|United States Department of Agriculture - Agricultural Research Service|
|Northwest Irrigation & Soils Research Lab, Kimberly, ID|
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
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.
Methods and Materials
Soil InformationAn irrigated study was conducted near Kimberly, Idaho. The soil was Porneuf silt loam (coarse-silty, mixed superactive, mesic Durinodic Xeric Haplocalcids). This soil has low organic matter, typically 10 to 13 g kg-1 and a moderate cation exchange capacity, typically 18 to 20 cmol(+) kg-1.
PAM Irrigation TreatmentTwo irrigation treatments were imposed on this study: untreated water and water containing 20 g m-3 PAM applied during the first hour of each 8-12 hr irrigation. Polyacrylamide (PAM) was Superfloc A836, provided by Cytec Industries, Wayne, New Jersey. This PAM has a molecular weight of 12 to 15 Mg mole-1 for approximately 150,000 monomer units per molecule, with a negative charge density of approximately 18%.
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)
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.
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