effects of organic compounds, water content and clay on the water repellency of a model sandy soil

Soil Science and Plant Nutrition (2007) 53, 711–719 doi: 10.1111/j.1747-0765.2007.00199.x

© 2007 Japanese Society of Soil Science and Plant Nutrition

Blackwell Publishing LtdORIGINAL ARTICLEWater repellency of sandy soilORIGINAL ARTICLE

Effects of organic compounds, water content and clay on the water repellency of a model sandy soil

D. A. L. LEELAMANIE1 and Jutaro KARUBE2

1United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology, Tokyo 183-8509 and 2Faculty of Agriculture, Ibaraki University, Ibaraki-ken 300-0393, Japan

Abstract

Soil water repellency is related to organic matter and clay, and varies non-linearly with soil water content.The purpose of this study is to assess the combined effects of organic compounds, water content and clayson water repellency of a model sandy soil under wetting and drying processes. Hydrophobic stearic acidand hydrophilic glucomannan were used as the organic compounds, and kaolinite or montmorillonite wasused as the clay conditioner. Water repellency was estimated using the water drop penetration time test.Repellency did not appear in samples free of stearic acid. Samples containing both stearic acid and glucomannanshowed higher repellency compared with samples containing stearic acid alone during the wetting process.Glucomannan with stearic acid increased the critical water content and widened the range of water contentat which soils showed slight repellency. During the wetting process, the repellency of most samplesincreased with increasing water content under relative humidity conditions ranging from 33 to 94%. Duringthe drying process, repellency appeared, reached a maximum and then decreased in samples containingstearic acid. Maximum repellency was observed not at oven-dried but at air-dried water content. Repellencywas highly sensitive to water content at around air-dried condition. The effects of organic compounds andclay on the water repellency of sandy soils were negligible in oven-dried condition. Repellency tended toincrease with the addition of a small amount of clay (1–2%) as a dry mix during the wetting process. Oncewetted, repellency disappeared with the addition of montmorillonite, but not with kaolinite. The higher thekaolinite content, the higher the critical water content.

Key words: clay, organic compounds, sandy soil, water content, water repellency.

INTRODUCTION

Soil water repellency is a dynamic phenomenon that iscaused by low-energy surfaces where the attractionbetween solid and liquid phases is weak (Heslot et al.1990; Roy and McGill 2002). If molecules of a liquidsurface have a stronger attraction to molecules of asolid surface than to each other, surface wetting occurs.Alternatively, if liquid molecules are more stronglyattracted to each other than to molecules of solid surface,liquid beads up and repellency occurs. Water repellencyhas an impact on preferential flow and soil moisturedynamics (Carrillo et al. 1999; DeBano 2000a; Dekker

and Ritsema 1994; Kobayashi et al. 1996). Soil organicmatter (SOM) and clay play an important role in makinga soil wettable or repellent (Dekker and Ritsema 1994).Water repellency often increases with strong drying orfire (DeBano 2000b; MacDonald and Huffmann 2004)and may break down when exposed to water for a longtime (Clothier et al. 2000).

The development of soil water repellency is related tothe content and the composition of SOM (Doerr andThomas 2000). The degree of hydrophobicity is positivelycorrelated with SOM (Mataix-Solera and Doerr 2004;Nakaya 1981). Repellency occurs when mineral particlesare hydrophobized by coatings of organic substances(Bachmann et al. 2000; Wallis and Horne 1992).Strongly hydrophobic organic coatings may preventwater entry into the soil and restrict infiltration, leadingto intensive surface runoff. Fatty acids, alcohols, estersand alkanes in SOM are associated with soil waterrepellency (Franco et al. 1995, 2000a). Ma’shum et al.

Correspondence: J. KARUBE, Faculty of Agriculture, IbarakiUniversity, 3-21-1 Chuo, Ami-machi, Ibaraki-ken 300-0393,Japan. Email: [email protected] 7 May 2007.Accepted for publication 23 July 2007.

712 D. A. L. Leelamanie and J. Karube


(1988) suggested that long-chain aliphatic compounds,including long-chain acids, alcohols and wax esterswith extended polymethylene chains, are responsiblefor producing water-repellent soils. Hurraß andSchaumann (2006) reported that the occurrence ofamphiphilic substances might be an important factorthat affects repellency. However, Doerr et al. (2005)reported that some compounds extracted from wettablesoils had induced hydrophobicity in wettable sand.Hydrophobic compounds (Doerr et al. 2005) or theamounts and proportions of hydrophobic functionalgroups in the SOM (McKissock et al. 2003) may notalways relate to the water repellency. In addition,hydrophobic and hydrophilic organic matter have dif-ferent effects on soil wettability (Piccolo and Mbagwu1999) and, thus, the relationship between water repellencyand SOM may be better explained by considering thedifferent types of organic compounds present in the soil.

Water repellency varies non-linearly with soil watercontent (Doerr et al. 2002; Goebel et al. 2004; de Jongeet al. 1999), although Poulenard et al. (2004) andLichner et al. (2006) observed monotonic increases inwater repellency with drying. Soils become wettable atwater contents above critical water content (CWC)(Dekker et al. 2001; Lichner et al. 2006). de Jonge et al.(1999) reported that the water repellency of differentsoils may vary differently with water content. Theypointed out that some soils have one or two peaks ofrepellencies with water content, whereas some othersoils are not repellent at any water content. Doerr et al.(2002) and Goebel et al. (2004) explained that soil waterrepellency increased with ambient relative humidity.King (1981) reported that water repellency increasedwith increasing water content from air-dried to nearwilting point. Regalado and Ritter (2005) reported thatwater repellency of volcanic soils increased with decreasingwater content from field capacity to permanent wiltingpoint. However, the non-linear behavior of water-dependent repellency has not been clearly explained.

Water repellency can reduce agricultural productionand contribute to land degradation resulting fromincreasing runoff and erosion (Dlapa et al. 2004;Lichner et al. 2006; Mckissock et al. 2000; Shakesbyet al. 2000; Ward and Oades 1993). It can causedelayed germination of pastures and crops, leaving thesoil prone to wind erosion (Mckissock et al. 2000). Theaddition of fine particles (e.g. clays) is useful as a remedialtreatment for these water-repellent soils (Lichner et al.2006; Mckissock et al. 2000; Ma’shum et al. 1989;Ward and Oades 1993). Prolonged shaking (Ma’shumand Farmer 1985; Wallis et al. 1990) and inducing thedegradation of hydrophobic organic carbon (Francoet al. 2000b) have been suggested as alternative methodsfor reducing repellency. Clay additions have long been

used as an effective way to reduce water repellency insandy soils. The addition of clay to the topsoil (claying)or into the subsoil (by deep plowing) has been successfullyused in many regions of the world. According toMcKissock et al. (2000, 2002), the addition of kaoliniticclays was the most successful in increasing the wettabilityof soils. They suggested that all clays may cause somereduction in water repellency and higher rates of clayapplication may cause greater reductions in waterrepellency. However, McKissock et al. (2003) laterreported that the clay content determined using thepipette method has no relationship with water repellencyin sandy soils. Instead, they reported that the kaolincontent is negatively related to water repellency.Dlapa et al. (2004) reported that the addition of Ca-montmorillonite caused an increase in water repellency inhighly repellent sand. The effects of clays on hydrophobicityof soil may be altered by the presence of organic matteras implied by Jouany (1991).

Soil organic matter, water content and clay can beconsidered as important factors affecting soil waterrepellency. Current knowledge on the individual effectsof these factors is not always consistent, which might bebecause previous studies have ignored one or two import-ant factors. The objective of this study is to assess thecombined effects of organic compounds, water contentand clays on the water repellency of a model sandy soil.

MATERIALS AND METHODS

Fine silica sand with a particle diameter of 94% of massranging from 45 to 150 μm (Table 1) was used as themodel soil. Stearic acid (molecular weight: 284.5) andkonjac-derived glucomannan (Wako Pure ChemicalIndustries, Osaka, Japan) were used as hydrophobic andhydrophilic organic compounds, respectively. Georgiankaolinite and Wyoming Na-montmorillonite (The ClayMinerals Society, Chantilly, VA, USA) were used as clayconditioners. Stearic acid was chosen because it is con-sidered to be a common organic acid in natural soil (Dengand Dixon 2002). Glucomannan was chosen because itis a neutral polysaccharide gum.

The sand was mixed with 1 g kg−1 stearic acid toprepare samples with a hydrophobic organic compound

Table 1 Particle size distribution of the silica sand used in theexperiment

Particle size (μm) Mass (%) Particle size (μm) Mass (%)

212–150 2.1 53–45 9.7150–106 13.4 45–38 2.5106–75 42.1 38–26 0.775–53 29.2 < 26 0.3

Water repellency of sandy soil 713


(S samples) because the total lipid content in naturalsoil is considered to be approximately 1 g kg−1 (Martenset al. 2003). As stearic acid is insoluble in water, it wasdissolved in diethyl ether and mixed with the sand in afume hood. The samples were kept open for 2 h in thehood to allow volatilization of the diethyl ether. Thesand was mixed with 10 g kg−1 glucomannan to preparesamples with a hydrophilic organic compound (Gsamples). Glucomannan dissolved in distilled water wasmixed with the sand and dried at 30°C for 1 day. Toprepare samples with both hydrophobic and hydrophilicorganic compounds (SG samples), the sand was firstmixed with stearic acid, kept for 1 day, and then mixedwith glucomannan.

Dry samples were placed in glass vials and air-driedkaolinite or Na-montmorillonite was added to the samplesto obtain 1, 2 and 5% clay content. The vials werethoroughly shaken for 2 min to mix the clay with the sand.

To accomplish the wetting process, the water contentof the samples was gently increased using water vapor.For this, 5 g of each sample (three replicates) wasexposed to relative humidity (RH) levels of 33, 57, 75and 94% at 25°C for 20–22 h (not equilibrated). TheseRH levels were obtained using saturated salt solutions(MgCl2·6H2O, NaBr, NaCl and KNO3, respectively) insealed chambers. The water content of the samples ateach RH was mass metrically measured to obtain therelationship between water content and water repellencyduring the wetting process. A limitation of the wettingprocess was the difficulty of achieving higher watercontents because longer incubation periods at higherRH may alter the repellency due to microbial effects(Jex et al. 1985).

The drying process was accomplished by gradualdrying of pre-wetted samples. After mixing with clay,200-g samples were wetted with distilled water(sprinkled onto the surface and mixed with a spatula)up to 150–160 g kg−1 of water content and kept sealed

for 1 day. Wet samples were gradually dried at 30°Cand about 5-g subsamples were taken during the dryingto determine repellency.

The water repellency of each sample was estimatedusing the water drop penetration time (WDPT) test.This test was chosen because it is the most indicative ofthe hydrological consequences of water repellency (Doerr1998), although it is not appropriate to distinguishbetween different extremely repellent soils (Annaka 2006).All measurements were done in a constant temperatureroom at 25°C. About 5-g samples in weighing bottleswith 30 mm diameter and height (12-mL volume)were used for the measurements. One drop of distilledwater with a volume of 50 ± 1 μL was placed on the soilsurface with a burette at a height of about 10-mm.Weighing bottles were covered with a lid to minimizeevaporation during the test. The time taken for the com-plete penetration of the water drop was measured usinga stopwatch. In the experiment, WDPT ≤ 1 s was con-sidered wettable, WDPT = 1–60 s slightly repellent,WDPT = 60–600 s strongly repellent, WDPT = 600–3600 s severely repellent, and WDPT ≥ 3600 sextremely repellent (Bisdom et al. 1993; Chenu et al.2000; King 1981). The WDPT measurements wereextended up to 5 h for extremely repellent soils. If thehighest WDPT was less than or nearly equal to 1 s thesample was considered to be wettable.

RESULTS AND DISCUSSION

Effects of organic compounds

Hydrophobic organic compoundsWater repellency appeared only in the S and SG samples(Fig. 1) during the wetting and drying processes. Repel-lency did not appear when the samples did not containstearic acid, regardless of water content (Fig. 1), claytype and clay content (results not shown).

Figure 1 Water repellency of samples (without clay) treated with 1 g kg−1 stearic acid (S), stearic acid and 10 g kg−1 glucomannan (SG),glucomannan (G) and untreated sand in (a) the wetting process and (b) the drying process. WDPT, water drop penetration time.



Hydrophilic organic compoundsRepellency did not appear (WDPT < 1 s) in G samplesduring the wetting and drying processes, regardless ofwater content (Fig. 1) and clay (results not shown).

Hydrophobic and hydrophilic organic compoundsDuring the wetting process, SG samples showed higherrepellency compared with S samples (Fig. 1a). Thisshows that although glucomannan does not induce soilwater repellency, it supports repellency together withhydrophobic stearic acid. Not only hydrophobic, butalso hydrophilic, organic compounds would increasethe water contact angle on soil because organic matterhas a lower surface free energy compared with minerals.Although hydrophilic organic compounds increase thecontact angle, it may not be sufficient to cause apparentwater repellency in a readily wettable soil, such as silicasand. Hydrophilic organic compounds may increase

water repellency when added to somewhat repellentsoils, such as S samples.

During the drying process, glucomannan with stearicacid (Fig. 1b) widened the range of water content atwhich soils show slight water repellency and increasedthe CWC (Table 2) compared with stearic acid alone.

Effects of water content

Wetting processIn the absence of clay, the water repellency of S samplestended to increase with increasing water content,whereas that of SG samples increased up to a maximumand decreased again (Fig. 1a) between 33 and 94% RH.

The water repellency of most S and SG samples withclay increased from wettable or slightly repellent toextremely repellent with increasing water content(Figs 2a–5a) between 33 and 94% RH.

These results agree with Doerr et al. (2002) and Goebelet al. (2004), who observed that the water repellency ofair-dried soils increased with increasing RH. Doerret al. (2002) explained that this might be owing to thedisplacement of hydrophobic organic moieties as mineraland organic bonds were disrupted by the energyreleased from water vapor condensation. Goebel et al.(2004), however, reported that these kinds of moleculereorientation processes are rather secondary becauserepellency of several samples decreased again at waterpotentials above –30 kJ kg−1. They further explained thatthe absence of adsorbed water molecules on high-energymineral surfaces leads to smaller contact angles. In apreliminary study, we found that the silica sand, used as themodel soil, would not be fully coated with hydrophobicorganic compounds at 1 g kg−1 stearic acid content.Accordingly, an increase in the adsorbed water moleculeson remaining high-energy mineral surfaces might haveincreased the contact angle and the repellency of themodel sandy soil.

Table 2 Critical water content of samples with 1 g kg−1 stearicacid and 10 g kg−1 glucomannan obtained in the drying process

Clay conditioner

Critical water content (g kg−1)

S S + G

0% clay 30.0 44.51% kaolinite 20.0 22.12% kaolinite 24.6 33.35% kaolinite 31.1 53.11% montmorillonite NR NR2% montmorillonite NR NR5% montmorillonite 18.2 NR

Critical water content is considered to be the point at which the water drop penetration time exceeds 1 s. G, glucomannan; NR, not repellent; S, stearic acid.

Figure 2 Water repellency of samples treated with 1 g kg−1 stearic acid and conditioned with kaolinite in (a) the wetting processand (b) the drying process. WDPT, water drop penetration time.



Drying processDuring the drying process, repellency appeared, increasedup to a maximum level, and decreased again in S andSG samples without clay (Fig. 1b) and with kaolinite

(Figs 2b,3b). Repellency did not appear in sampleswith montmorillonite (Figs 4b,5b).

Repellency of samples started to appear at a watercontent between 20 and 50 g kg−1, reached a maximumat a water content between 5 and 20 g kg−1, and started

Figure 3 Water repellency of samples treated with 1 g kg−1 stearic acid and 10 g kg−1 glucomannan and conditioned with kaolinitein (a) the wetting process and (b) the drying process. WDPT, water drop penetration time.

Figure 4 Water repellency of samples treated with 1 g kg−1 stearic acid and conditioned with montmorillonite in (a) the wettingprocess and (b) the drying process. WDPT, water drop penetration time.

Figure 5 Water repellency of samples treated with 1 g kg−1 stearic acid and 10 g kg−1 glucomannan and conditioned withmontmorillonite in (a) the wetting process and (b) the drying process. WDPT, water drop penetration time.



to decrease at water content of about 5 g kg−1. Soil waterrepellency was highly sensitive to water content at rela-tively dry conditions (Figs 1b–3b). Oven-dried sampleswere wettable with WDPTs less than or nearly equal to1 s, suggesting that the effects of SOM and clay on thewater repellency of sandy soils are negligible underoven-dried conditions.

At higher water contents above CWC, wettabilitywould be controlled by the cohesion of adsorbed watermolecules towards the additional water molecules, and thespecific properties of the solid surfaces would be lessimportant for the wetting behavior (Goebel et al. 2004;Vogler 1998). During drying, the number of adsorbedwater molecules will decrease and the surface maysubsequently reach a point at which the cohesion ofadsorbed water molecules towards the additionalwater molecules will no longer be sufficient to overcomethe surface hydrophobicity. The water content at thispoint can be identified as the CWC, beyond whichfurther drying increases the repellency. Decreasingrepellency with further drying (after reaching a maximum)might possibly be a result of increasing high-energy sur-faces with decreasing adsorbed water molecules.

Sandy soil samples reached a maximum repellencynot under oven-dried but under air-dried condition. deJonge et al. (1999), Goebel et al. (2004) and Ellerbrocket al. (2005) reported comparable findings for therepellency peak using natural soils. de Jonge et al. (1999)reported different repellency curves for different soils.They pointed out that some soils showed one or tworepellency peaks, whereas some other soils were notrepellent at all with drying. According to our results,soils that are wettable at any water content might beassumed as free of hydrophobic organic compounds oraffected by clay (e.g. presence of montmorillonite).

Effects of clay

Clay contentDuring the wetting process, water repellency tended toincrease with the addition of 1–2% clay as a dry mixand to decrease again with 5% clay (Figs 6,7) underRHs up to 75%. The addition of clay had no considerableeffect on water repellency at 94% RH. With 1–2% clayaddition, water vapor condensation on clay mayproduce isolated tiny clods by the surface tension ofwater (Koorevaar et al. 1983), drawing clay particlestogether and leaving air spaces around them. Thismight restrict the surface water entry into stearic acidtreated sand with hydrophobic organic coatings andincrease the persistence of water repellency. At higherclay content (5%), clay clods might not be isolated andmay provide a path for surface water entry, reducingthe persistence of water repellency.

During the drying process, the CWCs of sampleswere increased with increasing kaolinite content from1 to 5% (except samples with 0% clay) (Table 2). Sampleswith higher kaolinite content reached a maximumrepellency at higher water contents (Figs 2b,3b). Theseresults suggest that the water-dependent repellencycurve shifts towards higher water content with increasingclay content. This result agrees with Kawamoto andAung (2004) and Regalado and Ritter (2005), whoreported that volcanic ash soils with about 18% claycontent reached a repellency peak at about 40% watercontent.

Clay typeDuring the drying process, the samples conditionedwith montmorillonite did not show repellency (Figs 4b,5b),except samples with 5% montmorillonite. Once wetted,montmorillonite might form a layer on the solid surfacesenclosing the hydrophobic organic coatings (Fig. 8a).Therefore, repellency during drying did not appear insamples with montmorillonite, even though the samplescontained stearic acid. Samples with 5% montmorilloniteshowed very slight repellency (Figs 4b,5b), whichmight be due to the creation of a less permeable layerwith swelling of montmorillonite (WDPT of 100%

Figure 6 Effect of clay content on the water repellency ofsamples treated with 1 g kg−1 stearic acid in the wettingprocess. RH, exposure to each relative humidity for 20–22 h(not equilibrated); WDPT, water drop penetration time.



montmorillonite was about 30 s under the same con-ditions). Kaolinite did not hide the repellency(Figs 2b,3b) because it cannot enclose hydrophobicorganic coatings (Fig. 8b) owing to its morphology(Dixon 1989).

During the drying process, repellency peaks in the SGsamples were reduced with 1–2% kaolinite (Fig. 3b).Once wetted, polysaccharides can adsorb on kaolinite(Chenu et al. 1987) and might cover some parts of thehydrophobic surfaces with drying, reducing the repellency.

At higher clay contents, polysaccharides would adsorbmore kaolinite forming floccules and might not be ableto cover the hydrophobic surfaces. This might explainwhy 5% kaolinite did not reduce the repellency peak.

ConclusionsA small amount of hydrophobic stearic acid inducedwater repellency, whereas hydrophilic glucomannan didnot, regardless of water content, clay type and clay content.However, SG samples showed higher repellency comparedwith S samples during the wetting process, suggestingthat hydrophilic organic compounds may increasewater repellency when combined with hydrophobicorganic compounds. Glucomannan with stearic acidincreased the CWC and widened the range of water contentat which soils showed slight water repellency.

During the wetting process, repellency increased withincreasing water content under RH conditions rangingfrom 33 to 94%. During the drying process, repellencyappeared, increased up to a maximum, and decreasedagain in samples containing stearic acid and kaolinite.Maximum repellency was observed at air-dried watercontent. Water repellency was highly sensitive to watercontent under relatively dry conditions. Oven-driedsamples were wettable with WDPTs less than or nearlyequal to 1 s, suggesting that the effects of SOM and clayon water repellency of sandy soils are negligible underoven-dried condition.

Water repellency tended to increase with the additionof a small amount of clay (1–2%) as a dry mix duringthe wetting process. The effect of clay was changedonce the samples were wetted. Repellency disappearedwith montmorillonite, but not with kaolinite. Repellencypeaks were reduced with 1–2% kaolinite when samplescontained both stearic acid and glucomannan. Thehigher the kaolinite content, the higher the CWC. Thissuggests that the water-dependent repellency curveshifts towards higher water content with increasingclay content.

ACKNOWLEDGMENTS

The principal author D. A. L. Leelamanie gratefullyacknowledges the Ministry of Education, Science, Sportsand Culture, Japan, for awarding a PhD scholarship.This work was financially supported by a grant-in-aidfor scientific research (grant no. 18580239) from theJapan Society for the Promotion of Science.

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Figure 8 An illustration of the layering of (a) montmorilloniteand (b) kaolinite around a hydrophobic sand particle oncewetted.



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