Effect of Water Quality and Drying on Soil Crust Properties1

M. BEN-HUR, I. SHAINBERG, R. KEREN, AND M. GAL2

ABSTRACTThe effect of soil crust and drying on the infiltration rate of Calcic

Haploxeralfs (Loess) and Typic Rhodoxeralfs (Hamra) was studiedusing a rainfall simulator. The soils were exposed first to a rainfallof water with low electrolyte concentration [electrical conductivity(EC) = 0.01 dS.rn-'] until a steady state infiltration rate was ob-tained. Thereafter, the crusted soils were dried for different periodsof time (24, 48, 72 and 96 h). After drying, the soils were saturatedfrom beneath with tap water, then subjected to a second storm ofeither water with low electrolyte concentration (EC ~ 0.01 dS m~')or saline water (EC oeq 5 dS m~'). Drying the crust increased theinfiltration rates of the soil in the second storms, and increased itssensitivity to the salinity of the applied water. The results suggestthat drying the crust caused its' breakdown, due to both the for-mation of cracks and the formation of new structure at the soil sur-face. Drying the crust brings the soil particles, which form the crust,closer together creating a new structure. The new structure at thesoil surface makes the soil surface more permeable and more sen-sitive to the destructive action of the raindrops.

Additional Index Words: crusted soils, dispersion, runoff, salinity,irrigation.

Ben-Hur, M., I. Shainberg, R. Keren, and M. Gal. 1985. The effectof water quality and drying on soil and crust properties. Soil Sci.Soc. Am. J. 49:191-196.

THE PRESENCE of a crusted soil surface due to rain-fall is a common feature of many soils, particu-

larly in the arid and semi-arid regions.Soil crusts are known to reduce infiltration, increase

runoff (Morin and Benyamini, 1977), slow the soil-atmosphere gas exchange (Cowans et al., 1965), andinterfere with seed emergence (Sale and Harrison,1964).

The structure of the crust was investigated byMclntyre (1958), who found that it consisted of twodistinct parts: an upper skin seal attributable to com-paction by raindrop impact, and a "washed in" zoneof decreased porosity, attributed to the accumulationof small particles. Mclntyre (1958) measured thick-nesses of 0.1 and 2 mm for the skin seal and "washedin" zones, respectively. The permeability of the bulkof the soil was approximately 200 times that of the"washed in" zone and about 2000 times that of theskin seal. In other studies (Esptein and Grant, 1973;Chen et al., 1980), only one of the two layers wasfound. Chen et al., (1980), studying scanning electronmicrographs (SEM) of crust on loessial soil, found onlya thin seal skin (~0.1 mm thick) at the top layer ofsoil, not an accumulation of fine particles in the 0.1to 2.8 mm zone, as observed previously by Mclntyre(1958, the "washed in" zone). However, Gal et al.,(1984) found that on soils with an exchangeable so-dium percentage (ESP) of 1, the crust consisted only

1 Contribution from the Agricultural Research Organization, TheVolcani Center, Bet Dagan, Israel, no. 893-E 1983 series. Received22 Nov. 1983. Approved 28 Aug. 1984.2 Soil Scientists, Institute of Soils & Water, ARO, The VolcaniCenter, Bet Dagan.

of the compacted skin seal layer. Conversely, on soilswith ESP > 1 exposed to rain (distilled water), thecrust consisted of naked sand and silt grains over adense "washed in" layer.

The crust formation of soils exposed to rain is dueto two mechanisms; a physical breakdown of the soilsurface and a chemical dispersion of the soil clays.Raindrops impacting the soil surface destroy surfaceaggregates, compact the soil, and reduce the averagepore size of the top layer of soil. These factors pro-duced the thin skin seal (Farres, 1978; Morin and Ben-yamini, 1977; Epstein and Grant, 1967). Chemical dis-persion allows soil clay to migrate into the soil withthe infiltrating water, and clog the pores immediatelybeneath the surface ("washed in" zone) (Mclntyre,1958; Agassi, et al., 1981; Kazman et al. 1983). Thepermeability of a soil to water depends both on itsESP and on the electrolyte concentration. The effectof soil sodicity on infiltration rate (IR) of various soilswas studied by Kazman et al., (1983). They found thatthe application of distilled water (simulating rainwa-ter) resulted in a sharp decrease in the final infiltrationrate with an increase in soil sodicity at the exchange-able Na+ percentage ESP range of 1 to 5. In their studythe final IR of a sandy loam with ESP's of 1.0 and 4.6was 8 and 2 mm/h, respectively. The final IR wasalmost independent of the soil ESP at values above 5.Soil sodicity did, however, determine the rate of crustformation, increasing the rate of soil structure break-down as the percentage of sodium increased above 5.The effect of electrolyte concentration in the appliedwater on crust formation was studied by Agassi et al.,(1981), using a rainfall simulator. It was found thatwith an increase of the electrolyte concentration in theapplied water, the final IR of two loamy soils in-creased. The intensity of chemical dispersion and themovement of the clay to the "washed in" zone de-pended on the electrolyte concentration of the appliedwater. On a silty loam soil, with ESP's of 6.4 and 26,electrolyte concentration of 5.6 dS m~' in the appliedwater prevented clay dispersion, and the resulting crustformed by the impact action of the raindrops, main-tained a final IR of 7.5 mm h~'. Conversely, whendistilled water was used, the complementary chemicaldispersion occurred and the final IR was only 1.2 mmh-1.

The stability of the crust to the beating action ofthe raindrops, the chemistry of the applied waters andthe drying periods were studied by Farres (1978),Morin and Benyamini (1977) and Hardy et al., (1983).Farres (1978) proposed that the crust, when rainedupon, is not stable, its upper part is destroyed and acrust is formed below continuously. The upper partof the crust is destroyed by the impact of raindropsand the disintegrated particles are eroded by runoffwater, resulting in exposure of the underlying new soillayer to the impact energy of raindrops and the for-mation of a new crust in place of the eroded old one.Seginer and Morin (1970) also concluded that thebeating action of the raindrop destroys the crust at theimpact area.

191

192 SOIL SCI. SOC. AM. J., VOL. 49, 1985

Table 1. Some physical and chemical properties of the soils used.Particle size distribution

Classification

Typic Rhodoxeralf sCalcic Haploxeralfs

Texture

sandy loamloam

Sand

7938

Silt

1040

Clay% ————

1122

CaCO,

0.611.7

CECcmol( + )kg-'

8.015.5

ESP

2.23.6

Dominant clay t

M(60)M(70)

K(30)I (20)

t M—montmorillonite, K—kaolinite, I—illite.

The effect of drying on the crust of a sandy loamsoil was studied by Morin and Benyamini (1977). Thesoil was first rained on with tap water and the finalinfiltration rate was 8 mm/h. The crusted soil was ex-posed to drying periods of 24 h, 6 and l i d , afterwhich it was rained on again. The initial IR's were 50,160 and 170 mm h~' for the three drying periods,respectively, while the final IR in the second storm atdifferent drying periods was similar to the IR in thefirst storm. Their explanation was that the increase inthe initial IR's in the second storm was due to crackformation in the crust.

Hardy et al., (1983) studied the effect of water qual-ity and storm sequence on the IR's of two loamy soils.The soils were dried for 12 h at 50°C after each storm.They found that in the sandy loam the crust brokedown completely at the beginning of the second storm.The new crust that was formed in the second stormwas not affected by the earlier storm crust and wasaffected only by the quality of the water applied in thesecond storm. Conversely, in the silty loam soil theyfound that the crust that was formed in the first stormdid not break down in the subsequent storm. Appli-cation of saline water in subsequent storms did notincrease the low final infiltration rate obtained at theend of the first distilled water storm. It was concludedthat the beating action of the raindrops was enoughto break the crust only in the sandy loam. In the siltloam, exposed to 12 h drying, the crust was so strongthat the impact of the drops was not enough to destroyit.

Many studies examined the factors that influencedcrust formation and its characteristics in the first storm.However, few studies have been done regarding thecrust properties at subsequent storms and the influ-ence of the drying process between sequential stormson the crust's properties. From a practical point ofview, drying periods of various lengths between stormspredominate, and their effect on the crust propertiesdetermines processes such as runoff and erosion. Ef-fectiveness of supplemented irrigation depends alsoon the stability of the crust in a crusted soil. If thecrust formed during a rainstorm is stable, the electro-lyte concentration in the irrigation water does not in-crease the IR of this water. Also, if the crust is stable,the development of a plant canopy over a crust soildoes not increase rain infiltration.

The objective of this work was to study the effectof drying of the crusted soil on the stability of the crustand its properties.

MATERIALS AND METHODSThe < 4 mm fractions of a Typic Rhodoxeralf (Hamra)

and of a Calcic Haploxeralf (Loess) were used in this study.The Rhodoxeralf soil is from the coastal plain of Israel, where

Table 2. Water content for the upper layer (0.5 cm) of theCalcic Haploxeralfs (loess) and Typic Rhodoxerals

(Hamra) Soils after different periods of drying.Water content

Drying time Rhodoxeralfs Haploxeralfs-kg kg-

024487296t

0.3030.1280.0370.0250.025

0.450.340.240.120.04

t Air-dry soil.

the average annual precipitation is 600 mm. The Haplox-eralf soil is a loessial soil from the western Negev, where theaverage annual precipitation is 400 mm. Some properties ofthese soils, together with their classification, are given inTable 1.

Soil was packed 2 cm deep in a perforated box 30 by 50cm and placed in a rainfall simulator (described by Morinet al; 1967) over a layer of course sand at a slope of 5%. Thesoil was first saturated from the bottom with tap water andthen was exposed to simulated rainfall of distilled water (DW,EC — 0.01 dSm"1). The typical mechanical parameters ofthe applied rain were: rainfall intensity of 30 mm/h; rain-drop median diameter, 1.9 mm; median drop velocity, 6.02m sec"1; and total kinetic energy, 570 J m~2 h"1. The vol-umes of runoff and of water infiltration were recorded. Therain was stopped when steady state infiltration was stabi-lized.

The crusted soils after the first storm were dried at roomtemperature (35°C) for periods of 24, 48, 72 or 96 h. Themoisture content of the soil at the end of the drying periodwas determined gravimetrically.

After the drying, the soils were saturated from bottomwith tap water, and then subjected to a second storm ofeither distilled or saline water (SW) (EC of 5 dSm'1 andSAR of 5). The same typical mechanical parameters of theapplied rain in the first storm were obtained in the secondstorm. Distilled water (DW) was used to simulate rainwater,and the saline water (SW) was used to simulate saline irri-gation water.

RESULTS AND DISCUSSIONThe moisture content of the upper 0.5 cm of the

crusted soil after different drying periods at 35°C ispresented in Table 2. The results show that most ofthe drying in the Rhodoxeralf soil took place in thefirst 48 h and the moisture content dropped from 0.30kg kg"1 (immediately after the rain) to 0.04 kg kg"1.Additional drying of 24 h (72 drying h) decreased themoisture to 0.025 kg kg"1 which was the air-dry mois-ture content of the soil. Conversely, the drying of theHaploxeralf soil was more gradual, and the soil reachedthe air-dry moisture content at 96 h. The differencesin the rate of drying between the two soils were prob-ably the result of their texture. The clay + silt per-

BEN-HUR ET AL.: EFFECT OF WATER QUALITY AND DRYING ON SOIL AND CRUST PROPERTIES 193

Fig. 1—Crusted surfaces of Calcic Haploxeralfs (Loess) and TypicRhodoxeralfs (Hamra) soils after different periods of drying.

Fig. 2—Crusted surfaces of Calcic Haploxeralfs (Loess) and TypicRhodoxeralfs (Hamra) soils after different periods of drying andsubsequent wetting from beneath.

centage in the Haploxeralf soil is 62%, in comparisonwith 21% in Rhodoxeralf soil (Table 1), and the highfraction of fine particles slows the rate of drying.

Crusted soil surfaces of the Haploxeralf and Rho-doxeralf soils after different periods of drying, and ofthe same soils after being saturated from beneath withtap water, are presented in Fig. 1 and 2, respectively.In both soils, 24 h of drying had no visual effect onthe soil surface. After 48 h of drying, some slightcracking developed. These cracks grew in depth andwidth in the Haploxeralf soil with more drying (up to96 h).

Similar growth of cracks was not observed in theRhodoxeralf soil. It must be emphasized that the effectof drying time on soil cracking was obtained on thinlayers of disturbed soil samples in the laboratory. Itis likely that in the field, the time needed to reachsimilar cracking will be longer, because of watermovement from deeper layer.

When saturating the crusted soils from underneath(following each of the drying periods), closing of thecracks—due to swelling—took place (Fig. 2). In theRhodoxeralf, closing of the cracks seems to be betterthan in the Haploxeralf. The effect of drying and wet-ting of the crust on soil cracking should be remem-

bered in analyzing the infiltration rate curves of thesoils. The IR of the two soils, when subjected to sub-sequent storms of DW with different drying periodsbetween the storms, as a function of the cumulativerainfall is presented in Fig. 3. It is important to notethat the soil was saturated with tap water from thebottom before each storm. The following character-istics can be noted:

(a) There was a decrease in the IR with the amountof rainfall in the first storm of DW, until final IR or4.0 and 4.8 mm h~' were obtained from the Haplox-eralf and the Rhodoxeralf soils, respectively. The de-crease in IR was due to the formation of the crust onthe soil surface. The final IR values are in good agree-ment with those obtained by Agassi et al. (1981) andKazman et al. (1983).

(b) The IR curves of the Rhodoxeralf soil were higherthan those of the Haploxeralf soil at any given raindepth in the first storm. The greater sensitivity of theHaploxeralf soil to crust formation is probably due tothe higher ESP level. The ESP of Haploxeralf was 3.2,and that of sandy loam was 2.2. Kazman et al., (1983)showed that soils with higher ESP values (at the range5) were more sensitive to reduction in IR.

194 SOIL SCI. SOC. AM. J., VOL. 49, 1985

Ill(-<

zo

CE

5

——— ST. No.I DW

— — SI. No,2 DW - 2 4 h

— — ST.No.2 DW - 4 « k— — *ST.No.2 DW -72 h——— ST.No.2 OW -96h

5 10 15 20 25 30 35 40

CUMULATIVE R A I N F A L L , mm

Fig. 3—The infiltration rates of Calcic Haploxeralfs (Loess) andTypic Rhodoxeralfs (Hamra) soils as a function of the cumulativerainfall when subjected to subsequent storms of distilled waterafter different periods of drying.

(c) The IR values at the beginning of the secondstorm were higher than the final IR values of the firststorm for each drying period. The drier the crust, thehigher were the IR values at the beginning of the sec-ond storm. Drying the crust resulted in its breakdown,which was due both to crack formation and to thepossible formation of new structure at the soil surface.Drying the crust brought the soil particles whichformed the crust closer to form new aggregates. Theamount of the new aggregates increased with furtherdrying of the crust. Saturation of the soil from below,before subjecting it to the second storm, resulted inswelling and closing of some of the surface cracks. Thehigh IR values of the soil after 96 h of drying, in thebeginning of the second storm indicate that the newaggregates which forms during the drying remains rel-atively stable during the saturation process. Applica-tion of the second DW storm on the soil surface causeda physical and chemical destruction of the new aggre-gates, which, in turn, caused a decrease in the IR of

the soil until reaching a final IR. In general, the soilcrust after the first storm, the higher was the IR curveat the beginning of the second storm, the lower wasthe final IR.

(d) The final IR values of both soils at the end ofthe second storm were lower than the correspondingvalues at the end of the first storm (Table 3). More-over, the final IR of the second storm depended onthe drying degree of the crust which forms during thefirst storm. Similar results were obtained by Hardy etal., (1983). When the crust was wet and cohesive, itwas stable and no further crust forming processes couldtake place in the second storm, and the final IR of thesecond storm was maintained at a value similar tothat at the end of the first storm. As the crust dried,it lost its stability and the impact of the raindropscontinued the processes of crust formation. Thus, thefinal IR of the second storm was lower than that atthe end of the first storm.

(e) The IR curves of the second storm of the Rho-doxeralf soil, following each of the drying periods, werehigher than that of the Haploxeralf in spite of the factthat the cracks in the Haploxeralf were larger and wider(Fig. 1 and 2) and did not close when the soil wassaturated from below. These results indicate that thecracking of the soil is not the dominant factor in in-creasing the IR of the crusted soil.

The effect of drying on crust properties can also beevaluated from the IR curves of crusted soils exposedto saline water (SW) rain. In these experiments, thesoil was exposed first to distilled water rain, until steadystate IR was obtained. Then a second storm of SW(EC = 5dS • m~') was applied. In Fig. 4, the secondstorm of SW water was applied without any period ofdrying. It is evident that changing the quality of theapplied water from DW to SW in both soils causedan increase in the IR's. The final IR of the Rhodox-eralf increased from 4.8 to 6.0 mm/h upon switchingfrom DW to SW, and that of the Haploxeralf in-creased from 4.0 to 4.8 mm/h. It is evident that theeffect of the change in water quality on the final IRwas more pronounced in the Rhodoxeralf than in theHaploxeralf. Final IR of the Rhodoxeralf in the sec-ond SW storm reached the same value obtained whenSW was applied on a disturbed soil sample (first stormHardy et al., 1983). The final IR of the Haploxeralfexposed to SW rain on crusted soil did not recover,and the value was lower. It seems that different mech-anisms predominated in the two soils: in the Rho-doxeralf soil the cohesive forces between the particlesin the crust are weak (Hardy et al., 1983). Therefore,the increase in the IR of the soil with the change inthe quality of the applied water, was the result of the

Table 3. The final IR of the Calcic Haploxeralf (loess) and Typic Rhodoxeralf (Hamra) after various times of dryingprior to the second storm. __

-_____________^___Drying time before second storm, h_________________Distilled watert Saline waterf

Soil

HaploxeralfsRhodoxeralfs

First storm DW

4.0(0.2)4.8(0.1)

24

3.2 (O.l)t3.4 (0.2)

48

2.8(0.2)2.2 (0.5)

72

2.0 (0.1)2.2 (0.2)

96

nfiltration rates,2.0 (0.1)2.2 (0.2)

24

4.8 (0.4)6.4 (0.4)

48

5.0 (0.3)9.0 (0.2)

72

6.0 (0.2)9.0 (0.2)

96

8.0 (0.3)9.0 (0.2)

t Water quality in the second storm.t The numbers in the brackets are standard deviation.

BEN-HUR ET AL: EFFECT OF WATER QUALITY AND DRYING ON SOIL AND CRUST PROPERTIES 195

5 10 15 20 25 30 35 40

CUMULATIVE RAINFALL, mm

Fig. 4—Infiltration rates of Calcic Haploxeralfs (Loess) and TypicRhodoxeralfs (Hamra) soils as a function of the cumulative rain-fall upon replacing the DW with SW without drying.

raindrop impact which destroyed the crust which wasformed during the first DW storm, and a new crust-typical of saline water—was formed. Conversely, inthe Haploxeralf soil, the cohesive forces are stronger.Therefore, the destructive ability of the raindrops waslimited, and the increase in the IR's of this soil wasthe result of reflocculation and limited swelling of theclay, that was present in the crust.

This mechanism is further demonstrated by the datain Figure 5. In this case the two crusted soils wereexposed to SW rain after various periods of drying.The following characteristics should be noted: (i) TheIR values at the beginning of the second storm, foreach drying period, were higher than the final IR val-ues in the first storm. The drier the crust, the higherthe IR values at the beginning of the second storm(similar results with DW, are presented in Fig. 3). (ii)The final IR values in both soils at the end of thesecond storm (SW) were higher than the correspond-ing values at the end of the first storm (Table 3). Thepresence of electrolytes in the applied water that wassubjected in the second storm, prevented clay disper-sion at the soil surface and the new crust was formedonly due to the impact of the raindrops, and the break-down and compaction of the particles at the soil sur-face as found also by Agassi et al., (1981). It is evidentfrom these results, that the final IR in the second stormdepends on the drying degree of the crust after thefirst storm and on the water quality of the water sup-plied. It is clear that more time is needed to dry thecrust of the Haploxeralf and to render it a 'reversible'crust. Whereas in the Haploxeralf, a 96 h drying pe-

10 15 20 25 30 35 40 45 50 55

CUMULATIVE RAINFALL , mm

Fig. 5-Infiltration rates of Calcic Haploxeralfs (Loess) and TypicRhodoxeralfs (Hamra) soils as a function of the cumulative rain-fall when subjected to subsequent distilled water followed by sa-line water.

riod was needed to destroy either the crust formed inthe first storm, or to make it sensitive to the destruc-tion caused by the impact of the raindrops, only 48 hwere needed to maintain similar strength in the crustof the Rhodoxeralf.

SUMMARY AND CONCLUSIONSDrying the soil crust causes an increase in the crust

permeability. This increase is due to the formation ofcracks and the new structure at the soil surface. Bothprocesses intensify as the crust dries. Whereas the wetcrust is stable and maintains the original properties ofthe crust (low IR and the crust being almost indepen-dent of the salinity of the water in the subsequentstorm). The structure of the crust, upon drying, changesand both crust permeability and crust's response tothe salinity of the solution increase. The formation ofa new and less cohesive structure at the soil surfaceupon drying increases the permeability of the crustand causes it to be more sensitive to the destructiveeffect introduced by the impact of the raindrops.

ACKNOWLEDGEMENTSWe wish to thank Mr. Haim Tzodovnik for his help in

the rain simulator experiments.

196 SOIL SCI. SOC. AM. J., VOL. 49, 1985

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ERRATA

Effect of Water Quality and Drying on Soil CrustPropertiesM. BEN-HUR, I. SHAINBERG, R. KEREN, AND M. GALSoil Sci. Soc. Am. J. 49:191-156 (Jan.-Feb. 1985 is-sue)

Figures 4 and 5 on p. 195 should be transposed.