expressed as
 | [1] |
is the contact angle (°), 
l, s, and sl are the surface free energy of the liquid (mJ m–2), the surface free energy of the solid (mJ m–2), and the interfacial free energy between the liquid and the solid (mJ m–2), respectively. From Eq. [1] it follows, that perfect wetting, that is, cos = 1, is favored by a high solid surface free energy, a low interfacial free energy and a low liquid surface free energy.
According to Good and van Oss (1992), both the surface free energy of a liquid l and the surface free energy of a solid s can be expressed as the sum of two components of interaction; that is
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 | [2a] |
dl and pl are the dispersion (Lifshitz-van der Waals) and polar (acid-base) components of the surface free energy of the liquid (mJ m–2), and where ds and ps are the corresponding dispersion components of the surface free energy of the solid (mJ m–2).
Fowkes (1964) developed a concept to determine the dispersion and polar components of solid surfaces. He showed, that the interaction energies arising from dispersion forces at the interface could be reliably predicted by the geometric mean of the dispersion force components of the liquid and solid surface free energies. Owens and Wendt (1969) successfully extended this concept to the polar force components. Therefore, the interfacial free energy sl can be given as the sum of the energies of the liquid and solid phase; that is,
 | [3] |
By combining Eq. [1] and [3], Fowkes (1964) derived an expression for the contact angle of a liquid on a solid in terms of the dispersion force contributions. This approach was extended by Owens and Wendt (1969) to the polar forces. They derived the following expression:
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In practice the surface free energy of the solid can be obtained by transforming Eq. [4] into a linear equation of the type y = mx + b (Rabel, 1971); that is, as
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONTHEORYMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The RH values can be transformed to water potential values (
) (Pa) according to Eq. [6] (Campbell and Gardner, 1971):
 | [6] |
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Methods
To evaluate the wetting properties of the soil material, the physically based CRM was utilized. So far, CRM has not been applied to soil aggregates, but it has been used for the measurement of powders like silica flour and limestone (Siebold et al., 1997) as well as for peat material (Michel et al., 2001). The CRM was used because other techniques like the Wilhelmy plate method (Bachmann et al., 2004) and the sessile drop method (Bachmann et al., 2000) require homogenized material. The values obtained by the CRM are advancing (initial) contact angles, which are larger than equilibrium (intrinsic) contact angles measured on ideal smooth surfaces. However, to study the effect of different soil water potentials on wettability, it is essential to use the advancing contact angle, because this measure depends exclusively on the prescribed initial water potential. With use of contact angle measurements with different liquids (Table 3) it was possible to calculate the dispersion and polar components of the solid surface free energy. This gives a more precise measure for the description of soil–water interactions.
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The contact angle (°) is calculated with the Washburn equation, which is derived from Poiseuille's law of liquid flow through a capillary tube (Washburn, 1921):
 | [7] |
l is the surface tension of the liquid (J m–2), 
is the viscosity of the liquid (Pa s), and t is the time (s). From Eq. [7] an expression for the mass increase of the soil column during the capillary rise process can be derived (Siebold et al., 1997). This expression can be written as:
 | [8] |
is the liquid density (Mg m–3), and c is a geometry factor (m5) that reflects the porosity and tortuosity of the capillaries and depends on particle size and packing density. If a liquid with a nonzero contact angle is used, two unknown variables have to be determined in Eq. [8]. For this, the factor c has to be evaluated independently in a second run using the same soil with identical packing density and using a reference liquid that wets the soil particles completely ( = 0°). The precise and accurate determination of the c-factor is essential for a correct application of the modified Washburn equation. The difficulty in achieving a constant packing density to ensure that the c-factor is actually constant is the main source of error of the capillary rise method. It is noted that Washburn's equation only accounts for the capillary pressure and not for the hydrostatic pressure. However, considering only the early stages of capillary rise, the hydrostatic pressure can be approximately neglected, because its contribution is very small in comparison to capillary pressure (Washburn, 1921). Measurements used 2 g of the equilibrated soil material filled into a glass tube with a sintered glass plate at the base. The sintered glass plate was covered by a filter paper. The granular material was compacted by tapping the sample with 100 similar impacts to get nearly identical maximum packing densities (i.e., no further compaction). The aggregates were treated with only five impacts to achieve sufficient contact between the aggregates without disruption. After compaction, the tube was attached to an electronic balance (DCAT 11, DataPhysics, precision 10–5 g) and was brought into contact with the respective test liquid (Fig. 1) .
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l = 18.4 mJ m–2 for 20°C) were recorded at a rate of 30 measurements per second. The presumption that n-hexane is a liquid that completely wets the solid material ( = 0°) is reasonable for mineral soils with high surface free energies but may be a possible source of error when low-energy material is measured. This assumption was also verified by contact angle measurements of mineral soils with the Wilhelmy plate method (for details see Bachmann et al., 2004). According to Eq. [8] the c-factor was determined from the slope of the n-hexane absorption rate in the linear range of the w2(t)-function (Fig. 2)
. Preliminary experiments showed that differences in the slope of the n-hexane weight gain curves were small. Therefore, only one measurement was made to calculate the c-factor. Using intact aggregates, three stages of capillary rise can be distinguished. Experiments conducted with empty glass tubes showed that the initial weight gain is influenced by the sintered glass and the filter paper at the base of the glass tube. Consequently, this initial stage (2 s for water, 1 s for n-hexane) was not considered for the determination of the slope (Stage 1 in Fig. 2). Capillary rise measurements with glass beads showed that after the following Stage 2 no further weight gain was recorded, because of the absence of internal pores. Therefore, Stage 2 can be attributed to the wetting of the outer aggregate surfaces. Stage 3 resulted from the superimposition of the filling of inter- and intraaggregate pores. Except for Stage 1, it is usually not possible to distinguish different stages of the capillary rise process for the homogenized material.
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It is noted that the initial water content can affect the water potential gradient in the samples and therefore the capillary rise of water. For the samples equilibrated at high (less negative) water potentials this effect would lead to smaller infiltration rates and thus larger contact angles compared with the drier samples. Therefore, we used only the very first time period to evaluate the wetting properties, where the water potential gradient is less determinedly for the capillary rise.
The use of n-hexane may alter the surface characteristics of the pores when interacting with organic compounds. However, a significant change of the pore geometry due to possible interactions is not likely in mineral soils with low organic matter content as the investigated material. Michel et al. (2001) used the capillary rise method even for the measurement of peat material. As n-hexane is considered as a completely wetting liquid and the capillary rise experiment with n-hexane serves only for the evaluation of the pore geometry, small changes of the surface characteristics will not affect the proper determination of the c-factor for mineral soils.
Another difficulty of the CRM is the assumption that the geometry of the pores will not be affected by the wetting process. This is not always the case, especially for unstable material. However, for short-term measurements of mineral soils the error arising from a change of the pore geometry will be very small.
Each aggregate-size class was tested against four liquids having widely different physical properties (Table 3), to determine the surface free energy. Additionally, the surface tension of each test liquid was determined directly before the measurements. Despite the slightly higher temperatures, the measured values agree well with those published by Janczuk et al. (1989). The contact angles were determined in the same way as described for water. Because of the solubility of glycerol and ethylene glycol in water, the calculations of surface free energy were performed only with the oven-dried material.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONTHEORYMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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 | [9] |
y is the difference and n the number of the value pairs. Because of the nonlinearity of the cosine-function, the wetting coefficients k (k = cos) are given to make the values comparable (Table 5).
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The contact angle of the ignited material (1000°C), where the impact of SOM and water films are eliminated, were equal to zero degrees for all samples. This shows the reliability of CRM in producing uniform contact angle results, which are independent of aggregate size.
Impact of Soil Water Potential
The contact angles of aggregates and the corresponding homogenized material generally increase with increasing soil water potential as indicated by Fig. 3
. Despite the general observation, for several soil samples there seems to be a tendency for a contact angle maximum at –30 and –154 MPa, respectively. The wettability of the Al aggregates <1 mm is generally least affected by the soil water potential. In summary, the water potential induced shifting of contact angle can be considerable and reaches nearly 90° for the homogenized 2- to 4-mm aggregates of the Ap material. The shifting range seems to be slightly larger for the homogenized material. It is further noteworthy that the soil samples with small contact angles at low water potentials have large contact angles at high water potentials.
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To exclude any effects that may arise from microbial activity, the ignited homogenized material (1000°C) was subsequently equilibrated at 99.9% RH. The contact angles for all fractions are significantly larger compared with the dry soil samples, which are completely wettable ( = 0°). The increase of contact angles is particularly pronounced for the small fractions of both horizons (Fig. 4)
. It can be assumed that this effect is caused only by the impact of water films on the surface free energy of the solid particles as indicated by Derjaguin and Churaev (1986).
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= –1000 MPa) the absence of adsorbed water molecules on the high-energy mineral surfaces lead to small contact angles. With increasing water potential the amount of adsorbed water molecules increases and accordingly the surface free energy and the wettability is lowered. At a certain number of adsorbed water molecule layers the specific properties of the mineral surface become less important for the wetting behavior. This is the water potential at which the soil surface is beginning to behave again as wettable. Additional water molecules will undergo bonding typical for free water and the wettability is controlled by the cohesion of the water molecules (Vogler, 1998). The fact that the maximum contact angles were not found for the material in the oven-dry state but rather at higher soil water potential stands in contrast to the results of Scholl (1971) who found an increasing water repellency with decreasing water potential from –33 to –1500 kPa. This indicates that the oven-drying process has no intensifying effect on the water repellency of our soil material as this was reported for example by de Jonge et al. (1999). In contrast to King (1981), who found that water repellency is essentially unchanged between oven-dry and air-dry conditions, the increase of contact angles is the greatest in the very first shift from the oven-dry state toward higher potentials. The wetting behavior of the investigated material in relation to soil water potential seemed to be complex. However, the following trends can be summarized: (i) From the oven-dry state (–1000 MPa) to higher water potentials a decrease in wettability was observed; (ii) From water potentials of –154 and –30 MPa, respectively, the wettability of several samples increased again. The transition point seems to be specific for the individual material and aggregate-size class. The fact that for some samples this behavior cannot be observed may be explained by the small water potential range, where the measurements were made.
The dependence of wettability on soil water potential or water content may be caused by several processes. For a slightly water repellent soil, it seems that the wetting properties are primarily related to the formation of thin water films by the sorption of water molecules from the gaseous phase leading to a reduction of surface free energy of the material (Derjaguin and Churaev, 1986). However, as shown for the aggregates, also the content of organic C may affect the degree of shifting from wettable to water repellent behavior. This could be due to interactions between water and hydrophobic organic substances.
Impact of Aggregate Size
Different contact angles between the size fractions were found for the aggregates and for the homogenized material. However, the impact of the sieved size fraction on wettability is more pronounced for the intact aggregates especially for low soil water potentials (Fig. 5)
. The contact angles of the homogenized Ap material as a function of soil water potential are comparatively more similar for all size fractions. With the exception of –0.14 MPa water potential, the contact angles of the <1-mm aggregates are larger compared with the 1- to 2- and 2- to 4-mm aggregates (Fig. 3). This may indicate an accumulation of hydrophobic compounds on the outer aggregates surface due to eluviation impacts of dissolved organic C that will be greatest for the small aggregates because of the large surface area relative to aggregate volume. It may result in larger contact angles for the small aggregates compared with larger aggregates, when organic coatings are formed. Hence, not only the content but also the distribution and chemical composition of SOM may contribute to explain the different characteristics, which can be found for the contact angle—soil water potential relationship. However, in general, no obvious trend of the size—contact angle relationship can be observed for the aggregates as well as for the homogenized material.
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Horizon-Specific Differences
With the exception of the Ap aggregates at –30 MPa the arithmetic mean contact angles of all size fractions are comparatively similar for both materials. However, while the contact angles of the Ap aggregates exceed 80°, the values remain below 70° for the Al aggregates (Fig. 3). For the homogenized material, values above 80° were measured for both, the Ap and the Al horizon. The slightly smaller contact angles of the Al material with a considerable smaller organic C content as the Ap material (Table 1) could be expected, as organic matter is considered as the main factor for water repellency (Scholl, 1971; Bisdom et al., 1993).
Surface Free Energy
The determination of surface free energy is shown in Fig. 7
. All regressions are reasonable with r2 values above 0.97. The total surface free energy of all soil samples is between 55 and 65 mJ m–2 and rather similar for all fractions with a contribution of the dispersion and polar components of approximately 1/3 and 2/3, respectively (Table 6). While the dispersion components of the small aggregate fractions are similar, the polar components are notably smaller compared with the other fractions. This indicates the lower wettability of the small fractions as already revealed by larger contact angles. The absolute values of surface free energy indicate that already a small reduction in surface free energy can lead to contact angles greater than zero. It has to be mentioned that the surface free energies were calculated on the basis of advancing contact angles leading to a slight systematic underestimation of the values.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONTHEORYMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The results show further that the maximum of repellency is not necessarily in the oven-dry state, but rather at a specific water potential that varies from material to material. This means that material may become increasingly wettable again when it is in the oven-dry state. Hence, soils determined as wettable in the laboratory (oven-dry state) may be water repellent under natural conditions at less negative water potentials. Our measurements are in concert with the contradicting findings of several studies (e.g., King, 1981; Jex et al., 1985; de Jonge et al., 1999; Doerr and Thomas, 2000; Dekker et al., 2001; Doerr et al., 2002) indicating that it is necessary to be very careful when including the effect of soil water potential on wettability. A general description of the wettability with terms like "wettable", "non wettable" etc., seems to be nondiscriminating, because the range of contact angle for an individual and obviously wettable soil can vary between 0° and nearly 90°.
It was found that the CRM is an appropriate technique for analyzing the impact of small variations in soil water potential on the contact angle and the surface free energy of subcritical water repellent material. Slight differences in water repellency can be measured with a reasonable reproducibility. The determination of surface free energy as well as its dispersion and polar components provides a measure for a more general description of the wetting behavior. Unfortunately, the surface free energy calculation is restricted to dry material, otherwise three-phase systems (water-nonwater-air) may be created. However, especially for slightly water repellent soils the calculation of surface free energy can provide more detailed information about the wetting properties. Furthermore, the surface free energy allows a characterization of the solid, which is independent of the test liquid and in this way it is a measure that facilitate the translation toward natural conditions (e.g., for soil solution). Contact angles and surface free energies are physical quantities that can be easily introduced in transport models.
For a more accurate description of soil wettability, we recommend that the soil water potential (or water content) should be determined additionally. The water potential at which the mineral surface becomes completely wettable seems to be a specific characteristic of soils and a key factor for a better understanding of soil wetting. Consequently, there is a need of a physical description for the specific adsorption of water. To locate the point of transition from which a wettable soil turns to a water repellent soil, the water potential range should be refined and expanded.
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ACKNOWLEDGMENTS |
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Received for publication April 2, 2003.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONTHEORYMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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