Polymers' Effects on Infiltration and Soil Erosion during ConsecutiveSimulated Sprinkler Irrigations

G. J. Levy,* J. Levin, M. Gal, M. Ben-Hur, and I. Shainberg

ABSTRACTImpact energy of water drops from overhead sprinkler irrigation

can cause seal formation, and an increase in runoff and in soil erosion.The effects of low concentrations (5, 10, and 20 g m~3) of two poly-mers, an anionic polyacrylamide (PAM) and a cationic polysaccharide(PSD), on soil permeability and erosion from a grumusol (Typic Cbro-moxerert) and a loess (Typic Haploxeralf), were studied during fiveconsecutive irrigations of 60 mm each. The polymers were added tothe irrigation water during the first three consecutive irrigations, andthereafter the soils were subjected to two additional irrigations ofwater only. During the first three irrigations, the final infiltrationrates (FIR) of the soils were significantly higher than those of theuntreated samples (control). In the subsequent two irrigations withwater only, the FIR values of the treated samples decreased to valuessimilar to those of the control. The low residual effect of the polymerswas explained by erosion of the thin treated layer and an insufficientamount of the polymers. A lower concentration of PAM (10 g m~3)was needed for optimal effect on the FIR and cumulative infiltration,compared with PSD (20 g m~3). For the optimal treatments, infiltra-tion parameters were generally higher in the PAM- than in the PSD-treated soils. Soil losses in all the PAM treatments were significantlylower than those in the PSD treatments. Both polymers stabilized soilaggregates, but PAM also cemented aggregates together and increasedtheir resistance to erosion.

LOW WATER infiltration leading to runoff, erosion,and inefficient water use is a critical problem in

many soils from the arid and semiarid regions (Ben-Hur et al., 1989b; Carter, 1990; Oster and Singer,1984). A reduction in the soil IR is caused mainly byseal formation at soil surfaces exposed to the beatingaction of rain and sprinkler drops (Aarstad and Miller,1973; McIntyre, 1958). Surface seals are thin (<2-3mm) layers characterized by greater density, highershear strength, and lower saturated conductivity than

Institute of Soils and Water, Agricultural Research Organization,The Volcani Center, P.O. Box 6, Bet Dagan, Israel. Contributionfrom the Agricultural Research Organization, The Volcani Center,no 3237-E, 1991 series. Received 26 Apr. 1991. *Correspondingauthor.

Published in Soil Sci. Soc. Am. J. 56:902-907 (1992).

the underlying soil. Seal formation in soils exposedto drop impact is due to two mechanisms (Agassi etal., 1981; McIntyre, 1958): (i) physical disintegrationof soil aggregates and their compaction; and (ii) aphysicochemical dispersion and movement of clayparticles into a region of 0.1- to 0.5-mm depth, wherethey lodge and clog the conducting pores. The twomechanisms act simultaneously, as the first enhancesthe latter.

Interrill soil erosion is a function of soil detachmentby raindrop impact and water-transport capacity dur-ing thin sheet flow (Meyer et al., 1975). However, itis not clear how the development of the seal affectsinterrill soil erosion. Seal formation increases the shearstrength of the soil surface (Bradford et al., 1987),which reduces soil detachment (Moore and Singer,1990). This layer also decreases water infiltration andhence increases runoff, which in turn increases thetransport capacity for entrained material (Moore andSinger, 1990), and may initiate rill formation in ero-dible soils (Meyer and Harmon, 1989). Moore andSinger (1990) noted that the rate of soil erosion de-creased with seal development, indicating an apparentdecrease in soil credibility.

Preventing seal formation and reducing runoff and,possibly, soil erosion could be achieved by improvingthe structure and aggregate stability of the soil. Thepossibility of using organic polymers as soil condi-tioners and stabilizers was already explored in the early1950s (Allison, 1952; Weeks and Colter, 1952).However, most of the early studies used high rates ofpolymers, which were economically unfeasible for fieldagriculture. The use of small rates of organic polymers(20-40 kg ha-1), mainly PAM and PSD, for improv-ing the structure and permeability of the soil surfacehas been studied recently (Ben-Hur et al., 1989a; He-lalia and Letey, 1988; Shainberg et al., 1990; Smithet al., 1990). It was further reported that, in the caseof anionic PAM, the presence of electrolytes that as-Abbreviations: PAM, polyacrylarnide; PSD, polysaccharide; FIR,final infiltration rate; IR, infiltration rate; TW, tap water; SAR,sodium adsorption ratio; GIF, cumulative infiltration.

LEVY ET AL.: POLYMERS' EFFECTS ON INFILTRATION AND EROSION 903

sured the flocculation of the soil clay was essentialfor the effectiveness of PAM application (Shainberget al., 1990; Smith et al., 1990). With respect to PSDs,Ben Hur et al. (1989a) reported that both high- andlow-charge PSD were effective in maintaining highIR values. The PSDs tested were, however, relativelyineffective during subsequent water applications.

To date, studies on PAM and PSD efficiency as soilconditioners examined either polymer application byspraying it directly onto the soil surface (Azzam, 1980;Wallace and Wallace, 1986; Shainberg et al., 1990)or polymer addition to irrigation water of good quality(i.e., distilled water) (Ben-Hur et al., 1989a). Theseaspects are not applicable to common irrigation prac-tices. Our objectives were to study: (i) the effective-ness of PAM and PDS on both infiltration and erosionwhen applied via irrigation water, during consecutiveirrigations; and (ii) the long-term effect of the poly-mers during the irrigation season.

MATERIALS AND METHODSSamples from the surface 250 mm of two cultivated soils

were used for this study: a typic loess (Calcic Haploxeralf)from Bet Qama and a dark brown grumusol (Typic Chro-moxerert) from Negba, Israel. Some physical and chemicalproperties of the soils are given in Table 1.

Infiltration, runoff, and erosion were studied using a drip-type rainfall simulator (Smith et al., 1990). Briefly, thesimulator consisted of a 750 by 600 by 80 mm closed waterchamber that generated rainfall of a known constant dropsize through a set of hypodermic needles (approximately1000) arranged at spacings of 20 by 20 mm. The averagedroplet diameter was 2.97 ± 0.05 mm. A height of dropfall of 1.6 m was used to obtain drops with an impactvelocity of 4.98 m s-1 and a kinetic energy of 12.4 J m-2

mm-1 of rain (Epema and Riezebos, 1983). Applicationintensity was maintained at 31 mm h-1 using a peristalticpump.

Air-dried aggregates, crushed to pass through a 4.0-mmsieve, were packed in 200 by 400 mm trays, 20 mm deep,over a 5-mm-thick layer of coarse sand. The trays wereplaced in the simulator at a slope of 15%, saturated fromunderneath with TW, and exposed to an irrigation event of60 mm. During each event, the volume of water percolatingthrough the soil was collected and recorded as a functionof time. Runoff water was collected continuously through-out the irrigation event, after which the runoff water wasmixed thoroughly and a 0.25-L subsample taken. The sub-sample was placed on a water bath to dry, the weight ofthe eroded material determined, and total soil loss from theentire event calculated. Splash from the soil trays was notmeasured; however, it was found that soil carried by splashwas positively correlated with the soil removed by the run-off water (Young and Wiersma, 1973). Hence, soil lossfrom runoff water can serve as an indication of soil de-tachment.

The treatments studied included a control, and PAM andPSD treatments. In the control treatment, the soil was ex-posed to three consecutive irrigations, 60 mm each, usingTW with an electrical conductivity of 0.8 dS m-1 and SARof 2. In the PAM treatments, a PAM with an anionic lowcharge (20% hydrolysis) and a high molecular weight (=107

g mol-1) was used. In the PSD treatments, a cationic low-charge guar derivative with a molecular weight of 0.2 to 2x 106 g mol-1 was used. The soil in the boxes was sub-

jected to three consecutive irrigations (60 mm each) of TWto which PAM or PSD was added to obtain a concentrationof 5, 10, or 20 g m-3. Thereafter, the soil was exposed toan additional two consecutive rains (60 mm each) of TWonly. For all treatments, the soils was allowed to dry in anoven at 40 °C for 4 d between consecutive rains. Threereplicates were used concurrently for each treatment.

The infiltration data obtained from the rainfall simulatorwere analyzed using a nonlinear equation proposed by Morinand Benyamini (1977). A nonlinear regression program cal-culated the parameters of the equation that gave the bestcoefficient of determination (R2) between paired calculatedand measured IR values. The total depth of water that pen-etrated the soil during a single irrigation, designated GIF,was calculated using the instantaneous infiltration rate de-rived from Morin and Benyamini's equation (Morin andBenyamini, 1977). Significance of difference among treat-ments for the infiltration and erosion parameters was de-termined using Tukey's procedure for a multiple-range test(Steel and Torrie, 1960).

RESULTS AND DISCUSSIONInfiltration Parameters

Two parameters were used to describe the changesin the permeability of the soil when exposed to over-head sprinkler irrigation: (i) the measured IR at theend of each irrigation event, referred to as the FIRand (ii) the calculated GIF. The FIR values for thevarious treatments and soils obtained at the end of: (i)the first irrigation (60 mm of water), (ii) the thirdirrigation, which was the last one with polymers addedto the irrigation water (cumulative irrigation depth of180 mm), and (iii) the fifth (i.e., last) irrigation, whereonly TW was applied for the second consecutive time(cumulative irrigation depth of 300 mm), are pre-sented in Fig. 1. Results of a multifactor analysis ofvariance indicated significant interactions between typeof polymer, polymer concentration, and depth of ir-rigation (Table 2). Consequently, a multiple-range testwas used to determine differences among FIR valuesof individual treatments within each soil (Fig. 1).

When polymers were added to the irrigation water,the FIR of the treated Chromoxerert was significantlyhigher than that of the control (Fig. 1). This indicatedthat small amounts of the polymers in the irrigationwater (<20 g m~3) were enough to stabilize the ag-

Table 1. Physical and chemical properties of the surface horizon of the soils used.Particle-size distribution

Soil and site

Loess, Bet Qama

Grumusol, Negba

Classification

CalcicHaploxeralf

TypicChromoxerert

sand

50.0

36.0

silt

31.0

24.0

clay%

19.0

40.0

CaCo3content

11.4

25.1

Cation-exchangecapacity

cmolc kg-1

17.6

28.5

ExchangeableNa

percentage%3.7

5.2

904 SOIL SCI. SOC. AM. J., VOL. 56, MAY-JUNE 1992

0 g m 6 a m LJ 10 g m~3 • 20 g mTable 2. Analysis of variance for the final infiltration rates

obtained in the various polymer treatments of each soil.Sum of

Soil Source df squares F value Significance

ControldBO) 60 180 300 60 180 300

Cumulative irrigation depth (mm)Fig. 1. Final infiltration rates (FIR) obtained in soils treated

with polyacrylamide (PAM) or polysaccharide (PSD) for (A)Chromoxerert (grumusol) and (B) Haploxeralt (loess) soilsat the end of the first, third, and fifth irrigation events. Forthe control, the same FIR values were obtained after 60 and180 mm of irrigation. Within soils, bars with the same letterdo not differ significantly at the O.OS level.

gregates at the soil surface and to limit seal formation.At the end of the third irrigation, i.e., cumulativewater depth of 180 mm with polymers, there was aslight beneficial effect on the FIR in the case of thePAM and a detrimental effect for the PSD treatment(Fig. la). Following two more irrigations with TWonly (reaching a cumulative irrigation depth of 300mm), the FIRs, with the exception of the 20 g m~3

PAM treatment, decreased to values similar to that ofthe control (Fig. la). Except for the high-concentra-tion PAM treatment, the two polymers had no signif-icant residual effect on soil permeability. The lack ofresidual effect of the two polymers is attributed totheir adsorption characteristics, discussed below.

The PSD, being a cationic polymer, is highly ad-sorbed to the negatively charged clay be electrostatic(coulombic) forces. The amount of PSD adsorbed onmontmorillonite from a 10 g m~3 PSD solution is 70g kg-1 (Aly and Letey, 1988). In a 60-mm irrigationwith water containing 10 g m~3 of the polymer, theamount of PSD added to 1 m2 of soil is 0.6 g. Thisamount was adsorbed on 0.0086 kg of montmorillon-ite or on 0.02 kg of the Chromoxerert, which con-tained 40% montmorillonitic clay. The amount of soillost in the fourth irrigation was 0.27 kg m~2 (Table

Chromoxerert

Haploxeralf

polymer type (PT)polymer

concentration (PC)Irrigation depth

(IRD)PT x PCPT x IRDPC x IRDPT x PC x IRDErrorCorrected totalPTPCIRDPT x PCPT x IRDPC x IRDPT x PC x IRDErrorCorrected total

1 14.1072 71.503

2 1557.053

2 41.3432 252.9734 11.5274 121.507

36 23.18053 2093.1931 0.0472 90.1232 833.4182 169.0892 27.7054 34.2784 104.798

36 15.78053 1275.246

21.9155.52

1209.10

32.10196.44

4.4847.18

0.11102.81950.67192.8831.6019.5559.77

****

**

********

************

** Significant at the 0.01 probability level.

3). Evidently, soil loss exceeded the weight of soiltreated with the PSD by an order of magnitude. Thus,erosion of the treated layer may account for the lowresidual effect of the PSD. Another possible expla-nation for the poor residual efficacy of the PSD in theTW-only irrigations is the fact that the PSD is a nat-urally occurring polymer., and could have been partlydegraded in the course of the wetting and drying cyclesof these consecutive irrigation events (Barry et al.,1991).

The anionic PAM is adsorbed to the soil clay to amuch lesser degree than the PSD. The amount of PAMadsorbed on a montmorillonite from a 10 g m~3 PAMsolution is 2 g kg-1 (Stutzmann and Siffert, 1977).Thus, in a 60-mm irrigation containing 10 g m~3 ofPAM, the polymer was adsorbed on 0.75 kg m~2 ofthe Chromoxerert. The amount of soil lost in the fourthirrigation was only 0.13 kg m-2 (Table 3). Hence,the eroded material was only part of the entire treatedlayer, and PAM was expected to have some residualeffect (Fig. 1).

The two polymers differed in their effect on the FIRof the two soils. In the PSD treatments, with the ex-ception of the 5 g m~3 concentration in the Haplox-eralt, the highest FIR values were obtained at the endof the first irrigation. After the three consecutive ir-rigations with PSD, the FIRs dropped to 63 to 75%of those obtained at the first event (Fig. 1). On theother hand, in the PAM treatments, the FIR valuesafter 180 mm of water were similar to or higher thanthose after 60 mm.

The difference between the two polymers is attrib-uted to their being organic compounds and to theirrespective adsorption characteristics. Organic com-pounds adsorbed on soil particles may reduce thecohesion recovery and aggregate stability in soils(Kemper et al., 1987). These researchers postulatedthat mineral surfaces coated with organic compoundsmay reduce the number of mineral-to-mineral contactswhere bonds between adjacent mineral surfaces areformed during drying periods. Cationic PSD is stronglyadsorbed at the planar clay surfaces (Aly and Letey,

LEVY ET AL.: POLYMERS' EFFECTS ON INFILTRATION AND EROSION 905

Table 3. Mean soil losses from the irrigation events for the various treatments and soils.SoU loss

IrrigationSoil Polymerf depth

mmChromoxerert PAM 60

120180240300

PSD 60120180240300

Haploxeralf PAM 60120180240300

PSD 60120180240300

5gm- 3

Control of polymer———————————————— gm

337 ± 27.6$ 170 ± 15.3276 ± 25.4 279 ± 26.8222 ± 20.4 142 ± 11.6

162 ± 14.8201 ± 18.2266 ± 22.1357 ± 31.6292 ± 24.2255 ± 20.7263 ± 23.0

202 ± 13.1 319 ± 26.1242 ± 18.7 268 ± 25.3

227 ± 13.8179 ± 14.5247 ± 15.1321 ± 24.3398 ± 32.5229 ± 18.3224 ± 19.4261 ± 21.9

10 g m -3

of polymer-2

120 ± 10.998 ± 7.461 ± 5.9

131 ± 11.4193 ± 9.7237 ± 19.7233 ± 18.4150 ± 11.8266 ± 24.5245 ± 20.3142 ± 10.6112 ± 9.282 ± 5.997 ± 8.1

168 ± 13.4481 ± 33.9324 ± 30.1353 ± 31.8296 ± 26.1

20 g m-3

of polymer

83 ± 6.458 ± 4.128 ± 1.6

122 ± 9.6208 ± 19.3218 ± 16.9316 ± 28.3316 ± 27.4208 ± 18.5

178 ± 11.779 ± 6.141 ± 3.293 ± 8.0

131 ± 10.9369 ± 31.7251 ± 21.6373 ± 33.4319 ± 27.8

t PAM = polyacrylamide; PSD = polysaccharide.| Values followed by standard deviation.

1988), and could interfere with the development ofcohesion forces between primary particles. Conse-quently, during the second and third consecutive ir-rigations, although PSD was added to the irrigationwater, aggregates with lower stability were present atthe soil surface and a less permeable seal was formed.Conversely, the adsorption of anionic PAM is limitedand occurs mainly at the edges of clay minerals.Therefore, PAM does not interfere with cohesion re-covery and the formation of cohesion bonds betweenclay platelets. Thus, application of fresh solutions ofPAM in the second and third irrigations further sta-bilized the aggregates formed in the first irrigation.Also, it is likely that PAM in the irrigation waterpenetrated to a greater depth than did the PSD, andstabilized the soil layer underneath the crust.

The two polymers also differed in the concentrationthat was most efficient in maintaining the highest FIR.The PAM concentration of 10 g m~3 gave similar orhigher FIR values, than the 5 and 20 g m~3 concen-trations (Fig. 1). In the PSD treatments, a concentra-tion of 20 g m~3 gave the highest FIR values (Fig.1). It is possible that, for the soils studied, increasingthe PSD concentration above 20 g m~3 might furtherenhance aggregate stability and improve the soil'sFIR.The high concentration of PSD needed to preventseal formation could result from the high adsorptionof PSD onto the planar surfaces of the clay, whichlimits the interparticle bridging (Theng, 1982). Thus,larger amounts of PSD were needed for aggregate sta-bilization.

Comparing the response of the two soils to polymerapplication, it is evident that the FIR in the controltreatment was lower in the Haploxeralf (6.0 mm h"1)than in the Chromoxerert (8.5 mm h-1). Aggregatesof the Haploxeralf with lower clay content (19% clay)are weaker than those of the Chromoxerert (40% clay),which made the former soil more susceptible to sealformation. A trend was observed (Fig. 1) whereby the

FIR values in the polymer treatments were lower inthe Haploxeralf than in the Chromoxerert. A similarobservation, with respect to the efficiency of PAM inthese two soils, was reported by Levin et al. (1991).It seems that the polymers were more effective in asoil that contains more clay.

In accordance with their effect on the FIR, the 10and 20 g m~3 treatments in the PAM and PSD, re-spectively, resulted also in the highest GIF values.The GIF is an integrated value that reflects both therate at which the seal is formed and the final perme-ability of the soil. The accumulated GIF values duringthe five irrigation events for these two treatments arepresented in Fig. 2. In both soils, addition of polymersimproved water uptake by the soil, and the GIF valueswere at least 58 and 70% higher than those of thecontrol for the Chromoxerert and Haploxeralf soils,respectively. The total GIF value for the Chromoxerertwith PAM was significantly higher than that of thePSD (Fig. 2a). The total depth of water that penetratedthe PAM-treated soil was 236 mm (of the 300 mm ofwater applied), and hence only 22% of the appliedwater was lost as runoff. In the PSD-treated Chro-moxerert, 34% of the total applied water was lost asrunoff. For the Haploxeralf, no significant differencewas observed between the total GIF values of the twopolymers at the end of the five irrigation events (Fig.2b), and total runoff was 33% of the water applied.Thus, for both soils, PAM's efficacy in maintaininghigh IR was equal to or better than that of the PSD.A close inspection of the amount of GIF obtained atthe last two irrigations only, where only TW was used,revealed that, irrespecitve of the soil, the CIFs of thePAM-treated soils were always significantly higherthan those of the PSD-treated ones. For the PAM andPSD treatments, these CIFs were 74 and 48 mm, re-spectively, in the Chromoxerert and 68 and 44 mm,respectively, in the Haploxeralf. These results indicatethat the rate at which the infiltration decreased during

906 SOIL SCI. SOC. AM. J., VOL. 56, MAY-JUNE 1992

300A Control I PAM-10 g

B Control CD PAM-10 g m

Table 4. Total soil loss from five consecutive irrigation events.

60 120 180 240 300Cumulative irrigation depth (mm)

Fig. 2. Cumulative infiltration for five irrigation events for(A) Chromoxerert (grumusol) and (B) Haploxeralf (loess)soils treated with polyacrylamide (PAM) or polysaccharide(PSD). Within soils and a given cumulative irrigation depth,bars with the same letter do not differ significantly at the0.05 level.

water application in the last two irrigations was slowerin the PAM- than in the PSD-treated soil samples.The results further suggest that the residual effect ofPAM was significantly better than that of PSD.

Soil ErosionSoil losses from the various consecutive irrigations

for the two polymers and the control are presented inTable 3. In the PAM treatments of 10 and 20 g m-3

in both soils, soil losses were lower than those in thecontrol. Furthermore, soil losses in these treatmentsdecreased successively in the following two irriga-tions. In the last two irrigations (TW only, 240 and300 mm) soil loss increased, but to values lower thanthose in the control. The successive decrease in soilerosion in the first three irrigations with PAM was dueto the accumulation of PAM and the stabilization ofthe aggregates at the soil surface. Such stabilizationhas the following opposing effects on interrill soil er-iosion:

1. Seal formation is prevented and soil detachmentby water-drops' impact in the unsealed soil is highbecause of its low shear strength (Bradford et al.,1987; Moore and Singer, 1990).

2. Runoff level is reduced, and thus the capapcity

Soil

Chromoxerert

Haploxeralf

Polymerconcentration

gm-3

510205

1020

Soilpolyacrylamide

losspolysaccharide

—————— g m-2 ———————954 Abt 1433 Bb543 Aa 1131 Ba499 Aa 1058 Bat

1122 Ab 1433 Ba601 Aa 1454 Bat522 Aa 1312 Bat

f Within a row, means followed by the same capital letter do not differsignificantly at the 0.05 level. Within columns and soils, means followedby the same lowercase letter do not differ significantly at the 0.05 level.

t Total soil loss from the first four irrigation events.

of sheet flow to transport eroded material is also re-duced (Moore and Singer, 1990).

The relative importance of each of these two op-posing tendencies will determine the measured soilloss.

In the 10 and 20 g m~3 PAM treatments, infiltrationwas high (Fig. 1 and 2) and consequently runoff waslow compared with the control, and hence soil erosiondecreased. In addition, PAM increased the cohesiveforces between soil particles, which led to reducedsoil detachment. The combination of the two processescontrolled soil losses during the first three irrigations.In the fourth and fifth irrigaitons, where only TW wasused, aggregates stabilized by PAM were graduallyeroded, leading to the observed increase in soil losses.In the 5 g m~3 PAM treatment, soil erosion levels inboth soils were higher than and similar to, respec-tively, the erosion levels in the control. This occurredin spite of the fact that the infiltration and runoff weresimilar to those in the 10 g m~3 treatment. Hence, thehigh soil loss in the 5 g m~3 PAM treatment resultedmainly from a large increase in soil detachment, com-pared with the other two PAM concentrations. Evi-dently, the lower PAM concentration was not sufficientto bind together large and strong enough soil particlesto withstand detachment by the impact of the waterdrops.

Soil losses from the PSD-treated samples were greaterthan those from the PAM-treated soils (Table 3). Totalsoil losses from the 10 and 20 g m~3 PSD treatments,summed across all of the irrigation events, were morethan double the soil losses from the PAM treatments(Table 4). Also, unlike for PAM, no distinct patternin soil loss along the consecutive irrigations was ob-served in the PSD treatments (Table 3). The high soillosses in the PSD, relative to the PAM, should becompared with the effect of the polymers on infiltra-tion and GIF (Fig. 1 and 2), indicating similar IR andrunoff levels for both polymers. It seems that, like thelowest PAM treatment, PSD fails to form soil particlesof a size and strength that would make them less sus-ceptible to detachment, and hense erosion was high.Contrary to the low-concentration PAM treatment,however, the high adsorption of PSD on the planarsurfaces of the clay that limits the interparticle bridg-ing (Theng, 1982) is responsible for the relatively smallparticles formed. Another explanation related the ef-fect of PSD on aggregate-size distribution to changingit toward a size range favorable for transport by runoffwater (Ben-Hur et al., 1990).

LEVY ET AL.: POLYMERS' EFFECTS ON INFILTRATION AND EROSION 907

The larger soil losses in the PSD treatment, com-pared with the PAM treatment, could not have beenpredicted from the infiltration parameters (i.e., FIRand GIF). In most cases, the FIR and GIF were similarfor both polymers and, in the specific instances wherethe values of these parameters were higher in the PAMsamples, the differences were not of the same mag-nitude as those observed in the erosion studies. Levinet al. (1991), who studied the effects of PAM sprayedonto the soil surface on infiltration and erosion ofthree different soil types, also observed that soil lossesand infiltration parameters were not always directlyrelated. Our results suggest that, whereas infiltrationis sensitive to aggregate stability independent of ag-gregate size, erosion is prevented only when smallaggregates are cemented together to prevent their de-tachment by hydraulic shear.

SUMMARYAddition of low concentrations of PAM and PSD

to the irrigation water was found to be beneficial inmaintaining high infiltration rates. In subsequent ir-rigations with water only, however, the FIR waluesof the polymer-treated samples decreased to valuessimilar to those of the control. The low residual effectof the polymers was attributed to the erosion of thethin treated layer and, in the case of the PSD, to apossible partial degradation of the polymer in the courseof the wetting and drying cycles. A lower concentra-tion of PAM (10 g m~3) was needed for an optimaleffect on the infiltration parameters (FIR and GIF)than that needed of PSD (20 g m-3). For these optimaltreatments, the infiltration parameters were generallyhigher in the PAM than in the PSD treatments. Soillosses in all the PAM treatments were significantlylower than those for PSD. These large differences insoil losses between the two polymers could not havebeen predicted from the infiltration parameters. Bothpolymers stabilize soil aggregates, but PAM, becauseof its longer molecule and limited adsorption, is moreefficient in cementing aggregates together and in-creases their resistance to erosion.

ACKNOWLEDGMENTS

The authors wish to thank Mrs. H. Eisenberg for labo-ratory analysis. This research was supported by Grant no.US-1311-87 from BARD, the U.S.-Israel Binational Ag-ricultural Research and Development Fund. This support isgratefully acknowledged.