Scanning electron micrographs of polyacrylamide-treated soil in irrigation furrows

Polyacrylamide (PAM) is used at rates of 1 to 2 kg ha−1 per irrigation on a half million hectares of United States irrigated farmland to prevent 94% of irrigation-induced erosion and to enhance infiltration by 15% to 50% on medium to fine-textured soils. The polyacrylamides used for this application are large (12 to 15 megagrams per mole), water-soluble anion molecules applied in the irrigation stream. Erosion prevention has been shown to result from stabilized soil structure in the 1 to 5 mm veneer of surface soil that regulates infiltration, runoff, and sediment loss on water application. We hypothesized that this could be confirmed from scanning electron micrographs (SEMs) of PAM-treated soil. Both untreated and PAM-treated soils form surface seals in irrigation furrows, but the stable surface structure of PAM-treated furrows is more pervious. This is thought to result from a greater number of continuous unblocked pores at the soil-water interface. SEMs of PAM-treated and untreated soil microstructures are presented from thin surface samples of Portneuf silt loam, collected from furrows immediately following an irrigation, and freeze-dried. SEMs of PAM-treated soil showed net or web-like microstructural surface coatings about 1 ym thick on soil mineral particles, giving a glue-like porous appearance. Individual strands of PAM were about 0.2 ym in diameter. Strands of PAM aggregated the soil by ensnaring and bridging mineral particles while untreated soil had poorly aggregated, unconnected particles. Thus, microstructural differences between PAM-treated and untreated soil from irrigation furrows were consistent with erosion and infiltration results.