Water-Drop Kinetic Energy Effect on Infiltration in Sodium-Calcium-Magnesium SoilsR. Keren*

ABSTRACTThe effect of the complementary adsorbed ion (Ca vs. Mg) in the

presence of Na on seal formation and water infiltration in two soils(Calcic Haploxeralf and Typic Rhodoxeralf) at several kinetic ener-gies of water drops was studied using rain simulators. The infiltrationrate (IR) of the soils was lower for the Na-Mg soils than for theNa-Ca soils at the studied kinetic energy range of the water drops(3.2-22.9 kJ m 3). The higher the kinetic energy, the steeper thedrop in IR. The steady-state IR and the cumulative water depthrequired to reach steady-state IR were both decreased with increas-ing kinetic energy of the water drops. Adsorbed Mg on montmoril-lonitic soils had a specific effect on IR whether or not the soil con-tained CaCO3. Aggregates with adsorbed Na and Ca ions were morestable than those with adsorbed Na and Mg ions when they wereexposed to water drops having a kinetic energy in the range of 8.0to 12.5 kJ nr3. The specific effect of Mg on IR was explained bythe presence of Mg ions on the external surfaces of the clay tactoidsand the larger hydration shell of the Mg ion compared to Ca.

MANY SOILS AND WATERS in arid and semiaridregions have high Mg contents. The use of high-

Mg waters for irrigation increases the exchangeable Mgin the soil. Conflicting opinions regarding the influenceof Mg on soil properties have been published. For ex-ample, the U.S. Salinity Laboratory Staff (1954)grouped Ca and Mg together in their classifications,whereas Mg can cause deterioration of the soil struc-ture and the development of a Mg solonetz (Ellis andCaldwell, 1935). Laboratory studies also imply that Mgis a deleterious ion in some circumstances. Clay min-erals differ in their response to the presence of Mg.Montmorillonite and illite seem to be more easily dis-persed in the presence of Na and Mg, compared to Naand Ca (Bakker and Emerson, 1973; Emerson and Chi,1977; Shainberg et al., 1988). Similarly, McNeal et al.(1968) showed that mixed Na-Mg soils developedlower saturated hydraulic conductivity (HC) than Na-Ca soils under similar conditions. However, calcar-eous soils did not show a specific effect of Mg on claydispersion and HC losses (Alperovitch et al., 1981). Incalcareous soils, the exchangeable Mg enhances thedissolution of the carbonates and increases electrolyteconcentration, thus preventing clay dispersion and re-duction in soil HC. Recently, Keren (1989) showedthat the IR values were always lower for Mg soils thanfor Ca soils. These results indicated that exchangeableMg had a specific effect on IR when the soil was ex-posed to rain of deionized water, whether the soil con-tained CaCO3 or not.

The formation of a seal at the soil surface is a com-mon feature of many soils, particularly in the arid andInst. of Soils and Water, Agric. Res. Organization, the Volcani Cen-ter, Bet Dagan, Israel. This research was supported by grant no. I-743-84 from BARD (U.S.-Israel Binational Agric. Res. and De-velopment Fund). Contribution no. 2714-E, 1989 series, from theARO, the Volcani Center, Bet Dagan, Israel. Received 13 July 1989."Corresponding author.

Published in Soil Sci. Soc. Am. J. 54:983-987 (1990).

semiarid regions, and its HC controls the IR (Hilleland Gardner, 1970; Morin et al., 1989). This HC de-pends both on the adsorbed cation composition andthe electrolyte concentration of the percolating solu-tion (Quirk and Schofield, 1955; McNeal et al., 1968).Soil surfaces are especially susceptible to the compo-sition of the adsorbed ions, because of the mechanicalaction of the falling drops and the relative freedom ofsoil particle movement at the soil surface. Althoughmany reports on the effect of Na on soil permeabilityhave been published (e.g., Agassi et al., 1981; Kemperand Noonan, 1970; Keren et al., 1983), the effect ofthe complementary adsorbed ion (Ca vs. Mg) in thepresence of Na on seal formation and water infiltrationwas not considered.

Magnesium is a more hydrated ion than Ca (Bockrisand Reddy, 1970), and therefore may cause a thickerdiffuse layer (Shainberg and Kemper, 1966). Thus, itwould be expected that a Na-Mg soil would be lessstable than a Na-Ca soil for a given exchangeable so-dium percentage (ESP) at the low range (ESP < 15).This hypothesis was tested in the present study, theobjective of which was to determine the effect of theadsorbed Mg in the presence of Ca or Na ions on theIR of calcareous and noncalcareous montmorilloniticsoils at various kinetic energies of raindrops.

MATERIALS AND METHODSSoil Preparation

Two montmorillonitic soils from the coastal plain of Israelwere used in this study. The Calcic Haploxeralf (CH, loamsoil) from Nahal Oz was developed on loessial windblownmaterial from the Sinai Desert, and the Typic Rhodoxeralf(TR, sandy loam soil) from Morasha was formed on coastalsand and calcareous sandstone. Some properties of the twosoils are presented in Table 1. The main differences betweenthe soils are in their texture and CaCO3 content.

The air-dried soils from the A horizon were packed 0.02m deep in 0.3 by 0.5 m perforated trays and placed in arainfall simulator (Morin and Benyamini, 1977) over a layerof coarse sand at a slope of 5%. The soils were leached with200 molc m'3 solution of Nad + CaCl2, NaCl + MgCl2 orMgCl2 + CaQ2, using a rainfall simulator with a rain in-tensity of 10.4 /um s"1 (37.4 mm h~'). To prevent seal for-mation during leaching, the rain was applied as a mist for~2 h (three teachings of 0.66 h each). Appropriate Na/Caand Na/Mg ratios in solutions were used to obtain ESP 10in both the Ca and Mg soil systems. The CH soil was leachedalso with 200 molc m-3 MgCl2 + CaCl2 solution at ratiosappropriate to obtain exchangeable Mg fractions of 0, 0.2,

Table 1. Some chemical and physical characteristics of the studiedsoils.

Particle size

Soil

_______________ Cation-——————————————— exchange CaCO3

Clay Silt Sand pH capacity content

CalcicHaploxeralf 21.7 41.4 36.9 8.4 17.8 13.4

TypicRhodoxeralf 20.6 3.2 76.2 7.6 11.3 traces

983

984 SOIL SCI. SOC. AM. J.; VOL. 54, JULY-AUGUST 1990

0.4, 0.6, and 1.0. After three (cachings, the electrolytes insoil solution were removed from the soils by one more leach-ing with deionized water, after which the soils were air dried.The methods used to determine soil properties were the hy-drometer method for particle size (Black et al., 1965, p. 562),the volumetric calcimeter method for CaCO3 content (Blacket al., 1965, p. 1389), the NH4OAc method for cation ex-change capacity (Black et al., 1965, p. 891) and a 1:1 water/soil suspension for pH. The adsorbed ion composition of allsoils (after equilibration with the leaching solutions), wasdetermined using the NH4OAc method, to ensure that theadsorbed ion composition was as required (values are givenin Fig. 1-3).Rainfall Simulation Experiments

Two types of rainfall simulators were used. The first onewas a drip-type rainfall simulator with a closed water cham-ber (0.75 by 0.60 m) that generates rainfall through a set ofhypodermic needles (~1000 needles, spaced in a 0.02 by0.02 m grid) to form a fixed, known droplet size. The averagewater drop diameter was 2.97 ± 0.05 mm; the rainfall in-tensity of 9.2 /*m s-1 (33.1 mm Ir1) was controlled by a per-istaltic pump. The kinetic energy of the rainfall was variedby changing the height of fall of the droplets. The heightsused in these experiments were 0.4, 1.0 and 1.6 m, resultingin kinetic energies of 3.2, 8.0 and 12.5 kJ m'3, respectively(Epema and Riezebos, 1983). The dimensions of a tray were0.2 by 0.4 m and three repetitions were run simultaneously.

The second rainfall simulator was a nozzle type, describedby Morin and Benyamini (1977); it was used to generate arainfall with higher kinetic energy than the other. The maincharacteristics of the simulated rainfall were as follows: me-dian water drop diam., 2.3 mm; median water drop velocity,6.74 m s-1; sum of kinetic energies of drops, 22.9 kJ m-3;and rainfall intensity, 10.4 urn. s-' (37.4 mm h~')- The di-mensions of a tray were 0.3 by 0.5 m and four repetitionswere run simultaneously.

Soil aggregates <4 mm were packed 0.02 m thick in thetrays, over a layer of coarse sand. The trays were placed inthe rainfall simulator at a slope of 15%. The soil was firstsaturated from the bottom with tap water and then was ex-posed to simulated rainfall consisting of deionized water(electrical conductance < 0.01 dS m~'). All soils were ex-posed to simulated rainfall until a steady-state IR was ob-tained. Rainfall rate was checked and calibrated before andafter each rainfall event. During each rainfall event, bothinfiltrated and runoff waters were collected separately ingraduated cylinders and the water volume was recorded asa function of time.Computations

Analysis of each data set was carried out separately withnonlinear regression procedures in the Statistical AnalysisSystem (SAS) (Goodnight and Sail, 1982). The infiltrationequation used was described by Morin and Benyamini(1977):

= /f [1]where 7ti is the infiltration rate as a function of time; 7j and/f are the initial and steady state infiltration rates, respec-tively; r is a soil coefficient; p is the rainfall intensity; andti is the time from the beginning of the storm. Values forthe infiltration equation parameters were obtained by non-linear regression using the PROC SYSNLIN of the SAS package(Goodnight and Sail, 1982).

RESULTS AND DISCUSSIONMagnesium-Calcium Soil

The IR of the CH soil exposed to raindrops with akinetic energy of 8.0 kJ m-3 as a function of cumulative

rainfall and adsorbed Mg and Ca ratio is given in Fig.1. The IR of the Ca soil decreases gradually with theincrease in the cumulative rainfall, reaching a steady-state IR value of 3.3 ± 0.2 urn s-1 (11.9 ± 0.7 mmha"1)- This value is lower than that found for the samesoil exposed to rainfall of deionized water with verylow kinetic energy (0.2 J m"3) or to a rainfall of 25 molm~3 CaCl2 or MgCl2 solution with kinetic energy of8.0 kJ m-3 (Keren, 1989). The lower IR value for theCa soil exposed to this rainfall can be explained asfollows. The potential of the soil clay to disperse de-pends on the adsorbed ion composition and the soilsolution concentration. When the electrolyte concen-tration in soil solution on the surface is below theflocculation value, the tendency of the soil clay to dis-perse is high. Since the flocculation value of Ca mont-morillpnite at pH 7 is ~1 molc m-3 (Goldberg andGlaubig, 1987), the low IR of the Ca soil exposed torainfall of deionized water is probably due to two pro-cesses: (i) breakdown of soil aggregates by the impactenergy of the raindrops, and (ii) clay dispersion. Thehigher IR for the Ca soil exposed to rainfall with verylow kinetic energy or to a rainfall of CaCl2 solution(Keren, 1989) support the existence of these two pro-cesses.

A steeper drop in IR was observed by increasing thefraction of adsorbed Mg from 0 to 0.2 (Fig. 1). Thesteady-state IR was 1.4 ± 0.1 /mi s-1 (5.0 ± 0.4 mmrr1), a value ~60% lower than that of the Ca-soil.Increasing the adsorbed Mg fraction above 0.2 did notcause a further drop in IR. The effect of adsorbed Mgon IR, even at relatively low levels, can be explainedas follows: Calcium montmorillonite and Mg mont-morillonite exist in tactpids consisting of several clayplatelets each, with a thin film of water (0.96 and 0.99nm thick for Ca and Mg montmorillonite, respec-tively) between two parallel platelets (Blackmore andMiller, 1961; Norrish, 1954). The lower IR in the pres-ence of Mg (compared with Ca soil), and the negligibleeffect on IR of additional Mg above the Mg fractionof 0.2, suggest that demixing of cations exists whenMg ions are adsorbed on rnontmorillonite in the pres-ence of Ca ions. It is plausible that, as in Na-Ca mont-

10.0

05 7.5E

5.0

£2.5

CALCIC HAPLOXERALFKINETIC ENERGY 8.0 kJ en"3

Mg FRACTION

25 50 75 100CUMULATIVE RAINFALL (mm)

125

Fig. 1. Effect of various fractions of adsorbed Mg (with Ca as acomplementary ion) on the infiltration rate of Calcic Haploxeralfsoil exposed to raindrops of deionized water with a kinetic energyof 8.0 kJ m"3. Lines were calculated using Eq. [1]. Bars on datapoints indicate population SD.

ICEREN: DROPLET ENERGY AND INFILTRATION RATES IN TWO XERALFS 985

morillonite systems, Mg ions are concentrated on theexternal surfaces. This hypothesis is supported by thefact that the selectivity of montmorillonite for Mg ionsis less than that for Ca ions (Suarez and Zahow, 1989).Since the hydration number of the Mg ion is ~50%greater than that of the Ca ion (Bockris and Reddy,1970), it is expected that the thickness of the Sternlayer in the diffuse double layer on the external sur-faces of the tactoid would be greater for Mg than forCa ions. Because the linkage among the tactoids toform an aggregate is through the external surfaces, andbecause of the greater hydration shell of the Mg ionsthat concentrated on the external surfaces, it is ex-pected that less energy would be required to breakdown these linkages than those for Ca tactoids.

Sodium-Calcium and Sodium-Magnesium SoilsThe IR of the CH and TR soils at ESP 10 as a func-

tion of the cumulative rainfall at various kinetic ener-gies is given in Fig. 2 and 3, respectively. As expected,the presence of adsorbed Na ions affects the IR of thetwo soils significantly for both Ca and Mg systems.The IR values were lower than those found for the Caand Mg soils at any given kinetic energy. For example,the steady-state IR value of Ca and Mg CH soil in thepresence of 10% Na was 1.9 ± 0.2 and 1.4 ± 0 jims-1 (6.8 ± 0.7 and 5.0 ± 0 mm Ir1), respectively, whenthe soil was exposed to rainfall of deionized water witha kinetic energy of 8.0 kJ nr3. Higher values wereobserved for the soils in the absence of Na: 3.3 ± 0.2and 1.7 ± 0.2 Mm s~' (11.9 ± 0.7 and 6.1 ± 0.7 mmh-') for Ca and Mg soil, respectively (Keren, 1989).

The IR values of the Na-Mg soil were lower thanthose of the Na-Ca soil at any given cumulative watervolume and kinetic energy of the water drops. Theseresults indicate that the IR of the soils was influencedby the presence of Mg as a complementary cation(compared with Ca). Although the IR of the soils waslower when Mg was present, the effect of adsorbed Naon IR was more pronounced in the presence of Cathan in the presence of Mg. This difference in effec-tiveness is due to the greater stability of the Ca soil

10.0r

CALCIC HAPLOXERALF

jnergy

25 50 75 100CUMULATIVE RAINFALL (mm)

125 140

Fig. 2. Effect of kinetic energy of deionized water drops on theinfiltration rate of Calcic Haploxeralf soil saturated with Na-Caor Na-Mg at an exchangeable sodium percentage (ESP) of 10.Lines were calculated using Eq. [1]. Bars on data points indicatepopulation SD.

than the Mg soil (Keren, 1989) prior to the introduc-tion of Na.

The decrease in IR of the CH soil (Fig. 2) at theinitial stage of the rainfall was relatively moderate atthe lower energy levels but steeper at the higher energylevels, for both Na-Ca and Na-Mg soils. These resultsshow that the cumulative water depth to reach asteady-state IR decreases with increasing kinetic en-ergy of the water drops. When waters of low or highenergies (3.2 or 22.9 kJ nr3) were applied, the differ-ence between IR values of Na-Ca soil and Na-Mg soilwas relatively small. When waters of low energy areapplied, the mechanical breakdown of the soil aggre-gates at the soil surface, and the mechanical stirringof the clay suspension, are limited. Under these con-ditions the Na-Ca and Na-Mg soils are both muchless susceptible to dispersion at this level of ESP. Whenthe soils were exposed to water with high energy, theimpact forces of raindrops falling on the wetted soilwere enough to destroy the soil aggregates and to stirthe clay suspension, regardless of saturating cation.Under such conditions, the effect of the exchangeableNa at this energy level was more pronounced (com-pared with the low energy level), and, therefore, thedifference between Ca and Mg systems was small.

Although a relatively small difference in IR betweenCa and Mg systems was observed for low and highkinetic energies, a relatively large difference was ob-served when the soils were exposed to water drops withintermediate kinetic energies (8.0 and 12.5 kJ nr3; Fig.2). It seems that the Na-Ca aggregates are more stablethan the Na-Mg aggregates when exposed to waterdrops with a kinetic energy in this range. A similartrend was observed for the TR soil (Fig. 3), but thedrop in IR with the cumulative rainfall depth wasmuch steeper at any given kinetic energy for TR thanfor CR soils. These results suggest that the forces in-volved in stabilizing the aggregates of the Na-Mg soilat ESP 10 are weaker than those operating in the Na-Ca system.

The steady-state IR value of the soils decreased withincreasing kinetic energy of the water drops and ap-

10.0r

J2 7.5

ULII—oc-, 5.0

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ESP 10- Na-Ca Soil— Na-Mg Soil

TYPIC RHODOXERALF

Kinetic energykJ m-'

• 3.2• 8.0A 22.9

I i

•$=••

25 50 75 100CUMULATIVE RAINFALL (mm)

125

Fig. 3. Effect of kinetic energy of deionized water drops on theinfiltration rate of Typic Rhodoxeralf soil saturated with Na-Caor Na-Mg at an exchangeable sodium percentage (ESP) of 10.Lines were calculated using Eq. [1]. Bars on data points indicatepopulation SD.

986 SOIL SCI. SOC. AM. J., VOL. 54, JULY-AUGUST 1990

f~5.00CO

3=

< 3.75cc

<«Q<

2.50

1.25

- CALCIC HAPLOXERALF-TYPIC RHODOXERALF

ESP 10ADSORBED

IONCOMPOSITION

10 15KINETIC ENERGY (kj m~*)

20 25

Fig. 4. Effect of kinetic energy of deionized water drops on thesteady-state infiltration rate of Calcic Haploxeralf and Typic Rho-doxeralf soils saturated with Na-Ca or Na-Ca at an exchangeablesodium percentage (ESP) of 10. Bars on data points indicate pop-ulation SD.

peared to approach a constant value (Fig. 4). This de-crease was steeper for the TR soil than for the CH soil.As for homoionic Ca and Mg soils (Keren, 1989), theminimum steady-state IR was reached at a lower ki-netic energy in the Na-Mg TR soil than in the Na-Mg CH soil. These results show that the TR soil ismore susceptible to surface sealing than the CH soil.A similar phenomenon was observed for these twosoils saturated with either Ca or Mg ions (Keren,1989). The steady-state IR values of all four systemswere in the range of 0.5 to 0.8 fim s-1 (1.8-2.9 mmh-') when the kinetic energy of the raindrops was 22.9kJ m'3. Similar steady-state IR values were obtainedby Kazman et al. (1983) for similar Na-Ca soils at thishigh kinetic energy. It seems, therefore, that a raindropwith kinetic energy lower than the value obtained forthe same raindrop at terminal velocity (Epema andRiezebos, 1983) is enough to disintegrate the aggre-gates of Na-Ca and Na-Mg soils.

The specific effect of Mg in the presence of Na ionscan be explained in a way similar to the explanationfor the Mg-Ca soil system. The electrical double-layerrepulsion and the Van der Waals attraction forces arethe major forces acting between two clay platelets atdistances where the hydration energy of the ions is nolonger important. Israelachvili and Adams (1978),however, observed an extra short-range repulsiveforce. This force is due to hydration of the counterions(Low, 1987; Pashley, 1981). Since the hydration num-ber of the Mg ion is ~50% greater than that of the Caion (Bockris and Reddy, 1970), it would be expectedthat a thicker diffuse layer associated with the clayplatelets in the Stern layer is present. Therefore, lessenergy would be required to break down the linkagesbetween the aggregates of Mg clay than of Ca clay.

At ESP 10, Ca and Mg adsorb not only on sites ex-hibiting a strong preference for bivalent cations, suchas those in the interior of clay tactoids, but also onexternal surfaces. The attraction forces between clayplatelets are high at relatively short distances from theclay surface, such as in the range of the Stern layer of

the diffuse double layer (Keren et al., 1988). Thus, thepresence of Mg ions on external surfaces may lowerthe association strength among soil aggregates due tothe wider hydration shell and enhance aggregates' dis-integration and clay dispersion. These mechanismslead to the formation of a soil surface seal due to move-ment of clay particles into a region of decreased po-rosity (Mclntyre, 1958).

Although exchangeable Mg (in the presence of Na)did not have a specific effect on the HC of a calcareoussoil (Alperovitch et al., 1981), the above results showthat Mg has a significant effect on IR of the soil con-taining CaCO3. During leaching of calcareous soil, theexistence of exchangeable Mg enhances dissolution ofCaCO3, and the increase in electrolyte concentrationin the soil solution prevents both clay dispersion andreduction in HC. This effect of CaCO3 can occur inthe soil profile. The electrolyte concentration in soilsolution at the soil surface, however, is determinedsolely by the electrolyte concentration in the appliedwater, since the contact time between an incrementalvolume of water and the surface area of CaCO3 is tooshort. Thus, the low electrolyte concentration at thesoil surface exposed to rainfall, coupled with the en-ergy impact of the water drops, render the Na-Mg soilsurface more susceptible to sealing and crust forma-tion than the Na-Ca soil, regardless of the presence ofCaC03.

ACKNOWLEDGMENTSThe author wishes to thank Mr. Y. Gian for his technical

assistance.

WORKMAN & LINDSAY: DIVALENT CADMIUM ACTIVITIES OBTAINED BY CHELATION 987