|Ross, Craig - LANDCARE, NEW ZEALAND|
Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 1, 2003
Publication Date: October 1, 2003
Citation: Ross, C.W., Sojka, R.E., Foerster, J.A. 2003. Scanning electron micrographs of polyacrylamide-treated soil in irrigation furrows. Journal of Soil and Water Conservation. 58(5):327-331. Interpretive Summary: Polyacrylamide (PAM) is widely used for erosion control in irrigated agriculture. It is used at very low rates, usually reduces erosion by 90% or better, is environmentally safe, is relatively easy to apply and has very low cost. For these reasons about two million acres were estimated in 2003 to have been PAM-treated for erosion control. Although research on PAM-use for irrigation-induced erosion control has accumulated rapidly in the last decade, questions have remained about how PAM actually achieves soil stabilization in irrigation furrows. This series of images, obtained at high power and high resolution using scanning electron micrographs, shows the network of filamentous PAM strands that are created on the soil surface by applying dissolved PAM solutions in the irrigation water. These wispy polymer strands act like a fine net, binding particles and aggregates together, and preventing them from being detached and eroded by the flowing irrigation water. These scanning electron micrographs (SEMs) give the first direct evidence of how PAM does its job in treated irrigation furrows. The fine net is strong but porous, allowing water to infiltrate into the soil. The filaments can also be seen ensnaring microorganisms, whch PAM has been shown to sequester from runoff as well as mineral particles. These SEMS provide an excellent visual teaching tool for conveying the attributes of PAM that make it an excellent soil conservation tool.
Technical Abstract: Polyacrylamide (PAM) is used at rates of 1 to 2 kg/ha per irrigation on half a million hectares of US irrigated farmland to prevent 94% of irrigation-induced erosion and to enhance infiltration by 15 to 50% on medium- to fine- textured soils. The PAMs used for this application are large (12-15 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 hypothesised 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 micro-structural surface coatings about 1 µm thick on soil mineral particles, giving a glue-like porous appearance. Individual strands of PAM were about 0.2 µm in diameter. Strands of PAM aggregated the soil by ensnaring and bridging mineral particles. 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.