Soil Science Society of America Journal 66:401-405 (2002)
© 2002 Soil Science Society of America
DIVISION S-1—NOTES
Water repellency of soils
The influence of ambient relative humidity
S. H. Doerr*,a,
L. W. Dekkerb,
C. J. Ritsemab,
R. A. Shakesbya and
R. Bryantc
a Dep. of Geography, Univ. of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK
b Alterra, Land Use and Soil Processes Team, P.O. Box 47, 6700 AA Wageningen, The Netherlands
c Dep. of Chemical and Biological Process Engineering, Univ. of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK
* Corresponding author (S.Doerr{at}Swansea.ac.uk)
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ABSTRACT
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Adverse effects of soil water repellency (hydrophobicity) are of concern during or following rainfall or irrigation, and are often preceded by conditions of high atmospheric relative humidity (RH). Assessments of repellency are, however, commonly conducted on air-dried samples at ambient laboratory conditions. This study explores the effects of differing antecedent RHs (32–98%) on the water repellency of air-dried soils of wide ranging characteristics under laboratory conditions using water drop penetration time (WDPT) and ethanol-percentage tests. Most samples exhibited considerably higher water repellency after exposure (<1 d) to 98% RH compared with lower RHs, typical of ambient laboratory conditions. This work suggests that previous studies may have incorrectly classified some soils, likely to exhibit water repellency in the field, as wettable, and that tests carried out following exposure of samples to high RH provide assessments that best reflect critical field conditions.
Abbreviations: RH, relative humidity • WDPT, water drop penetration time
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INTRODUCTION
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ASSESSMENTS OF WATER REPELLENCY (hydrophobicity) of soils are sometimes made on samples with their original moisture content, but are more commonly carried out on air- or oven-dried samples exposed to ambient laboratory conditions prior to testing. It has been established that ambient temperature (King, 1981), drying temperature (Franco et al., 1995; Dekker et al., 1998), actual water content (Berglund and Persson, 1996; DeJonge et al., 1999; Dekker and Ritsema, 2000), and the wetting and drying history of samples (Doerr and Thomas, 2000) can all strongly affect water repellency. Little is known, however, about the effects of RH. Roberts and Carbon (1971) and Jex et al. (1985) observed that long-term incubation (several days to weeks) at high RH (90–100%) can alter water repellency in soils because of the effects of microbial activity, but the effect of short-term exposure (<1 d) of air-dried soil samples to different levels of RH has remained essentially unexplored. Such variations occur not only in the field, but may affect laboratories in which atmospheres are uncontrolled and thus dependent on local daily and seasonal conditions.
Anecdotal evidence (Hammond and Yuan, 1969) suggests that penetration times of water droplets in sandy soils increase sharply at high RH (95%). The objective of this study is to investigate the effect of different antecedent RHs on water repellency assessments of air-dried soils. Thus, twelve sandy and medium-textured soil samples taken from under a range of vegetation types and from diverse locations were tested at four different RHs at a constant ambient temperature of 20°C using the water drop penetration time (WDPT) method (Letey, 1969) and ethanol-percentage techniques. The latter is also termed molarity of an ethanol droplet (MED), critical surface tension, or ninety-degree surface tension method (Watson and Letey, 1970; King, 1981).
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Materials and Methods
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Soil samples (Table 1) were dried at room temperature (20–25°C) to constant weight and then stored at this temperature in sealed plastic bags or glass jars. Prior to testing, samples were sieved through a 2-mm mesh screen and the coarser material was discarded. The remaining fine-earth fraction was then gently mixed until it appeared to be homogeneous. Subsamples (8 g) from each soil were placed in separate 50-mm diam. plastic dishes and then exposed to air for 16.5 to 19.5 h, at prescribed RHs of 32, 50, 73, or 98% at 20°C, in a climate chamber (SANYO Gallenkamp PLC, Loughborough, UK; Model No. HCCO3I.PF4.J). For WDPT tests, drops of distilled water (
0.05 ml) were placed on the subsamples, using a mechanical arm, and the times for their complete penetration (WDPTs) were recorded and classified according to Bisdom et al. (1993) (Table 2). Observations were made through the cabinet window, ensuring that samples remained under constant environmental conditions.
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Table 2. Water drop penetration time (WDPT) class increments used in this study and corresponding descriptive repellency rating. Numbers denote the upper time limits (in seconds) for individual repellency classes.
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A duplicate set of subsamples was prepared and conditioned in the cabinet as described above. They were then removed and, within 1 min, subjected (in triplicate) to the ethanol-percentage test at ambient laboratory conditions (20°C, 42–51% RH). The increments in ethanol concentration (Table 3) were those used in previous studies (Doerr, 1998; Doerr and Thomas, 2000), which are similar to those used by Crockford et al. (1991) and correspond to surface tension increments of 0.05 ± 0.015 N m-1. The overall apparent water repellency of the soil is considered to lie between that of the weakest solution penetrating the soil within 5 s and the preceding weaker solution in the series. It should be noted that the thresholds used to discriminate between different levels of water repellency for both WDPT and ethanol-percentage tests are essentially arbitrary. The associated severity classifications vary between authors and are often based on convenience or field experience rather than being based on factors of particular physical or environmental importance.
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Table 3. Ethanol classes (concentration increments) used in this study, together with associated molarity of an ethanol droplet (MED) values, apparent surface tension ( ), and two associated descriptive classifications of water repellency.
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Results
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WDPT Test
The median penetration times for three droplets1 of the different soils adjusted to various RHs (Table 4) range over three orders of magnitude with soil type and nearly two orders of magnitude for 32 to 98% RH. Some droplets, exposed to RH 
50%, suffered evaporation rather than penetration within the period shown (Table 4). For those that did penetrate the soils, there is a general tendency of increasing WDPT with increasing RH. Compared with the values measured at 50% RH, these variations result in changes in soil classification of 1 repellency class for RHs of 32 or 73% for all except Soils V and XI, but a rise of >1 class at 98% RH for eight of the twelve soils (for classes see Table 2). The deviation in WDPT relative to that measured at 50% RH (a value commonly found in many controlled laboratories) shows that of the soils unaffected by droplet evaporation, more than half suffered a twofold or higher increase in WDPT after exposure to 98% RH, with Soil X exhibiting an increase of 5900%.
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Table 4. WDPT classes in seconds (based on the median of three droplets) for samples I–XII measured after exposure to different levels of relative humidity (RH).
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Ethanol-Percentage Test
Most soils exhibited increasing resistance to ethanol solution penetration with increasing RH so that a higher concentration of ethanol was required for droplet penetration within 5 s (Table 5) 2. For RHs < 98% RH, results vary little (no more than ±1 ethanol class, Table 3). At 98% RH, ten of the twelve soils require an increase in ethanol concentration to induce penetration, and for six of these this amounts to two to three ethanol classes. Two soils (V and XI) required no increase. The deviation in apparent surface tension from that at 50% RH, expressed in increments of 0.05 ± 0.015 N m-1 (Fig. 1)
, shows the considerable reduction in apparent surface tension of the soil surface (i.e., increase in repellency) after exposure to 98% RH.
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Table 5. Percent-ethanol classes of the soils I–XII measured after exposure to different levels of relative humidity (RH) (classes are explained in Table 3).
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Fig. 1. Differences in approximate apparent surface tension, , in N m 10-3 for soils I–XII following adjustment to relative humidities (RH) of 32, 73, and 98% at 20°C compared with the apparent surface tension of the soils adjusted to 50% RH (median of three tests). No bar indicates no difference between RHs. Details concerning the apparent surface classes are given in Table 3.
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Discussion and Conclusion
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The marked influence of exposure to high RH on the results of both tests for such diverse soils in terms of origins, characteristics, and histories of use (Tables 1, 4, and 5) clearly has implications for both soil testing in the laboratory and its behavior in the field.
For the water repellency classification systems used here (Tables 2 and 3), the variations in results (±1 class) that occur after exposure to RH 73% might be considered insignificant because of the ordinal nature of classification. The considerable increase in repellency detected by both methods for soil exposed to 98% RH over short time-scales (<1 d), however, confirms the anecdotal evidence of Hammond and Yuan (1969). As increases in RH often precede precipitation (whether natural or as irrigation) or wetting front advances in the soil, the implications and underlying mechanisms of this effect warrant investigation.
The fact that such short exposure to high RH has such a marked effect on water repellency indicates that physicochemical rather than microbiological processes as suggested in studies with much longer exposure times (Roberts and Carbon, 1971; Jex et al., 1985) are likely to be responsible for changes in soil behavior. It seems reasonable to assume that the penetration into or the condensation, and adsorption of water vapor on to soil associated with an increase in RH, would enhance or encourage its wettability. Surface adsorption of water molecules at RH approaching saturation, however, appear to have the opposite effect. It is commonly accepted that water repellency is caused by the presence of hydrophobic components of amphiphilic organic coatings on soil particles (Wallis and Horne, 1992; Doerr et al., 2000). Ma'shum and Farmer (1985) suggest that liquid water interacts directly with these components. The present study suggests that prior interactions between soil components and water vapor at high RH may also be of significant influence. Vapor is relatively mobile and so more able to penetrate to hydrophilic mineral surfaces. Its condensation and adsorption is accompanied by release of (considerable) energy available for the local disruption of mineral and organic hydrophobic bonds. This could cause the displaced hydrophobic organic moieties to expand into pore spaces, further obstructing the penetration of liquid water. It is possible that the observed increases in water repellency reported here arise from these, or similar processes. As the uptake of water vapor by soils at near saturation conditions may be rapid in comparison with subsequent release to an atmosphere of low RH, the effect of such prior exposure may be comparatively enduring.
Irrespective of whether or not the above speculation is correct, the results presented here may have considerable implications for land management. For if these results apply to field conditions, then the water repellency of dry soils will tend to increase at times when the RH in the atmosphere and the soil increases; i.e., immediately prior to rainfall events, or in the vicinity of irrigated areas or advancing wetting fronts.
Furthermore, since the least repellent sample (X), which might be classified by some as wettable (McGhie and Posner, 1980), exhibits by far the greatest increase in repellency with increasing RH, some soils, currently classified as wettable, could well reach water repellency levels of environmental concern as local RH increases. This possibility clearly requires further investigation.
The following conclusions for water repellency levels in soils can be drawn: (i) measurements carried out on air-dried samples adjusted to moderate RH in the laboratory may underestimate considerably actual field water repellency values of soils following high RH conditions; (ii) considerable variations can be expected for field measurements depending on antecedent atmospheric conditions; (iii) direct comparison between test results should be treated with caution if antecedent RH has not been monitored; and (iv) laboratory testing should ideally be carried out after subjecting samples to a controlled, high RH atmosphere to best reflect the most critical field conditions (atmospheric RH at the very moment of measurement may be less critical).
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ACKNOWLEDGMENTS
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We are indebted to Robert Inkpen and Paul Farres at the Department of Geography, University of Portsmouth for providing access to the environmental cabinet, to Gemma Leighton-Boyce for establishing our contact with Portsmouth, to Paul Blackwell at Agriculture Western Australia for advice with sampling soil at the Australian site and to the editor and referees for their useful comments on the manuscript. This study was supported by NATO travel grant CRG.CRG.973169 and EU grant FAIR-CT98-4027. The project does not necessarily reflect the European Commission's views and in no way anticipates its future policy in this area.
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NOTES
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1 Thorough homogenization of the soil samples prior to the experiments resulted in low variability in WDPT between the droplets applied to a sample. Mean standard deviation of WDPTs for all samples was 0.178 classes. For 33 of the 48 WDPT tests carried out, there was no variation in WDPT classes between the three droplets. For the remaining 15 tests, WDPTs ranged over no more than two classes. 
2 Median class of three tests. Thorough homogenization of the soil samples prior to the experiments resulted in zero variability (ethanol-percentage classes) among the three tests for all samples.
Received for publication November 16, 2000.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
