polyacrylamide effect on infiltration and erosion in furrows

Polyacrylamide Effect on Infiltration and Erosion in FurrowsX. C. Zhang* and W. P. Miller

ABSTRACTSurface sealing and crusting are important factors affecting runoff

and erosion in many cultivated soils. The objective of this studywas to evaluate the effects of low rates of surface-applied anionicPolyacrylamide (PAM) on surface sealing and crusting, water infiltra-tion, and soil erosion. A series of experiments was conducted on aCecil sandy loam soil (clayey, kaolinitic, thermic Typic Kanhapludult)in ridge-furrow-type field plots (3.5 by 0.92 m). Polyacrylamide solu-tions containing 1 kg solids m ~3 and 2.5 mol CaSO4 m~ 3 were sprayedonto dry soil surfaces at 15 and 30 kg ha"1 rates with two replicateseach. Three simulated rains, 31 min each, were applied at an intensityof 85 mm h'1 at 2-wk intervals. Final infiltration rates were 30 mmh~' for the control treatment for all three rains and were >85 nunh~' for the first rain and 45 mm h"1 for the two following rains forthe PAM treatments. Total soil loss of the control was 1.62 kg m~2

for the first rain, while the average soil loss of the two PAM treatmentswas only 0.03 kg m~2. For two subsequent rains, although the differ-ences were narrowed, soil loss from PAM was 48 to 66% less thanfrom the control. The percentage of sediments >0.5 mm in the PAMtreatments was 45% greater than in the control, indicating high aggre-gate stability with PAM addition. The sediment reduction was attrib-uted to the significant reduction of runoff and the prevention of rillformation due to PAM addition. Based on this study, surface applica-tion of PAM at a rate of 15 kg ha~> controlled surface sealing andcrusting and therefore reduced runoff and erosion.

SURFACE SEALING AND CRUSTING reduce water infiltra-tion, increase soil loss, and impede seedling emer-

gence (Mclntyre, 1958). Surface seals formed in interrillareas may either increase or decrease interrill erosion,depending on the relative magnitude of increased runoffshear and shear strength of newly formed seals; however,the larger amount of runoff produced in the sealed areasis expected to cause severe rill erosion (Poesen andCovers, 1985). Cropland fields with bedded rows arecommon for row crops in the USA and are prone to rillerosion. A single bedded row acts as a small subwater-shed, in which runoff from the side slopes or interrillelements drains into the row furrow that conveys theconcentrated flow down slope. The concentrated flow inthe furrow tends to cause severe rilling, especially undercrusted conditions (Mutchler and Murphree, 1980).

Surface seals are induced by two major complementarymechanisms: (a) physical disintegration of aggregatesand (b) chemical dispersion of soil particles (Mclntyre,1958). Physical processes predominate in soils with highEC or low ESP; otherwise, chemical processes supple-ment the physical processes (Agassi et al., 1981). Inaddition, the interaction of these two processes, or me-chanically induced clay dispersion, becomes significantfor the soils with low ESP, low EC, or a high contentof 1:1 clay when energy such as raindrop impact is input

X.C. Zhang, USDA-ARS, National Soil Erosion Research Lab., PurdueUniv., West Lafayette, IN 47907; W.P. Miller, Dep. of Crop and SoilScience, Univ. of Georgia, Athens, GA 30602-7272. Correspondingauthor ([email protected]).

Published in Soil Sci. Soc. Am. J. 60:866-872 (1996).

into the system. These processes may be completely orat least partially controlled by organic polymers thatstabilize aggregates by flocculating the clay particles.Polyacrylamide reduced runoff and erosion on certainsoils (Lentz and Sojka, 1994; Levy et al., 1992; El-Amiret al., 1985; Levin et al., 1991; Shainberg et al., 1990)and increased hydraulic conductivity and the rate ofwetting front advance (El-Amir et al., 1985; Pla, 1975).These effects resulted from the increased soil aggregationand porosity and reduced clay dispersion.

The use of polymers to amend soil physical propertieswas studied extensively in the 1950s. In most earlystudies, polymer was applied in very large quantitiesand was typically mixed into the cultivated layer (12-25 cm) to improve soil structure. High cost due to highapplication rates and difficulties in mixing this applicationresulted in lack of acceptance in agriculture. New appli-cation strategies devoted to controlling surface sealingand crusting have revived the interest (Norton et al.,1993). Small amounts of polymer, either sprayed ontosoil surfaces or dissolved in irrigation water, consider-ably reduced runoff and erosion by reducing surfacesealing and crusting (Shainberg et al., 1990; Lentz etal., 1992; Levin et al., 1991).

Polymer adsorption by clay minerals largely dependson molecular size, conformation, charge properties ofthe polymer, and characteristics of soil colloids such asexternal surface area and pore sizes (Letey, 1994). An-ionic polymers tend to be repelled by negatively chargedclay surfaces, and little adsorption occurs; however, con-siderable adsorption may take place when cation bridgesare present (Theng, 1982). Likewise, adsorption of an-ionic polymers on soils depends on polymer characteris-tics, soil properties, and water quality. The effectivenessof anionic PAM in stabilizing soil aggregates dependson the nature and extent of the adsorption. Malik andLetey (1991) found PAM polymer did not penetrate soilaggregates and was mainly adsorbed onto the aggregatesurfaces due to the high molecular weight of the polymer.The adsorption by soil constituents is irreversible whenthe system is allowed to dry because this allows theshort-range van der Waals force to become effective(Nadler et al., 1992). As a result, PAM applicationsstabilize soil aggregates that exist at the time of applica-tion and do not cause formation of new aggregates (Cookand Nelson, 1986). The effectiveness of preserving soilaggregates can be unproved with presence of salts suchas gypsum and with dehydration of the system (Shainberget al., 1990; Levin et al., 1991).

Although some promising results of using PAM tocontrol surface sealing and soil crusting have been re-ported, more experiments on a range of soils underdifferent conditions are needed to better understand these

Abbreviations: PAM, polyacrylamide; CEC, cation-exchange capacity;EC, electrical conductivity; ESP, exchangeable sodium percentage; TDR,time domain reflectometry.

866

ZHANG & MILLER: POLYACRYLAMIDE EFFECT ON INFILTRATION AND EROSION 867

mechanisms. The objectives of this study were to evaluatethe effectiveness of PAM under field conditions in con-trolling surface sealing and soil crusting and reducingrunoff and erosion, especially rill erosion on a kaoliniticCecil soil in Georgia.

MATERIALS AND METHODSThe experimental site was at the University of Georgia Plant

Science Farm in Oconee County, Georgia. A Cecil soil wasstudied because it is the most common soil series in the Pied-mont region in the southeastern USA. The Ap horizon contained77% sand, 14% silt, 9% clay, and 15 g kg"1 organic matterand had a CEC of 5.3 cmolc kg"1 and an ESP of 1.9%. Waterdispersable clay on a clay basis was =90% (determined byshaking for 24 h in deionized water). The soil is easily crusted(Chiang et al., 1993) and aggregates are typically unstableunder field conditions.

Prior to plot setup, the site was disked twice to a 15-cmdepth to create a well-mixed tillage layer. Six plots, 3.5 mlong by 0.92 m wide, were laid out with metal borders.Time domain reflectometry rods (32 cm long) were installedhorizontally at six depths (2.5, 5, 10, 15, 20, and 25 cm) inthe center of each plot. The soil surfaces of each plot wereshaped manually into a bedded row with a V-shaped metalmold. Row sideslopes and row furrow gradients were adjustedto 0.2 and 0.11 m m"1, respectively. The plots were allowedto stabilize for =2 mo. under natural rainfall conditions toreduce heterogeneities between the plots. Prior to initiation ofthis experiment, the soil surfaces were shallow raked by handto a 3-cm depth to break surface crusts, and then they weresmoothed with the mold to eliminate microtopography bycrushing surface clods.

The PAM polymer used was Magnifloc 836A (AmericanCyanamide Corp., Stamford, CT). It is a medium-charge an-ionic PAM (20 % hydrolysis) with a high molecular weight of 15million g mol~'. The PAM was slowly hydrated and dissolved in2.5 mol CaSCX m~3 solution (which aided in reducing polymersolution viscosity) to obtain 1 kg m"3 concentration. Threetreatments (0, 15, and 30 kg ha"1) with two replicates eachwere randomly assigned to the six plots. Solutions were thenuniformly sprayed onto the prepared dry soil surfaces with apesticide sprayer. To obtain the 15 kg ha"1 (PAM-15) and 30kg ha"1 (PAM-30) application rates, 1.5 and 3 L m"2 of thePAM solution were needed, respectively. Assuming that theporosity of the freshly tilled soil was 0.47 cm3 cm"3 and thesoil was fully saturated in the wetting zone, the wetting depthswould be =3.2 mm for the 15 kg ha"1 rate and 6.4 mm forthe 30 kg ha"1 rate. After spraying, plots were allowed to airdry.

A rainfall simulator with oscillating nozzles was used (Meyerand Harmon, 1979). Electrical conductivity of rainwater was= 0.2 dS m"1. Three simulated rains were made at 2-wkintervals with a rainfall intensity of 85 mm h"1. Each runlasted for 31 min, and runoff samples were collected at 3-minintervals. After each rain, plots were protected from naturalrainfall with a clear plastic cover positioned 60 cm aboveground. Volumetric water contents from TDR probes at sixdepths were measured immediately before each rain and at5-min intervals during the rain.

Sand-sized sediments including soil aggregates were sepa-rated from collected runoff by sieving at 53 u.m, then ovendried and dry sieved on a motorized sieving machine for 2 minto obtain sediment size distribution. The clay + silt fraction wasdetermined by drying an aliquot of the sieved and well-mixedsuspension. Rill width and depth measured from the apex of

the sloped metal mold (20% side slope), which was used toform the V-shaped soil surfaces, were determined after eachrain at 76-mm intervals along the plot length. Rill cross-sectionareas were calculated as the sum of a base rectangular areaand two overlying triangular areas. An average bulk densityof 1.40 g cm"3 under freshly tilled conditions was used toconvert rill volumes to soil masses. Following the third rain,approximately six surface crust samples, <10 mm thick, weretaken from the sideslope areas of each plot after soil surfacehad dried, and the bulk densities were determined by the clodmethod (Blake and Hartge, 1986). The Duncan Multiple Rangetest was used to test for the significance between the treatments.

RESULTS AND DISCUSSIONCrust Density and Sediment Size DistributionThe average bulk densities of 10-mm crust-soil layer

after three consecutive simulated rains were 1.54 g cm"3

for PAM-15, 1.55 g cm"3 for PAM-30, and 1.66 gcm"3 for control. The Duncan test showed that the crustdensities of both PAM treatments were different fromthe control at the P — 0.05 level, but no statisticaldiiference was found between the two PAM treatments.The higher bulk density in the control treatment wasattributed to surface crust formation following disruptionof soil aggregates. Well-aggregated surfaces were ob-served on the PAM-treated plots after the experiment,while smoothed surfaces with clean and well-sorted sandgrains occurred on the control plots, indicating a severeclay dispersion and aggregate disintegration.

Differences in the soil surfaces were also reflected bythe sediment size distribution. Aggregates and soil grainsin sand-size fractions >0.05 mm accounted for 85% ofthe total sediment for the control treatment and « 91 % forthe PAM treatments (Table 1). The percentage sedimentfrom very coarse and coarse size classes (>0.5 mm)was 34% for the control treatment and 49% for the PAMtreatment. This indicates that a larger portion of sedimentwas transported hi the larger size classes in the PAMtreatments. Because greater runoff and soil loss ratesalong with severe rilling were observed on the controlplots, selective erosion for finer particles was less likelyto occur on the control plots than on the PAM-treatedplots. Therefore, the coarser sediment from the PAMplots indicated that surface-applied PAM increased ag-gregate stability. This result is consistent with earlierfindings (Allison, 1952; Shainberg et al., 1990; Kijne,1967; Gabriels, 1990).

Infiltration and Water PenetrabilityInfiltration rates for the control and PAM treatments

are plotted with time in Fig. 1. During Event 1, infiltra-tion rates in both PAM treatments were close to therainfall intensity (85 mm h"1), while the infiltration ratein the control treatment declined rapidly to 30 mm h"1

due to the rapid formation of surface seals. For the twofollowing events, a similar trend as in Event 1 wasexhibited for the control, indicating mat the crusts formedduring the previous rain were disrupted by drying andwetting actions between and during the following rains,and new seals were formed rapidly. This was observed

868 SOIL SCI. SOC. AM. J., VOL. 60, MAY-JUNE 1996

Table 1. Average sediment size distribution of aggregates and sand grains across three consecutive rains.t

TreatmentVery coarse

>1 mmCoarse

0.5-1 mm

Sand size fractions

Medium0.25-0.5 mm

Fine0.1-0.25 mm

Very fine0.05-0.1 mm

Silt + clay<0.05 mm

ControlPAM (15 + 30)

13.719.5

20.429.3

25.5 20.023.7 14.7

5.34.0

15.08.7

t Values averaged across all sampling times for replicate plots for three rains; treatments are significantly different at P = 0.05 for all size groups exceptfor very fine sand class.

in another study on the same soil (Zhang and Miller,1993). For the PAM treatments, infiltration rates inEvents 2 and 3 declined slowly to 40 to 45 mm h"1, butthey were still 50% greater than the control.

The decline in infiltration rate during subsequent rainswas probably due to failure of the polymer nets thatsurrounded the aggregates. Since polymers are irrevers-ibly adsorbed onto the outer surfaces of soil aggregates(Malik and Letey, 1991; Letey, 1994) and just stabilize,rather than improve, the physical conditions at the timeof application, the persistence of these polymer netsseems to be the main factor affecting the effectivenessof the PAM treatment. Disruption of aggregates reducedthe polymer effectiveness because the unprotected inte-rior of the aggregate was exposed and susceptible todispersion (Letey, 1994). As aggregates were dried afterthe first rain and rewetted during the subsequent rains,slaking of the aggregates due to shrinking-swelling andair escaping might have caused breakage of the protectivenets. Thus, the network would have been less effectivein protecting the aggregates during the subsequent rains.In addition, the microbial decomposition might havereduced the effectiveness of the PAM treatments, whichwas not investigated in this study. A parallel study undernatural rainfall conditions showed that PAM reducedsurface runoff significantly following 5 mo of surfaceapplication, indicating a slow microbial decompositionrate (data not presented). Interestingly, the infiltrationrates from the two PAM treatments were not significantlydifferent from each other. This suggests that 15 kg ha"1

is a sufficient rate to use for control of crust formationin this soil.

Since the difference between the two PAM treatmentswas relatively small, pooled data were tested againstthe control. Average infiltration rates across the entirerainfall duration for PAM were higher than for controlfor all three rainfall events at the 5% level (Table 2).The opposite was true for the average runoff rates. Runoffcoefficients (runoff to rainfall ratios) were greater in thecontrol than in the PAM treatments at the 5% level,showing the significant effect of the PAM treatments.

Water content profiles measured by TDR from controland PAM-15 plots during the first rain are shown as anexample in Fig. 2. For the PAM-15 treatment (Fig.2a), the wetting front was relatively sharp and movedgradually downwards. At the end of rain, the top 25 cmsoil profile was almost saturated and equally wetted withthe maximum volumetric water content of 0.35 to 0.38cm3 cm"3. For the control plot in Fig. 2b, the wettingfront was diffuse with the maximum volumetric watercontent <0.18 cm3 cm"3 near the soil surface. Thisindicates that the rapidly formed soil seal limited thewater intake rate. With this limiting layer, there was nota sufficient water movement into the soil matrix to pro-duce a clear wetting front advance, as was found in thePAM treatments.

Sediment ConcentrationSediment concentrations for control and PAM treat-

ments are plotted in Fig. 3. Sediment concentration of

• Control» PAM 15• PAM 30

30 45TIME (min)

Fig. 1. Replicate mean infiltration rates for control, PAM-15 (15 kg ha~'), and PAM-30 (30 kg ha'1) treatments across three rainfall eventswith soil surface dried between rains.


Table 2. Comparison of observed variable means averaged across entire rainfall duration by treatment and event.

Event

1

2 + 3

Mean

Treatment

ControlPAM(15 + 30)DifferenceControlPAM(15 + 30)DifferenceControlPAM(15 + 30)Difference

Infiltrationrate

————— mm h~' —38.883.6

*31.851.3

*35.367.5

*

Runoffrate

48.62.8*

50.330

*49.516.4

*

Runoffcoefficient

m m"1

0.560.03

*0.610.37

*0.580.2

*

Sedimentconcentration

gL-64.617.3

#

56.440.5NS60.528.9

*

Erosionrate

g m"2 min"1

52.30.8*

47.320.2

*49.810.5*

* Significantly different treatment means at P = 0.05; NS = nonsignificant at P = 0.05.

the control in Event 1 increased rapidly to ==65 g L"1,whereas it remained low in both PAM treatments. Thisis because little runoff was produced and large aggregateswere present at the soil surfaces. For the two followingrainfall events, the sediment concentrations of the controlstabilized at a similar level as Event 1, while considerableincreases were exhibited for the two PAM treatments,especially for PAM at 30 kg ha"1. Average sedimentconcentration for PAM was significantly different fromthe control at the 5 % level for Event 1 but not for Events2 or 3 (Table 2). Interestingly, sediment concentrationof PAM-30 was somewhat greater than PAM-15 acrossthree consecutive rains, which was difficult to explainsince the runoff rate and sediment size distribution weresimilar. Further research is needed to determine if thissoil was over-dosed with PAM at the higher rate, causingreduction of inter-aggregate cohesion.

The increase in sediment concentration for the PAMtreatments during Events 2 and 3 was caused by thegradual breakdown of aggregates that tended to increasesediment transportability, increase in runoff and conse-quent flow shear, and relatively lower surface shearstrength due to the improved soil aggregation comparedwith the severe crusts in the control treatment (Wood

and Oster, 1983; Page, 1979). Because PAM treatmentpreserves aggregation, PAM application results in lowerbulk density, less soil crusting, and therefore, a smallerpenetrometer resistance compared with crusted surfaces(Cook and Nelson, 1986; Terry and Nelson, 1986).Bradford et al. (1986) measured seal shear strength under—0.5 J kg"1 matric potential with a fall-cone techniqueand found shear strength increased 2.5 times for theCecil soil after seal formation. The higher shear strengthunder sealed conditions is consistent with higher bulkdensity measured in this study for the control treatment.The sealed soil surface had less loose materials availablefor transport compared with well-aggregated soil sur-faces. Thus, for a given erosive force, soil loss fromthe sealed surfaces would be less than from the unsealedsurfaces. This explains why sediment concentrations dur-ing the first 10 min of Events 2 and 3 were higher onthe PAM-treated plots than on the control plots for thesame runoff rate. Although PAM treatment increasessoil cohesion and resistance to transport, reduction inshear stress and transport capacity of surface runoffthrough enhancing infiltration with PAM application ap-peared to be the main mechanism resulting in reducedsoil loss in this study.

VOLUMETRIC WATER CONTENT (cm3 cmJ)

0.0 0.1

5 -

X 10

15

20

25

30

0.2 0.3 0.4 0.00

0.1 0.2 0.3 0.4

(a) PAM 15

10 -

15 -

20

25

30

0 minS1015202530

(b) Control

Fig. 2. Changes of volumetric water contents with depth and time during the first event for (a) 15 kg ha'1 PAM and (b) control treatments.


100

• Control» PAM15• PAM30

TIME (min)

Fig. 3. Replicate mean sediment concentrations of the control, PAM-15, and PAM-30 treatments across three rainfall events with soil surfacedried between rains.

Soil Erosion and its PartitioningAverage soil loss rates for the PAM treatments were

less than for the control treatment at the 5% level forall three events (Table 2). Changes of soil loss rateswith time across three events are shown in Fig. 4. ForEvent 1, soil loss rate increased rapidly to 60 g m~2

min"1 for control, while little soil loss was observed forPAM. For Events 2 and 3, soil loss rate of the controlwas similar to Event 1, whereas it increased to 30 gnr2 min"1 for PAM-30 and 20 g m"2 min"1 for PAM-15.The rapid increase compared with Event 1 was attributedto the increase in runoff (Table 2), which was inducedby progressive aggregate breakdown due to breakage ofthe protective polymer nets caused by slaking and rain-drop impact. Furthermore, the breakdown of aggregatesenhanced sediment delivery (sediment transportability)and therefore soil loss rate. The similar levels of soilloss and runoff rates in Events 2 and 3 may indicate,at least to some extent, the residual effects of PAMamendment.

100

The measured total and rill soil losses are presentedin Table 3. Interrill soil loss, calculated as the difference,is also tabulated. The rill volume of Event 3 was missingbecause the plots were affected by an unexpected rain-storm. Rill erosion for the control was 1.03 kg m"2 forEvent 1 and 0.40 kg m~2 for Event 2, which accountedfor 60 and 30% of the total soil loss, respectively. Therill erosion was probably underestimated for Event 2,because the change of the surface elevation due to interrillerosion and consolidation was not accounted for. Also,rill measurement becomes more subjective as the rillwidens. For both PAM treatments, no visible rilling orscouring holes were observed after three rains. Basedon this observation, the total soil losses of PAM treat-ments were only from interrill areas or sideslopes, whichwere much less than those of the control. This indicatesthat the surface application of a small amount of PAMcan reduce or even eliminate rill formation-erosion. Thisfinding is consistent with the results reported in earlierstudies for other soils. Lentz et al. (1992) found that

60 75 9045TIME (min)

Fig. 4. Replicate mean soil loss rates of the control, PAM-15, and PAM-30 treatments across three rainfall events with soil surface dried betweenrains.


Table 3. Replicate means of measured total and rill soil lossesand calculated interrill soil loss by event and treatment.

Measured

Event

1

2

3

Treatment

ControlPAM 15PAM30ControlPAM 15PAM 30ControlPAM 15PAM 30

Total

1.620.010.041.390.620.721.540.520.64

Rill

—— kg m-2 -1.03000.4000-i00

Calculatedinterrfflt

0.590.010.041.000.620.72-t

0.520.64

t Difference between measured total and rill soil losses.| Data missing.

sediment losses from furrows that were pre-treated with<1 kg ha"1 PAM were significantly less than those fromthe control furrows. Shainberg et al. (1994) conducteda laboratory experiment with mini-flumes and concludedthat PAM applied at the rate of 0.4 kg ha"1 preventedrill erosion. They attributed the sediment reduction to theincreased soil cohesiveness for PAM treatment comparedwith noncrusted control treatment. Larger aggregates inPAM treatment, being more resistant to transport, mighthave also contributed to the reduction.

CONCLUSIONSurface sealing and crusting were largely controlled

by the surface application of small amounts of PAM.The mean infiltration rates across three consecutive rainswere =68 mm h"1 for two PAM treatments and 35 mmh"1 for the control. Considerable runoff reduction in thePAM treatments led to a significant reduction in soilloss. Average soil loss rates across three rains were 50g m~2 mur1 for control and only 11 g m~2 min"1 forPAM. Compared with the control treatments, rill erosionwas prevented in the PAM treatments even under rilling-prone conditions. This observation should be furtherexplored for more soils under different conditions.

Based on this study, the surface application of PAMwas effective in reducing surface sealing and crusting inthe ridge-furrow tillage systems on Cecil soil followingseveral weeks of the application. The PAM applicationmay be more attractive under circumstances when baresoil surfaces are unavoidable such as on constructionsites or during seedbed preparation. The 15 kg ha"1 rateseemed to be sufficient for surface application to controlsurface sealing and crusting for this soil. A similarapplication rate (10-20 kg ha"1) was also recommendedby Shainberg and Levy (1994) for surface applicationon Israeli soils. This rate may be economically feasiblefor certain crops and soils where surface sealing andcrusting severely affect production or where reductionin sediment delivery to surface water is critical.


quality and PAM interactions in reducing surface sealing. Soil Sci.149:301-307.

Terry, R.E., and S.D. Nelson. 1986. Effects of polyacrylamide andirrigation method on soil physical properties. Soil Sci. 141:317-320.

Theng, B.K.G. 1982. Clay-polymer interactions: Summary and per-spectives. Clays Clay Miner. 30:1-10.

Wood, J.D., and J.D. Oster. 1983. The effect of Cellulose Xanthateand polyviny! alcohol on infiltration, erosion, and crusting at differ-ent sodium levels. Soil Sci. Soc. Am. J. 47:243-250.

Zhang, X.C., and W.P. Miller. 1993. The effect of drying on runoffand interrill erosion of crusted soils, p. 103-114. In J.W.A. Poesenand M.A. Nearing (ed.) Soil surface sealing and crusting. CatenaSupplement 24.

polyacrylamide effect on infiltration and erosion in furrows

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