Scanning Electron Micrographs of Polyacrylamide-Treated Soil in Irrigation Furrows

C.W. Ross¹, R.E. Sojka² and J.A. Foerster²

¹Manaaki Whenua, Landcare Research, New Zealand Limited, Palmerston North, New Zealand
²USDA, Agricultural Research Service, Northwest Irrigation and Soils Research Laboratory, Kimberly, Idaho

Introduction

Polyacrylamide (PAM) for erosion control in irrigated agriculture in the U.S. began in 1991 (Lentz et al., 1992). By1998 PAM was used on about half a million irrigated hectares. Studies have verified the effectiveness of PAM for reducing erosion and enhancing infiltration. The mechanism by which PAM alters soil erosion, infiltration, and runoff is through alteration of soil surface seal formation (Shainberg et al., 1992; Smith et al., 1990). PAM-treated soil forms stabilized aggregates and has better pore continuity through the surface seals that result from the process of wetting, detachment, dispersion, transport and redeposition that accompany irrigation.

We felt that PAM applied in the field to soil surface seals via irrigation water might be visible at a sub-microscopic level. Since PAM applied in the field is "sieved" from infiltrating water by the surface few millimeters of soil, its accumulation might be denser and appear more netlike than in image of clay flocs. Images of PAM adsorbed on field-treated soil samples, taken from furrow seals, can also provide insight into PAM formation.

Objectives

• To examine the sub-microscopic structures of PAM-treated and untreated surface soil in irrigation furrows using scanning electron microscopy (SEM).
• To relate SEM microstructures to soil structural development and stabilization by PAM.

Methods and Materials

   Soil Information

   PAM Irrigation Treatment

   SEM Methodology

Surface samples for SEM imaging were collected from PAM-treated and untreated furrows 12 hours after their fifth irrigation. We estimate that soil samples with PAM had a total undisturbed seasonal application of 2.24 grams of PAM per square meter of soil at the time of sampling. Disks of surface soil 25 mm in diameter by 3-5 mm thick were randomly collected from the central areas of irrigation furrows using a sharpened cork-borer-type tube.

Soil disks were freeze-dried from the field-wet state and mounted onto aluminum sample holders. The surfaces of soil disks were given a conductive coating of 10 nm carbon followed by 10 nm 60/40 gold/palladium before viewing in a Cambridge Stereoscan 250 Mark 3 scanning electron microscope.

Results and Discussion

Significant sub-microstructural differences are seen between PAM-treated and untreated surface soil seals in the irrigation furrows. PAM-treated soil seals have semi-continuous net- or web-like surface polyacrylamide coatings on mineral particles. Coatings are about 1 µm thick, giving a glue-like appearance at lower magnifications (1500x-2000x). Individual strands of polyacrylamide of about 0.2 µm diameter, which form the webs, are distinguishable at higher magnifications (7400x).

The sub-microstructure of PAM-treated soil seals in the furrows provide protective, yet pervious, coatings against disaggregation and dispersion of soil mineral particles in the irrigation stream. The micrographs also indicate polyacrylamide bridging between particles as a mechanism for coagulating or ensnaring soil particles into a temporary water-stable surface sealing structure.

Untreated soil in the irrigation furrows, in contrast, is very poorly aggregated. These images show a paucity of organic material or muscilages available for binding primary mineral particles. There is little evidence of durable particle adhesion or binding via visible organic or mineral compound bridging or any other soil aggregating mechanisms. The soil surface seals without PAM treatment show sub-microstructures that would be prone to slaking and dispersion in the irrigation stream.

Comparative PAM-treated (left) and untreated (right) surface soil sub-microstructures from irrigation furrows (nominal magnification of 1500x). Note the glue-like porous appearance with PAM, and the poor visual micro-aggregation in the untreated soil. (click on image to enlarge)

Comparative PAM-treated (left) and untreated (right) surface soil sub-microstructures from irrigation furrows (nominal magnification of 2000x). A near complete coating of polyacrylamide compares with poorly bound, uncoated particles on untreated soil. (click on image to enlarge)

High magnification (nominal 7400x) view of net- or web-like polyacrylamide strands coating mineral particles (left) compared with uncoated grains from untreated soil (right). Individual strands of PAM appear to be about 0.1 µm in diameter. (click on image to enlarge)

Conclusions

• PAM-treated soil seals have semi-continuous net- or web-like coating of polymer on, and bridging between, mineral particles. This contrasts with poorly aggregated particles on untreated soil seals.
• Thin (about 1 µm), porous, surface soil veneers of protective polyacrylamide coatings account for soil stabilization against furrow irrigation erosion and improved infiltration rates.
• Unstable soil microstructures of surface soil seals on untreated furrow account for slaking and dispersion under irrigation, which leads to erosion losses and reduced infiltration rates.

References

1. Lentz, R.D., I. Shainberg, R.E. Sojka and D.L. Carter. 1992. Preventing irrigation furrow erosion with small applications of polymers. Soil Sci. Soc. Am. J. 56:1926-1932.
2. Shainberg. I., G.J. Levy, P. Rengasamy and H. Frenkel. 1992. Aggregate stability and seal formation as affected by drops' impact energy and soil amendments. Soil Sci. 154:113-119.
3. Smith, H.J.C., G.J. Levy and I. Shainberg. 1990. Water-droplet energy and soil amendments: effect on infiltration and erosion. Soil Sci. Soc. Am. J. 54:1084-1087.

Acknowledgments

Thanks to Rick Lentz, USDA-ARS and Kay Card, Industrial Research NZ Ltd for their assistance. This project was partly funded by the USDA-ARS and by the New Zealand Foundation for Research, Science and Technology.