Effect of Electrolyte Concentration and Soil Sodicity onInfiltration Rate and Crust Formation1

M. AGASSP, I. SHAINBERG3, AND J. MORIN2

ABSTRACTThe effects of electrolyte concentration and soil sodicity on the in-

filtration rate and extent of crust formation of a calcareous and a non-calcareous soil were studied using a rain simulator. The infiltrationrate was more sensitive to the sodicity of the soil and to the electrolyteconcentration of the applied water than was the permeability of theunderlying soil. The mechanical impact of the raindrops and therelative freedom for particle movement at the soil surface may ac-count for the greater sensitivity of the infiltration rate. These observa-tions suggest that crust formation is due to two mechanisms: (i) aphysical dispersion of soil aggregates caused by the impact action ofthe raindrops, and 00 a chemical dispersion which depends on the soilexchangeable sodium percentage (ESP) and the electrolyte concentra-tion of the applied water.

Additional index words: aggregate stability, dispersion, runoff, salinity.

Agassi, M., I. Shainberg, and J. Morin. 1981. Effect of electrolyteconcentration and soil sodicity on infiltration rate and crust forma-tion. Soil Sci. Soc. Am. J. 45:848-851.

THE PERMEABILITYof a soil to water depends bothon the exchangeable sodium percentage (ESP) of the

soil and on the salt concentration of the percolatingsolution, tending to decrease with increasing ESP anddecreasing salt concentration (Quirk and Schofield,1955; McNeal et al., 1968). Soil hydraulic conductivity(HC) can be maintained at a high ESP provided that theelectrical conductivity (EC) of the infiltrating water isabove a critical (threshold) level (Quirk and Schofield,1955). Intermittent applications of rainwater may lowerthe electrolyte concentration below the threshold value.Even at low ESP values where little soil swelling is ex-pected, Shainberg et al. (198la) observed that claydispersion and reductions in soil permeability tookplace. When the salt concentration was 3.0 meq/liter,decreases in HC and clay dispersion occurred if the ESPexceeded 12. Conversely, in distilled water, clay disper-sion and HC reductions occurred at an ESP as low as 1to 2 (Shainberg et al., 1981a).

Several studies have been conducted on the effect ofrainfall on the structure and hydraulic properties of soilcrusts (Mclntyre, 1958; Evans and Buol, 1968; Chen etal., 1980; Keren and Shainberg, 1981; Morin et al.,1981). Mclntyre (1958) found the crust to consist of twodistinct parts: (i) an upper skin seal attributed to com-paction by raindrop impact, and (ii) a "washed in"region of decreased porosity, attributed to the ac-cumulation of particles. Mclntyre (1958) measuredthicknesses of 0.1 and 2 mm for the skin seal and thewashed in zone, respectively. The washed in layer wasformed only in soils that were easily dispersed. Thepermeability of the deeper layer was about 800 timesthat of the washed in layer and about 2,000 times that of

' Contribution no. 107-E, 1981 series, from the AgriculturalResearch Organization, Bet Dagen, Israel. Received 12 Mar. 1981.Approved 29 Apr. 1981.2 Soil Scientists, Soil Erosion Research Station, Emeq Hefer, Israel.J Soil Scientist, Inst. of Soils and Water, ARO, The Volcani Center,Bet Dagen.

the skin seal. Chen et al. (1980) examined scanning elec-tron micrographs (SEM) of crusts of loessial soil andalso found a thin seal skin, about 0.1 mm in thickness,that had formed at the uppermost layer of the soil. Theydid not however, find accumulation of fine particles inthe 0.1 to 2.8-mm zone, as observed previously byMclntyre (1958; the washed in layer). The sealing effi-ciency of the crust is achieved by suction forces whichhold the clay particles together in a continuous denseskin (Morin et al., 1981). The suction forces at thesoil-crust interface are created as a result of the largedifferences in HC between the crust and the underlyingsoil. The suction mechanism accounts for the stabilityof the crust HC and the similarity in HC of the crust ofsoils varying greatly in their texture and mineralogy(Morin et al., 1981).

It is evident from the above discussion that crust for-mation is associated with clay dispersion and movementin the soil. Soil surfaces are especially susceptible to thechemistry (electrolyte concentration and cationic com-position) of the applied water because of the mechanicalaction of the falling drops and the relative freedom forparticle movement at the soil surface. Indeed, Oster andSchroer (1979), studying the infiltration rate of un-disturbed loam soil columns, found that the effects ofthe chemistry of the applied water were far greater thanexpected. When the ESP of the surface soil layer was 8,the infiltration rates decreased from 15 to 1 mm/hour asthe concentration of the irrigation water decreased from28 to 8 meq/liter. Comparable reductions in saturatedhydraulic conductivity of saturated soils with an ESP of10 occur only when the concentration of the percolatingsolution decreases below 2 to 3 meq/liter (Shainberg etal., 1981a).

The objectives of this study were: (i) to evaluate theeffects of soil sodicity and concentration of appliedwater on the infiltration rate of two loamy soils (onecalcareous and one noncalcareous) under conditionssimulating sprinkler application of irrigation water andrainstorms; and (ii) to demonstrate the illadvice of usingtapwater in rain simulation studies.

MATERIALS AND METHODSSoils

The <4-mm fractions of two sandy loam soils from Netanya andNahal Oz were used in this study. The Netanya soil is from the coastalplain in Israel where the annual precipitation is 600 mm. It containsonly trace amounts of CaCO3. The Nahal Oz soil is a loessial soil fromwestern Negev, where the annual precipitation is 400 mm. It contains13% CaCO3. Some other properties of these soils, together with theirclassification, are given in Table 1.

Two samples of each soil, with two levels of exchangeable sodium,were studied. In the loessial soil, one sample was taken from a fieldwhich has never been irrigated. The ESP of this soil was 6.4. Anothersample was taken from a field which has been irrigated for 7 yearswith saline water. [The EC and sodium adsorption ratio (SAR) of thesaline water was 4.6 mmho/cm and 26, respectively.] The ESP of thissoil was 26: The ESP of the natural Netanya soil was 1.0. To prepare asoil sample with higher ESP, a 1-m2 plot was irrigated with 120 litersof 0.2M solution of SAR 20 (three applications of 40 liters each, with

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AGASSI ET AL.: ELECTROLYTE CONCENTRATION-SOIL SODICITY: EFFECT ON INFILTRATION 849

1 week between irrigations) and followed by a final irrigation with 40liters of O.OlAf solution of SAR 20. After the soil had been air-dried, asoil sample was taken for the study. It had an ESP of 13.6. Thistechnique of leaching the soil in the field was used in order to preventsoil structure breakdown which might take place when a disturbed soilsample is leached in the laboratory with the desired salt solutions.

Rain Simulation ExperimentsSoil was packed in 30- by 50-cm perforated trays, 2.0 cm deep, over

a layer of coarse sand. The trays were placed in a rainfall simulator(Morin et al., 1967) at a slope of 5% and subjected to a rainfall inten-sity of 26 mm/hour. The concentration of electrolytes in the simulatedrain was distilled water, 0.005AT, 0.020JV, and 0.05N (the correspon-ding ECs of the solutions were 0, 0.5, 2.3, and 5.6 mmho/cm). TheSAR of each of the simulated rain solutions was prepared and ad-justed to be equal to the ESP of the test soil (i.e., the soil samples withESP 26 were watered with a solution of SAR 26, and the Netanya soilwith ESP 13 was watered with a solution of SAR 13, etc.). This adjust-ment in the simulated rain composition was made in order to preventthe exchangeable cation composition from changing during thesimulated rain application. Typical mechanical parameters of the ap-plied rain were median raindrop diameter of 1.9 mm; median dropvelocity of 6.02 m/second; and total kinetic energy of 470J/hourxm2. The volumes of runoff water and of percolation weremeasured, and the infiltration rate was calculated.

RESULTS AND DISCUSSIONThe infiltration rates of the loessial soil at two ESP

values as a function of the depth of applied water arepresented in Fig. 1. It is evident that both the electrolyteconcentration of the applied water and the ESP of thesoil have a very pronounced effect on the infiltrationrate of the soil. The final infiltration rate (IR) of the soilexposed to distilled water rain was independent of theESP of this soil. In both ESP's (6.4 and 26) the final IRwas maintained at 1.2 mm/hour. Similarly, the final IRof the loessial soil irrigated with water of EC of 5.6mmho/cm was 7.5 ±0.3 mm/hour, independent of theESP (and SAR) of the soil. It is concluded that, whendistilled water is applied, even low ESP values (> 6.4)are enough to cause dispersion, crust formation, and avery sharp decrease in IR. Conversely, when water ofEC 5.6 mmho/cm is used, the concentration of elec-trolyte is so high that only limited clay dispersion ispossible, independent of the soil ESP (at ESP < 26),and the final IR is maintained at a relatively high value.When solutions of high concentration are used in therain simulator, the impact energy of the drops is themain force causing breakdown of the soil aggregates,and a compacted layer with low permeability is produc-ed at the soil surface. When distilled water is applied toa soil, even with low levels of exchangeable sodium,chemical dispersion of the soil clay also occurs, thedispersed clay particles are washed into the soil with theinfiltrating water, and the pores immediately beneaththe surface become clogged. The final IR drops tovalues which are 0.16 of those obtained with the mostsaline water. From a practical point of view, the dif-ference in IR between a value of 7.5 and 1.2 mm/hour isvery important. When the IR is maintained at values of7.5 mm/hour, very little runoff is expected. Conversely,when the final IR reaches 1.2 mm/hour, a lot of runoffis expected (Agassi et al., 1981).

The effect of the interaction between the electrolyteconcentration and the ESP is also seen when waters ofintermediate salinities are used (Fig. 1). A concentration

Table 1—Some physical and chemical propertiesof the soils used.

SoilInternationalclassification

Mechanicalcomposition

CaCO, Sand Silt Clay CEC——————— % ——————— meq/

100 gHamra-Netanya Typic Rhodoxeralf 0.2 79 10 11 8.0Loess-NahalOz Calcic Haploxeralf 13.4 37.7 40.6 21.7 15.4

of 0.5 mmho/cm is enough to decrease the chemicaldispersion in soil with an ESP of 6.4, and the final IR ismaintained at 5 mm/hour. In soil with an ESP of 26,however, this concentration was not enough to preventthe chemical dispersion caused by the adsorbed sodium,and the IR dropped to 1.2 mm/hour. When a solutionof EC = 2.3 mmho/cm is used, there is less chemicaldispersion in both ESP values, and the final IR reachesvalues of 6.0 and 2.6 mm/hour for soil samples withESP of 6.4 and 26, respectively.

Whereas the final IR in the distilled water rain was in-dependent of the ESP of the soil, the rate of decrease inwater intake was sensitive to the ESP of the soil. For soilwith an ESP of 6.4, the infiltration rates were 8.0, 4.0,and 2.8 mm/hour for rain depths of 10, 20, and 30 mm,whereas the corresponding infiltration rates for soil withan ESP of 26 were 3.5, 1.7, and 1.2 mm/hour, respec-tively. Since the mechanical energy of the rain was thesame in both cases, the differences between the two weredue to the rate of chemical dispersion. It seems that, inthis range of exchangeable sodium, the main effect ofNa was to enhance the rate of soil aggregate breakdown,and when steady state is maintained, the final IR issimilar for both ESPs watered with distilled water.

24

£ 2°E 16.sfTo* 12co

1 8^c

4

0

24

^ 20

E 16oTTo<*• 12

LOESS -Nahal OzESP 26

aggregate size 0-4 mmRain intensity- 26mm/h

30 40 50 60 70 80

LOESS-Nahal OzESP 6.4

aggregate size 0-4mmRain intensity- 26mm/h

" 10 20 30 40 50 60 70 80Cumulative Rainfall, mm

Fig. 1—Effect of electrolyte concentration in rain simulation ex-periments on the infiltration rate of loess soil.

850 SOIL SCI. SOC. AM. J., VOL. 45, 1981

24

HAMRA-NETANYAESP- 13.6

aggregate sizeO-4mm

Rain Intensity-26mm/h

40 50 60 70 80

HAMRA - NETANYAE S P - 1.0

aggregate sizeo-4mm

Rain Intensity — 26 mm/h

10 20 30 40 50 60

Cumulative Rainfall,mm

Fig. 2—Effect of electrolyte concentration in rain simulation ex-periments on the infiltration rate of Hamra soil.

When water of EC = 5.6 mmho/cm is used, the ef-fect of soil sodicity on the rate at which IR decreases (asa function of water depth) diminishes. The electrolyteconcentration in the applied water is high enough to pre-vent the deleterious effect of exchangeable sodium. Atthe intermediate concentrations of electrolytes in the ap-plied water, intermediate effects of exchangeablesodium on the rate of decline in IR are evident.

The infiltration rates of the Netanya soil at two ESPvalues as a function of the depth of applied water andconcentration of electrolytes in the waters are presentedin Fig. 2. Also in Fig. 2, the IR of a Netanya soil with anESP of 4.6 watered with distilled water is presented. Thefollowing characteristics should be noted:

1) The effect of ESP on the IR curve. The final IRs ofthe Netanya soils with ESPs of 1.0, 4.6, and 13.6 are7.6, 1.0, and 0.4 mm/hour, respectively, when wateredwith distilled water. The effect of low levels of ex-changeable sodium on the IR is surprisingly high. In aprevious study on the effect of ESP and electrolyte con-centration on the hydraulic conductivity of this soil, dif-ferent results were obtained (Felhendler et al., 1974).The hydraulic conductivity of the Netanya soil was notaffected by the ESP (< 20), as long as the concentrationof the percolating solution exceeded 10 meq/liter. Whenthe 0.01N percolating solutions (of SAR =, 10, and 20)were displaced with distilled water, however, the relativehydraulic conductivity of the Netanya soil dropped to80, 5, and 0%, respectively. It is evident that the IR is

much more sensitive to the ESP of the soil than is thehydraulic conductivity. The greater sensitivity of thesurface of this soil to low levels of exchangeable sodiumis probably due to several reasons: (i) the mechanical ac-tion of the raindrops which enhance dispersion; (ii) theabsence of the soil matrix (sand particles) which slowsclay dispersion and clay movement; and (iii) the lowconcentration of electrolytes in the soil solution at thesurface. In the soil profile, the concentration of elec-trolytes in the soil solution is affected also by thedissolution of the minerals in the soil. The effect of soilparticles' dissolution on clay dispersion and hydraulicconductivity of soils was determined recently (Shainberget al., 1981b). At the soil surface, the concentration ofelectrolytes in the soil solution is determined by the ap-plied water only, and when distilled water is used, theelectrolyte concentration in the soil solution is very low,and dispersion occurs even at very low levels of ex-changeable sodium (< 5%).

2) The IR curve of the Netanya soil, with an ESP of4.6, was slightly lower than that of the loessial soil withan ESP of 6.4 when watered with distilled water (Fig. 1).The IR values of the Netanya soil were 7.6,2.5,1.5, and1.0 mm/hour for rain depths of 10, 20, 30, and 40 mm,whereas the corresponding values for the loessial soilswere 8.0, 4.0, 2.8, and 2.0 mm/hour. Similar resultswere obtained for the hydraulic conductivity of thesetwo soils (Felhendler et al., 1974), and the Netanya soilwas found to be more susceptible to exchangeablesodium when leached with distilled water. It should benoted that the Netanya soil is more sensitive to sodicconditions in spite of two factors which generallystabilize the soil structure: (i) the Netanya soil contains ahigh percentage of sesquioxides which act as cementingagents stabilizing the soil structure; and (ii) the loessialsoil contains a high level of silt (37.8% compared with5.0% in the Netanya soil), and it is generally accepted(Cary and Evans, 1974) that a high content of siltweakens the soil structure.

The differences between the two soils in their HCresponse to sodic conditions was explained (Shainberget al., 1981a, b) by the potential of the soils to releasesalt When leached with distilled water. The calcareousloessial soil released electrolytes at a rate sufficient toslow down the dispersion of the clay. The same explana-tion may hold for the differences between two soils inthe IR curves.

3) Even the Netanya soil with an ESP of 1.0 respond-ed to the electrolyte concentration in the applied water.As expected, however, soil with an ESP of 13.6 wasmore responsive to the EC of the applied water than soilwith an ESP of 1.0. The presence of electrolytes in thesolution prevented the chemical dispersion of the soilaggregates, and the IR was maintained at higher values.

4) In the Netanya soil, the high concentration of elec-trolytes in the 5.6 mmho/cm treatment was not enoughto prevent the dispersive effect of exchangeable sodium,and the final IR values were 14.2 and 7.6 mm/hour forsoils with ESP of 1.0 and 13.6, respectively. Evidentlythe presence of exchangeable sodium weakened thestructure of the Netanya soil, and the IR dropped to lowvalues as a result of the beating action of the waterdrops. This phenomenon was not evident in the loessialsoil, with its poor structure, where the exchangeablesodium had no effect when saline water was used.

AGASSI ET AL.: ELECTROLYTE CONCENTRATION-SOIL SODICITY: EFFECT ON INFILTRATION 851

100

2 4 6 0 2 4 6ELECTRICAL CONDUCTIVITY, mmho /cm

Fig. 3— Runoff percentage as a function of the soil ESP and appliedwater concentration of loess-Nahal Oz and Hamra-Netanya.

The fraction of the applied rain that infiltrated thesoil or that appeared as runoff may be estimated fromthe area under (or above) the infiltration rate curves(Fig. 1 and 2). The percent of runoff, up to 60-mm raindepth, as a function of the EC of the applied solution ispresented in Fig. 3. It is evident that the amount ofrunoff increases with the ESP of the soil and decreaseswith electrolyte concentration. Whereas the percent ofrunoff from the Netanya soil with an ESP of 1.0 isrelatively small (25%) and insensitive to electrolyte con-centration (at this low ESP, no chemical dispersiontakes place even in distilled water, and thus the effect ofelectrolytes is minimal), the percent of runoff from thesame soil with an ESP of 13.6 is very sensitive to elec-trolyte concentration, and the runoff dropped graduallyfrom 91 to 45% in distilled water and in the high con-centration treatment, respectively. Similarly, in theloessial soil, at an ESP of 26, the amount of runoffdropped gradually with the increase in electrolyte con-centration. Conversely, in the loessial soil with an ESPof 6.4, a sharp drop in runoff took place between distill-ed water and the solution with an EC of 0.5 mmho/cm.Whereas an electrolyte concentration of 5 meq/liter isenough to prevent clay dispersion in soils with ESP of6.4, a very intense chemical dispersion occurred indistilled water, and the runoff increased to 82% of theapplied rain.

SUMMARY AND CONCLUSIONSCrust formation in soils exposed to rain is due to two

mechanisms: (i) a physical dispersion caused by the im-pact action of the raindrops, and (ii) a chemical disper-sion which depends on the ESP of the soil and the elec-trolyte concentration in the applied water. The physicalmechanism alone operates in soils with no sodium in the

exchange complex or when high electrolytes are presentin the applied water. When only the physical mechanismoperates, the infiltration rate of the two soils dropped tovalues around 8 mm/hour. In soils with low ESP valuesCv57) watered with distilled water, chemical dispersionalso operated, and the final infiltration rate dropped tovalues around 2 mm/hour. The intensity of the chemicaldispersion depended on both the soil ESP and elec-trolyte concentration in the applied water. In soils withlow ESP (^5), an increase in electrolyte concentrationto 5 meq/liter reduced the chemical dispersion sharply.Conversely, in soils with moderate-to-high ESP, agradual change in infiltration rate occured as the EC ofthe applied water was increased from ~ 0.1 to 5.6mmho/cm.

ACKNOWLEDGMENTSThis research was supported in part by a grant from the United

States-Israel (Binational) Agricultural Research and DevelopmentFund (BARD), and in part by a grant from the Israel FertilizerResearch Center.