Aggregate stability as affected by polyacrylamide molecular weight, soil texture and water quality

Authors: Iliasson, Amrax; BECKMANN, S - UNIV OF REGENSBURG, GERMA; Huang, Chi Hua; LEVY, G J - AGRIC RES ORG, ISRAEL

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/18/2007
Publication Date: 11/12/2007
Citation: Mamedov, A.I., Beckmann, S., Huang, C., Levy, G.J. 2007. Aggregate stability as affected by polyacrylamide molecular weight, soil texture and water quality. Soil Science of America Journal. 71:1909-1918.

Interpretive Summary: Non toxic, anionic polyacrylamide (PAM) together with a source of electrolytes, such as gypsum, was found to be very effective in controlling seal formation, runoff and erosion because it slows both the physical disintegration of surface aggregates and the chemical dispersion of the soil clays. These potential reimbursements from using PAM for soil aggregate stability are affected due to complex relations among PAM properties (molecular weight) and soil properties (texture, solution chemistry). Some uncertainty exists regarding the issue of how PAM stabilizes soil aggregates: whether PAM penetrates into aggregates or whether it adsorbs on the aggregates’ exterior surfaces, and thus stabilizes only these exterior surfaces. We assumed that (i) the effects of PAM on soil aggregate stability may depend on the ability of PAM to penetrate into aggregates and thus stabilize both exterior and interior aggregate surfaces; and (ii) penetration of PAM into aggregates depends on PAM molecular weight (MW) and solution chemistry. We examined the impact of two PAM polymers having medium and high MW on the stability of soil aggregates from four semi arid soils varying in clay content. Aggregate stability, estimated from the sensitivity of the aggregates to slaking and was determined using distilled water and Gypsum solution. Two sizes of aggregates were used: 0.5-1.0 and 1.0-2.0 mm. Penetration of PAM into the aggregates was estimated comparing the stability of the small size aggregates to that of the large size aggregates. The results indicated that aggregate stability (i) increased with the increase in soil clay content, (ii) maintained a greater level with gypsum solution than distilled water, and (iii) was greater for the PAM-treated aggregates than the control. Aggregate stability was found to depend on the combined effects of PAM MW, solution salinity, and aggregate size. Because most of the PAM added to the aggregates was adsorbed on the exterior surfaces of the aggregates, the small fraction of the PAM that entered into the aggregates’ pores, had no significant impact on aggregate stability. Thus, to enhance aggregates’ resistance to slaking, it is enough to stabilize the exterior surfaces of the aggregates with PAM. Treating aggregates with PAM, irrespective of its MW, improved their stability in comparison to that of untreated aggregates. No specific PAM was selected as a preferable, because the effects of the PAM varied among the soils tested and depended on initial aggregate size and water type. The choice of PAM to be used as a soil amendment, however, needs to be made in accordance with the soil to be treated and the specific conditions prevailing in the field.

Technical Abstract: The favorable effects of the environmentally friendly, non toxic, anionic polyacrylamide (PAM) as a soil conditioner have long been established. However, some uncertainties exist regarding the effects of PAM molecular weight (MW) on its performance as a soil amendment and the ability of PAM to penetrate into aggregates and stabilize interior aggregates surfaces. We studied the effects of two anionic PAM polymers, a high-molecular-weight (12x106 Da) and a medium-molecular-weight (2x105 Da) PAM, using deionized water (electrical conductivity of 0.004 dS m-1) or a 15 mM gypsum solution, on the stability of aggregates from four smectitic soils varying in clay content. Penetration of PAM into the aggregates was estimated from treating 0.5-1.0 and 1.0-2.0 mm aggregates with PAM and thereafter comparing the stability of the small size aggregates to that of the large size aggregates after the latter had been crushed and sieved to obtain 0.5-1.0 mm aggregates. Aggregate stability, expressed as stability ratio (SR), ranged from 0.090 to 0.900. The SR tended to (i) increase with the increase in soil clay content, (ii) maintain, in the absence of PAM, greater level when electrolyte solution was used compared with the use of deionized water, and (iii) be greater for the PAM-treated aggregates compared with the control. In the finer- textured soils SR of the initially small aggregates was generally greater than that of the initially large aggregates, indicating that most of the PAM added to the aggregates was adsorbed on the exterior surfaces of the aggregates and only a small fraction of the PAM added, if any, entered into pores within the aggregates. A significant interaction among the treatments tested (PAM MW, aggregate size and solution ionic strength), with respect to their effect on the SR, was identified. Consequently, no specific PAM could have been singled out as the preferable.