|Huang, Chi hua|
Submitted to: World Congress of Soil Science
Publication Type: Abstract only
Publication Acceptance Date: 5/1/2006
Publication Date: 7/9/2006
Citation: Mamedov, A., Beckman, S., Huang, C., Levy, G.J. 2006. Effects of Polyacrylamide Molecular Weight, Soil Texture and Electrolyte Concentration on Drainable Porosity and Aggregate Stability. 18th World Congress of Soil Science. July 9-15, 2006. Philadelphia, PA. CDROM. Interpretive Summary:
Technical Abstract: The literature reports on the intricate relations between soil type and molecular weight (MW) of polyacrylamide (PAM) with respect to PAM efficacy as a soil conditioner. This relation may depend on the ability of PAM to penetrate into aggregates and thus stabilize both outer and inner aggregate surfaces or inter aggregate porosity. Our objective was to study the effects of the MW of PAM, soil clay content and electrolyte concentration of the soil solution on (i) the ability of PAM to stabilize inner aggregate surfaces, drainable porosity and (ii) total aggregate stability. Aggregates in the size of 0.5-1 or 1-2 mm from four Israeli smectitic soils (loam, Calcic Haploxeralf; sandy clay, Chromic Haploxerert and two clay soils, Typic Haploxerert) varying in clay content (22-65%), were studied. The aggregates were soaked in a polymer solution (200 mg L-1) for 24 h. Two types of anionic PAM were studied, a high-molecular-weight (12X106 Da) PAM designated PAM(H), and a medium-molecular-weight (2X105 Da) PAM designated PAM(M). After completion of the soaking, the aggregates were air-dried and the 1-2 mm aggregates were crushed to a size of 0.5-1 mm. Aggregate stability and drainable porosity was studied using the high-energy moisture characteristic (HEMC). In this method, accurately controlled wetting of the aggregates (i.e., the driving force for slaking) was the only force exerted on the aggregates. An index of aggregate stability, termed "stability ratio" (SR), was obtained by quantifying differences in moisture content curves (or pore size distribution up to 50 cm H2O tension) for wetting with distilled water (DW), or gypsum solution (GS, 2 mS cm-1) relative to wetting with ethanol and is presented on a relative scale of zero to one. In general, the aggregate stability of the soils increased with the increase in clay content, however the magnitude of the rise depended on aggregate size, MW of PAM and the electrolyte concentration of the solution used. The increase in aggregate stability was also coupled with a rise in moisture content of the saturated samples, serving as additional evidence that the stability of the aggregates was closely linked to prevention of aggregate slaking. Application of both types of PAM increased drainable porosity and aggregate stability compared with the control in all soils. Furthermore, for each soil, the stability of the initially small aggregates was significantly higher than that of crushed aggregates for the two polymers. This phenomenon was more distinct in DW than in GS treatments. The greater stability and drainable porosity of the PAM treated small aggregates indicated that the polymers stabilized mainly the outer surfaces of the aggregates. In the clay soils PAM(M) was more effective in stabilizing the small aggregates than PAM(H) when DW was used, suggesting that in the case of the PAM(M), some penetration into the aggregates and stabilization of inner aggregate surfaces occurred. When GS was used the effects of the two polymers were comparable. Increasing the salt content of the water had two effects on aggregate stability; use of GS increased the stability of crushed aggregates with initially lower stability, but decreased the stability of the small aggregates compared with DW. Our results indicate that the stability and drainable porosity of PAM-treated aggregates depended on the MW of PAM, clay content and the salinity of the soil solution. Stabilizing the aggregates under our experimental conditions involved complex relations among stabilization of aggregates' inner surfaces, impact of water salinity on the degree of PAM coiling, and the effect of electrolyte concentration on prevention of aggregate slaking.