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

ABSTRACTThe effect of adsorbed Mg and Ca ions on the infiltration rate

(IR) of two montmorillonitic soils (Calcic Haploxeralf and TypicRhodoxeralf) at several kinetic energies of water drops was studied.The kinetic energy of the water drops, the type of adsorbed ion andthe electrolyte concentration in the applied solution all have a strongeffect on the IR of the soils saturated by Mg or Ca ions. The higherIR values of the soils exposed to electrolyte solution suggests that •clay dispersion takes place in Mg and Ca soils exposed to deionizedwater from simulated rainfall. The IR of the soils decreased withincreasing kinetic energy and the values were always lower for Mgsoils than for Ca soils. This difference was attributed to the higherwidth of the hydration shell of the adsorbed Mg than the Ca ion.Breakdown of the soil aggregates and clay dispersion are the twomechanisms that appear to be operative in seal formation when soilsare saturated by either Ca or Mg ions exposed to deionized water.Magnesium adsorbed by montmorillonitic soils has a specific effecton IR, whether the soils contain CaCO, or not.

MANY SOILS in arid and semiarid regions havehigh Mg contents. Conflicting opinions regard-

ing the importance of Mg in soils have previouslybeen mentioned. For example, the U.S. Salinity Lab-oratory Staff (1954) grouped Ca and Mg together intheir classifications, whereas Mg can cause the soilstructure to deteriorate and to develop a "magnesiumsolonetz" (Ellis and Caldwell, 1935). Laboratory stud-ies also imply that Mg is deleterious in some circum-stances. Clay minerals differ in their response to Mg.Montmorillonite and illite seem to be more easily dis-Institute of Soils and Water, Agric. Res. Organization, The VolcaniCenter, Bet Dagan, Israel. This research was supported by grant no.1-743-84 from BARD—The U.S.-Israel Binational Agric. Res. andDevelopment Fund. Contribution from the ARO, The Volcani Cen-ter, Bet Dagan, Israel, no. 2633-E 1989 series. Received 31 Mar.1989. 'Corresponding author.

Published in Soil Sci. Soc. Am. J. 53:1624-1628 (1989).

persed in the presence of Na and Mg compared withNa and Ca (Bakker and Emerson, 1973; Emerson andChi, 1977; Shainberg et al., 1988). McNeal et al. (1968)showed that mixed Na-Mg soils developed lower hy-draulic conductivity (HC) than Na-Ca soils undersimilar conditions. In more recent work, Rowell andShainberg (1979) showed that exchangeable Mg ap-peared to have no specific effect on soil HC of a sodicsandy loam soil dominated by mqntmorillpnite andkaolinite clay minerals leached with solutions withelectrolyte concentration >10 mmolc dm'3. However,there was evidence of a specific effect of Mg when thesoils were subsequently leached with distilled water(conditions favorable for clay dispersion). Magnesiumwas shown to have a specific effect on the HC of non-calcareous montmorillonitic soils. Alperovitch et al.(1981) found that noncalcareous, chemically stablesoils were more sensitive to exchangeable Mg than toCa, and showed a specific effect of exchangeable Mgon clay dispersion and losses in HC. The calcareoussoils, however, did not show a specific effect of Mg onclay dispersion and HC losses. In these soils, theexchangeable Mg enhances the dissolution of carbon-ates and increases electrolyte concentration, thus pre-venting clay dispersion and reduction in soil HC.

Whereas only a few reports on the effect of Mg onsoil permeability have been published, the effect of Mgon seal formation (a thin soil layer on the surface char-acterized by greater density, finer pores, and lower sat-urated HC than the underlying soil) and water infil-tration is not known. The formation of a seal at thesoil surface is a common feature of many soils, par-ticularly in the arid and semiarid regions. The reduc-tion in the HC of a soil seal controls the IR (Hilleland Gardner, 1970; Morin et al., 1989). Seal formationis a kinetic process that depends on the electrolyteconcentration in the applied water, the exchangeableion composition, and the disturbance caused by the

KEREN: WATER INFILTRATION IN CALCIUM AND MAGNESIUM SOILS 1625

applied water at the soil surface (Agassi et al., 1981;Mclntyre, 1958). Soil surfaces are particularly suscep-tible to the adsorbed ion composition because of themechanical action of the falling drops and the relativefreedom of soil-particle movement at the soil surface.

Magnesium is a more hydrated ion than Ca and,therefore, may cause a thicker diffuse layer (Shainbergand Kemper, 1966). Thus, it would be expected thatMg soil would deteriorate more than Ca soil whensubjected to the same low solution concentration.This hypothesis was tested in the present study. In theworks referred to above, Mg was shown to have aspecific effect on the HC of noncalcareous soils, butnot on calcareous soils. Since the IR is controlled bythe soil surface of a sealed soil, it is expected that theMg ions will exert a specific effect on the IR, despitethe presence of CaCO3 in the soil. This is expectedsince the contact time between an incremental volumeof water and the surface area of CaCO3 in the soilsurface is too short to elevate the electrolyte concen-tration by CaCO3 dissolution to a level that preventsseal formation. This hypothesis was considered aswell. The objective of this study was to determine theeffect of adsorbed Mg on the IR of calcareous andnoncalcareous soils at various impact energies of rain-drops.

MATERIALS AND METHODSSoil Preparation

Two soils from the coastal plain of Israel were used inthis study. The Calcic Haploxeralf (loamy soil) from NahalOz developed on loessial windblown material from the SinaiDesert and the Typic Rhodoxeralf (sandy loam soil) fromMorasha formed on coastal sand and calcareous sandstone.Some properties of both soils are presented in Table 1. Themain difference between the two soils is in their texture, andCaCO3 and sesquioxide contents.

The soils were packed 2 cm deep in 30- by 50-cm perfo-rated trays and placed in a rainfall simulator (Morin andBenyamini, 1977) over a layer of coarse sand at a slope of5%. The soils were leached with a 0.1 mol dnr3 solution ofeither CaCl2 or MgCl2, using a rainfall simulator with a rain-fall intensity of 10.4 fim s~'. In order to prevent seal for-mation during leaching, the rainfall was applied as a mistfor approximately 2 h (three leachings of 40 min each). Afterthe three leachings, the electrolytes in the soil solution wereremoved by one more leaching with deionized water andthe soils were air dried.

Rainfall Simulation ExperimentsTwo types of rainfall simulators were used. The first one

was a drip-type rainfall simulator with a closed water cham-ber (75 by 60 cm) that generates rainfall through a set ofhypodermic needles (more than 1000 spaced in a 2- by 2-cm grid) to form a fixed, known droplet size. The average

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

Soil property Calcic Haploxeralf Typic RhodoxeralfTexture, %

claysiltsand

PHCEC, mole kg-'CaC03, %

21.741.436.98.4

17.813.4

20.63.2

76.27.6

11.3traces

water-drop diameter was 2.97 ± 0.05 mm and the rainfallintensity of 9.2 pm s-'was controlled by a peristaltic pump.The kinetic energy of a raindrop was varied by changing theheight of fall of the droplets. The heights used in these ex-periments were 0.4, 1.0, and 1.6 m, resulting in a kineticenergy of the water drops of 3.2, 8.0, and 12.5. kJ m-3, re-spectively (Epema and Riezebos, 1983). The dimensions ofthe soil trays were 20 by 40 cm and three repetitions wererun simultaneously. The second rainfall simulator was anozzle type described by Morin and Benyamini (1977). Thisrainfall simulator was used for generating rainfall with ahigher kinetic energy. The main characteristics of the sim-ulated rainfall were as follows: median water-drop diameter2.3 mm; median water-drop velocity, 6.74 m s-'; the sum ofthe kinetic energy of the water drop, 22.9 kJ nr3; and arainfall intensity of 10.4 ̂ m s-'. The dimensions of a traywere 30 by 50 cm and four repetitions were run simulta-neously.

Soil aggregates <4 mm were packed in the trays in a layer2 cm thick over a layer of coarse sand. The trays were placedin the rainfall simulator at a slope of 15% and the soil wasfirst saturated from the bottom with tap water and then ex-posed to simulated rainfall consisting of deionized water(electrical conductance <0.01 dS nr') and to 25 mmoldnr3 MgCl2 and CaCl2 solutions for the Mg soils and Casoils, respectively. These latter soils were exposed to dropshaving a kinetic energy of 12.5 kJ m-3 only.

All soils were exposed to simulated rainfall until a steady-state IR was obtained. Standard operating conditions foreach rainfall event were to calibrate and to check the rainfallrate before and after the run. During each rainfall event,both infiltrated and runoff waters were collected separatelyin graduated cylinders and the water volume was recordedas a function of time.

ComputationsAnalysis of each data set was carried out separately with

nonlinear regression using the method proposed by Mar-quardt (1963) and computerized by Goodnight and Sail(1982). The infiltration equation was described by Morinand Benyamini (1977):

7ti = 7f [1]where 7ti is IR as a function of time, I, and 7f are the initialand steady-state IRs, respectively, y is a soil coefficient, p isthe rainfall intensity and /; is the time from the beginningof the storm. Values for the infiltration equation parameterswere obtained by nonlinear regression using PROC SY-SNLIN of the SAS package. The lines in the figures (exceptFig. 7) were calculated using this procedure, and the bars onthe dots (experimental results) indicate population standarddeviations as determined by the three or four replicationsfor each treatment.

RESULTS AND DISCUSSIONThe IR of the Ca and Mg Calcic Haploxeralf (CH)

soil as a function of the cumulative rainfall and kineticenergy of the water drops is given in Fig. 1 and 2,respectively. The results indicate that the kinetic en-ergy of the water drops and the type of adsorbed ionboth have a strong effect on the IR. In general, the IRof the soil decreased with the rise in kinetic energy ofthe water drops and the values were always lower forthe Mg soil than for the Ca soil. The decrease in IRat the initial stage of the rainstorm for both cationswas relatively moderate at the lower energy levels, butsteeper at the higher energies. These results also in-dicate that the cumulative water depth applied to

1626 SOIL SCI. SOC. AM. J., VOL. 53, NOVEMBER-DECEMBER 1989

reach a steady-state IR decreases with an increase inkinetic energy of the water drops. Whereas the IR ofthe soils decreased with the cumulative rain in thekinetic-energy range 3.2 to 22.9 kJ nr3, an insignificantchange in IR was measured when the kinetic energyof the water drops was very small (mist). This suggeststhat the kinetic energy of the water drops is an im-portant factor in seal formation of both Ca and Mgsoils.

The IR of the Ca and Mg Typic Rhodoxeralf (TR)as a function of cumulative rainfall and kinetic energyof the water drops is given in Fig. 3 and 4, respec-tively. The kinetic energy of the water drops had astrong effect on the IR of this soil, similar to that onthe CH soil. However, the values were lower thanthose found for the CH soil, for both ions at any givenenergy. Unlike the results with the CH soil, the changein IR with the amount of cumulative rainfall was notaffected by the kinetic energy of the water drops inthe higher range of energies (8.0-22.9 kJ m3). The IRvalues of the Mg-soil were always lower than those ofthe CA-soil, as was also true for the CH soil.

The sharp drop in IR with the increase in the cu-mulative rainfall and the low steady-state IR (Fig. 1-4) both indicate that seal formation is taking place on

the soil surface even when the soils are saturated byeither Ca or Mg ions. Seal formation could be ac-counted for by at least three possible mechanisms: (i)breakdown of soil aggregates by raindrop impact; (ii)compaction of the soil surface to form a thin soil filmon the surface, which restricts further entry of waterand movement of particles in the soil pores; and (iii)clay dispersion and movement of fine particles intothe upper layer (few millimeters) and deposition intosoil pores (Mclntyre, 1958). The potential of the soilclay to disperse depends on the adsorbed-ion com-position and the soil-solution concentration. Whenthe electrolyte concentration in soil solution at the soilsurface is sufficiently high (above the flocculationvalue), the tendency for the soil clay to disperse iszero.

To test the hypothesis that clay dispersion is one ofthe mechanisms that plays a role in seal formation ofCa and Mg soils, the Ca and Mg soils were exposedto rain drops of 25 mmol dnr3 CaCl2 and MgCl2 so-lution, respectively, at a kinetic energy of 12.5 kJ m~3

(Fig. 5 and 6). This concentration is well above theflocculation value of both Ca and Mg montmorillonite(Goldberg and Glaubig. 1987). The results showclearly that the IR increases in the presence of elec-

10.0 r CALCIC HAPLOXERALF, Ca-SOIL 10.0 r TYPIC RHODOXERALF, Ca-SOIL

0.025 0.050 0.075 0.100CUMULATIVE RAINFALL (m)

0.025 0.050 0.075 0.100CUMULATIVE RAINFALL (m)

0.125

Fig. 1. Effect of kinetic energy of distilled-water drops on the infil-tration rate of a Calcic Haploxeralf soil saturated with Ca ions.

Fig. 3. Effect of kinetic energy of distilled-water drops on the infil-tration rate of a Typic Rhodoxeralf soil saturated with Ca ions.

10.0r CALCIC HAPLOXERALF, Mg-SOIL

Kinetic energyMnrs

10.0TYPIC RHODOXERALF, Mg-SOIL

0.025 0.100 0.1250.050 0.075CUMULATIVE RAINFALL (m)

Fig. 2. Effect of kinetic energy of distilled-water drops on the infil-tration of a Calcic Haploxeralf soil saturated with Mg ions.

0.100 0.1250.025 0.050 0.075CUMULATIVE RAINFALL (m)

Fig. 4. Effect of kinetic energy of distilled-water drops on the infil-tration rate of a Typic Rhodoxeralf soil saturated with Mg ions.

KEREN: WATER INFILTRATION IN CALCIUM AND MAGNESIUM SOILS 1627

trolyte in solution in comparison with deionizedwater. The steady-state IR value of the Ca and Mg CHsoils increased from 1.94 and 1.05 /urn s-' to 4.60 and2.55 /urn s'1, respectively, when the electrolyte solu-tions were used (Fig. 5). Similar behavior was ob-served for the TR soil (Fig. 6). Since the kinetic energyof the raindrops and the rain intensity for both elec-trolyte solutions and deionized water were the same,the higher IR values of the soils exposed to electrolytesolution suggests that clay dispersion takes place inCa and Mg soils exposed to deionized water. It is alsoimportant to note that a significant reduction in IRdue to clay dispersion occurs only when the mechan-ical impact of the raindrops is above a certain value.The steady-state IR of the various systems exposed toelectrolyte solution at a concentration well above theflocculation value was lower than that obtained whenthe soils were exposed to mist (kinetic energy of 0.2 Jnr3, Figs. 1-4). The experimental results suggest thatall three mechanisms mentioned above occur in Caand Mg soils.

Based on the above possible mechanisms, the lowerIR values of the Mg soil, in comparison with those

10.0

7.5

3.UJ

O

cc.

u.z

CALCIC HAPLOXERALFKINETIC ENERGY I2.5 kj m''

AdsorbedIon

0.025 0.050 0.075CUMULATIVE RAINFALL (m)

0.100 0.125

Fig. 5. Effect of CaCl2 and MgCl2 solution on the infiltration rateof a Calcic Haploxeralf soil saturated with Ca and Mg ions. Thekinetic energy of the water drops was 12.5 kj m-3.

found for Ca soil, can be explained as follows. Theelectrical double-layer repulsion and the Van derWaals attraction forces are the two major forces actingbetween two clay platelets at distances where the hy-dration energy of the ions is no longer important. Is-raelachvili and Adams (1978), however, observed anadditional short-range repulsion force. This force isdue to the hydration of the counterions (Low, 1987;Pashley, 1981). It is expected, therefore, that an ad-ditional repulsive force would be required to removesome fraction of the hydration shell around the ad-sorbed ions, and thus to increase the degree of clayplatelets association. Since the hydration number ofthe Mg ion is approximately 50% greater than that ofthe Ca ion (Bockris and Reddy, 1970), it would beexpected that less energy would be required to breakdown the linkages among the aggregates of Mg tac-toids than of Ca tactoids. This is because of a thickerdiffuse layer associated with the clay platelets in theStern layer on the external surfaces of the tactoidswhen Mg is present. This hypothesis is supported bythe fact that Mg-soil aggregates are weaker than thosecontaining adsorbed Ca ions (Emerson and Smith,1970).

The steady-state IR value of the soils decreased withthe increase in kinetic energy of the water drops usingdistilled water and each system appeared to approacha constant minimum value (Fig. 7). The mechanicalimpact of the water drops on the steady-state IR ofthe soil was more pronounced in the lower-energyrange. The impact forces of raindrops falling on wet-ted soil aggregates can destroy the soil structure of thesurface almost completely (Moldenhauer and Kem-per, 1969). The results in Fig. 7 show that kinetic en-ergy lower than the value obtained for the terminalvelocity of the raindrops is enough to disintegrate theaggregates of the Ca- and Mg-CH soils and to form aseal. The stead-state IR value of the Ca soil at thehighest kinetic energy is very close to the value foundby Kazman et al. (1983) for a similar soil at exchange-able sodium percentage (ESP) 1.8. The lower steady-state IR values of the Mg soil may suggest that theaverage aggregate size of Mg soil in the seal is smallerthan that of Ca soil. This hypothesis is supported by

10.0r

~7.5(0

E

<5.0

g5

TYPIC RHODOXERALFKINETIC ENERGY I2.5 kj m''

IS.D.

0.025 0.050 0.075CUMULATIVE RAINFALL (m)

0.100

Fig. 6. Effect of CaCl2 and MgCl2 solution on the infiltration rateof a Typic Rhodoxeralf soil saturated with Ca and Mg ions. Thekinetic energy of the water drops was 12.5 kJ nr3.

~5.00pin

<3.75

2.50\LzUJI-1*1.25

Q

Calcic HaploxeralfTypic Rhodoxeralf

8 12 16KINETIC ENERGY (kj m-

20 24

Fig. 7. Effect of kinetic energy of the distilled-water drops on thesteady-state infiltration rate of Calcic Haploxeralf and Typic Rho-doxeralf soils saturated with Ca and Mg ions.

1628 SOIL SCI. SOC. AM. J., VOL. 53, NOVEMBER-DECEMBER 1989

the fact that the average number of platelets in a tac-toid of Mg montmorillonite is smaller than that of Camontmorillonite (Banin and Lahav, 1968).

The steady-state IR values of the TR soil, as a func-tion of the kinetic energy of the water drops, is alsogiven in Fig. 7. These results show that the values weresignificantly lower for the Mg soil than for the Ca soil,but the values for the two ions were relatively closerto each other at the low kinetic energy (3.2 kJ nr3)than at the higher range. A specific effect of Mg on IRwas observed in this soil, similar to the situation inthe CH soil. The steady-state values of the CH soildecreased with the kinetic energy in the entire energyrange, while the steady-state IR dropped sharply forthe TR soil to a low value and only a very smallchange (the change was even smaller for the Mg form)was observed for the kinetic energy range of 8.0 to22.9 kJ nr3. This suggests that the TR soil is moresusceptible to IR reduction than is the CH soil whenthe soils are saturated with either Ca or Mg ions. Thisgreater susceptibility exists despite the fact that thesesquioxide content in the former soil is higher.

Although exchangeable Mg (in the presence of Na)does not have a specific effect on the HC of a calcar-eous soil (Alperovitch et al., 1981), the results aboveshow that Mg has a significant effect on IR despite thefact that the CH soil contains CaCO3. During leachingof a calcareous soil, the existence of exchangeable Mgenhances dissolution of CaCO3, and the increase ofelectrolyte concentration in the soil solution preventsclay dispersion and reduction in the soil's HC. Thiseffect of CaCO3 is possible in the soil profile belowthe surface. The concentration of the soil solution atthe soil surface, however, is determined solely by theconcentration of electrolytes in the applied water,since the contact time between an incremental volumeof water and the surface area of CaCO3 is too short.Thus, the low electrolyte concentration in the soil sur-face exposed to rainfall, coupled with the energy im-pact of the water drops, renders the Mg soil surfacemore susceptible to sealing than it does the Ca soil,regardless of the presence of CaCO3.

CONCLUSIONThe results presented herein show that seal forma-

tion is taking place in the soil surface when soils, areexposed to simulated rainfall of distilled water, evenwhen the soils are saturated by either Ca or Mg ions.The IR of the soils decreased with the rise in kineticenergy of'the water drops and the values were lowerfor the Mg soils than for the Ca soils. Seal formationin Ca and Mg soils exposed to distilled water is dueto the breakdown of soil aggregates by raindrop im-pact, compaction of the soil surface, and clay disper-sion and movement of fine particles near the soil sur-face with their deposition into soil pores. A significantreduction in IR due to clay dispersion occurs onlywhen the mechanical impact energy of the water dropsis greater than 4 kJ nr3. Magnesium has a significanteffect on IR despite the fact that the soil containsCaCO3. The concentration of the soil solution at thesoil surface is determined solely by the concentrationof electrolyte in the applied water. Thus, the low elec-trolyte concentration near the soil surface, exposed to

rainfall and coupled with the energy impact of thewater drops, render the Mg soil surface more suscep-tible to sealing than it does the Ca soil, regardless ofthe presence of CaCO3.

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

assistance.