Effect of Polysaccharides, Clay Dispersion, and Impact Energy on Water InfiltrationM. Ben-Hur* and J. Letey

ABSTRACTEffects of clay dispersion, impact energy of water drops, and

chemical amendments on crust formation and infiltration rate (IR)of Haplic Durixeralf soil were studied using a sprinkler infiltro-meter. The soil was subjected to two impact energies, high energy= 240 J m 2 h ' and low energy = 0, and to three waters: (i) distilledwater [DW, electrical conductivity (EC) < 0.05 dS nr'], (ii) salinewater (SW, EC = 5.0 dS m~'), and (iii) regular water (RW, EC =1.0 dS m ')• The chemical amendments, polysaccharides, guar de-rivatives were supplied with the DW and RW and with high impactenergy. Crust formation by the impact energy was found as a dom-inant factor in the IR reduction. This factor reduced the total soilinfiltration capacity by more than 40%. Conversely, clay dispersionin the soil surface reduced the total soil infiltration capacity by 24%.Likewise, it was found that clay dispersion in the soil surface de-creased the hydraulic conductivity of the crust and sharply increasedits hydraulic resistance. The polymers had an amendatory effect onthe IR. The polymers apparently adsorbed on the particle surfacesand acted as a cementing material holding primary particles togetheragainst the destructive forces of the water drops. The order of themaximum effect of the chemical amendments (0.01 kg m'3), thatwere supplied in DW, on the maintenance of IR was: high chargecationic polymer (HCCP) > low charge cationic polymer (LCCP)» nonionic polymer > anionic polymer (had no effect). A concen-tration of 0.01 kg m-3 of HCCP and LCCP in RW prevented crustformation and preserved the high initial IR of the soil throughoutthe water application period. The HCCP and LCCP under sprinkledDW and RW conditions apparently adsorbed at the soil surface anddid not move with the water into the soil layer.

Low WATER INFILTRATION resulting in runoff, ero-sion, inefficient water use, and plant injury due

to water ponding is a significant problem in some ir-rigated lands. For example, the infiltration rates of over1 million ha of irrigated land on the east side of theSan Joaquin Valley of California are reduced duringthe irrigation season to rates as low as 1 mm h-1 (Os-ter and Singer, 1984). Low infiltration rates are par-ticularly acute for irrigation by systems that have ahigh instantaneous rate of water application like amoving sprinkler (Gilley and Mielke, 1980).

The reduction of the infiltration rate is caused mainlyby the formation of crust on the soil surface and/orby the reduction of the hydraulic conductivity of thebulk soil (Ben-Hur et al., 1987; Shainberg and Letey,1984).

Surface crusts are thin and characterized by greaterdensity, higher strength, finer pores, and lower satu-rated conductivity than the underlying soil (Gal et al.,1984; Mclntyre, 1958). Impact energy of the waterdrops and water surface stream break down the sur-face aggregates, compact the upper soil layer, and formthe crust (Morin and Benyamini, 1977). In additionDep. of Soil and Environmental Sciences, Univ. of California, Riv-erside, CA 92521. Research was supported by the Univ. of Califor-nia Kearney Foundation of Soil Science. Received 1 Apr. 1988.'Corresponding author and visiting soil scientist from AgriculturalResearch Organization (ARO), Volcani Center, Israel.

Published in Soil Sci. Soc. Am. J. 53:233-238 (1989).

to physical breakdown of the soil aggregates, physical-chemical dispersion of soil clays can cause clogging ofthe pores immediately beneath the surface, which isfrequently referred to as the "washed in" zone (Agassiet al., 1981; Kazman et al., 1983). On the other hand,swelling and dispersion of clay from aggregates thatmigrate and lodge in pore spaces greatly reduce thehydraulic conductivity of the bulk soil (McNeal andColeman, 1966; Park and O'Connor, 1980; Shainberget al., 1981a,b). Felhendler et al. (1974) found thatwater with intermediate sodium adsorption ratio(SAR) values of 5 to 10 and a low solution electrolyteconcentration caused soil clay particle dispersion andreduced the hydraulic conductivity to near zero.

Treatment of soils with chemical amendments toimprove or maintain soil structure and aggregate sta-bility may be one means of maintaining high waterinfiltration. Polymers have been shown to be effectivein increasing hydraulic conductivity and porosity, im-proving water-holding capacity (Shanmuganathan andOades, 1982), and reducing erosion and weakeningcrust strength (Wood and Oster, 1985). The effects ofpolymers as soil conditioners were reviewed by Harriset al. (1966).

However, most of the initial studies with polymersdealt with applying copious amounts of polymers eitherdry or by spraying and then mixing to create soil ag-gregates. Consequently, the polymers used for agri-culture were too expensive and not economically fea-sible. Therefore, it is important to study potentiallyactive polymers that can be easily applied (such aswith irrigation water) at relatively low amounts andyet have a significant positive effect on the soil phys-ical properties.

The objectives of this study were: (i) to study theeffects of clay dispersion and impact energy of the waterdrops on crust formation and water infiltration; (ii) todetermine the effect of low concentration of some typesof guar derivatives in irrigation water on the infiltra-tion rates; and (iii) to study the interaction betweenthe polymers and water quality in crust formation.

MATERIALS AND METHODSThe <4-mm fraction of a Haplic Durixeralf soil classified

as Arlington sandy loam was used in this study. The soilwas collected from the upper layer (0-20 cm) in RiversideCounty, CA, where the average annual precipitation is ~250mm. Some properties of this soil are given in Table 1. Themineralogical composition of the soil was determined byFrenkel et al. (1978).

Soil was packed 2-cm deep at a 1.47 Mg m~3 bulk densityin 12- by 20-cm perforated trays over two fiberglass sheetsplaced on the tray bottom. Trays were placed in a sprinklerinfiltrometer (SI) at a slope of 5% with four replicates foreach treatment. The SI was 78 by 78 cm and 82-cm high. Ithad a rotating disk at the top with speed of 7 rpm. Therewere 10 slots in the disk and each slot contained a unitholding four hypodermic needles. The unit holding theneedles moved in and put as the disk rotated so that dropsdid not continuously hit the same spot. A diagram of the SIand trays is presented in Helalia and Letey (1988b). The

233

234 SOIL SCI. SOC. AM. J., VOL. 53, JANUARY-FEBRUARY 1989

Table 1. Some physical and chemical properties of Arlington sandyloam (Riverside County, CA).

Mechanicalcomposition Cation

Sand Silt

58.6 31.8

Clay capacity

cmolc kg" '9.5 18.0

Mineralogicalcomposition!

ESP CaCO3 M V Q+F

2.0 t4 tr 72 14

K

14

f Composition of clay fraction where the following minerals are identified bythe symbols: M = montmorillonite, Q = quartz, V = vermiculite, F =feldspar, and K = kaolinite.

|tr = trace amount, < 0.1%.

mechanical parameters of the applied sprinkler water were:instantaneous application rate of 30 mm h~', water dropaverage diameter of 3.5 mm, water drop velocity of 4.0 ms~', and total kinetic energy of 240 J m~2 h~'.

The soil was first saturated from the bottom with tap water[electrical conductivity (EC) = 0.7 dS m~'] and then re-ceived 50 mm of various treated waters by the SI. The vol-umes of water percolating through the soil were recorded foreach 2.5 mm of water application to compute infiltrationrate. The average clay concentration in the total collectedeffluent was determined by gravimetric procedure.

Three waters were synthesized and applied in the sprin-kler infiltrometer: (i) distilled water (DW) with EC < 0.05dS m~' that represented snow water; (ii) saline water (SW)with EC of 5.0 dS m~' that represented saline irrigation water;(iii) regular water (RW) with EC of 1.0 dS m"' that repre-sented the common water in arid and semiarid regions. TheSAR of the RW and SW was 2.0, and were prepared bymixing NaCl and CaCl2 in appropriate amounts. The DWand the SW were applied with high and low impact energyof water drops (240 J rrr2 h~' and 0). The low impact energywas obtained by placing two fiberglass sheets over the soilsurface at a 0.5-cm height. The RW and following polymerstreatments were carried out under high impact energy con-ditions only.

The amendatory compounds tested were derivatized guarprovided by Celanese Corp. (Louisville, KY). The molecularweight of the compounds is relatively low (200 000-2 mil-lion) and they are soluble in water. The polymer compoundswere either nonionic, anionic, or cationic with differing chargedensity. The type and density of the charge of the com-pounds were determined by the types and the amounts ofthe substitutional groups. Schematic structure of the com-pounds regarding their charge is given in Fig. 1.

A source solution of 0.5 kg m~3 of each polymer was pre-pared in DW and appropriate amounts of these solutionswere added to the appropriate water to be tested by the ISto form concentrations of 0.0025, 0.005, 0.01, and 0.02 kgm-3.

RESULTS AND DISCUSSIONThe best fit curves obtained from least squares anal-

ysis of the infiltration rate (IR) values as a functionof water application depth for DW and SW for bothcovered and uncovered soil and the R2 of the regres-sion are presented in Fig. 2 and Table 2, respectively.The IR of the covered soil that was subjected to SWwas maintained at the initial infiltration value duringthe entire run. Coverings and saline water have beenshown to prevent clay dispersion (Shainberg and Le-tey, 1984) and the physical breakdown of the soil ag-gregates (Morin and Benyamini, 1977). Consequently,in this treatment a crust was not formed, the hydraulicconductivity of the soil layer was not likely reducedthus maintaining the IR. When the covered soil was

CH20(R)nH-o

CHgOH= CH2CH(CH3)0

H(R),H H H H

HYDROXYPROPYL GUAR (NONIONIC)

CHpOCHgCOO'Na* CHgOHR=CH2CH(CH3)0

H H H H H / HCH2COO~Na+

ANIONIC GUAR

H H

CHgOH

Q = Quaternaryammonium group

H H H H H ' HCATIONIC GUAR

Fig. 1. Schematic structure of the polymers studied.

subjected to DW, the IR decreased with increasingamount of the water application. Likewise, the aver-age value of the clay concentration in the total col-lected effluent was 0.29 mg mL~' in this treatmentcompared to 0 in the SW treatment (Table 2). Eventhough the covering prevented crust formation fromimpact (a visual observation after the run indicatedthat the surface aggregates did not break down), leach-ing the soil layer with DW caused clay dispersion, mi-gration, and relodgment in the pore spaces that re-duced the hydraulic conductivity, and consequentlydecreased the IR and increased the clay concentrationin the effluent.

In the uncovered soils, there was a sharp decreasein IR with the amount of both the SW and DW ap-plications, to final values of 14.6 and 4.0 mm h"1,respectively (Fig. 2 and Table 2). The decrease of IR

BEN-HUR & LETEY: POLYSACCHARIDES, CLAY DISPERSION, IMPACT ENERGY AND WATER INFILTRATION 235

32

eE 24

<(X.

z2 16

- 8

SW

sw

COVERED SOIL——— UNCOVERED SOIL DW

10 20 30 40CUM. WATER APPLICATION, mm

50

Fig. 2. Infiltration rate as a function of cumulative (cum.) waterapplication for two water qualities (SW = saline water, DW =distilled water) and covered and uncovered soil.

with SW was due to crust formation from drop impactand the decrease in IR with DW was due to both crustformation from drop impact and clay dispersion. WhenDW was applied, physical disintegration of the surfacesoil aggregates was the first step followed by chemicaldispersion of the clay particles in the soil surface. Thedispersed clay migrated with the infiltrating water andclogged the pores immediately beneath the surface.Hence, a greater density and thicker crust was formed(Gal et al, 1984; Mclntyre, 1958) and the IR in thistreatment dropped to 4.0 mm h~'.

In order to determine the relative quantity effect(RQE) of the different factors that are responsible forIR reduction, the areas below the IR curves in Fig. 2were measured, and the RQE were calculated as fol-lows: (i) the effect of the clay dispersion in the bulksoil (uncrusted soil) is equal to the difference betweenthe covered SW and DW curves; (ii) the effect of thewater drop impact energy is equal to the differencebetween uncovered and covered soil curves for SW;(iii) the effect of the chemical clay dispersion in thesoil surface during impact energy is equal to the dif-ference between SW and DW uncovered soil curves.To get a relative value all the area differences weredivided by the area below the SW covered soil curve.This area represents the total soil infiltration capacitywithout any change of the soil structure. The RQEvalues of the vermiculite soil (Durixeralf) and mont-morillonite soil (Calcic Haploxeralf) with differentexchangeable sodium percentage (ESP) values are rep-resented in Fig. 3. The RQE values of the montmo-rillonitic soil were calculated from data presented byAgassi et al. (1985).

It is evident from Fig. 3 that crust formation by theimpact energy of the water drops is the dominant fac-tor in the reduction of the IR in both soils and eachESP value. This factor reduced the total soil infiltra-tion capacity of the montmorillonitic soil with ESP6.5 and 21 by 62 and 67%, respectively, and of theyermiculitic soil by 38%. The large value of the RQEin the montmorillonitic soil was apparently due to the

Table 2. Final infiltration rates, clay concentrations in the effluentand their standard deviation (SD), and the R2 of the regressionfor different treatments.

Treatment

Final Effluent clayinfiltration rate concentration

Mean SD Mean SDR2 of theregression

— mm h~' — — mg mL~' —

SW, CSfDW, CSsw, ustDW, USRW, USDW, HCCP 2.5 g m-3, USDW, HCCP 5.0 g m-3, USDW, HCCP 10 g m-3, USDW, LCCP 5.0 g m-3, USDW, LCCP 10 g m-3, USDW, nonionic 10 g m~3, USDW, anionic 10 g m~3, USRW, HCCP 10 g m~3, USRW, LCCP 10 g m-3, US

30.019.014.64.0

12.06.0

10.419.07.4

14.25.24.0

30.030.0

3.42.82.40.31.82.32.43.00.91.91.00.43.22.9

0.000.290.00NDt0.00NDND0.30ND0.36NDND0.000.00

0.000.030.00

—0.00

——

0.09—

0.04——

0.000.00

_0.930.920.900.900.910.940.930.950.920.940.94_-

f Covered soil (CS), uncovered soil (US).| ND = not determined.

WinV) "-0

0.8

UJ

OUJ

0.2

CLAY DISPERSION IN THE SOIL LAYER

I I IMPACT ENERGY OF THE WATER DROPS

Fllil CLAY DISPERSION IN THE SOIL SURFACE

VERMICULITIC MONTMORILLONITIC MONTMORILLONITICSOIL SOIL SOIL

ESP = 2 ESP = 6.5 ESP = 21Fig. 3. Relative quantity effect values of vermiculitic soil and mont-

morillonitic soil with different ESP values.

weaker structure of the montmorillonitic soil thanvermiculitic soil, and/or probably, since the more clayavailable to form a crust in the Calcic Haploxeralf soil(21.%) than in the Durixeralf soil (9.5%) (Ben-Hur etal., 1985).

The effect of chemical dispersion of the clay parti-cles at the soil surface during impact energy was rel-atively low and was not affected by the soil ESP athigh ESP values (>6.5) and not by the clay type atlow ESP values (<6.5). Apparently, it is because thesoil surface, under impact energy, is sensitive to claydispersion, and most of clay dispersion occurs at thelow ESP (<5.0) (Kazman et al., 1983). The RQE val-ues of this factor were about 24% (Fig. 3). It is im-portant to note that the chemical clay dispersion hasan additive effect to the physical disintegration in thereduction of the IR. The crust formation by the phys-ical disintegration and chemical dispersion togetherreduced the total infiltration capacity by 61, 80, and91% for the vermiculitic soil and the montmorilloniticsoil with ESP 6.5 and 21, respectively (Fig. 3).

236 SOIL SCI. SOC. AM. J., VOL. 53, JANUARY-FEBRUARY 1989

Table 3. The hydraulic conductivity and the hydraulic resistance ofthe crust for vermiculitic (ver.) and montmorillonitic (mon.) soils.

Treatment

Ver. soil, ESP 2, SWVer. soil, ESP 2, DWMon.f soil, ESP 2.5, SWMon. soil, ESP 2.5, DWMon. soil, ESP 6.5, SWMon. soil, ESP 6.5, DWMon. soil, ESP 21, SWMon. soil, ESP 21, DW

Hydraulicconductivity

Crust layer(*,)

2.60.491.060.530.780.150.660.096

Sublayer(jy

h-' ———

30194443.64428.843.911.5

Hydraulicresistance of the

crust (LJK,)

——— h ———

0.73.951.83.72.53

13.62.99

20.5

t The values for montmorillonitic soil were calculated from data that werepresented by Agassi et al. (1985).

The effect of the clay dispersion in the bulk soil onthe IR was low (RQE ~25%) and it was similar to theeffect of the chemical clay dispersion in the soil surfacefor the montmorillonitic and vermiculitic soils withlow ESP (<6.5, Fig. 3). Whereas, at ESP 21 the effectof the clay dispersion in the soil layer was great, RQE= 58%.

Crust formation creates a two-layer system for waterflow. The effective hydraulic conductivity (Ke) of thetwo-layer system is related to the K values of the in-dividual layers by

K, = (L, + L2)/(Ll/Kl + L2/K2) [1]where L is the individual layer thickness and the sub-scripts 1 and 2 refer to the crust and underlying layer,respectively. Under steady state conditions the mea-sured IR equals Ke

The covered condition prevented crust formationso that the steady state IR equals the K value for thesoil without crust and this value was assumed to equalK2 for the crusted condition. The crust thickness (Li)was assumed to be 2 mm to calculate A\. Since the KIvalue was sensitive to the assumed value of L{, theratio Li/K, that represents the hydraulic resistance ofthe crust (Hillel, 1971) was also computed. Inasmuchas L2 » LI, L2 is not greatly affected by the value ofLL

The Kt and the hydraulic resistance of the crust val-ues for vermiculitic soil and montmorillonitic soil withdifferent ESP values are presented in Table 3. Thephysical disintegration of the surface soil aggregatesand the compaction of the upper layer by the impactenergy of the water drops, reflected by the uncovered,SW treatment appear to be a dominant factor in thereduction of the K of the crust, similar to IR values(Fig. 2). For vermiculitic soil, the AT of the crust underthese conditions is 2.6 vs. 30 mm h~' of underlyingsoil, and for montmorillonitic soil with different ESPvalues the K of the crust are 1.06 to 0.66 mm h~' incomparison with 44 mm h~' of the underlying soil(Table 3). Likewise, the chemical dispersion of the clayparticles in the soil surface decreased the AT of the crustand sharply increased the hydraulic resistance of thecrust. This effect was correlated with the soil ESP, thehigher ESP the higher hydraulic resistance of the crustand lower K of the crust.

The effects of the polymer type and concentration

40 500 10 20 30 40 50Fig. 4. Infiltration rate as a function of cumulative (cum.) water

application for four polymer types and their different concentra-tions in the water applied (for anionic polymer the control and10 g m~3 polymer treatments have the same line).

in DW on IR as a function of the water applicationdepth for uncovered soil are presented in Fig. 4. Thelines represented best fit curves obtained from leastsquares analysis, and the R2 of the regression are pre-sented in Table 2.

The following characteristics are noted:1. All the polymers except the anionic polymer had

an amendatory effect. The IR values of the poly-mer treatments, at any given water applicationdepth, were greater than the IR values of the un-treated soil (Fig. 4). This amendatory effect in-creased as concentration of the polymers in theapplied water increased up to 10 g m~3. For ex-ample, the final IR values of the soil that wasexposed to 2.5, 5, and 10 g m~3 of high chargecationic polymer (HCCP) were 6.0,10.4, and 19.0mm h~', respectively, compared to 4.0 mm h"1

for the untreated soil (Table 2).The polymers were apparently adsorbed on the

particles at the soil surface and acted as acementing material holding primary particles to-gether against the destructive forces of the waterdrops. Thus, the destruction of the surface soilstructure diminished and a high IR was main-tained.

2. The type of polymer had a significant effect onthe IR of the soil under sprinkler conditions. Theorder of beneficial effect of the polymers on themaintenance of IR was HCCP > low charge cat-ionic polymer (LCCP) » nonionic polymer >anionic polymer (had no effect), and the corre-sponding final IR values were 19.0, 14.2, 5.2, and4 mm h~', respectively, compared to 4.0 mm h"1

of the untreated soil (Table 2). These results sug-gest that the electrostatic adsorption of the poly-mers on the negative clay surface was the dom-inant factor in providing stability to theaggregates.

Helalia and Letey (1988a) studied the effect ofthe same derivatized guar compounds on theflocculation of Arlington soil in a suspension of

BEN-HUR & LETEY: POLYSACCHARIDES, CLAY DISPERSION, IMPACT ENERGY AND WATER INFILTRATION 237

0.5 kg soil per 25-L solution. They found that allthe compounds had a large effect on the clay floc-culation, and the effectiveness of the differentpolymer types were similar. The sorption mech-anisms of the nonionic and anionic polymerssuch as "polyvalent ion bridge" and dipole bondsrepresent weak attractive forces that can affectclay flocculation when the system is at rest asmeasured by Helalia and Letey (1988a). How-ever, when there is a physical interference, likethe impact energy of the water drops, these at-tractive forces are not strong enough to preventthe destruction of the aggregates to prevent crustformation.

3. The IR values of the soil that was exposed to0.01 kg m~3 of HCCP as a function of the waterapplication (Fig. 4) were similar to the IR valuesof DW, covered soil treatment (Fig. 2). Likewise,high clay concentration was found in the effluentwater from this treatment (Table 2). It can beconcluded from these results that 0.01 kg m~3 ofHCCP prevented physical destruction of the soilaggregates by impact energy, chemical dispersionof the clay at the soil surface, and crust forma-tion, but did not prevent clay dispersion in thesoil layer and the reduction of the hydraulic con-ductivity of the bulk soil. The apparent high ad-sorption of HCCP at the soil surface preventedits movement with water into the soil layer. Con-sequently, the chemical dispersion of the clay inthe soil layer was similar to the untreated soil.

Conversely, the IR values of the 0.01 kg m"3 LCCPtreatment (Fig. 4) were similar to the SW, uncoveredsoil treatment (Fig. 2). The low charge of the polymer,and consequently its low adsorption on the clay sur-face, did not prevent the crust formation, but the de-struction and the dispersion of the aggregates at thesoil surface were limited and the IR values maintainedhigh, 14.2 mm h~' (Table 2). However, this polymer,like the HCCP, adsorbed on the soil surface and didnot prevent the dispersion of the clay in the soil layer(Table 2).

The IR values of the exposed soil as a function ofRW (EC = 1.0 dS m-1) application depth for 0 and0.01 kg m~3 of HCCP and LCCP in the water applied,and the R2 of the lines fit are represented in Fig. 5 andTable 2, respectively. In the control treatment (poly-mer amount = 0) the IR decreased with increasingamount of water applied until a final IR was obtained.This decrease was caused by the formation of a cruston the soil surface. However, at any given water ap-plied depth, the IR values in this treatment were greaterthan the IR values in DW treatment but lower thanthe IR values in the SW treatment (Fig. 2). The finalIR values for the DW, RW, and SW treatments were4, 12, and 14.6 mm h~', respectively (Table 2). It isevident from these results that a chemical dispersionof the clay in the soil surface had occurred but to alesser extent than in DW treatment. Similar resultswere obtained for a montmorillonitic soil (Agassi etal., 1981).

Conversely, in the HCCP and LCCP treatments theinitial IR was maintained throughout the run (Fig. 5).The HCCP prevented the crust formation, like in DW

32

24

<cc

I 16\-<a:H_l

\

\

— — CONTROL

——— HCCP, LCCP I0gr/m3

10 20 30 40 50

CUM. WATER APPLICATION, mm

Fig. 5. Infiltration rate as a function of regular water application fortwo cationic polymer types (HCCP = high charge cationic poly-mer, LCCP = low charge cationic polymer).

treatment (Fig. 4), and the electrolyte concentrationin the RW prevented the chemical clay dispersion inthe soil layer (the effluent clay concentration = 0, Ta-ble 2). These two factors prevented the IR decrease.However, comparison of the curve of treatments DW,0.01 kg m-3 LCCP (Fig. 4), to RW, 0.01 kg nr3 LCCP(Fig. 5), indicates that the electrolyte concentration inthe RW increased the effectiveness of the LCCP inpreventing crust formation.

Concentration of 0.01 kg m~3 in the irrigation waterduring the entire season that receives 500 mm of water,which is common for many crops in arid and semiaridregions, is equal to 50 kg ha~'. The amounts suggestedby many studies for other polymer types to maintainsoil structure are 1000 to 4000 kg ha"1 (Harris et al.,1966; Mitchell, 1986). Continued application of apolymer throughput the irrigation season may not benecessary to maintain soil stability and this factorwould reduce the amount of required polymer.

The high efficiency of the HCCP and LCCP at lowrates in prevention of the crust formation and the re-duction of the IR on the one hand, and the convenientapplication of the polymers in the irrigation water, onthe other hand, give them potential commercial use.

238 SOIL SCI. SOC. AM. J., VOL. 53, JANUARY-FEBRUARY 1989