Soil Water Repellency: Effects of Water Content, Temperature, and Particle Size L. W. de Jonge,* O. H. Jacobsen, and P. Moldrup ABSTRACT
(Letey, 1969) and the molarity of an ethanol droplet test (King, 1981). The WR test is most commonly performed on a dry soil sample. Brandt (1969) stated that water repellent soils exhibit hydrophobic properties when dry, and Richardson (1984) stated that the simplest way to recognize a soil with a water repellency problem is by adding a drop of water to the surface of a fairly dry soil. Dekker and Ritsema (1994) concluded that air-dry soils repel water the most, whereas wet soils might not be water repellent at all. In contrast, King (1981) found that WR increased rapidly with increasing moisture content between air-dry and wilting point; it reached a maximum near wilting point, and then decreased rapidly to zero as moisture contents approached field capacity. King also found WR was essentially unchanged as soil moisture contents were increased from oven-dry to air-dry, whereas Berglund and Perrson (1996) found that all the organic soils tested showed almost no repellency at a soil water content of ovendry, while WR rapidly increased with increasing water contents up to 0.20 to 0.30 m3 m~ 3 . Wallis et al. (1990) found a highly significant rise in WR as soil water content increased from 0.03 g g"1 to 0.05 g g"1. Before determination of WR, soils are often dried at 60°C followed by two days of equilibration at room temperature. The degree of WR might, however, be
Water repellency (WR) of soils is a common problem in many countries. It can cause a reduction in the rate of water infiltration into soils as well as an unstable water flow within the soil matrix. Water repellency has typically been related to dry soils. We investigated the effect of soil water content and soil pretreatment temperature on the degree of WR in soils and soil size fractions. Water repellency was measured with an ethanol test as the minimum liquid surface tension of an aqueous ethanol droplet that can stay on the soil surface for at least 5 sec. The WR varied greatly with water content. The lowest water contents were achieved by drying soil at different temperatures between 40 and 105°C. Hence, the apparent effect of water content on WR for low water contents may be a combination of a direct water content effect and the effect of pretreatment temperature. High WR was observed at soil water contents up to 0.08 g g '. Out of 14 soil samples, three did not show WR regardless of temperature treatment or soil water content, four had single peaks in WR as a function of water content, and seven had double peaks in WR, one peak at very low and one peak at higher soil water content. Results from comparison experiments with freeze-dried soil samples implied that the WR peak at lower soil water contents was caused mainly by temperature effects, while the peak at higher soil water contents was related to water content only. In water repellent soil the smaller soil size fractions exhibited the highest degree of water repellency. This can partly be explained by higher organic matter content in the fractions with smaller particle size. As water repellency is dependent on soil water content, performing the WR test solely on dry soils can lead to the wrong classification regarding whether a soil is water repellent or not.
W
ATER REPELLENCY (WR)
dependent on this pretreatment. Franco et al. (1995)
reported differences in WR of samples dried at 25, 70, and 105°C and found that the WR of samples were highest when dried at 105°C. However, Ritsema et al. (1997) found no changes in WDPT after drying dune sand samples at 25°C, 45°C, or 65°C, whereas Dekker and Ritsema (1996) found for peaty clay and clayey peat soils that temperature may influence the WR measurement, since in some cases they found severe WR after drying at 25°C and extreme WR after drying at 65°C. Drying at different oven temperatures will cause different soil water contents in the soil samples before determination of WR, but as indicated above, authors do not agree on whether this has an effect on WR or not. Heat might also create changes in the soil organic matter, which in turn might lead to changes in WR. It appears that there is conflicting evidence regarding the effect of temperature and water content on WR. The present paper will focus on this subject. The objectives of the present work were to determine (i) how WR depends on soil water content, (ii) whether heat during pretreatment influences WR, and (iii) whether WR is dependent on particle size, and if so, if WR of specific soil size fractions is more sensitive to differences in pretreatment temperature or to those in soil water content.
IS A WIDESPREAD PHENOM-
ENON in soils (e.g., Wallis and Home, 1992). Water repellency can lead to erosion (Wallis and Home, 1992) and to unstable wetting fronts, also called finger flow (e.g., Ritsema and Dekker, 1996), which is in turn related to increased risk of ground water contamination. Water repellency can, among other things, be caused by an organic coating of the particles produced by the growth of microorganisms (Bond and Harris, 1964), or be induced by plants (e.g., Ma'shum and Farmer, 1985; Wallis and Home, 1992). Many plants produce waxes in order to keep their leaves water repellent because this facilitates the removal of particulate depositions (dust, spores, etc.) and results in a purification of plant surfaces through rain, fog, or dew (Neinhuis and Barthlott, 1997). These waxes are introduced to the soil environment after plants die and are decomposed. The WR can be determined by several methods, such as the water drop penetration time (WDPT) method L.W. de Jonge and O.H. Jacobsen, Danish Institute of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark; and P. Moldrup, Environmental Engineering Lab., Dep. of Civil Engineering, Aalborg Univ., Sohngaardsholmsvej 57, DK9000 Aalborg, Denmark. Received 29 Oct. 1997. *Corresponding author (
[email protected]).
Abbreviations: CEC, cation-exchange capacity; LST; liquid surface tension; WDPT, water drop penetration time; WR, water repellency.
Published in Soil Sci. Soc. Am. J. 63:437-442 (1999).
437
438
SOIL SCI. SOC. AM. J., VOL. 63, MAY-JUNE 1999
Table 1. Soil characteristics. Silt
Clay
Sand IIP 6 Kko" & K
Fladerne C (not classified)?! Fladerne A (same) Fladerne B (same) Tylstrup (Cumulic Haplumbrept) Poulstnip forest (not classified) Hornum (Typic Haplumbrept) Lnndgaard (Typic Haplohumod) Jyndevad grass (same) Jyndevad barley (same) Borris (same) Poulstnip (not classified) Foulum (Typic Hapludult) 0dum (Typic Agrudalf) R0gen (Typic Hapludalf)
0.954 0.896 0.915 0.800 0.736 0.775 0.802 0.859 0.860 0.742 0.668 0.693 0.575 0.652
0.015 0.033 0.024 0.134 0.146 0.137 0.132 0.540 0.540
0.025 0.036 0.036 0.037 0.041 0.044 0.048 0.055 0.056 0.076 0.087 0.095 0.137 0.149
0.155 0.213 0.186 0.264 0.171
Organic C
pH
0.004 0.020 0.016 0.017 0.045 0.026 0.011 0.019 0.018 0.016 0.019 0.015 0.014 0.018
5.5 6.4
1
6.0 5.7
4.1 5.8 6.1 5.8 6.3 5.5 6.2 7.1 7.1 7.3
CECt cmol, kg ' 3.2 8.6 12.0 7.1 15.0 10.2 8.8 9.5 9.9 7.6 10.5 15.2 14.0 15.5
NP| 0 2 2 2 2 2 1 2
2 0 1
0
1
1
t CEC, cation-exchange capacity. i NP denotes the number of peaks on a curve where water repellency (WR) is plotted against soil water content (Fig. 1). § Soil classification is given in parentheses.
MATERIALS AND METHODS Fourteen soils were used in this study (Table 1). Except as otherwise noted in Table 1, all soil samples were collected under grass cover. Two batches of soil were taken from the Jyndevad site, one from a permanent grass field, and one from a barley field. Soil samples from Poulstrup were collected from a grass field and a mixed hardwood forest. At the Fladerne site, soils were taken from three different depths: 0 to 5 cm (A), 20 to 25 cm (B), and 30 to 35 cm (C), where A is within the plow layer, B within a spodic horizon, and C within the subsoil. All soils were air-dried at room temperature, passed through a 2-mm sieve and mixed thoroughly. Particle-size analysis to determine sand, silt, and clay size fractions was performed by hydrometer (Gee and Bauder, 1986) and sieve. The organic C was determined by combustion (Tabatabai and Bremner, 1970) in a Leco CNS 1000 oven (Leco, St. Joseph, MI). The cation-exchange capacity (CEC) was determined by extraction with ammonium acetate (Rhoades, 1986). The degree of water repellency was determined by an ethanol test and given as the minimum liquid surface tension (LST) of a 60-jxL aqueous ethanol droplet that can stay on the soil surface for at least 5 s. Pure water and ethanol solutions of 0.01 to 0.60 m3 m~ 3 with steps of 0.01 m3 m~ 3 were used. Lowering the surface tension of the droplet used in this test (by increasing the concentration of ethanol) can cause the droplet to enter the soil in
